Article
Pseudotachylyte and Fluid Alteration at Seismogenic Depths (Glacier Lakes and Granite Pass Faults, Central Sierra Nevada, USA)
Pure and Applied Geophysics
(2015)
Abstract
We present evidence for ancient seismicity in the form
of tectonic pseudotachylyte and coeval, cyclic hydrothermal alteration,
and cataclasis along fault zones exhumed from 2.4 to 6.0 km in the
central Sierra Nevada, CA. The Glacier Lakes fault (GLF) and Granite
Pass fault (GPF) are exhumed left-lateral to left-lateral oblique, strikeslip
faults with up to 125 m of left-lateral separation exposed in
Mesozoic granite and granodiorite plutons. Precipitation of epidote
along fault slip-surfaces, chloritization of biotite, saussurite and sericite
alteration of plagioclase, and quartz- and-calcite filled veins are present
in the GLF and GPF zones. One difficulty encountered in studying
exhumed fault zones is providing convincing evidence for a frictional
melt origin of pseudotachylyte. Rocks in the field may preserve convincing
evidence for frictional melt (i.e., aphanitic, dark, injection
structures) that are later shown to be related to cataclasis or injection of
hydrothermal fluids. Another challenge results from the low preservation
potential of several of the microscopic features that are
convincing evidence of a frictional melt origin (microlites, amygdules,
and glassy matrix). Here we test the usefulness of grain shape and
nearest neighbor distribution analysis of pseudotachylyte and cataclasites
from the GLF and GPF to discriminate between these fault rocks
and to determine a frictional melt origin for pseudotachylyte. Fabric
analyses of the clasts within the pseudotachylytes examined are more
circular and exhibit a random nearest neighbor clast distribution relative
to adjacent cataclasites. With increased comminution and melting
the mean clast circularity increases and the nearest neighbor distances
approach a random distribution. We conclude that this observed pattern
can be applied to other fault zones as an indicator of a frictional
melt origin for fault-related rocks. Mutually cross-cutting zones of
hydrothermal alteration and calcite deformation twins constrain the
ambient temperature conditions in the fault zone to between 170 and
320 C during pseudotachylyte formation. Based on previous thermochronologic
studies, the temperature conditions of the country rock
during faulting were between 110 and 220 C. The overlapping to
elevated temperatures in the fault zone can be explained by: (1)
infiltration of hydrothermal fluids into the fault zone; or (2) residual
elevated temperatures as a result of frictional heating in the fault zone
during seismic slip.
Keywords
- Pseudotachylyte,
- hydrothermal alteration,
- earthquake cycle,
- fault,
- fault rocks,
- seismicity.
Disciplines
Publication Date
2015
Citation Information
James P. Evans. "Pseudotachylyte and Fluid Alteration at Seismogenic Depths (Glacier Lakes and Granite Pass Faults, Central Sierra Nevada, USA)" Pure and Applied Geophysics Vol. 172 (2015) p. 1203 - 1227 Available at: http://works.bepress.com/james_evans/132/
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This work is licensed under a Creative Commons CC_BY International License.