20 Micro to Macro: Linking Structural Geology, Petrology, Geochronology, and Tectonics Across Scales

Monday, 18 March 2024
8:00 AM - 12:00 PM
Piscataquog Room

RHEOLOGICAL BRIDGE ZONE, STRESS AMPLIFICATION, AND INITIALIZATION OF STRAIN LOCALIZATION

Paper number: 20-9
Program: Theme Sessions
Time: 10:20 AM – 10:40 AM

Authors

Abstract

The phenomenon of strain localization occurs throughout the crust, in both brittle and viscous regimes. However, the precise underlying causes remain a subject of ongoing debate. Observations in natural rocks suggest that alterations in phase properties, encompassing physical attributes, phase distribution, and grain geometry, wield a substantial influence on weakening processes. We evaluate that claim through a comprehensive exploration of the initial stages of strain accumulation at various pressure-temperature conditions. Our study focuses on three instances of weakly deformed rocks that exhibit localized zones, which we term "bridge zones," on a millimeter or smaller scale. These localized zones function as mechanical connectors between weak domains and consistently feature relatively small grain sizes within a narrow band. Rather than having a dominantly mechanical origin, we find that these zones appear to have developed largely from chemical processes, inducing phase mixing and reactions, with mechanical processes emerging as a secondary deformation mechanism. The dominant influence of chemical processes is substantiated by core-rim cathodoluminescence (CL) variations observed in fine-grained plagioclase and the occurrence of new phases (e.g., K-feldspar, quartz, titanite, etc.) at the triple-junctions of fine-grained feldspar. These findings suggest that reactions, dissolution-precipitation, and neocrystallization are pivotal in the development of fine grains within the bridge zones. Crucially, these zones form both within shear zones and in adjacent less-deformed rocks. Numerical modeling illustrates a spatial coincidence of high-stress areas with bridge zones, emphasizing their impact on reducing rock strength. These observations lead to a conceptual model in which far-field loading and microscale alterations catalyze the development of these bridge zones. Bridge zone characteristics likely scale up and relate to macroscale deformation localization, a fundamental component of plate tectonics, metamorphism, seismic activity, and other first order lithospheric processes.

Geological Society of America Abstracts with Programs. Vol. 56, No. 1

doi: 10.1130/abs/2024NE-397338

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