March 10, 2017 @ 12:00 pm - 12:50 pm CST
John Cornthwaite & Xu Chu will present the Departmental Seminar this Friday, March 10th
Where: Room 100, KWGL
Who: John Cornthwaite
What: “Flat slab subduction of the Carribean plate beneath northwest South America”
Abstract: About 10% of all subduction occurs as flat subduction, defined as slab dips at angles ≤ 30°. Flat slab subduction is an important phenomenon in that it causes deformation, and associated seismicity, in continental lithosphere hundreds of kilometers from the subduction zone. In this study I examine the subduction zone of northwest South America (SA), where the Caribbean (CAR) plate and its over-thickened crust subducts beneath Venezuela and Colombia at angles between 17° and 30°. I address several questions: (1) Over what extent does the CAR plate subduct shallowly, at what angle, and where does it dip steeply? (2) Where does the subducting slab transition from positive buoyancy to negative buoyancy? (3) Does the CAR plate tear under Lake Maracaibo and is a tear responsible for the Bucaramanga seismic nest? (4) What will seismic observations tell us about the coupling between the upper and lower plates? I will interpret images derived from receiver functions and finite frequency teleseismic tomography of the CAR and SA plates and the surrounding mantle. I will determine the S and P velocity structures and associate them with lithologies and hydration states at the plate interfaces and mantle wedge. I will combine these results with a local seismicity study using accurate microearthquake locations. This project provides an opportunity to study an active subduction zone where flat slab subduction is responsible for recent and ongoing Laramide-style basement uplifts.
Who: Xu Chu
What: “Ultra-fast metamorphism: unconventional petro-tectonic wisdom”
Abstract: The conventional wisdom holds that orogenic metamorphism takes place under lithostatic pressures in response to internal heating in variably overthickened crusts. The conductive thermal relaxation typically lasts millions of years. Ultra-fast metamorphism, on the other hand, requires additional heat sources and/or alternative geodynamic schemes. I use garnet zonation and simulations to explore metamorphism in a temporal dimension and trace petrologic records of transient geologic processes.
Eclogite in a Taconic thrust slice in Northwest Connecticut records an eclogite facies pressure-temperature (P-T) anomaly superimposed on the regional amphibolite facies background. Sharp compositional zones are preserved in the garnet porphyroblasts. Thermobarometry and diffusion simulations indicate that a 5 kbar compression lasted only 500 years. The remarkably ultra-fast compression, if due to burial along a lithostatic pressure gradient, requires a descent rate >30 m/yr, orders of magnitude faster than any long-term tectonic rates. “Autoclave” overpressure due to near-isochoric anataxis induced by transient shear heating, explains the ultra-fast compression without necessitating burial to great depth. Dynamic simulation (Kelemen & Hirth, 2007) shows that shear heating events can occur with a period of 200-250 years in convergent zones, and the thermally-activated rapid stress drops can be associated with episodic earthquake cycles at intermediate depth. The garnet zonation documents a similar timescale and, thus, may reflect ancient interseismic stress transients in the orogenic belt.