Geophysics and Tectonics Seminar - winter-2019

Jan. 9, 2019
noon - 1 p.m.
Geology 1707

Presented By:

• An Yin - UCLA

Seminar Description coming soon.

Jan. 16, 2019
noon - 1 p.m.
Geology 1707

Presented By:

• Clément Perrin - IPGP

Seminar Description coming soon.

Jan. 30, 2019
noon - 1 p.m.
Geology 1707

Presented By:

• Tushar Mittal - UC Berkeley

Large igneous provinces represent very large volcanic events during which millions of cubic kilometers of magma erupted in a short time interval (less than 1 Myr). LIPs are also often correlated with severe environmental impacts. Although much progress has been made to understanding LIPs and their climatic implications, many fundamental questions regarding flood basalt volcanism still remain -- What is the eruptive tempo of the flood basalt on sub-Myr timescale? What is the crustal and lithospheric magmatic system that fed these large eruptions? What is the primary mechanism through which a flood basalt event causes climate perturbations? In order to address some of these questions, I have been working on the Deccan Traps LIP as an archetypical example. The Deccan Traps are one of the largest, well exposed, and geochemically and chronologically characterized continental flood basalt province. I will discuss results from our group combining different methodologies including high-precision geochronology, lava flow morphologies & thicknesses, statistical models for paleo-secular variation, model inversion of high-resolution sedimentary Hg records, constraints on Intrusive/Extrusive ratio for the Deccan Traps, as well as theoretical models for magma chamber dynamics and passive degassing. These results have enabled us to characterize the eruptive rate of Deccan Traps on sub 50kyr resolution, infer the structure of sub-surface magmatic plumbing system, and illustrate the potential role of passive degassing in the climatic influence of flood basalts.

Feb. 6, 2019
noon - 1 p.m.
Geology 1707

Presented By:

• Kiran Chotalia - UCL
• Antoniette Grima - UCL

Seminar Description coming soon.

Feb. 13, 2019
noon - 1 p.m.
Geology 1707

Presented By:

• Daniel Frost - UC Berkeley

The Earth’s inner core shows variability in the speed of seismic waves travelling through it depending on the direction that the wave travels, a feature known as anisotropy. Seismic waves passing through the inner core along paths propagating roughly north-south (near-parallel to the Earth’s rotation axis) travel faster than those traversing the inner core roughly east-west (in the plane of the equator). Since the discovery of anisotropy 30 years ago, further patterns of the variation in seismic velocity have been discovered. Inner core anisotropy is proposed to result from preferred alignment of iron crystals. However, the strength and distribution of anisotropy are difficult to reconcile with predictions from mineral physics and dynamical models of inner core growth. We examine the trends of travel times on newly acquired polar paths from recent deployments of seismic arrays in Alaska and Antarctica. We observe large travel time anomalies in Alaska associated with the Alaskan subduction zone, indicating upper mantle contamination of these paths. To match our observations, we propose a model of asymmetric growth at the inner core boundary resulting in net translation and flow from viscous relaxation at the equator. The flow from the equator to the poles results in a strong alignment of iron crystals, which is laterally offset from the rotation axis, explaining the patterns of seismic anisotropy observed in our data. The asymmetric growth of the inner core may have influenced or been influenced by outer core and mantle dynamics.

Feb. 20, 2019
noon - 1 p.m.
Geology 1707

Presented By:

• Roby Douilly - UC Riverside

The 2010 M7.0 Haiti earthquake was the first major earthquake in southern Haiti in 250 years. As this event could represent the beginning of a new period of active seismicity in the region, and in consideration of how vulnerable the population is to earthquake damage, it is important to understand the nature of this event and how it has influenced seismic hazards in the region. Most significantly, geodetic data showed that the 2010 earthquake occurred on the secondary Léogâne thrust fault (two fault segments), not the Enriquillo Fault, the major strike-slip fault in the region, despite it being only a few kilometers away. Following the earthquake, several groups had installed temporary seismic stations to record aftershocks and we used this combined dataset to clearly delineate the Léogâne fault, with a geometry close to that inferred from geodetic data. Its strike and dip closely agree with the global centroid moment tensor solution of the mainshock but with a steeper dip than inferred from previous finite fault inversions. The aftershocks also delineate a structure with shallower southward dip offshore and to the west of the rupture zone, which could indicate triggered seismicity on the offshore Trois Baies reverse fault. Using this dataset, we also investigated a detailed 3D crustal structure of this region. Our results showed a pronounced low velocity zone across the Léogâne fault, which is consistent with the sedimentary basin location from the geologic map. We also observed a southeast low velocity zone, which is consistent with a predefined structure in the morphology. In addition, we used a finite element model to simulate scenarios of the 2010 Haiti earthquake to understand why the rupture did not jump to the nearby Enriquillo fault. Our model successfully replicated rupture propagation along the two segments of the Léogâne fault, and indicated that a significant increase in stress had occurred on the top and to the west of the Enriquillo fault. These work provide information that can be used in future studies focusing on how changes in material properties can affect rupture propagation, which is useful to assess the seismic hazard that Haiti is currently facing.

March 6, 2019
noon - 1 p.m.
Geology 1707

Presented By:

• Saikiran (Sai) Tharimena - JPL

Tectonic plate motions cause earthquakes, tsunamis, and volcanos, and also impact climate change estimates. However, plate thicknesses and what defines them and allows them to move over the underlying asthenosphere remains unclear. Tight constraints on observables that could distinguish the defining mechanism, such as the depth and seismic velocity gradient at the base of tectonic plates, the lithosphere asthenosphere boundary (LAB), have proven challenging. It is well established that oceanic lithosphere cools, thickens, and subsides as it ages according to conductive cooling models. Yet, this simple realization fails to explain various observations, for example, lower than predicted subsidence of old oceanic lithosphere. Although previous studies use seismic surface and body waves to image oceanic lithospheric discontinuities, directly connecting them to the LAB have proven challenging. Here seismic SS precursors were used to image a sharp pervasive velocity discontinuity structure beneath the Pacific lithosphere. The discontinuity increases in depth with age from the ridge to at least ~36 My along 1100°C conductive cooling isotherm. Beneath seafloor > 36 My there is no age-depth dependence, and the discontinuity is imaged at an average depth of 60 km. The amplitude and sharpness of the discontinuity suggests that a compositional and/or layered carbonatitic melt may be required to explain observations rather than temperature alone. On a regional scale, seismic receiver functions, tomography, anisotropy and magnetotellurics were used to image variations with age in the ocean lithosphere and asthenosphere. The equatorial mid-Atlantic ocean plate thickness increases with age in some cases, while in other it undulates. In the asthenosphere, melt was imaged in long and thin channels and also in broader and more punctuated zones. The variability suggests that the plate definition is dynamic with a thickness and character that depend on mantle dynamics and melt generation and migration

March 13, 2019
noon - 1 p.m.
Geology 1707

Presented By:

• Vamsi Ganti - UCSB

The origin of low-gradient meandering rivers — the primary conduits of water, carbon and nutrients in present-day terrestrial landscapes — is considered coeval with Silurian-age plant evolution. It is hypothesized that pre-Silurian rivers lacked bank strength and were dominantly steep and braided, implying vastly different transport capacities of water and sediment. This hypothesis, however, is inconsistent with the super-continental-scale drainage of Neoproterozoic rivers, which requires unrealistically high mountains to achieve the necessary river gradients. In this talk, I will provide geologic evidence that pre-Silurian rivers were low gradient, deep, and single-threaded, similar to modern meandering rivers. These results are built upon a well-developed quantitative framework of river bar and dune formation, global compilation of geometries of Proterozoic fluvial deposits, and original field measurements of the scale, texture and structure of fluvial deposits in Proterozoic-age Torridonian Group, Scotland—a type-example of pancontinental, pre-vegetation fluvial systems. These results point to the abundance of low-sloping, single-threaded rivers in the Proterozoic eon, at a time well prior to the evolution and radiation of land plants, and demonstrate uniformity of fluvial morphology despite a global revolution in Earth’s terrestrial biota, with ramifications for the topographic signature of life on Earth and other planets.