Fluid-Control of the Crustal Brittle-Plastic Transition (BPT) During Extension: Permanent Embrittlement at High Temperatures

Tuesday, 16 December 2014: 10:20 AM
Gary J Axen, New Mexico Institute of Mining and Technology, Socorro, NM, United States and Jane Selverstone, University of New Mexico, Albuquerque, NM, United States
The footwalls of the Whipple detachment (SE California) and Brenner and Simplon lines (Alps) passed up through the BPT and were carried to the surface with minimal overprints, allowing study of products formed there and inference about BPT processes (Selverstone et al. 1995; Axen et al. 1995; Wawrzyniec et al. 1999; Axen et al. 2001; Selverstone et al. 2012).

In all cases embrittlement (mm – 100 m scale) was due to fluid effects and was at higher T (380 to >500°C) than expected for quartzose rocks, not due to monotonic cooling. Depths of embrittlement are typical (~9.5-18 km), so heat was advected with the footwalls. Strain rates are poorly known but may have been high. High fluid pressure (Pf) during mylonitic flow apparently aided embrittlement: fractures formed and drained fluids, Pf dropped rapidly, and plastic deformation shut off. Evidence of return to plastic processes is absent or sparse; if embrittlement was related to the seismic cycle, plasticity was not renewed in interseismic periods. Specifics differ: in the Whipple footwall, reaction strengthening preceded embrittlement, sealed and locked up mylonites, and Pf increased, aiding embrittlement. In the Brenner footwall, a thin (~50 m) hydrously altered mylonite zone persisted above deeper brittle mylonites, suggesting that deeper cracks delivered hydrous fluids upward to a weakened, overlying plastic shear zone, locally inverting the BPT. In the Simplon footwall, along-strike structural changes correspond to a change from hydrous to hydrous + carbonic fluids: non-wetting carbonic fluids blocked fluid migration and Pf rose until brittle failure occurred.

Including transtensile (e.g., Griffith) failure, fluid pressure and/or fluid-rock reactions will improve BPT models. Thrust upper plates may behave similarly but the stress field should favor subhorizontal cracks and opening may be subdued so the BPT may evolve differently. Cracking in the upper part of a subhorizontally shearing BPT may deepen it.