Seismologic evidence has consistently shown that displacement on normal faults occurs at a steep (>45°) angle, so geologists were surprised when, in the 1980s, they first identified major, low-angle extensional faults separating exposures of rocks from different crustal depths. These so-called detachment faults are associated with core complexes: distinctive dome-shaped bodies of metamorphic rocks that were brought to the surface as the result of extreme extension in Earth’s crust. Since their discovery, a debate has raged as to whether these detachment faults initially formed as steeper faults that were subsequently tilted or as gently dipping faults, which would make them a fundamentally different type of normal fault.
In the ensuing decades, the discovery of detachment faults around the globe and on other planets has bolstered the interpretation that detachments represent a different type of fault. However, after revisiting southeastern California’s Whipple Mountains, an archetypal example of a core complex whose detachment fault is widely believed to have initially slipped at a low angle, Gans and Gentry have reached the opposite conclusion—and reignited the debate.
By collecting a suite of new structural and geochronologic data centered upon a swarm of dikes intruded into the western half of the Whipple Detachment Fault, the team was able to better constrain the initial slip along this portion of the fault. Their findings offer strong evidence that this feature began its life not as a regional, low-angle detachment surface but rather as a high-angle normal fault that was later cut by a succession of younger, high-angle faults that rotated it as much as 60° to its current low angle. Although this conclusion differs radically from previous interpretations, it is consistent with standard rock mechanics as well as seismologic observations of active normal faults.
Because progressively greater slip on several generations of faults can rotate a structure to a lower angle, the main difference between normal and detachment faults may be the rate and magnitude of accumulated slip rather than their initial failure conditions, according to the authors. At least in the case of the archetypal Whipple Mountains, they argue, the detachment is not an inherently different type of fault. (Tectonics, https://doi.org/10.1002/2016TC004215, 2016)
—Terri Cook, Freelance Writer