Dike emplacement, footwall rotation, and the transition from magmatic to tectonic extension in the Whipple Mountains metamorphic core complex, southeastern California

Abstract

The Chambers Well dike swarm and associated plutonic/volcanic rocks in the western footwall of the Whipple Detachment Fault (WDF) provide key insight into the evolution of this metamorphic core complex. New structural and geochronologic data suggest that the western 12–15 km of exposed footwall is steeply tilted to the SW, providing a cross‐sectional view of the upper crust, from the Miocene erosion surface to the top of the coeval mylonitization. Ages and compositions of dikes are indistinguishable from adjacent thick volcanic successions. Several kilometers of early Miocene extension (~20.5 to 19.0 Ma) were accommodated by magmatic accretion but transitioned to rapid extensional faulting and tilting at 19.0–18.5 Ma. The subhorizontal WDF in this area initiated as a northeast dipping high‐angle (50–60°) normal fault that breached the surface locally, not in a breakaway tens of kilometers to the west. Large‐scale tilting and differential uplift of the western footwall was in part coeval with mylonitization and dike emplacement and was accomplished by block rotation in the hanging wall of additional normal faults, isostatic uplift, and flow of lower crust from beneath less extended regions to the west. The WDF is likely a composite surface with a western segment that had ceased moving by ~18.5 Ma, cut by successively younger and steeper fault(s) to the east. Perhaps, the most important difference between seismogenic high‐angle normal faults and low‐angle “detachment faults” characteristic of metamorphic core complexes is one of magnitude and rate of total accumulated slip, not of initial failure conditions.

Structural and Geomorphic Evidence for Rolling‐Hinge Style Deformation of an Active Continental Low‐Angle Normal Fault, SE Papua New Guinea

“…During rolling‐hinge activity, the hangingwall may be crosscut by new, steeply dipping splay faults that merge downdip into deeper and still active part of the original fault resulting in the capture of original hangingwall rocks into the footwall (rider block; e.g., Reston & Ranero, ). The shallow dip of some detachment faults might have also resulted from “domino”‐like tilting of more nearly rigid fault blocks, followed by their intersection by younger faults, leading to a composite detachment fault of different‐aged faults (Figure b, “domino block rotation” model, e.g., Proffett, ; Lister & Davis, ; Gans & Gentry, ). Differentiating between these processes—which are not mutually exclusive—is nontrivial (see Gans & Gentry, ).…”

“…The shallow dip of some detachment faults might have also resulted from “domino”‐like tilting of more nearly rigid fault blocks, followed by their intersection by younger faults, leading to a composite detachment fault of different‐aged faults (Figure b, “domino block rotation” model, e.g., Proffett, ; Lister & Davis, ; Gans & Gentry, ). Differentiating between these processes—which are not mutually exclusive—is nontrivial (see Gans & Gentry, ). Despite some geometrical similarities between these two models, the main processes forming them are quite different.…”

“…At low dips, faults may be crosscut by younger high‐angle normal faults that themselves eventually tilt to shallower dips. The final geometry may include a composite of different‐aged fault segments (the older “Detachment 1” and the younger “Detachment 2”) that resemble a single‐detachment fault (“composite detachment fault”; Gans & Gentry, ).…”

Synextensional dike emplacement across the footwall of a continental core complex, Chemehuevi Mountains, southeastern California

“…Assuming no preexisting anisotropies, dikes are predicted to intrude roughly orthogonal to the extension direction (Anderson, 1951), as they do in the Newberry Mountains (Spencer, 1985), Whipple Mountains (Gans and Gentry, 2016), Harquahala Mountains (Richard, et al, 1990), and Buckskin Mountains (Singleton, 2015). Large populations of Miocene dikes in the Chemehuevi Mountains strike east-west or northeast-southwest, parallel or oblique to the extension direction.…”

“…In the Whipple Mountains, the ca. 20.5-19.0 Ma Chambers Well dike swarm intruded prior to rapid tectonic extension beginning at 19.0-18.5 Ma (Gans and Gentry, 2016). In the adjacent Buckskin-Rawhide core complex, the ca.…”