March 31, 2017 @ 12:00 pm - 12:50 pm CDT
Who: Dr. Min Chen & Dr. Jonathan Delph
When: Noon, Friday, March 31, 2017
Where: Room 100, KWGL
Chen: Lithospheric foundering and underthrusting imaged beneath Tibet
Long-standing debates exist over the timing and mechanism of uplift of Tibetan Plateau, and more specifically, over the connection between lithospheric evolution and the surface expressions of plateau uplift and volcanism. Our new tomographic model reveals a T-shaped high wave speed structure beneath South-Central Tibet, interpreted as remnant in the upper mantle from an earlier lithospheric foundering event. Its spatial correlation with ultrapotassic and adakitic magmatism supports the hypothesis of convective removal of thickened Tibetan lithosphere causing major uplift of Southern Tibet during the Oligocene. Lithospheric foundering induces an asthenospheric drag force, which drives continued underthrusting of the Indian continental lithosphere and shortening and thickening of the Northern Tibetan lithosphere. Surface uplift of Northern Tibet is subject to more recent asthenospheric upwelling and thermal erosion of thickened lithosphere, which is spatially consistent with the recent potassic volcanism and an imaged narrow low wave speed zone in the uppermost mantle.
Delph: The effects of subduction termination on the continental lithosphere: Linking volcanism, deformation, surface uplift, and slab tearing in central Anatolia
Subduction beneath central Anatolia represents the transition between continuous subduction along the Aegean trench in the west and slab breakoff/subduction termination at the Arabian-Eurasian collision zone in the east. In this study, we apply a recently developed approach to the joint inversion of receiver functions and surface wave dispersion data to newly collected seismic data from the Continental Dynamics – Central Anatolian Tectonics project to create a 3D shear wave velocity model of central Anatolia. This new model allows for the proposal of a unified model of central Anatolian evolution that ties together observable tectonic structures with seismic characteristics during the terminal stages of subduction.
The results of this study indicate that the lithospheric mantle of central Anatolia is thin and variable (<50 to 80 km) due to the accretion of continental terranes from the downgoing to overriding plate during the late Cretaceous-Paleocene. The accretion process decoupled the crust of the downgoing plate from its associated mantle lithosphere, which continued to subduct at a shallow angle until foundering in early-mid Miocene times. The resulting lithospheric thickness variations appear to control neotectonic (i.e., Neogene – Quaternary) deformation as well as the distribution of volcanism throughout the region. The thinnest mantle lithosphere underlies the Central Anatolian Volcanic Province, home to voluminous (>1000 km^3) Miocene-recent ignimbrite volcanism. The uppermost mantle in this region is characterized by very slow shear velocities (<4.2 km/s), consistent with the presence of melt. The fastest shear velocities observed in this study (>4.5 km/s) are located along the southern margin of central Anatolia beneath the Central Taurus Mountains, which have experienced ~2 km of uplift in the past ~8 Ma. These velocities are consistent with lithospheric mantle, and we interpret that the recent uplift of these mountains is due to a rebound of the subducting slab after slab breakoff/fragmentation as opposed to past hypotheses invoking support via upwelling asthenosphere through a slab tear.