Fault localization controlled by fluid infiltration into mylonites: Formation and strength of low-angle normal faults in the midcrustal brittle-plastic transition

摘要

Minidetachments (MDs) found in the uppermost footwall of the Whipple low-angle normal fault record physical and chemical conditions of LANF formation and early history. MDs are subparallel to the Whipple LANF and mimic features of that fault on a small scale. Principal slip surfaces and R _1 Riedel shear fractures parallel C and C planes, respectively, in adjacent mylonites. Thus, MDs likely formed subparallel to planes of maximum shear stress and were not severely misoriented during initial rupture of intact rock. Damage zones contain secondary epidote, titanite, chlorite, calcite, and felspars. Breccias record volume gains via enrichment in all elements relative to immobile Fe-Ti-Zr-P, and ultracataclasites record volume losses. Epidote and titanite are locally porphyroclastic in mylonites; cataclasites contain both old shattered fragments and new euhedral grains of these minerals. Pseudosections constrain alteration, the end of mylonitization, and cataclasis to T = 380-420°C. Fluid inclusions with 17-20 wt% CaCl _2 were entrapped at 270-290, 170-200, and 80-130 MPa, consistent with a drop from lithostatic toward hydrostatic P _(fluid) at ～9.5 km depth. MDs thus record (1) infiltration of reactive fluids into a mid-crustal shear zone; (2) reaction strengthening at the locus of maximum infiltration and sealing; (3) brittle fault slip triggered by fluid overpressure; and (4) permanent embrittlement following reduction of P _(fluid). The brittle-plastic transition and crustal strength maximum were strongly modified by fluid- and reaction-driven mineralogical changes. At any given point in space or time, this "transition" may thus be very thin, corresponding to the thickness of the altered zones surrounding nascent LANFs.