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

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.