Geophysics and Tectonics Seminar - fall-2019
Intracontinental Deformation: Seismic attenuation sheds new light on an old problem
Oct. 2, 2019
noon - 1 p.m.
Geology 1707
Presented By:
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Contrary to a strict interpretation of plate tectonics, continental interiors experience seismicity and deformation. These events are often concentrated in distinct zones and are occasionally very damaging. After roughly six decades of research, there is no consensus on the mechanisms that produce intracontinental earthquakes, with a range of diverse hypotheses currently being debated. In my talk I will present evidence that suggests that lateral changes in lithospheric rheology play a major role in determining where intracontinental seismicity occurs. I use observations of seismic attenuation as a proxy for lithospheric strength and show how it relates to intracontinental seismicity and deformation in 3 different locations. In Iberia, high attenuation regions were more affected by the Alpine orogeny and show higher seismicity than low attenuation regions. In Australia there is a quantitative correlation between seismicity rates and attenuation. In Oklahoma, I explore a possible connection between lateral strength variations in the lithosphere and the extraordinary rise of induced seismicity. I furthermore discuss a conceptual model that serves as a working hypothesis to interpret these findings.
The influence of melting and viscosity on the thermo-chemical evolution of Earth and other rocky planets
Oct. 9, 2019
noon - 1 p.m.
Geology 1707
Presented By:
- Diogo Lourenco - UC Davis
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In this presentation I will firstly show results from numerical simulations of global mantle convection to explore the effects of melting on the thermo-chemical evolution of terrestrial bodies. I applied the models to investigate (i) how does melting-induced crustal production affects the interior state and surface behavior of an Earth-like planet, and (ii) the effects of intrusive versus extrusive magmatism on the surface tectonics and mantle cooling of a terrestrial planet. Results show that (i) melting-induced crustal production helps plate tectonics on Earth-like planets by strongly enhancing the mobility of the lid; (ii) high intrusion efficiencies (i.e. dominance of intrusion versus extrusion) lead to a new tectonic regime, named “plutonic-squishy lid” characterized by a set of strong plates separated by warm and weak regions generated by plutonism, and can cool the mantle more efficiently than volcanic eruptions for planets with no subduction in their history. In the second part of the talk I will focus on the present-day structure and dynamics of the Earth. Seismic images of Earth’s mantle have revealed changes in mantle structure between 400-1000 km depth. The structures at these depths appear to be different in nature from the lowermost mantle or the lithosphere. I demonstrate that the changes in structure are driven primarily by the reduced rate of sinking of subducted oceanic plate material in the western Pacific basin. Next, I use numerical models of mantle convection to demonstrate that the observed structures can be best explained by a relatively large increase in mantle viscosity between the upper mantle and lower mantle at 660 km depth or perhaps somewhat deeper, near 1000 km.
New insights into the uplift of the Himalayas through advances in metamorphic petrology
Oct. 16, 2019
noon - 1 p.m.
Geology 1707
Presented By:
- Elizabeth Catlos - UT Austin
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The Main Central Thrust (MCT) shear zone is one of the major Himalayan fault systems that is largely responsible for the generation of its high topography. Garnets collected across the MCT record their growth history in the crust through changes in their chemistry. These chemical changes can be extracted and modeled. Here, we report detailed pressure-temperature paths recorded by garnets collected across the MCT, which is exposed along the Marsyangdi River in central Nepal. The paths track evolving conditions in the Earth’s crust when the MCT was active during the growth of the Himalayas. The results suggest the fault system formed as individual rock packages moved at distinct times. Further modeling of the P-T paths makes predictions about how the Himalayas developed, including that the MCT have may have ceased motion 18-15 million years ago, as other faults closer to the Indian subcontinent became active, and that it re-activated 8-2 million years ago, leading to the generation of high Himalayan topography. In addition, the modeling suggests very high erosion rates occurred within the range after re-activation. Although garnets have long been used to understand how fault systems evolve, we provide details of an approach that allows higher-resolution data to be extracted from them, and show how they could be used to track rates of large-scale erosion.
Abijah Simon: Cenozoic development of eastern Tibet by pure-shear shortening; Valeria Jaramillo: Insights into the emplacement of the Kathmandu Klippe from quartz microstructures and titanite petrochronology
Oct. 23, 2019
noon - 1 p.m.
Geology 1707
Presented By:
- Abijah Simon - UCLA EPSS
- Valeria Jaramillo - UCLA EPSS
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Abijah Simon: A key issue in the studies of the eastern Tibetan plateau is that its high relief (>4 km) and thick crust (>50 km) cannot be adequately explained by the slow geodetic slip rates (<3 mm/yr), late Cenozoic initiation (~30 Ma), and 10s km Cenozoic shortening along its easternmost edge. The channel flow and thrust belt end-member models have been proposed to address this discrepancy, but they discount the role of deformation in the upper mantle and the upper crust in the interior of eastern Tibet. Here we address this problem by conducting field mapping, interpreting crustal-scale seismic-reflection profiles, and constructing balanced cross sections in the Longmen Shan and Min Shan of western Sichuan. Our mapping reveals a distinct contrast in the deformational styles affecting this region. In the frontal part of this margin, widespread cleavage development in Triassic-folded Silurian and Devonian strata contrast with brittle faults and discrete shear planes defined by slickensides. The parallelism, similar kinematics, and similar brittle mode of deformation between the mapped faults and the seismically active plateau-bounding structures suggest a Cenozoic age. In the northern part of eastern Tibet, Triassic E-W trending folds are cross-cut by N-S trending faults that parallel the seismically active Minjiang fault in this region, also suggesting a Cenozoic age. Our seismic interpretations consistent with regional field relationships suggest: (1) two-phase deformation first by Triassic thin-skinned folding and ductile deformation followed by later Cenozoic thick-skinned faulting and brittle deformation, (2) the presence of a west-directed thrust wedge in the Min Shan with the east-directed Minjiang thrust as its roof structure, and (3) pure-shear ductile shortening in the middle and lower crust in contrast to brittle faulting in the upper crust and the uppermost mantle. A preliminary balanced cross section across the Min Shan reveals >67% shortening accommodated by brittle thrusting. Integrating field observations and seismic data leads to a tectonic model that involves pure-shear thick-skinned crustal thickening for the Cenozoic development of eastern Tibet. If the 67% shortening from the Min Shan represents the average Cenozoic strain in eastern Tibet, this would be sufficient to explain the current crustal thickness and high relief. Valeria Jaramillo: The Himalayan Orogen is one of the largest continent-continent collision zones in the world and serves as the preeminent example of an actively evolving mountain belt. Several fundamental questions persist, however, as to how the Himalaya has tectonically evolved through time, including the origin of the Lesser Himalayan Crystallines—a series of fault bound outliers of metamorphic rocks that occur to the south of the main Himalayan structural sequence. Several tectonic models have been proposed to explain the emplacement of these thrust sheets and klippen, wherein different configurations of major faults are invoked to have resulted in the southward transport of these metamorphic rocks. As an initial attempt to answer some of these questions this study investigated the geometry and timing of deformation in one of these tectonic outliers, the Kathmandu Klippe in central Nepal. Microstructural analysis of rocks collected along four transects across the klippe allows for identification of major structures and quantification of internal strain. Microstructural data collected from several quartzites across the klippe using the Rf-φ and Fry methods yields strain ellipse ratios (Rs values) that range from 1.10 to 2.28 (YZ) and 1.67 to 2.89 (XZ). Strain magnitudes increase systematically with proximity to several major mapped and unmapped structures consistent with their occurrence as broad shear zones and not discrete faults. Among the samples displaying the largest Rs values, are those collected from the Mahabharat Thrust—a large top-south shear zone and possible analogue to the Main Central Thrust. U-Pb dating of strained titanite in these samples yields a date of ~16-18 Ma, overlapping with prior estimates for the timing of slip along the southern MCT. Petrographic study of the titanites indicates these young dates primarily reflect subgrain formation at grain tips and may thus represent the first direct determination of the timing of deformation within the klippe. When combined with information provided by the bulk strain ellipses and shear sense indicators, these results provide important insights into the various models on the development of the Himalaya and, more broadly, how high-grade metamorphic rocks are juxtaposed over low-grade metamorphic rocks in convergent orogens.
Sarah Brownlee - Seismic anisotropy in the continental crust: Using rocks to improve seismic interpretations
Oct. 30, 2019
noon - 1 p.m.
Geology 1707
Presented By:
- Sarah Brownlee - Wayne State University
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Seismic anisotropy, the directional dependence of seismic velocity, has been an invaluable tool for understanding strain and flow in the upper mantle. The utility of seismic anisotropy in the upper mantle can be attributed in part to a wealth of studies characterizing the properties of mantle rocks and minerals. In contrast, the continental crust is not nearly as well-characterized due in large part to its very small volume in relation to global seismic raypaths. The continental crust also poses numerous complexities in mineralogy and structure making it significantly more difficult to characterize. Recently, a number of studies have been focused on characterizing the full anisotropic elasticity of rocks from the continental crust. These studies have motivated efforts to predict how these rocks will appear in seismic observations, and thus to recalibrate the assumptions used in seismic inversions in order to improve our ability to distinguish various rock types and deformation in the continental crust. I will begin by discussing the basics of seismic anisotropy and how it is observed. Then we will delve into the catalogue of crustal rock properties, reviewing some of the trends in elastic symmetry with deformation and rock type in the continental crust. I will present a simple scaling scheme to allow for more realistic non-elliptical hexagonal elastic tensors in seismic inversions, and discuss how real crustal rocks might appear in seismic data. The take home message is that while the continental crust is complicated, it cannot be ignored, because even when the focus of study is the mantle, most of our observations are made through the window of the continental crust. Further characterization of the elastic properties of crustal rocks, and how these rocks are expressed in seismic data will improve our ability to use seismic methods to understand deformation in and beyond the continental crust.
Zachary Ross - The structural architecture of fault zones and its role in earthquake physics
Nov. 6, 2019
noon - 1 p.m.
Geology 1707
Presented By:
- Zachary Ross - Caltech
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The geometric and mechanical properties of fault zones are believed to affect many aspects of earthquake physics, including the seismic energy budget, aseismic processes, and earthquake triggering potential. Fault zones are multi-scale structures that encode a long history of deformation within them, providing additional constraints on how they form and evolve over their life cycle. Most of our knowledge about these structural properties comes from observations on the surface, but at depth, our understanding is far more incomplete. To improve on these shortcomings, we produced a comprehensive seismicity catalog (2008-2017) for the whole of Southern California with a template matching detection technique, which expanded the number of events by a factor of ten to 1.8 million. This detailed catalog illuminates the complex 3D geometries of fault zones at depth, enabling a better understanding of characteristics like damage zones, fault zone variations with depth, and orthogonal faulting. This information can be used together with estimates of earthquake source properties, observations of postseismic deformation and intricate spatiotemporal patterns of seismicity, to better understand the role of fault zone structure in earthquake physics.
Small-scale topography of the 660-km discontinuity and its implications in mantle geodynamics
Nov. 13, 2019
noon - 1 p.m.
Geology 1707
Presented By:
- Wenbo Wu - Caltech
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As a major interface in the solid-Earth, the 660-km discontinuity plays a critical role in our understanding of mantle geodynamics. Various seismic methods have been exploited to detect its properties, including sharpness, impedance contrast and topography. In this talk, I will present the observations of back-scattering seismic waves P'●660●P' and explain the constraints they provide on small-scale topography of the 660-km discontinuity. This small-scale topography must be caused by chemical heterogeneities, because any thermal anomalies in the deep earth would be smoothed out at a geological scale. I will discuss the possible interpretations of these chemical heterogeneities and their implications in the mantle material recycling.
Turning noise into signal and listening to the environment with seismic waves
Nov. 20, 2019
noon - 1 p.m.
Geology 1707
Presented By:
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The last decade has witnessed a renaissance in the breadth of applications of seismology, which extended into non-traditional areas with the aim of studying environmental processes. A unique kind of seismic sources, identified by the coupling of different Earth systems into the solid Earth, provides a new way for monitoring the global environment and for exploring the Earth’s interior. In this talk, I will present novel findings on the study of mass-wasting events and atmospheric-driven ocean storms through the analysis and modeling of seismic signals. In the first part of my talk, I will show how we can infer dynamics and main characteristics of remote landslides by using seismic data analysis and numerical modeling. In the second part of my talk, I will show how we can model seismic signals generated by ocean storms by numerical simulations, and how the time-varying behavior and strength of atmospheric and oceanic events can be estimated from decades of ambient seismic noise records.