T121 Tectonic Inheritance in Orogenic Systems (Posters)

Monday, 23 September 2024
8:00 AM - 5:30 PM
Hall D

WIDESPREAD CATACLASIS AND PSEUDOTACHYLITE (?) FORMATION IN THE ENCHANTED ROCK BATHOLITH, CENTRAL TX

Paper number: T121-4
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

We present new field data for widespread cataclasis, ultra-cataclasis and possibly pseudotachylite formation in the 1.08 Ga Enchanted Rock batholith (ERb). These rocks were previously mapped (quite precisely) by Hutchinson (1956) and named mylonites. Investigation of m to 10’s of m’s wide “mylonite” belts that cut the ERb granite in two localities indicate that the belts are actually brittle fault zones that contain a variety of fault rocks from breccias to ultracataclasites and possibly pseudotachylite. We present a detailed outcrop mapping that records the following observations: (1) The fault rocks are characterized by large magnitudes of clast size changes (cm to < microns), anastomosing and braided hairline fractures filled with black, amorphous material, and dark grey, ultrafinegrained belts several centimeters wide that cut discordantly across coarse-crystalline granite at high angles and appear to be dike-like intrusions. (2) 790 subvertical faults (dip average of 79°) define two maxima with average strikes of 146/326° and 39/219°, forming conjugate sets ; (3) the fault sets cross-cut each other at a variety of scales; (4) the NE-SW striking faults are more abundant and have a larger maximum zone width of approximately .7 m; (5) 80% of the NE-SW striking faults are right-lateral, whereas 57% of the NW-SE striking faults are left-lateral; (6) other lithologic structures (i.e., joints, magmatic foliation, enclaves) are oriented parallel to the strikes of one or both of the fault sets, implying some lithologic control on strain localization. . (7) The fault density in one outcrop per m for 36 m’s of the NE-SW striking faults is 2, with a high of up to 7 faults within a single m. On-going work will attempt to determine whether or not the fine grained intrusive material is pseudotachylyte and if so, attempt to date its formation.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-404316

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