HOW DOES HETEROGENEITY IMPACT THE RHEOLOGY OF THE DEEP SEISMOGENIC ZONE?

Drew Levy, University of Washington
Sophie Johnson, University of Washington
Emberlee McPherron, University of Washington
Cailey Condit, University of Washington
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Abstract

Crustal deformation along tectonic plate boundaries is accommodated by fault systems that shape our landscapes over millions of years and generate geologic hazards over seconds to decades. The primary hazard posed by these faults are earthquakes, which occur in the brittle upper crust. Below this seismogenic zone, faults transition into a ductile regime where deformation occurs by aseismic, viscous shear. The temperature-dependent depth of the brittle-ductile transition zone limits the depth of seismic slip, which dictates rupture area, energy release, and earthquake magnitude. Constraining the geological mechanisms that are responsible for the transition from seismic to aseismic slip in this broad region is key to understanding where and why this transition occurs. This has significant implications for resultant earthquake magnitudes and hazards. In this presentation, we explore the influence of lithological heterogeneity on the nucleation and propagation of earthquakes within an ancient, exhumed brittle-ductile transition zone along the Norumbega fault system, Maine, USA. This exposure of the deep seismogenic zone allows us to study the geology of the earthquake source, which is otherwise inaccessible along active faults such as the San Andreas. Our study focuses on the Great Common fault zone – a late Paleozoic strike-slip fault now exposed along coastal wavecut terraces in southern Maine and New Hampshire. Using outcrop-scale geological mapping and microstructural analysis, we investigate the spatial distribution of lithologic units, deformation fabrics and evidence of fossilized earthquakes (i.e., pseudotachylyte). We find broad heterogeneity across the km-wide fault zone, as well as at the outcrop scale, influences the location of earthquake rupture. High-strain ductile fabrics (ultramylonites) and pseudotachylyte veins are localized along lithologic contacts where viscosity contrasts locally amplified shear stress. The core of the fault zone is a 50–100 m-wide amphibolitic ultramylonite that is dominated by recrystallized and sheared pseudotachylyte, which we interpret as the primary mechanism of grain size reduction. A switch to grain-size sensitive deformation mechanisms within the ultramylonite would have significantly altered the strength of the shear zone, potentially increasing the strain rate and drawing the brittle-ductile transition to greater depth. Over seismic timescales, stress amplification due to lithologic heterogeneity may expand the width of the seismogenic zone. The modification of fault rocks during the seismic cycle may play a significant role in the rheologic evolution of faults over tectonic timescales.

Session

Deformation in the upper crust