The role of calcite-rich metasedimentary mylonites in localizing detachment fault strain and influencing the structural evolution of the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Abstract

Although crystalline rocks dominate the footwall of the Buckskin-Rawhide detachment fault in west-central Arizona (USA), we estimate that thin (<1 to 100 m thick) calcite-rich metasedimentary mylonites were present along 25%-35% of the detachment fault, and in parts of the footwall they were continuous for ~30 km in the slip direction. New field observations, geochronology, and detailed microstructural data provide insight into the origin of these metasedimentary rocks and their role in the structural evolution of the detachment fault system. We propose that calc-mylonites along the Ives Peak footwall corrugation were derived from locally overturned Pennsylvanian-Permian strata that were buried to mid-crustal depths beneath a southeast-vergent Cretaceous thrust fault, which was reactivated in the Miocene by the parallel Buckskin detachment fault shear zone. In some areas these laterally persistent calc-mylonites were smeared out along the detachment fault during incisement into the crystalline footwall, forming a thin carapace of rheologically weak rocks structurally below the original weak zone. Metasedimentary mylonites consistently record top-to-the-northeast simple shear parallel to the detachment fault slip direction. Strain, synmylonitic veins, and paleostress recorded in these mylonites increase toward the detachment fault. Marble mylonites <1 m below the detachment fault preserve strong calcite crystallographic preferred orientations and lack cataclastic deformation that characterizes quartz-rich rocks along the detachment fault. In addition, unlike quartzofeldspathic mylonites, calc-mylonites typically lack extension via postmylonitic normal faults and associated horizontal axis rotation. Paleopiezometry and rheological modeling of the metasedimentary mylonites suggest that when quartzite layers were being sheared at ~100 MPa and 10 −13 to 10 −14 s −1 near the brittle-plastic transition, marble layers could have been strained ~100× faster at ~20 MPa. Detachment fault strain localized within the metasedimentary rocks, and the calcite marbles exerted significant control on the rheology of the footwall shear zone. This study highlights the important role that inherited weak zones may play in influencing the location, geometry, rheology, and style of deformation associated with detachment fault systems.

Distributed Neogene faulting across the western to central Arizona metamorphic core complex belt: Synextensional constriction and superposition of the Pacific–North America plate boundary on the southern Basin and Range

“…1). This belt of core complexes parallels the WNW-trending margin of the Maria fold-and-thrust belt, a zone of Cretaceous thick-skinned shortening that likely influenced the geometry and location of Miocene extension (Spencer and Reynolds, 1991;Singleton et al, 2018;Spencer et al, 2018). Located ~60-100 km west-northwest of the White Tank Mountains, these core complexes were unroofed by ~45 km of top-to-the-NE displacement across the Eagle Eye-Bullard detachment fault system in the early to middle Miocene (ca.…”

Reconstruction of Mid‐Cenozoic Extension in the Rincon Mountains Area, Southeastern Arizona, USA, and Geodynamic Implications

“…As noted above, mylonitic fabrics in lower plate rocks in the Tanque Verde Ridge area are exposed over ~25 km parallel to lineation, a greater lineation‐parallel distance than in any other core complex in southwestern North America except the Harcuvar complex in western Arizona (Singleton & Mosher, ; Spencer et al, ). As with the Rincon Mountains, the geometry of the detachment‐fault system in the Buckskin and Rawhide Mountains of the Harcuvar complex was apparently influenced by the distribution of carbonate tectonites that form slivers and sheets within or above quartzo‐feldspathic rocks in the detachment‐fault footwall (Singleton et al, ). This association suggests that these weak zones led to greater extension‐parallel exposures of mylonitic quartzo‐feldspathic footwall rocks than in other southwestern core complexes.…”

“…These carbonates would have been a zone of weakness near the base of an otherwise brittle, quartzo-feldspathic upper crust (Brodie & Rutter, 2000). Furthermore, such a zone of weakness would be most significant at the strongest part of the brittle upper crust, at temperatures of~250-400°C (e.g., compare dislocation-creep flow-law parameters of Hirth et al (2001) for quartz and Renner et al (2002) for calcite, as plotted by Singleton et al (2018, Figure 14)). Such weakening would be especially effective in the cratonic setting of Arizona's core complexes because of the abundance of quartzo-feldspathic crystalline rocks.…”

Tectonic Inheritance Following Failed Continental Subduction: A Model for Core Complex Formation in Cold, Strong Lithosphere

“…One leading concept for LANF activity is that these faults slip at low angles in the brittle crust due to the presence of intrinsically low‐friction gouge minerals and/or high pore fluid pressure, and geological observations suggest such faults on the continents sole into low‐angle mylonitic shear zones at or below the brittle‐ductile transition (BDT; e.g., Lister & Davis, ; Axen, ; Collettini et al, ). In this case, the LANF may have been reactivated from a preexisting low‐angle structure, such as a thrust fault (e.g., Singleton et al, ), or it potentially may have nucleated in intact rock at a primary low dip, with the latter phenomenon being harder to explain mechanically (e.g., Collettini & Sibson, ; Sibson, ). An alternative concept is that most LANFs initiate as high‐angle normal faults through the brittle crust and are later rotated back via a “rolling‐hinge” process to low angles at shallow depths as a result of tectonic unloading and consequent flexural and isostatic footwall uplift, possibly aided by focused lower crustal flow or asthenospheric upwelling (e.g., Brun et al, ; Lavier et al, , ; Lister & Davis, ; Platt et al, ).…”