‘MINIDETACHMENT' LOCALIZATION CONTROLLED BY FLUID INFILTRATION IN THE FOOTWALL OF THE WHIPPLE MOUNTAINS CORE COMPLEX: WEAK MATERIALS OR HIGH PORE PRESSURE?

Low-angle normal faults apparently slip at high angles to σ₁, leading to the hypothesis that they represent weak faults. Possible weakening mechanisms include generation of high P_f within the fault zone or of weak materials that allow slip on unfavorably oriented surfaces. Here we use data from footwall ‘minidetachment’ zones (MDs) to assess the timing and consequences of fluid-rock interaction relative to MD rupture and slip. MDs mimic the orientation, architecture, and kinematics of the main Whipple fault, but have trace lengths of only 1-50 m; hence, fault core rocks were derived from immediately adjacent mylonites. Petrographic and chemical transects across six MDs show that fluid infiltration occurred during mylonitization and prior to cataclasis: 1) Footwall and hangingwall mylonites show increasing abundances of secondary minerals, chlorite or epidote + sphene ± chl, towards MD zones. 2) These minerals line mylonite S–C planes, epid shows grain-size reduction in ductile shears, and sph has deformation lamellae. Both sph and epid are truncated at breccia clast boundaries and are dismembered in breccia matrices. 3) Chl forms an axial planar foliation to folds in one MD that shows evidence for synchronous cataclasis and folding of mylonites. Chl breccia truncates the folds, but both breccia and folds die out into the mylonitic foliation. Opening-mode fluid inclusion trails in the mylonites are subperpendicular to the stretching lineation but are reoriented around the folds. 4) Whole-rock data show progressive decreases in Si and Na and increases in Fe, Ca, and Ti towards the fault cores; these chemical modifications are consistent with formation of the observed secondary minerals, so must also predate cataclasis. The observation that minidetachments are localized where fluid infiltrated during mylonitization and prior to cataclasis and fault slip suggests a causative relationship between fluid channelways and faulting. Formation of interconnected chl during mylonitization could have provided weak layers that localized slip, but epid+sph would not promote weakening. Opening-mode microcrack data are consistent with locally elevated P_f prior to cataclasis. The data do not require a causative relationship between high P_f and low-angle normal faulting, but they support such a link during minidetachment formation.