Article
Constraints on paleofluid sources using the clumped-isotope thermometry of carbonate veins from the SAFOD (San Andreas Fault Observatory at Depth) borehole
Tectonophysics
(2016)
Abstract
The San Andreas Fault Observatory at Depth (SAFOD), near Parkfield, California, is a borehole drilled through two active deforming zones of the San Andreas fault, the Southwest Deforming Zone (SDZ) and the Central Deforming Zone (CDZ). These zones accommodate displacement by seismic slip and aseismic creep. Elevated fluid pressures and fluid-rock interactions have been proposed to explain the low apparent strength and aseismic creep observed, but the origin of the fluids and existence of high fluid pressures remains uncertain. We use clumped-isotope thermometry and delta (super 18) O-delta (super 13) C compositions of calcite in veins to constrain the origin of paleofluids and compare these results to the isotopic composition of modern-day pore fluids from the SAFOD borehole and nearby areas. We observe that: (1) calcite vein temperatures vary from 81 to 134 degrees C, which overlaps the current ambient borehole temperatures of 110-115 degrees C at sampled depths; (2) vein calcite is not in carbon isotope equilibrium with modern-day pore fluids; (3) the delta (super 18) O values of paleofluids close to the SDZ and CDZ, calculated from vein delta (super 18) O and temperature data, are not in equilibrium with local modern-day pore waters but approach equilibrium with modern pore waters far from these zones; and (4) syntectonic vein calcite is only in C- and O-isotopic equilibrium with their host rocks within the SDZ and CDZ. Spatial patterns of delta (super 18) O and delta (super 13) C show little evidence for across-fault fluid-flow. Clumped isotope temperatures are consistent with locally-derived fluid sources, but not with continuous or episodic replenishment of fluids from shallow sedimentary brines or deep fluid sources. Our findings are compatible with flow of meteoric fluids from the southwestern damage zone into the SDZ and CDZ, which would have favored the formation of weak phyllosilicates and contributed to the present day weakness of the two actively deforming zones.
Keywords
- fault zone hydrology,
- alteration,
- San Andreas Fault,
- SAFOD,
- geochemistry,
- stable isotopes
Disciplines
Publication Date
2016
DOI
http://dx.doi.org.dist.lib.usu.edu/10.1016/j.tecto.2016.05.024
Citation Information
Benjamin Luetkemeyer, David Kirchner, Katherine Huntington, Judith S. Chester, et al.. "Constraints on paleofluid sources using the clumped-isotope thermometry of carbonate veins from the SAFOD (San Andreas Fault Observatory at Depth) borehole" Tectonophysics Vol. 690 (2016) p. 174 - 189 ISSN: 0040-1951 Available at: http://works.bepress.com/james_evans/126/
Creative Commons license
This work is licensed under a Creative Commons CC_BY International License.