131 Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems

This session explores key structural processes that govern the temporal, mechanical, and rheological evolution of crustal/lithospheric extensional and strike-slip systems with a focus on novel field observations, microstructural and analytical methodologies, and numerical modeling.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 217D

Detailed Fluid Records from the Whipple Mountains Core Complex.

Paper number: 131-5
Program: Technical Sessions
Time: 2:45 PM – 3:05 PM

Authors

Abstract

Extensional detachment fault systems of metamorphic core complexes (MCC) have the potential to facilitate significant fluid movement between Earth’s surface and the ductile middle crust. Oxygen isotopes have long been recognized as potential tracers of surface-to-depth fluid flow in MCCs, and could provide a record of fault-controlled hydrothermal activity and fluid-assisted deformation. However, typical sampling methods and the competing effects between temperature and fluid composition have made oxygen isotope signatures of meteoric fluid infiltration difficult to detect and/or interpret in the MCC rock record. This study uses in situ secondary ion mass spectrometry (SIMS) δ18O analysis as a tool to investigate fluid-rock interactions in the Whipple Mountains MCC footwall mylonites at a new level of detail. Here we present two datasets: (1) the rock record of meteoric fluid interactions and temperature variation beneath the Whipple detachment fault (WDF) using SIMS δ18O measurements in quartz and epidote; (2) in situ SIMS δ18O measurements in feldspar porphyroclasts from quartzofeldspathic mylonites with multiple crosscutting brittle and ductile deformation microstructures to characterize intragrain oxygen isotope zoning, and relate δ18O signatures with deformation microstructures and fluids effects.   All of the phases investigated by SIMS – epidote, quartz, feldspars – record interactions with low- δ18O fluids during deformation-related recrystallization, though the relative timing and extent of interaction varies as a function of depth. Quartz and feldspars are modally and rheologically important phases in the Whipple Mountains detachment system, and the low-δ18O values preserved within plastically deformed domains of these minerals implicates fluid infiltration in fault weakening. Feldspars in particular preserve a rich history of alternating brittle and ductile deformation behavior that suggests initial meteoric fluid infiltration via pervasive microcracking and a subsequent switch to creep deformation mechanisms.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-411958

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