Haller Hall (102), 24 Oxford St., Cambridge

EPS Colloquium

Jason Phipps Morgan, Chair Professor, Department of Ocean Science and Engineering, SUSTech, will present “Two-Phase Lithospheric Growth at Oceanic Transform Plate Boundaries."

Oceanic transform faults are seismically and tectonically active major plate boundaries. They leave fracture zones on oceanic plates that can cross entire ocean basins. Plate tectonics idealizes transforms to be conservative two-dimensional strike-slip boundaries, where lithosphere is neither created nor destroyed, and along which the lithosphere cools and deepens as a function of plate age. A new compilation of high-resolution data shows that this simple idealization is incorrect. We find that transform valleys are systematically deeper (by up to 1.6 km) than their adjacent fracture zones, in contrast to previous expectations based on plate cooling arguments. Lithospheric accretion at ridge-transform intersections (RTI) is strongly asymmetric, with outside corners showing shallower relief and more extensive magmatism while inside corners have deep nodal basins and appear magmatically starved. Numerical modelling suggests that these features result from transform faults becoming extensional structures below their rectilinear surface strike-slip motion. Plastic shear failure within the inside corner is shown to create a slightly oblique and extensional shear zone at depth that is offset with respect to axial magma supply and tectonically thins the lithosphere thereby creating the transform valley. Seafloor bathymetry shows that, at all spreading rates, thinned transform lithosphere is augmented by magma as it passes the opposing ridge axis. Lithosphere production near fracture zones therefore differs fundamentally from that elsewhere on mid-ocean ridges, with magmatically starved and tectonically thinned inside-corner crust augmented millions of years later by a second phase of outside corner magmatism. The seismic predictions of this scenario appear to be testable with further seismic studies, both local and global.

Contact Name: 

Katrina Blanch

Research Areas: 

Harvard University
Center for the Environment

Address: 26 Oxford Street, 4th Floor, Cambridge
Phone: (617) 495-0368

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