HOW THE WHIPPLE DETACHMENT FAULT FORMED IN THE CONTINENTAL BRITTLE-PLASTIC TRANSITION

Low-angle normal faults (LANFs) are common and tectonically significant in highly extended continental regions and passive margins. However, they remain enigmatic, being the mechanically least-understood major fault type: how they form is uncertain, especially if a gently dipping anisotropy is lacking in the brittle crust; even explanations for how they slip are contentious. Paleoseismic slip on LANFs is well documented but the historical record of significant LANF earthquakes is contentious.

The Whipple detachment fault (WDF) is a well-studied Oligo-Miocene LANF that developed in normal, strong, quartzo-feldspathic crust, slipped ~50 km top-NE (at least partly seismogenically), and displays most features of classic Cordilleran metamorphic core complexes. Combining published results from the WDF footwall allows characterization of the full stress tensor during WDF formation in the brittle-plastic transition (BPT), yielding a simple, Mohr diagram-based, mechanical model of WDF formation that is consistent with experimental fault mechanics and geological relations. The WDF formed by frictional slip on mica-rich mylonitic C planes in the BPT, which formed "minidetachments" that I infer coalesced into a through-going WDF. Embrittlement probably was driven by metamorphic crystallization of strong epidote. The WDF formed at a gentle dip (0° - 30° NE), at ~9.5 km depth and 380°-420°C, under differential stress of ~136 MPa. Maximum shear stress acted on the C planes, minidetachments, and the nascent WDF. Thus, all were oriented ~45° to the maximum principal stress, which plunged ~45°-75° NE in the BPT. Upon embrittlement and before temperature dropped, pore-fluid pressure dropped rapidly from lithostatic to hydrostatic values, presumably due to newly formed fracture permeability. Embrittlement occurred in the same temperature range as the first incursion of meteoric water into the footwall. WDF formation and early slip did not require pore-fluid pressure elevated above hydrostatic, anomalously low effective normal stress, nor anomalously low friction. I hypothesize that the WDF formed in the BPT as described above, and propagated upward in-plane, into and through the brittle crust, possibly steepening somewhat as the maximum principal stress steepened upward from moderate plunge in the BPT.