May 12, 2023 — Coastal Institute Auditorium, 2 p.m.

Dr. Tianze Liu, Green Scholar Postdoc, Scripps Institution of Oceanography

Tracking Fluids and Melts in Earth’s Interior with Seismology: From Oceanic Transform Faults to the Lithosphere-asthenosphere System

Abstract

Fluids and melts play critical roles in many processes in the solid Earth due to their great mobilities and potent effects on medium properties. For example, increased fluid-filled porosity in a fault zone could inhibit earthquake ruptures through dilatancy strengthening, and the migration of fluids and melts could cause earthquake swarms on tectonic faults and in magmatic systems. At a greater scale, the accumulation of partial melts at the lithosphere-asthenosphere boundary beneath tectonically active regions may have formed a low-viscosity layer, effectively decoupling the lithosphere from the mantle below, whereas beneath stable cratons, infiltration of reactive melts and fluids from below could have caused widespread metasomatism in the mantle lithosphere, which may have planted the seeds for future craton destruction by reducing the strength of the lithosphere and causing density inversions in it. Despite the heavy involvement of fluids and melts in numerous solid-Earth processes, characterizing them in situ remains challenging because Earth’s interior is generally inaccessible for direct sampling, and traditional geophysical techniques usually lack the temporal and spatial resolutions to capture the complicated behaviors of fluids and melts in Earth’s interior. In the talk, I will present recent advancements in seismically characterizing fluids and melts in Earth’s interior through two examples. In the first example, I will show how we used differential travel times of nearby small earthquakes to reveal the spatial-temporal variation of the Vp/Vs in the fault zone of the Gofar transform fault near the East Pacific Rise during a one-year period at the end of a seismic cycle. Our observations, combined with rock physics models, suggest that the rupture barrier zone and the down-dip edge of the mainshock rupture zone likely differ in both pore-fluid content and chemical composition, which could explain their distinct slip behaviors. Moreover, we showed that the pore-fluid content and the percentage of thin cracks in the rupture barrier zone likely increased in the nine months before the main shock, which could be evidence of increased medium damage. In the second example, I will show how melts and fluids are at work at a greater scale by presenting a new high-resolution image of lithospheric discontinuities beneath the contiguous US constructed using the teleseismic SH-reverberation technique. In the tectonically active Western US, the image shows a lithosphere-asthenosphere boundary whose characters correlate well with ongoing tectonic processes, e.g., the concentrated extensions at the boundaries of the Basin and Range Province, and likely results from the accumulation of partial melts at the base of the plate. In the stable Central US, the image shows mid-lithosphere discontinuities with various depths and amplitudes, confirming their detections by previous studies. By conducting a detailed analysis of the data from two stations in the eastern Wyoming craton and the southwestern Superior craton, we showed that the MLDs beneath the two stations are likely formed by melt-assisted mantle metasomatism. These two examples demonstrate seismic imaging as a promising tool for unraveling the mystery around fluids and melts in Earth’s interior.