Insights into low-angle normal fault initiation near the brittle-plastic transition; A micro- to macroscale documentation of the Mohave Wash Fault system, Chemehuevi Mountains, Southeastern CA.

摘要

Rupture and continued slip on low-angle (≤30° dip) normal faults (LANFs) remain a mechanical paradox within our current understanding of deformation in the brittle crust. However, LANFs may form under distinct mechanical conditions, as weak faults or with atypical in-situ stresses, to allow formation within our current mechanical theory. In this thesis, I document a denuded LANF hosted in crystalline basement, the Mohave Wash Fault (MWF), active initially at the base of the seismogenic zone, in an effort to understand the evolution and mechanical properties of such faults during initiation and early slip.;Denuded exposures of the Miocene Chemehuevi detachment fault (CDF) system exposed in the Colorado River extensional corridor (CREC) provide a natural laboratory to study LANF initiation near the base of the seismogenic zone (~5 to >10 km paleodepth). The regional fault system formed with a gentle dip (≤30°) in heterogeneous gneissic and granitoid rocks, and is characterized by three stacked LANFs that initiated at 23 +/- 1 Ma. Originally, the CDF system spanned temperature conditions from ~150°C to >400°C, rooted to the brittle-plastic transition (BPT). The CDF preferentially localized ≥18 km of NE-directed slip rendering the deepest fault, the MWF, inactive after 1--2 km of slip preserving its original fault properties.;Across its 23-km down-dip exposure, the brittle MWF is a 10- to 60-meter thick zone dominated by cataclasite series fault rocks defined by discontinuous or anastomosing, altered principal slip zones within a damage zone of dense fractures that rarely host pseudotachylite. Altered portions of the principal slip zones were likely produced from fluid flow after initial rupture. The footwall to the MWF hosts localized quartz mylonites and Miocene dikes (of the Chemehuevi dikes swarm) bearing a penetrative mylonitic fabric that increase in volume, intensity, and estimated deformation temperature down-dip. Footwall mylonites are absent in the westernmost exposures of the fault to 9 km down dip; they are locally preserved from 9 to 23 km down dip, and widely distributed at ≥23 km down dip in the deepest exposures of the fault. Mylonitic deformation was accommodated by dislocation creep in quartz mylonites and by diffusion creep with grain boundary sliding in syntectonic dikes. These data support that initial extension occurred across the upper limit of the quartz brittle-plastic transition in a semi-brittle fashion through coeval brittle (seismogenic) slip on the MWF and localized to distributed footwall mylonitization.;The compositionally heterogeneous, calc-alkalic to alkali-calcic Miocene Chemehuevi dike swarm intruded into the footwall and damage zone of the CDF system during the first ~ 1.5--3.8 Ma of extension. Intermediate to felsic dikes truncated by the MWF were emplaced from 21.45 +/- 0.19 Ma to 19.21 +/- 0.15 Ma. These intermediate-to-felsic dikes do not exhibit a mylonitic fabric 0--18km down dip; in the deepest exposures (≥18 km down dip), they are gently folded, rotated, and host a well-developed mylonitic foliation at, even when hosted by non-mylonitic country rock. In contrast, mafic dikes were intruded episodically during early fault slip and, as such, are preserved within the MWF zone. The relative timing of dike emplacement implies that the CDF system operated in an "active rifting" environment with the main pulses of diking/magmatism prior to rapid, denudation-related CDF slip. Whereas the style of intrusion, deformation, and active rifting evolution of the Chemehuevi Dike swarm is well constrained, the mechanical influence of synextensional diking are less obvious.;I conclude that the MWF accommodated extension by coeval seismogenic rupture/brittle deformation at shallow depths, with plastic deformation at structurally deeper levels in the crust. This history was overprinted by a complex pattern of brittle slip, fluid flow, intrusive magmatism, and continued localization of strain manifested as footwall mylonitization in syntectonic dikes. Following these phases of MWF slip, regional slip localized onto the structurally shallower CDF rendering the MWF inactive. Despite the evidence of the CDF system forming in an active rifting environment, the MWF, a representative precursor to the CDF, lacks clear evidence of significant fault-weakening mechanisms or stress rotation during slip, and thus remains unexplained by Andersonian fault mechanics.