39.Calvert, AJ; Doublier, MP.Migration of reflector orientation attributes in deep seismic profiles: evidence for decoupling of the Yilgarn Craton lower crust.Solid Earth, 2019, 10: 637-645 Migration of reflector orientation attributes in deep seismic profiles: evidence for decoupling of the Yilgarn Craton lower crust
Interpretation of deep seismic data is challenging due to the lack of direct geological constraints from drilling and the more limited amount of data available from 2-D profiles in comparison to hydrocarbon exploration surveys. Thus other constraints that can be derived from the seismic data themselves can be of great value. Though the origin of most deep seismic reflections remains ambiguous, an association between seismic reflections and crustal strain, e.g. shear zones, underlies many interpretations. Estimates of the 3-D orientation of reflectors may help associate specific reflections, or regions of the crust, with geological structures mapped at the surface whose orientation and tectonic history are known. In the case of crooked 2-D onshore seismic lines, the orientation of reflections can be estimated when the range of azimuths in a common midpoint gather is greater than approximately 20 degrees, but integration of these local orientation attributes into an interpretation of migrated seismic data requires that they also be migrated. Here we present a simple approach to the 2-D migration of these orientation attributes that utilizes the apparent dip in reflections on the unmigrated stack and maps reflector strike, for example, to a short linear segment depending on its original position and a migration velocity. This interpretation approach has been applied to a seismic line shot across the Younami Terrane of the Australian Yilgarn Craton and indicates that the lower crust behaved differently from the overlying middle crust as the newly assembled crust collapsed during the Late Archean. Some structures related to approximately east-directed shortening are preserved in the middle crust, but the lower crust is characterized by reflectors that suggest N-NNE-oriented ductile flow. Deployment of off-line receivers during seismic acquisition allows the recording of a larger range of source-receiver azimuths and should produce more reliable future estimates of these reflector attributes. DOI
38.Calvert, AJ; Doublier, MP.Archaean continental spreading inferred from seismic images of the Yilgarn Craton.Nat. Geosci., 2018, 11: 526-U93 Archaean continental spreading inferred from seismic images of the Yilgarn Craton
On the early Earth, oceanic plateaux similar to present-day Iceland are thought to have evolved into less dense microcontinents as they thickened by continued melt intrusion and crustal fractionation. These earliest continents may have been so weak on a hotter Earth that they collapsed laterally in response to thickening by further magmatic growth or tectonic imbrication. This continental spreading is likely to have resulted in the development of pervasive ductile strain fabrics in the deeper crust, which, if preserved, could generate seismic reflections. Here we present seismic images from the ancient core of the Archaean Yilgarn Craton of Australia that reveal shallowly dipping to horizontal reflections that pervade the middle and lower crust. We interpret these reflective fabrics as the result of widespread lateral crustal flow during the late stage of craton evolution approximately 2.66 to 2.61billion years ago, which coincided with the widespread intrusion of high-temperature crustal melts, as thickened early continental crust collapsed. The consequent subsidence of large regions of the upper crust, including volcanic and sedimentary greenstone rocks, in the hanging walls of listric mid-lower crustal ductile flow fabrics caused these rocks to drop beneath the granitic melts rising towards the surface, and did not involve Rayleigh-Taylor instabilities within a mostly mobile crust. DOI
37.Calvert, AJ.Continuous estimation of 3-D reflector orientations along 2-D deep seismic reflection profiles.Tectonophysics, 2017, 718: 61-71 Continuous estimation of 3-D reflector orientations along 2-D deep seismic reflection profiles
Seismic reflection; Crustal seismology; Archean tectonics
Interpretation of 2-D deep seismic reflection data can be limited by the recording of out-of-plane reflections that cannot be readily distinguished from those originating beneath a seismic line. Here I present a method analogous to semblance velocity analysis that utilizes varying source-receiver azimuths to derive continuous estimates of 3-D reflector orientations along onshore 2-D reflection profiles. For each zero-offset time within a common depth point supergather, the semblance is calculated along 3-D travel time curves, and the dip and strike of the most coherent reflection is determined. Relative errors in these angles are derived from the range of travel time curves that have semblance values greater than a specified fraction, for example 90%, of the maximum. The method is illustrated Using a section from line 10GA-YU1 from the Youanmi terrane of the Yilgarn craton in Australia in which the original field data have been replaced with synthetic in-line and cross-line reflections. Reflector orientations are generally well recovered where the range of available source-receiver azimuths is greater than 20 degrees, but the method fails at lower ranges where the seismic line is almost linear, and this behavior is also observed in analysis of the field data. Nevertheless when using a realistic 1-D velocity function the orientation of upper crustal shear zones can be readily determined, and on unmigrated sections subhorizontal sills can be distinguished on the basis of their geometry from the mid-crustal reflectivity. In future surveys, reflector orientations can be determined along the near-linear sections of a seismic profile by deploying additional receivers, perhaps as cross-line recording spreads, to supplement the limited range of azimuths available from the in-line acquisition. The method can in principle be extended to marine reflection surveys, and more complex sub-surface velocity models. (C) 2016 Elsevier B.V. All rights reserved. DOI
36.Calvert, AJ; Hayward, N; Vayavur, R; Colpron, M.Seismic and gravity constraints on the crustal architecture of the Intermontane terranes, central Yukon.Can. J. Earth Sci., 2017, 54: 798-811 Seismic and gravity constraints on the crustal architecture of the Intermontane terranes, central Yukon
In 2004, two seismic reflection lines were shot across the Mesozoic Whitehorse trough and adjacent terranes. Three-dimensional first-arrival tomographic inversion is used to constrain lithology to 800-1200 m depth, and surface structures are extrapolated into the middle crust using the coincident reflection data. In the Yukon-Tanana terrane, the metasedimentary Snowcap assemblage is characterized by velocities of 4.5-5.5 km/s, while in Quesnellia, velocities of 5.0-6.0 km/s occur at 500 m depth, and probably represent igneous rocks of the Tatchun batholith. Across the Whitehorse trough, velocities >4.0 km/s correspond to clastic rocks of the Jurassic Laberge and Triassic Lewes River groups; velocities <4.0 km/s probably present the clastic Jurassic to Cretaceous Tantalus Formation. Several near-surface units with velocities of 2.0-3.0 km/s are identified; some correlate well with volcanic rocks of the Upper Cretaceous Carmacks Group, but others could be attributable to alluvial deposits or faulting. The Big Salmon fault is interpreted to dip southwest, implying that rocks of the Yukon-Tanana terrane extend beneath Quesnellia. Stikinia and Quesnellia underlie up to 5-8 km of Triassic to Early Cretaceous sedimentary strata, and appear to be a single allochthon within an 18-20 km deep synform above the Yukon-Tanana terrane, which we name the Northern Intermontane synform. In general, reflection geometries in the upper crust are complex, but are consistent with large-scale imbricate structures that have been dissected into numerous blocks by displacement along moderately to steeply dipping strike-slip faults, which may be part of a crustal-scale flower structure extending to the base of the crust. DOI
35. Roots, E; Calvert, AJ; Craven, J.Interferometric seismic imaging around the active Lalor mine in the Flin Flon greenstone belt, Canada.Tectonophysics, 2017, 718: 92-104 Interferometric seismic imaging around the active Lalor mine in the Flin Flon greenstone belt, Canada
Seismic interferometry; Seismic reflection imaging; Lalor mining camp; Flin Flon greenstone belt; Volcanogenic massive sulphide
Seismic interferometry, which recovers the impulse response of the Earth by cross-correlation of ambient noise recorded at sets of two receivers, has found several applications, including the generation of virtual shot gathers for use in seismic reflection processing. To evaluate the effectiveness of this passive recording technique in mineral exploration in a hard-rock environment, 336 receivers recorded 300 h of ambient noise over the volcanogenic massive sulphide deposit of the recently discovered Lalor mine in the Canadian Flin Flon greenstone belt. A novel time-domain beamforming algorithm was developed to search for individual source locations, demonstrating that the vast majority of noise originated from the mine and ventilation shafts of the Lalor mine. The results of the beamforming were utilized in conjunction with frequency-wave number filtering to remove undesirable, mostly monochromatic surface wave noise originating from nearby sources. Virtual shot gathers were generated along three receiver lines, each of which was processed as a separate 2-D reflection line. Two of the resulting unmigrated reflection profiles are compared against coincident dipmoveout-stacked data from a larger, coincident 3-D dynamite seismic survey that was also acquired over the Lalor mine in 2013. Using knowledge of the local geology derived from numerous boreholes, coherent events recovered in the passive reflection profiles are inferred to be either spurious arrivals or real reflections, some of which can be interpreted in terms of geological contacts, indicating the future potential of passive recording surveys in hard rock settings. (C) 2017 Elsevier B.V. All rights reserved. DOI
34.Calvert, AJ.Seismic interpretation of crustal-scale extension in the Intermontane Belt of the northern Canadian Cordillera.Geology, 2016, 44: 447-450 Seismic interpretation of crustal-scale extension in the Intermontane Belt of the northern Canadian Cordillera
The crustal thickness of the Canadian Cordillera, which may have reached 50-65 km during the Cretaceous, is now only 32-38 km along its length. In the south, postorogenic extension during the Eocene resulted in the formation of core complexes and crustal-scale extensional shear zones, whereas in the north evidence for postorogenic extension is either limited to relatively minor basin formation in a transtensional environment or has not been recognized. Using new images of Lithoprobe seismic reflection line 2a in northern British Columbia, which has been previously interpreted in terms of terrane accretion, I make an alternative interpretation of crustal-scale postaccretion extension in the middle and upper crust. As early as the late Permian to Early Triassic, the Quesnellia and Stikinia arc terranes were accreted to ancestral North America along a crustal ramp, which is imaged in the lower crust. An extensional shear zone, which extends from close to the surface to 20 km depth, is identified from the geometry of reflections in the overlying structural basin and a lateral change in seismic velocity. The oceanic Cache Creek terrane, which is <4 km thick in the vicinity of line 2a, was preserved, because it was imbricated with its overlap assemblage and dropped into the 10-km-deep structural half-graben, which may correspond approximately to the combined extent of the Whitehorse trough and the Cache Creek terrane in southern Yukon and northern British Columbia. DOI
33. Farahbod, AM; Calvert, AJ; Cassidy, JF; Brillon, C.Coda Q in the Northern Cascadia Subduction Zone.Bull. Seismol. Soc. Amer., 2016, 106: 1939-1947 Coda Q in the Northern Cascadia Subduction Zone
Using seismograms recorded at 66 Canadian seismic stations, coda Q was estimated from earthquakes in southwestern British Columbia and northern Washington State, employing the single backscattering approximation. A total of 580 earthquakes with magnitudes ranging from 1.2 to 6.4, depths from 0 to 67 km, and epicentral distances of 5-110 km were selected to obtain 3022 high signal-to-noise ratio traces for analysis. An average of all the data yields a relationship for coda Q of Q(C) = 72 f(0.91). There is little variation of this coda Q relationship when using either crustal or in-slab sources, which represent uniform sampling of the crust and upper mantle. Crustal earthquakes result in a relationship of Q(C) = 73 f(0.89), and for in-slab events Q(C) can be expressed as Q(C) = 69 f(0.94). In general, Q(0) (Q(C) at 1 Hz) increases from the west coast of Vancouver Island to the east-southeast within the Coast belt. Stations on west-central Vancouver Island closest to the landward projection of the Nootka fault zone, and the location of the only two known large crustal earthquakes (1918 M similar to 7 and 1946 M similar to 7: 3) on Vancouver Island, have the lowest Q(0) values in our study area, suggesting a contrast in Q between the north and south of the island. DOI
32. Vayavur, R; Calvert, AJ.Mitigation of guided wave contamination in waveform tomography of marine seismic reflection data from southwestern Alaska.Geophysics, 2016, 81: B101-B118 Mitigation of guided wave contamination in waveform tomography of marine seismic reflection data from southwestern Alaska
We have applied 2D frequency-domain acoustic waveform tomography to two different sections of a marine seismic reflection line from southwest Alaska: one section with a deep igneous basement overlain by a thick pile of sediments and the other section with a shallow basement and a thin sedimentary cover. We have evaluated the appearance of dispersive guided waves on both sections, and we have determined that with appropriate data preconditioning it is possible to invert the data using 2D acoustic waveform tomography. Where the basement is deep, we first reduced the dispersive wave contamination of the seismic field data by trace editing, band-pass filtering, and careful choice of the data window for inversion. We then tested different objective functions and inversion scheduling before selecting an approach based on the logarithmic phase, which could be followed by joint phase and amplitude inversion. Where the basement is shallow, the starting model itself, which was generated by ray-based first-arrival tomography, generated acoustic guided waves, necessitating the use of an absorbing boundary condition at the free surface. Logarithmic phase inversion was used, but the amplitude inversion did not converge. To invert seismic data from both sections, we used a layer stripping strategy in which the gradient was used at each stage of the inversion process to check the corresponding model updates. Our results were validated by comparison between synthetic and observed waveforms, comparison of residual phase error plots for the initial and final velocity models, and comparison of waveform tomography velocity models with migrated images. Waveform tomography permits interpretation of the subsurface close to the seafloor where reflection images are contaminated by water-layer multiples, and we inferred the existence of a fault zone from a low-velocity anomaly within the igneous basement. DOI
31. Gazel, E; Hayes, JL; Hoernle, K; Kelemen, P; Everson, E; Holbrook, WS; Hauff, F; van den Bogaard, P; Vance, EA; Chu, SY; Calvert, AJ; Carr, MJ; Yogodzinski, GM.Continental crust generated in oceanic arcs.Nat. Geosci., 2015, 8: 321-327 Continental crust generated in oceanic arcs
Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intra-oceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10Myr ago-coincident with the onset of subduction of more oceanic crust that originally formed above the Galapagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone-a process probably more common in the Archaean-can produce juvenile continental crust. DOI
30.Calvert, AJ; Talinga, D.Deep seismic reflection constraints on Paleogene crustal extension in the south-central Intermontane belt, British Columbia.Can. J. Earth Sci., 2014, 51: 393-406 Deep seismic reflection constraints on Paleogene crustal extension in the south-central Intermontane belt, British Columbia
Following growth of the Canadian Cordillera during the Mesozoic, the southern Cordillera was subject to extension during the Paleocene and Eocene that correlated with widespread volcanic activity in south- central British Columbia, including across much of the Nechako- Chilcotin plateau. In 2008, Geoscience BC acquired 330 km of deep vibroseis reflection profiles on the plateau, mostly over the Stikinia arc terrane, but also over its eastern contact with the oceanic Cache Creek terrane. All seven seismic reflection lines reveal a strongly reflective lower crust that extends from 7 to 9 s down to the Moho, which is defined by the downward termination of reflectivity at 11- 12 s. In the uppermost crust, extension occurred by block faulting with faults soling into subhorizontal to shallowly dipping detachments above 10 km depth. Extension in the deeper upper and middle crust, which was partly controlled by antiforms likely related to earlier shortening, was accommodated on a network of anastomosing shear zones that sole out into the top of the reflective lower crust. The lower crustal reflections correlate with seismic P- wave velocities of 6.45- 6.98 km/ s, indicating that the reflective lower crust has a more mafic composition than the middle crust. As in other extensional settings, we suggest that this pervasive fabric of reflectors arises from the intrusion of mantle- derived basaltic magma into zones of ductile shearing, and that differentiation of these melts resulted in the widespread Paleocene to Eocene volcanism. Reflector dips indicate that extension was approximately east- west, consistent with north- northwest- trending horsts separated by basins filled with Paleocene to Eocene volcanic and volcaniclastic rocks. DOI
29. Talinga, D; Calvert, AJ.Distribution of Paleogene and Cretaceous rocks around the Nazko River belt of central British Columbia from 3-D long-offset first-arrival seismic tomography.Can. J. Earth Sci., 2014, 51: 358-372 Distribution of Paleogene and Cretaceous rocks around the Nazko River belt of central British Columbia from 3-D long-offset first-arrival seismic tomography
Across the Nechako-Chilcotin plateau of British Columbia, the distribution of Cretaceous sedimentary rocks, which are considered prospective for hydrocarbon exploration, is poorly known due to the surface cover of glacial deposits and Tertiary volcanic rocks. To constrain the subsurface distribution of these Cretaceous rocks, in 2008 Geoscience BC acquired seven long, up to 14.4 km, offset vibroseis seismic reflection lines across a north-northwest-trending belt of exhumed sedimentary rocks inferred to be part of the Taylor Creek Group. P-wave velocity models, which are consistent with sonic logs from nearby wells, have been estimated using three-dimensional first-arrival tomography to depths ranging from 1 to 4 km. Igneous basement can be identified on most lines using the 5.5 km/s isovelocity contour, which locates the top of the basement to an accuracy of similar to 400 m where its depth is known in exploration wells. There is no general distinction on the basis of seismic velocity between Cretaceous sedimentary and Paleocene-Eocene volcanic-volcaniclastic rocks, both of which appear to be characterized in the tomographic models by velocities of 3.0-5.0 km/s. The geometry of the igneous basement inferred from the velocity models identifies north-trending basins and ridges, which correlate with exposed rocks of the Jurassic Hazelton Group. Identified Cretaceous sedimentary rocks occur beneath less negative Bouguer gravity anomalies, but the original distribution of these rocks has been disrupted by later Tertiary extension that created north-trending basins associated with the most negative gravity anomalies. We suggest that Cretaceous sedimentary rocks, if deposited, could be preserved within these basins if the rocks had not been eroded prior to Tertiary extension. DOI
28.Calvert, AJ; McGeary, SE.Seismic reflection imaging of ultradeep roots beneath the eastern Aleutian island arc.Geology, 2013, 41: 203-206 Seismic reflection imaging of ultradeep roots beneath the eastern Aleutian island arc
Seismic reflection data show that the eastern Aleutian Arc is characterized by reflectors that extend continuously from the lower arc crust to >50 km depth, which is considerably deeper than the crustal thickness of 27-35 km previously inferred from coincident wide-angle seismic surveys. Because the upper mantle is commonly homogeneous, and therefore nonreflective, relative to the overlying crust, we interpret these reflectors to be gabbro, garnet gabbro, and pyroxenite intrusions within two 50-km-wide roots that represent a >25-km-thick heterogeneous transition from mafic lower crustal rocks to ultramafic mantle rocks. We suggest that the reflectivity is linked to repeated differentiation and intrusion of mantle-derived melts into the subarc lithosphere, and that the depth of these roots shows that fractionation of arc crust can extend well below the seismically determined Moho. Because these deep roots are not evident beneath the central Aleutian Arc, either the roots form sporadically, perhaps as a consequence of an elevated magmatic supply, or such roots ultimately founder into the underlying mantle due to their relatively high mass density. DOI
27. Takougang, EMT; Calvert, AJ.Seismic waveform tomography across the Seattle Fault Zone in Puget Sound: resolution analysis and effectiveness of visco-acoustic inversion of viscoelastic data.Geophys. J. Int., 2013, 193: 763-787 Seismic waveform tomography across the Seattle Fault Zone in Puget Sound: resolution analysis and effectiveness of visco-acoustic inversion of viscoelastic data
Tomography; Seismic attenuation; Wave propagation; Acoustic properties
Visco-acoustic waveform tomography was applied to marine seismic reflection data across the Seattle fault zone in Puget Sound in the northwestern USA. Using the recovered velocity and attenuation models, we performed a set of synthetic visco-acoustic and viscoelastic checkerboard tests, and compared the results to verify the effectiveness of applying visco-acoustic waveform tomography to viscoelastic field data. Visco-acoustic waveform tomography produces higher resolution velocity and attenuation models than ray-based tomography, but artefacts due to elastic effects such as mode conversion are present at layer interfaces where the velocity contrast is high. Elastic effects also affect attenuation values, which can be too high or too low in places because visco-acoustic inversion compensates the loss of amplitude due to mode conversion by inadequately estimating the attenuation. A comparison of the attenuation models inverted from viscoelastic and visco-acoustic synthetic data suggests that inverted attenuation values can be reliable when the velocity gradient is low, and the quality of the inversion improves in a highly attenuating medium or in a medium with high attenuation contrasts. Joint interpretation of the derived velocity and attenuation models enables us to identify Quaternary (glacial and postglacial Pleistocene) sedimentary, Tertiary sedimentary and Eocene volcanic rocks. Several shallow faults, anticlines and a syncline are identified across the Seattle uplift and the Seattle fault zone. Our interpretation of faults using the velocity model, attenuation model and migrated seismic section is consistent with two possible published models of the Seattle Fault Zone: either a thrust fault that accommodates north-south shortening by forming a fault-propagation fold with a forelimb breakthrough, or part of a passive roof duplex in which the Seattle Fault Zone is located at the leading edge of a triangle zone that is propagating into the Seattle basin. DOI
26. Singh, SC; Chauhan, APS; Calvert, AJ; Hananto, ND; Ghosal, D; Rai, A; Carton, H.Seismic evidence of bending and unbending of subducting oceanic crust and the presence of mantle megathrust in the 2004 Great Sumatra earthquake rupture zone.Earth Planet. Sci. Lett., 2012, 321: 166-176 Seismic evidence of bending and unbending of subducting oceanic crust and the presence of mantle megathrust in the 2004 Great Sumatra earthquake rupture zone
megathrust; subduction zone; Sumatra earthquake; seismogenic zone; underplating; thrust faulting
In subduction zones the plate interface (megathrust) is typically poorly imaged at depths> 12 km, however its precise geometry and nature as well as the positions of updip and downdip limits of the seismogenic zone are important elements to understand the generation of megathrust earthquakes. Using deep marine seismic reflection and refraction data, we observed discontinuous reflections off the top of the subducting oceanic crust down to 60 km depth in the 2004 great Sumatra-Andaman earthquake rupture zone. We find that the top of the downgoing plate does not dip gently into the subduction zone but instead displays a staircase geometry with three successive, 5-15 km vertical steps, spaced similar to 50 km apart. Micro-earthquake data indicate that most of the seismicity lies below this interface, suggesting that the oceanic plate is deforming actively. Along part of the profile, we also image a second reflector located 8-10 km below the top of the oceanic crust. The forward modelling of the gravity data along the profile supports the presence of a high-density material above this reflector. The presence of a staircase shape for the top of the oceanic crust together with constraints from gravity data and earthquake data, require that the megathrust goes through this second reflector. This leads us to conclude that the megathrust is at least partly located in the oceanic mantle and that underplating of oceanic crust beneath the wedge and underplating of upper mantle beneath the forearc basin are taking place in this region. (C) 2011 Elsevier B.V. All rights reserved. DOI
25. Takougang, EMT; Calvert, AJ.Seismic velocity and attenuation structures of the Queen Charlotte Basin from full-waveform tomography of seismic reflection data.Geophysics, 2012, 77: B107-B124 Seismic velocity and attenuation structures of the Queen Charlotte Basin from full-waveform tomography of seismic reflection data
We applied viscoacoustic waveform tomography to four seismic reflection lines from the central and northern part of the Queen Charlotte sedimentary basin and, using frequencies of 7-12 Hz, we estimated the compressional velocity and attenuation above a depth of approximately 1.2 km. We refined our previously published inversion strategy by alternating between phase-only and amplitude-plus-phase velocity inversion for the first two pairs of frequencies used, and added a second step, in which we inverted for attenuation from the lowest frequency using the final recovered velocity model and an initial homogeneous Q(p)-model. Our recovered velocity and attenuation models demonstrated an overall good correlation with the available sonic and gamma-ray logs. Modeled seismic data matches the field data well and ID velocity and attenuation profiles extracted at line intersections show a good correlation, thus demonstrating the robust nature of the results. Recovered velocities aid in interpreting shallow structures not readily identifiable on the conventional migration such as Quaternary strata and Pliocene faulting. Recovered attenuation values in the sedimentary rocks are generally consistent with saturated sandstones and consistent with the geology interpreted from well logs. Localized regions of elevated attenuation and associated low velocities correlate with siltstones and shales, the presence of hydrocarbons, or inferred increases in porosity due to fracturing. Seafloor pockmarks, where venting of gas occurs, are underlain by low velocities and an anomalous attenuation variation, and pipe-like gas chimneys are interpreted in two other areas of Hecate Strait. Igneous basement is associated with high velocity and high attenuation in its uppermost part, suggesting the presence of volcanic rocks, but the elevated attenuation may also be clue to scattering and elastic mode conversions not included in the viscoacoustic inversion. DOI
24.Calvert, AJ; Hayward, N; Spratt, JE; Craven, JA.Seismic reflection constraints on upper crustal structures in the volcanic-covered central Nechako basin, British Columbia.Can. J. Earth Sci., 2011, 48: 1021-1037 Seismic reflection constraints on upper crustal structures in the volcanic-covered central Nechako basin, British Columbia
In 2008, a Vibroseis seismic reflection survey was acquired by Geoscience BC across the eastern part of the volcanic-covered Nechako basin in central British Columbia, where Cretaceous sedimentary rocks have been exhumed along a NNW trend. Good signal penetration through the volcanic cover is indicated by lower crustal reflections at 8-12 s, which were recorded by the entire seismic survey. Comparison of the 2008 seismic survey with data from a previous survey indicates that the lack of reflectivity in the earlier surveys is generally representative of the subsurface geology. The seismic data show that similar to 1700 and similar to 2900 m thick sub-basins are present at the northern and southern ends of this trend, but the intervening Cretaceous rocks are discontinuous and relatively thin. The creation of a passive-roof duplex by Campanian or later low-angle thrusting is inferred within the thickest Cretaceous strata, but elsewhere faulting is likely related to Eocene extension or transtension. Seismic reflections are also recorded from folded volcanic stratigraphy, the base of the surface volcanic rocks, an underlying volcaniclastic stratigraphy, and intrusions projecting into a Quaternary volcanic cone. Seismic interpretation is complemented by coincident audiofrequency magnetotelluric surveys, from which faulting is inferred at offsets in a regional conductor. No regionally extensive stratigraphy can be identified within the seismic data, and the central Nechako basin appears to be a complex network of small, deformed sub-basins, rather than a single large basin. DOI
23.Calvert, AJ; Preston, LA; Farahbod, AM.Sedimentary underplating at the Cascadia mantle-wedge corner revealed by seismic imaging.Nat. Geosci., 2011, 4: 545-548 Sedimentary underplating at the Cascadia mantle-wedge corner revealed by seismic imaging
Earth's largest earthquakes occur in subduction zones, along the boundary between the subducting and overriding plates(1). Non-volcanic tremor generated by slow slip between the plates is thought to originate on, or near, this boundary(2,3). Earthquakes also occur in the down-going plate as fluids are released(4), and zones of anomalously low seismic velocities observed beneath several subduction zones are interpreted to be the subducting oceanic crust(5-10). Yet, the exact location of the plate boundary remains uncertain(5). Here we interpret a three-dimensional seismic tomography model from the northern Cascadia subduction zone in the northwest USA. We find that the low-velocity zone varies considerably along the Cascadia margin. In places, we observe the low-velocity zone to crop out at the surface and separate from the descending plate at depths of 35-40 km. We argue that the low-velocity zone here cannot represent oceanic crust as previously suggested, and instead the zone mostly represents sediments that have been subducted and underplated beneath the North American continent. We also find that tremor signals correlate with the position of the low-velocity zone, implying that slow slip and tremor may be facilitated by trapped fluids and high pore fluid pressures in subducted sedimentary rocks at, or close to the plate boundary. Our results also imply that the plate boundary beneath Cascadia is much deeper than previously thought. DOI
22. Hayward, N; Calvert, AJ.Interpretation of structures in the southeastern Nechako Basin, British Columbia, from seismic reflection, well log, and potential field data.Can. J. Earth Sci., 2011, 48: 1000-1020 Interpretation of structures in the southeastern Nechako Basin, British Columbia, from seismic reflection, well log, and potential field data
The structure and stratigraphy of the southeast Nechako Basin, which are poorly understood primarily because of substantial volcanic cover, are investigated in an analysis of seismic reflection, well, and potential field data. Formation and development of the SE Nechako Basin resulted in sub-basins containing Cretaceous and Eocene rocks. Interpretation reveals that dextral transtension in the Early to Middle Eocene created NNW-trending, en echelon, strike-slip faults linked by pull-apart basins, which locally contain a thickness of Eocene volcaniclastic rocks of >3 km. This structural pattern is consistent with regional observations that suggest the transfer of slip from the Yalakom fault to the north via a series of en echelon strike-slip faults. In the Middle to Late Eocene, faults associated with a change in the direction of stress, echoed by the north-trending right-lateral Fraser fault, reactivated and cut earlier structures. A simple model agrees with local observations, that northeast-directed compression was subparallel to the relic Cretaceous grain. Cretaceous rocks are discontinuous throughout the basin and may be remnants of a broader basin, or a number of contemporaneous basins, formed in a regional transpressional tectonic setting that caused northeast-directed thrusting along the eastern side of the Coast Plutonic Complex. Results suggest that thrusting affected most of the SE Nechako Basin, as observed across the Intermontane Belt to the northwest and southeast. The pattern of deposition of Neogene volcanic rocks of the Chilcotin Group was in part controlled by the Eocene structural grain, but we find no evidence of Neogene deformation. DOI
21. Takougang, EMT; Calvert, AJ.Application of waveform tomography to marine seismic reflection data from the Queen Charlotte Basin of western Canada.Geophysics, 2011, 76: B55-B70 Application of waveform tomography to marine seismic reflection data from the Queen Charlotte Basin of western Canada
To obtain a higher resolution quantitative P-wave velocity model, 2D waveform tomography was applied to seismic reflection data from the Queen Charlotte sedimentary basin off the west coast of Canada. The forward modeling and inversion were implemented in the frequency domain using the visco-acoustic wave equation. Field data preconditioning consisted of f-k filtering, 2D amplitude scaling, shot-to-shot amplitude balancing, and time windowing. The field data were inverted between 7 and 13.66 Hz, with attenuation introduced for frequencies >= 10.5 Hz to improve the final velocity model; two different approaches to sampling the frequencies were evaluated. The limited maximum offset of the marine data (3770 m) and the relatively high starting frequency (7 Hz) were the main challenges encountered during the inversion. An inversion strategy that successively recovered shallow-to-deep structures was designed to mitigate these issues. The inclusion of later arrivals in the waveform tomography resulted in a velocity model that extends to a depth of approximately 1200 m, twice the maximum depth of ray coverage in the ray-based tomography. Overall, there is a good agreement between the velocity model and a sonic log from a well on the seismic line, as well as between modeled shot gathers and field data. Anomalous zones of low velocity in the model correspond to previously identified faults or their upward continuation into the shallow Pliocene section where they are not readily identifiable in the conventional migration. DOI
20. Hayward, N; Calvert, AJ.Eocene and Neogene volcanic rocks in the southeastern Nechako Basin, British Columbia: interpretation of the Canadian Hunter seismic reflection surveys using first-arrival tomography.Can. J. Earth Sci., 2009, 46: 707-720 Eocene and Neogene volcanic rocks in the southeastern Nechako Basin, British Columbia: interpretation of the Canadian Hunter seismic reflection surveys using first-arrival tomography
Il est difficile d'effectuer une etude geologique a faible profondeur dans le sud-est du bassin de Nechako. La pietre qualite de l'imagerie de la sismique reflexion a faible profondeur est en partie due a la grande couverture par des roches volcaniques datant de l'Eocene et du Neogene. Les affleurements de ces roches volcaniques et du socle, datant principalement du Cretace, sont souvent caches par des depots du Quaternaire et de la vegetation. Les estimes de la vitesse de l'onde P a faible profondeur proviennent d'une inversion tomographique des premieres ondes arrivees, un outil efficace lorsque l'imagerie sismique est de faible qualite. Les vitesses du modele tomographique concordent avec la diagraphie sonique et les echantillons de laboratoire, sauf pour ceux provenant du Groupe de Chilcotin (Neogene). Des roches sedimentaires datant du Cretace ont des vitesses d'environ 2800-4200 ms(-1). Les roches des groupes de Endako et d'Ootsa Lake (Eocene), lesquelles ont des vitesses d'environ 3000-4200 ms(-1), ne peuvent pas etre differenciees en se basant sur les vitesses. La vitesse, le caractere (densite, foyer et profondeur de penetration) des rayons et les liens avec la geologie de surface et des puits limitent l'etendue sous la surface du Groupe d'Endako a proximite du puits b-82-C. Le Groupe de Chilcotin donne habituellement des vitesses (similar to 2400-3000 ms(-1)) inferieures aux vitesses correspondantes provenant de diagraphies soniques (4500-5200 ms(-1)) et de mesures en laboratoire (5000-5200 ms(-1)). Ces basses vitesses pour le modele peuvent etre causees par les roches brechiques a porosite elevee pres de la surface par rapport aux autres mesures qui ciblaient des laves massives a porosite moindre. Les vitesses moyennes les plus basses, situees au sud-est, sont reliees aux depots brechiques anormalement epais a porosite elevee du Groupe de Chilcotin. Cette conclusion concorde avec l'interpretation que le Groupe de Chilcotin est plus epais dans les vallees des anciennes rivieres. DOI
19.Calvert, AJ; Klemperer, SL; Takahashi, N; Kerr, BC.Three-dimensional crustal structure of the Mariana island arc from seismic tomography.J. Geophys. Res.-Solid Earth, 2008, 113 Three-dimensional crustal structure of the Mariana island arc from seismic tomography
[1] A three-dimensional (3-D) seismic refraction survey was acquired over the Mariana volcanic arc at 14.5-18.5 degrees N and 145-147 degrees E. First-arrival traveltimes from this survey and from a separate 2-D survey acquired approximately perpendicular to the arc have been simultaneously inverted for a 3-D P wave velocity model using seismic tomography subject to smoothness constraints. The active arc, which initiated only 3-4 Ma ago, has an average crustal thickness of 18 km. Approximately 40 km to the east the inactive remnant of the rifted Eocene arc has an average crustal thickness of 21 km, due primarily to a thicker lower-crustal layer with velocities of 6.5-7.0 km s(-1). Crustal production clearly varies both temporally and spatially, with some crustal layers, including the igneous fore-arc crust, varying in thickness by a factor of up to 2 along strike. Average P wave velocities within the upper crust of the modern arc are 240-360 m s(-1) lower than in the Eocene arc but are 280 m s(-1) higher within the lower crust. Middle crust with velocities of 6.0-6.5 km s(-1) is best developed beneath the Eocene arc. These results suggest an evolution of arc structure with increasing age: We infer closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the middle to lower crust as a result of intracrustal differentiation. Although tonalitic rocks may predominate in the transition from upper to middle crust, the bulk of the crust is essentially basaltic. DOI
18. Hayward, N; Calvert, AJ.Seismic reflection and tomographic velocity model constraints on the evolution of the Tofino forearc basin, British Columbia.Geophys. J. Int., 2007, 168: 634-U2 Seismic reflection and tomographic velocity model constraints on the evolution of the Tofino forearc basin, British Columbia
Cascadia forearc; seismic stratigraphy; Tofino Basin; tomography
The Tofino Basin is a sedimentary forearc basin that overlies the continental shelf of the Cascadia margin to the southwest of Vancouver Island. The basin, which contains up to similar to 4 km of marine clastic sedimentary rocks, formed following accretion in the Early Eocene of the Crescent and Pacific Rim Terranes, and subsequent accretionary wedge basement. Subduction of the Juan de Fuca plate has since been the primary tectonic driving force in the development of the basin's structure. Investigations using coincident seismic reflection profiles, tomographic velocity models and recently reassessed biostratigraphic well data show that basement composition has largely controlled deformation of the overlying Tofino Basin sediments. Anticlinal folds overlying the accretionary wedge exhibit low P-wave velocities at the apex of the fold, which may be related to fracturing of older, more lithified sediments accompanied by fluid expulsion from the accretionary wedge. In contrast the velocity variation across folds over the Crescent Terrane mimics the fold geometry, and does not appear anomalous. A sub-basin (containing up to similar to 3 km of Oligocene to Holocene sediment) has developed in the central part of the Tofino Basin at the boundary between the Crescent and Pacific Rim Terranes. Seismic interpretation suggests that deposition has increased more rapidly in the Late Miocene to Holocene. Subsidence within the sub-basin is likely to have been controlled by sediment loading, flexure and regional tectonic forces, localized by pre-existing zones of weakness such as the Tofino Fault. The development of the sub-basin may also have been influenced by the displacement landward of part of the lower forearc crust during subduction erosion. Diapiric structures along the axis of the sub-basin suggest that fluid expulsion into the Tofino Basin from the deeper accreted terranes is localized by the terrane-bounding fault. Further seaward, fluid expulsion from the accretionary wedge may be more pervasive. The seismic data demonstrate that subsidence of the sub-basin beneath the inner continental shelf has been occurring relatively late in the history of the Tofino Basin, and that the Cascadia accretionary complex cannot be viewed as growing gradually seaward with no inboard deformation. DOI
17.Calvert, AJ; Ramachandran, K; Kao, H; Fisher, MA.Local thickening of the Cascadia forearc crust and the origin of seismic reflectors in the uppermost mantle.Tectonophysics, 2006, 420: 175-188 Local thickening of the Cascadia forearc crust and the origin of seismic reflectors in the uppermost mantle
cascadia; subduction; erosion; underplating; seismic reflection; tremor; duplex
Seismic reflection profiles from three different surveys of the Cascadia forearc are interpreted using P wave velocities and relocated hypocentres, which were both derived from the first arrival travel time inversion of wide-angle seismic data and local earthquakes. The subduction decollement, which is characterized beneath the continental shelf by a reflection of 0.5 s duration, can be traced landward into a large duplex structure in the lower forearc crust near southern Vancouver Island. Beneath Vancouver Island, the roof thrust of the duplex is revealed by a 5-12 km thick zone, identified previously as the E reflectors, and the floor thrust is defined by a short duration reflection from a < 2-km-thick interface at the top of the subducting plate. We show that another zone of reflectors exists east of Vancouver Island that is approximately 8 kin thick, and identified as the D reflectors. These overlie the E reflectors; together the two zones define the landward part of the duplex. The combined zones reach depths as great as 50 km. The duplex structure extends for more than 120 km perpendicular to the margin, has an along-strike extent of 80 kin, and at depths between 30 km and 50 km the duplex structure correlates with a region of anomalously deep seismicity, where velocities are less than 7000 m s(-1). We suggest that these relatively low velocities indicate the presence of either crustal rocks from the oceanic plate that have been underplated to the continent or crustal rocks from the forearc that have been transported downward by subduction erosion. The absence of seismicity from within the E reflectors implies that they are significantly weaker than the overlying crust, and the reflectors may be a zone of active ductile shear. In contrast, seismicity in parts of the D reflectors can be interpreted to mean that ductile shearing no longer occurs in the landward part of the duplex. Merging of the D and E reflectors at 42-46 km depth creates reflectivity in the uppermost mantle with a vertical thickness of at least 15 km. We suggest that pervasive reflectivity in the upper mantle elsewhere beneath Puget Sound and the Strait of Georgia arises from similar shear zones. (c) 2006 Elsevier B.V. All rights reserved. DOI
16. Hayward, N; Nedimovic, MR; Cleary, M; Calvert, AJ.Structural variation along the Devil's Mountain fault zone, northwestern Washington.Can. J. Earth Sci., 2006, 43: 433-446 Structural variation along the Devil's Mountain fault zone, northwestern Washington
The eastern Juan de Fuca Strait is subject to long-term, north-south-oriented shortening. The observed deformation is interpreted to result from the northward motion of the Oregon block, which is being driven north by oblique subduction of the oceanic Juan de Fuca plate. Seismic data, acquired during the Seismic Hazards Investigation in Puget Sound survey are used, with coincident first-arrival tomographic velocities, to interpret structural variation along the Devil's Mountain fault zone in the eastern Juan de Fuca Strait. The Primary fault of the Devil's Mountain fault zone developed at the northern boundary of the Everett basin, during north-south-oriented Tertiary compression. Interpretation of seismic reflection data suggests that, based on their similar geometry including the large magnitude of pre-Tertiary basement offset, the Primary fault of the Devil's Mountain fault west of similar to 122.95 degrees W and the Utsalady Point fault represent the main fault of the Tertiary Devil's Mountain fault zone. The Tertiary Primary fault west of similar to 122.95 degrees W was probably kinematically linked to faults to the east:(Utsalady Point, Devil's Mountain, and another to the south), by an oblique north-northeast-trending transfer zone or ramp. Left-lateral transpression controlled the Quaternary evolution of the Devil's Mountain fault zone. Quaternary Primary fault offsets are smaller to the east of similar to 122.95 degrees W, suggesting that stress here was in part accommodated by the prevalent oblique compressional structures to the north. Holocene deformation has focussed on the Devil's Mountain, Utsalady Point, and Strawberry Point faults to the east of similar to 122.8 degrees but has not affected the Utsalady Point fault to the west of similar to 122.8 degrees W. DOI
15. Royle, GT; Calvert, AJ; Kao, H.Observations of non-volcanic tremor during the northern Cascadia slow-slip event in February 2002.Geophys. Res. Lett., 2006, 33 Observations of non-volcanic tremor during the northern Cascadia slow-slip event in February 2002
We locate in the Cascadia subduction zone non-volcanic tremors from an episodic tremor and slip event in February 2002. The tremors occurred during two 10- day periods separated by a short lull. Tremors that occurred during the first period are broadly distributed between 10 km and 40 km depth at the southern end of Vancouver Island, and over a > 50 km wide region measured normal to the margin. During the second period of tremor activity, most tremors were concentrated in a narrower zone, and many tremors occurred near the subduction megathrust at approximately 35 km depth. This relatively large number of tremors near the subduction megathrust is in contrast to the broad depth distribution of the subsequent March 2003 tremor sequence. The change in the pattern of tremor activity observed during the February 2002 sequence may indicate a change in the slow slip process, or in its migration along the margin. DOI
14. Zeng, FF; Calvert, AJ.Imaging the upper part of the Red Lake greenstone belt, northwestern Ontario, with 3-D traveltime tomography.Can. J. Earth Sci., 2006, 43: 849-863 Imaging the upper part of the Red Lake greenstone belt, northwestern Ontario, with 3-D traveltime tomography
Seismic reflection line 2B was shot across the Archean Red Lake greenstone belt and Sydney Lake fault zone that marks the northern boundary of the English River metasedimentary belt, as part of the Western Superior Lithoprobe transect. Three-dimensional tomographic inversion of first arrival traveltimes recorded in this survey delineate the subsurface to depths as great as 1.5 km around this crooked two-dimensional seismic line. Within the Red Lake greenstone belt, P-wave velocities of 6.2-7.0 km s(-1) occur at 500 m depth in the Mesoarchean Balmer assemblage, clearly distinguishable from the lower velocities of 5.1-6.1 km s(-1) of the Neoarchean Confederation assemblage. Although the overall range of velocities in the metasedimentary rocks of the English River subprovince is similar to that found in the Confederation assemblage, lower velocities of 5.1-5.4 km s(-1) are found in the upper 300 m of the metasedimentary rocks. In particular, two 2-3 km wide, east-northeast-striking zones of low velocity are associated with the Sydney Lake fault zone and the Pakwash Lake fault zone. Correlation of the velocities with the coincident reflection section suggests that these two faults delineate a fault-bounded block in the hanging wall of a more northerly fault zone that crops out within the Uchi subprovince. Anomalous regions of low velocity, which occur at the boundary between the Confederation and Balmer assemblages, and within the Balmer assemblage, may also be related to shear zones that have minimal near-surface expression, felsic lithologies, or hydrothermal alteration of the basalts. DOI
13.Calvert, AJ.A method for avoiding artifacts in the migration of deep seismic reflection data.Tectonophysics, 2004, 388: 201-212 A method for avoiding artifacts in the migration of deep seismic reflection data
migration; seismic reflection; artifacts; smiles
Conventional wave-equation-based migration of deep seismic reflection data can produce severe artifacts, which appear as broad circular arcs or "smiles", due to the existence of apparent truncations of reflections on the stack section arising from poor signal penetration, changes in orientation of the acquisition profile, and the existence of strong overlying lateral velocity variations. These artifacts limit the interpretation of deep seismic profiles, because they obscure weak reflections and reflection truncations that may, e.g., indicate the presence of subsurface faults. Here I present a new migration algorithm, in which each sample of the stack is migrated to a short linear segment whose position and dip are determined by its original position on the stack, an estimate of the local apparent dip at that point, and a user-specified migration velocity. No subjective interpretation of reflections on the stack section is required, and the algorithm produces no arc-like migration artifacts. The degree of lateral smearing can be easily controlled, allowing reflection truncations to be revealed. In practice, the algorithm is most effectively applied to data that have been coherency-filtered to remove low amplitude noise, which would otherwise be preserved. (C) 2004 Elsevier B.V. All rights reserved. DOI
12.Calvert, AJ.Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone.Nature, 2004, 428: 163-167 Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone
At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust(1). Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together(2). Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent(3-7). Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, similar to110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there(8,9). DOI PubMed
11.Calvert, AJ; Cruden, AR; Hynes, A.Seismic evidence for preservation of the Archean Uchi granite-greenstone belt by crustal-scale extension.Tectonophysics, 2004, 388: 135-143 Seismic evidence for preservation of the Archean Uchi granite-greenstone belt by crustal-scale extension
Uchi; greenstone belt; extension; seismic reflection; superior province
In the westernmost Superior Province of Canada, the east-west alignment of granite-greenstone belts and the adjacent, highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2.5 Ga ago, with the belts thrust against one another by east-west-oriented subduction zones. Here, we present seismic reflection data, which demonstrate that in this region the present juxtaposition of the Uchi granite-greenstone belt and the North Caribou gneiss terrane occurred along a late southeast-dipping extensional shear zone that extends from the surface into the lower crust. The preservation of the Uchi belt and probably the English River metasedimentary belt is directly reiated to their dropping along extensional shear zones, which limited subsequent erosion. The relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust, nor preserved by vertical crustal movements as might occur in the absence of plate tectonics. Extension may have been associated with the emplacement of mantle-derived magmas at 2700 Ma, which has been linked to slab break-off or lithospheric delamination, making the extension approximately coeval with local gold mineralisation. Since crustal-scale faults can facilitate the circulation of gold-bearing fluids, we suggest that greenstone rocks preserved in the hanging walls of syn- to post-accretion extensional shear zones may preferentially host Archean lode-gold deposits. In the westernmost Superior Province, our seismic observations imply that some of the late structures in the well-developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes. (C) 2004 Elsevier B.V. All rights reserved. DOI
10. Fisher, MA; Normark, WR; Langenheim, VE; Calvert, AJ; Sliter, R.Offshore Palos Verdes fault zone near San Pedro, southern California.Bull. Seismol. Soc. Amer., 2004, 94: 506-530 Offshore Palos Verdes fault zone near San Pedro, southern California
High-resolution seismic-reflection data are combined with a variety of other geophysical and geological data to interpret the offshore structure and earthquake hazards of the San Pedro shelf, near Los Angeles, California. Prominent structures investigated include the Wilmington graben, the Palos Verdes fault zone, various faults below the west part of the San Pedro shelf and slope, and the deep-water San Pedro basin. The structure of the Palos Verdes fault zone changes markedly along strike southeastward across the San Pedro shelf and slope. Under the north part of the shelf, this fault zone includes several strands, with the main strand dipping west. Under the slope, the main fault strands exhibit normal separation and mostly dip east. To the southeast near Lasuen Knoll, the Palos Verdes fault zone locally is low angle, but elsewhere near this knoll, the fault dips steeply. Fresh seafloor scarps near Lasuen Knoll indicate recent fault movement. We explain the observed structural variation along the Palos Verdes fault zone as the result of changes in strike and fault geometry along a master right-lateral strike-slip fault at depth. Complicated movement along this deep fault zone is suggested by the possible wave-cut terraces on Lasuen Knoll, which indicate subaerial exposure during the last sea level lowstand and subsequent subsidence of the knoll. Modeling of aeromagnetic data indicates a large magnetic body under the west part of the San Pedro shelf and upper slope. We interpret this body to be thick basalt of probable Miocene age. This basalt mass appears to have affected the pattern of rock deformation, perhaps because the basalt was more competent during deformation than the sedimentary rocks that encased the basalt. West of the Palos Verdes fault zone, other northwest-striking faults deform the outer shelf and slope. Evidence for recent movement along these faults is equivocal, because we lack age dates on deformed or offset sediment. DOI
9.Calvert, AJ; Fisher, MA; Johnson, SY.Along-strike variations in the shallow seismic velocity structure of the Seattle fault zone: Evidence for fault segmentation beneath Puget Sound.J. Geophys. Res.-Solid Earth, 2003, 108 Along-strike variations in the shallow seismic velocity structure of the Seattle fault zone: Evidence for fault segmentation beneath Puget Sound
Seattle Fault; seismic reflection; first arrival tomography; seismic velocity; seismic hazard
Around 1100 years ago, the Seattle fault, which trends east-west beneath Puget Sound and the greater Seattle metropolitan area, experienced a M>7 earthquake. We present high-resolution images of the shallow P wave velocity variation across the fault zone. These images were obtained by tomographic inversion of the first arrivals recorded along two north-south oriented seismic reflection lines shot within Puget Sound near Seattle. Just beneath the seafloor, the fault zone includes uplifted Tertiary rocks with seismic velocities in the range of 2300 to 2600 m s(-1). These velocities contrast markedly with values of similar to1600 m s(-1) in shallow Holocene sediments. South of the Seattle fault zone volcanic rocks of the Crescent Formation, which exhibit velocities >3700 m s(-1),are identified at depths of only 900 m. Seismic velocities of around 2600 m s(-1), which represent Oligocene rocks, are found in the hanging wall of the Seattle fault beneath eastern Puget Sound. In the west, lower, 2300 m s(-1) seismic velocities occur, probably due to the presence of Miocene rocks, which are not found in the east. Along-strike velocity variations arise from the folding of Tertiary rocks and the presence of distinct fault splays, including a north striking tear fault characterized by depressed seismic velocities that was intersected by the eastern seismic line. Along-strike differences in the uplift of Tertiary rocks beneath Puget Sound are likely associated with the existence of a segment boundary of the Seattle fault system. DOI
8.Calvert, AJ; Fisher, MA; Ramachandran, K; Trehu, AM.Possible emplacement of crustal rocks into the forearc mantle of the Cascadia Subduction Zone.Geophys. Res. Lett., 2003, 30 Possible emplacement of crustal rocks into the forearc mantle of the Cascadia Subduction Zone
Seismic reflection profiles shot across the Cascadia forearc show that a 5-15 km thick band of reflections, previously interpreted as a lower crustal shear zone above the subducting Juan de Fuca plate, extends into the upper mantle of the North American plate, reaching depths of at least 50 km. In the extreme western corner of the mantle wedge, these reflectors occur in rocks with P wave velocities of 6750-7000 ms(-1). Elsewhere, the forearc mantle, which is probably partially serpentinized, exhibits velocities of approximately 7500 ms(-1). The rocks with velocities of 6750-7000 ms(-1) are anomalous with respect to the surrounding mantle, and may represent either: (1) locally high mantle serpentinization, (2) oceanic crust trapped by backstepping of the subduction zone, or (3) rocks from the lower continental crust that have been transported into the uppermost mantle by subduction erosion. The association of subparallel seismic reflectors with these anomalously low velocities favours the tectonic emplacement of crustal rocks. DOI
7. Fisher, MA; Normark, WR; Bohannon, RG; Sliter, RW; Calvert, AJ.Geology of the continental margin beneath Santa Monica Bay, Southern California, from seismic-reflection data.Bull. Seismol. Soc. Amer., 2003, 93: 1955-1983 Geology of the continental margin beneath Santa Monica Bay, Southern California, from seismic-reflection data
We interpret seismic-reflection data, which were collected in Santa Monica Bay using a 70-in(3) generator-injector air gun, to show the geologic structure of the continental shelf and slope and of the deep-water, Santa Monica and San Pedro Basins. The goal of this research is to investigate the earthquake hazard posed to urban areas by offshore faults. These data reveal that northwest of the Palos Verdes Peninsula, the Palos Verdes Fault neither offsets the seafloor nor cuts through an undeformed sediment apron that postdates the last sea level rise. Other evidence indicates that this fault extends northwest beneath the shelf in the deep subsurface. However, other major faults in the study area, such as the Dume and San Pedro Basin Faults, were active recently, as indicated by an arched seafloor and offset shallow sediment. Rocks under the lower continental slope are deformed to differing degrees on opposite sides of Santa Monica Canyon. Northwest of this canyon, the continental slope is underlain by a little-deformed sediment apron; the main structures that deform this apron are two lower-slope anticlines that extend toward Point Dume and are cored by faults showing reverse or thrust separation. Southeast of Santa Monica Canyon, lower-slope rocks are deformed by a complex arrangement of strike-slip, normal, and reverse faults. The San Pedro Escarpment rises abruptly along the southeast side of Santa Monica Canyon. Reverse faults and folds underpinning this escarpment steepen progressively southeastward. Locally they form flower structures and cut downward into basement rocks. These faults merge downward with the San Pedro Basin fault zone, which is nearly vertical and strike slip. The escarpment and its attendant structures diverge from this strike-slip fault zone and extend for 60 km along the margin, separating the continental shelf from the deep-water basins. The deep-water Santa Monica Basin has large extent but is filled with only a thin (less than 1.5-km) section of what are probably post-Miocene rocks and sediment. Extrapolating ages obtained from Ocean Drilling Program site 1015 indicates that this sedimentary cover is Quaternary, possibly no older than 600 ka. Folds and faults along the base of the San Pedro Escarpment began to form during 8-13 ka ago. Refraction-velocity data show that high-velocity rocks, probably the Catalina Schist or Miocene volcanic rocks, underlie the sedimentary section. The San Pedro Basin developed along a strike-slip fault, widens to the southeast, and is deformed by faults having apparent reverse separation and by folds near Redondo Canyon and the Palos Verdes Peninsula. DOI
6.Calvert, AJ; Fisher, MA.Imaging the Seattle fault zone with high-resolution seismic tomography.Geophys. Res. Lett., 2001, 28: 2337-2340 Imaging the Seattle fault zone with high-resolution seismic tomography
The Seattle fault, which trends east-west through the greater Seattle metropolitan area, is a thrust fault that, around 1100 years ago, produced a major earthquake believed to have had a magnitude greater than 7. We present the first high resolution image of the shallow P wave velocity variation across the fault zone obtained by tomographic inversion of first arrivals recorded on a seismic reflection profile shot through Puget Sound adjacent to Seattle. The velocity image shows that above 500 in depth the fault zone extending beneath Seattle comprises three distinct fault splays, the northernmost of which dips to the south at around 60 degrees. The degree of uplift of Tertiary rocks within the fault zone suggests that the slip-rate along the northernmost splay during the Quaternary is 0.5 mm a(-1), which is twice the average slip-rate of the Seattle fault over the last 40 Ma. DOI
5. Adam, E; Perron, G; Milkereit, B; Wu, JJ; Calvert, AJ; Salisbury, M; Verpaelst, P; Dion, DJ.A review of high-resolution seismic profiling across the Sudbury, Selbaie, Noranda, and Matagami mining camps.Can. J. Earth Sci., 2000, 37: 503-516 A review of high-resolution seismic profiling across the Sudbury, Selbaie, Noranda, and Matagami mining camps
Lithoprobe high-resolution seismic surveys have provided the first systematic images of the deep stratigraphy in four major Canadian mining camps (Noranda, Matagami, Sudbury, and Selbaie). Systematic compressional wave velocity and density measurements in deep boreholes have established that lithological contacts were the main impedance contrast imaged, although reflections from faults and deformation zones have also been observed. The strongest reflections are attributed to mafic intrusions and some sulphides and oxides. Integrating seismic, physical rock property measurements, and geological data has resulted in the revision of several geological models with direct impact on local strategies for deep mineral exploration. Mining companies have shown an interest in seismic reflection methods and this has led to several follow-up studies. The application of seismic methods to the direct detection of massive sulphides, based on physical rock property measurements, has been studied through two-dimensional and three-dimensional (3D) seismic imaging and vertical seismic profiling technologies. The challenge will now be to optimize 3D seismic imaging for mineral exploration and to improve seismic data processing by enhancing the seismic response from deep, lenticular orebodies. DOI
4. Fullagar, PK; Livelybrooks, DW; Zhang, P; Calvert, AJ; Wu, YR.Radio tomography and borehole radar delineation of the McConnell nickel sulfide deposit, Sudbury, Ontario, Canada.Geophysics, 2000, 65: 1920-1930 Radio tomography and borehole radar delineation of the McConnell nickel sulfide deposit, Sudbury, Ontario, Canada
In an effort to reduce costs and increase revenues at mines, there is a strong incentive to develop high-resolution techniques both for near-mine exploration and for delineation of known orebodies To investigate the potential of high-frequency EM techniques for exploration and delineation of massive sulfide orebodies, radio frequency electromagnetic (RFEM) and ground-penetrating radar (GPR) surveys were conducted in boreholes through the McConnell massive nickel-copper sulfide body near Sudbury, Ontario, from 1993-1996. Crosshole RFEM data were acquired with a JW-4 electric dipole system between two boreholes on section 2720W. Ten frequencies between 0.5 and 5.0 MHz were recorded. Radio signals propagated through the Sudbury Breccia over ranges of at least 150 m at all frequencies. The resulting radio absorption tomogram clearly imaged the McConnell deposit over 110 m downdip. Signal was extinguished when either antenna entered the sulfide body. However, the expected radio shadow did not eventuate when transmitter and receiver were on opposite sides of the deposit. Two-dimensional modeling suggested that diffraction around the edges of the sulfide body could not account for the observed held amplitudes. It was concluded at the time that the sulfide body is discontinuous; according to modeling, a gap as small as 5 m could have explained the observations. Subsequent investigations by INCO established that pick-up in the metal-cored downhole cables was actually responsible for the elevated signal levels. Both single-hole reflection profiles and crosshole measurements were acquired using RAMAC borehole radar systems, operating at 60 MHz. Detection of radar reflections from the sulfide contact was problematic. One coherent reflection was observed from the hanging-wall contact in single-hole reflection mode. This reflection could be traced about 25 m uphole from the contact. In addition to unfavorable survey geometry, factors which may have suppressed reflections included host rock heterogeneity, disseminated sulfides, and contact irregularity. Velocity and absorption tomograms were generated in the Sudbury Breccia host rock from the crosshole radar. Radar velocity was variable, averaging 125 m/mus, while absorption was typically 0.8 dB/m at 60 MHz. Kirchhoff-style 2-D migration of later arrivals in the crosshole radargrams defined reflective zones that roughly parallel the inferred edge of the sulfide body. The McConnell high-frequency EM surveys established that radio tomography and simple radio shadowing are potentially valuable for near- and in-mine exploration and orebody delineation in the Sudbury Breccia. The effectiveness of borehole radar in this particular environment is less certain. DOI
3. Martignole, J; Calvert, AJ; Friedman, R; Reynolds, P.Crustal evolution along a seismic section across the Grenville Province (western Quebec).Can. J. Earth Sci., 2000, 37: 291-306 Crustal evolution along a seismic section across the Grenville Province (western Quebec)
Results of deep seismic reflection survey along a 375 km long transect of the Grenville Province in western Quebec are combined with a review of geological observations and published isotopic ages. The seismic profile offers a remarkably clear image of the crust-mantle boundary and a good definition of the various crustal blocks. Crust about 44 km thick beneath the Grenville Front zone thins abruptly to ca. 36 km southeastward, perhaps the result of extension on southeast-dipping surfaces extending to the Moho. Other zones of relatively thin crust, although less pronounced, occur where Proterozoic crust overlies Archean crust, and beneath the Morin anorthosite complex. The thickest crust is found at the extreme southeast of the transect, east of the Morin anorthosite. From northwest to southeast, three main crustal subdivisions are (1) deformed Archean rocks with southeast-dipping reflectors in the Grenville Front zone, (2) an Archean parautochthon with northwest-dipping reflectors extending to the lower crust, and (3) an overlying three-layer crust interpreted as accreted Proterozoic terranes. The boundary between (2) and (3) is a major, southeast-dipping, crustal-scale ramp (Baskatong ramp) interpreted to have accommodated strain during and after accretion. U-Pb and Pb-Pb ages on detrital zircons show that metasedimentary rocks of the allochthons (Mont-Laurier, Reservoir Cabonga, and Lac Dumoine terranes) range from Archean to as young as 1.21 Ga. A single zone with 1.4 Ga old Sm-Nd model ages appears to lack Archean components and may be considered as a fragment of juvenile Mesoproterozoic crust pinched in a shear zone (Renzy shear zone) that could be raised to the status of terrane (Renzy terrane). In the allochthons, U-Pb ages of metamorphic zircon and monazite cluster around 1.17 Ga (Mont-Laurier and Reservoir Cabonga terrane) and 1.07 Ga (Renzy and Lac Dumoine terrane) and are interpreted to record late and post-accretion crustal reworking, a common feature of the Grenville orogen. A final high-grade metamorphic event (ca. 1.0 Ga) documented only in the parautochthon and the Grenville Front zone records large-scale, piggyback-style thrusting of allochthonous slabs onto the parautochthon. The age of transcurrent displacement following peak metamorphism affecting both the allochthons and the parautochthon decreases northwestward from 1.07 to 1.00 Ga. Dating thus shows that Grenvillian deformation in western Quebec occurred in pulses over an interval of 180 million years, with a tendency to propagate from the inner part of the orogen toward the Grenville Front. Reworked migmatites from the parautochthon cooled from the ca. 1.0 Ga peak of metamorphism through about 450%C (Ar closure in hornblende) at ca. 0.96 Ga with calculated cooling rates of about 6%C per million years, and unroofing rates of 0.33 km per million years. The cooling-unroofing history of the allochthons is not so straightforward, probably due to tectonic disturbances related to allochthon emplacement. Cooling through 450%C occurred between 1.04 and 1.01 Ga, at least 50 million years earlier than cooling in the parautochthon; this contrast agrees with the northwestward propagation of the orogen. DOI
2.Calvert, AJ; Li, YX.Seismic reflection imaging over a massive sulfide deposit at the Matagami mining camp, Quebec.Geophysics, 1999, 64: 24-32 Seismic reflection imaging over a massive sulfide deposit at the Matagami mining camp, Quebec
A 2-D seismic reflection profile was shot across the southern flank of the Matagami mining camp, almost directly above the recently discovered Bell Allard massive sulfide deposit, now estimated at more than 6 million metric tons. All orebodies found in the southern part of the mining camp, including Bell Allard, are located at the contact between the primarily basaltic Wabassee Group and the underlying rhyolitic Watson Lake Group. Seismic reflections were recorded from the basalt-rhyolite contacts of the lower Wabassee Group, as well as from gabbro sills that intrude much of the volcanic stratigraphy. A strong reflection from the top of the Bell Allard orebody was also detected, but the reflection does not extend over the full width of the deposit as defined by drilling, appearing to correlate with the lower pyrite-rich zone. Faulting, which can be interpreted from discontinuities in the observed reflections, probably controlled the formation of the Bell Allard deposit. If the interpreted gabbro sills are accepted as isotime markers, then faulting of the deeper sill complex defines a series of half grabens within the rhyolitic Watson Lake Group. The Bell Allard deposit is found at the intersection of one of these apparently low-angle normal faults with the top of the Watson Lake Group, indicating that sulfide mineralization may have been associated with fluid flow along the fault, which likely penetrates to the underlying mafic intrusion. Although the precise geometry of subsurface faulting cannot be estimated from a single 2-D seismic profile, these results indicate that a full 3-D seismic survey should allow the mapping of many of the subsurface fault systems and the verification of hypotheses of fault-controlled deposit formation. DOI
1.Calvert, AJ; Ludden, JN.Archean continental assembly in the southeastern Superior Province of Canada.Tectonics, 1999, 18: 412-429 Archean continental assembly in the southeastern Superior Province of Canada
Between 1988 and 1993, seismic reflection and refraction surveys were acquired across the medium- to high-grade Opatica plutonic gneiss belt, the low-grade Abitibi greenstone belt, and the Pontiac metasedimentary belt, all of which form part of the late Archean Superior Province. Shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 Ma of various exotic terranes against a backstop provided by the Opatica belt. This rapid southward growth of the Archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65-km depth in the upper mantle below the Opatica belt. In contrast to the mainly orthogneissic Opatica and Pontiac belts, the midcrust of the Abitibi belt comprises metasedimentary and igneous rocks, plus imbricated units of unknown affinity. Relict midcrustal accretionary complexes of substantial size, which are indicative of primary suture zones, are interpreted near the northern and southern limits of the Abitibi belt. An interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous. Evidence of large-scale extension appears to be confined to the Southern Volcanic Zone of the Abitibi, which developed into a half graben as the original suture zone was reactivated in extension. Unusually high seismic P wave velocities, 7.5-8.2 kms(-1), are present in the lower 8 km of the Abitibi crust, and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust. Crustal xenolith studies suggest that this process may be linked to early Proterozoic magmatism. DOI
