University of Minnesota - Twin Cities
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.
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.
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.
Wayne State University
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.
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.