3. 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
2.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
1.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
