T271 Constraints on Strain Rates, Stresses, and Deformation Processes During Shear Zone Localization at Different Lithospheric Levels

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room 200DE

USING HETEROGENEOUS QUARTZ CPO DATA TO INTERPRET THE DEFORMATION HISTORY OF A SHEAR ZONE AT THE BASE OF THE SEISMOGENIC ZONE

Paper number: T271-2
Program: Technical Sessions
Time: 1:45 PM – 2:00 PM

Authors

Abstract

Grain size data, CPO data, and other microstructural observations of quartz can be used to estimate deformation conditions within a shear zone, including differential stress, vorticity, and temperature. Using quartz data requires that the quartz microstructure records the deformation conditions of interest rather than a preserved stage of earlier deformation or a later overprint. For rocks with a multi-stage deformation history or rocks that experienced changing deformation conditions, quartz data can be particularly ambiguous and difficult to interpret. The frictional-to-viscous transition within a seismogenic shear zone is one such environment, where deformation behavior is both transitional with depth as well as variable with the seismic cycle. Using quartz CPO data from the Norumbega fault system (NFS; a long-lived, subvertical, transpressive, seismogenic fault system in Maine), we document the influence of a strong pre-existing fabric on CPO development and use across strike variations in CPO data to interpret deformation history.

The NFS is characterized by focused mylonitic shear zones exhumed from depths of ~10-15kms, some of which preserve mutually-overprinting pseudotachylyte and mylonite. Using a large dataset collected across a well-exposed section of one of these shear zones, we trace the transformation of the pre-existing fold fabric to the new shear fabric. In many samples, the pre-existing fabric (a C-axis peak in one hemisphere of the pole figure) persisted to create asymmetric and/or ambiguous shear CPO patterns. In other cases, transposition of the folded quartz ribbons placed the inherited c-axis peak into an orientation coincident with rhomb-a slip in quartz, facilitating the development of the new fabric and influencing quartz rheology. This suggests that a pre-existing fabric may facilitate or hinder the development of a new CPO depending on its orientation relative to the dominant slip system.

There is significant heterogeneity in quartz CPO patterns and microstructure across the shear zone. We propose that this heterogeneity reflects both (1) differences in the deformation behavior between rock types as the earlier fold generation was sheared and (2) changes associated with the complex deformation history of the seismogenic shear zone.