STRUCTURAL OBSERVATIONS IN UPPER-PLATE ROCKS OF THE CENTRAL MOJAVE METAMORPHIC CORE COMPLEX DO NOT SUPPORT MODELS OF LARGE-MAGNITUDE EXTENSION

In the central Mojave Desert of California, Miocene volcanic and volcaniclastic rocks overlie sheared metamorphic rocks and a Miocene pluton. The area has been interpreted to be a metamorphic core complex with 40 to 60 km of NE-directed extension along a low-angle normal fault between 22 and 18 Ma (Glazner et al., 1989; Fletcher et al., 1995). However, recent mapping and structural observations in the Miocene volcanic and volcaniclastic rocks (previously inferred to be “upper plate”) and their contact with the metamorphic rocks below (“lower plate”) reveal structural relationships that conflict with models of large-magnitude extension.

Observations and mapping in the Waterman Hills and Mount General areas show that: 1) “Upper-plate” volcaniclastic sedimentary rocks are uniformly tilted to the southwest and show no fanning relationships or other evidence of syn-depositional tilting. 2) There are no major normal faults within these sedimentary rocks. 3) The contact between the “upper plate” rocks and the metamorphic rocks below is complex but is not a fault in many places, where it is instead a depositional or intrusive contact. 4) Where the contact is a fault, the orientation and kinematics (both present-day and possible restorations) do not agree with what the core-complex model predicts. 5) Pre-, Syn-, and Post-extensional volcanic rocks in the “upper plate” occur in the same area and the syn-extensional rocks are the same age as, and likely sourced from, a Miocene pluton that intruded the “lower plate” directly beneath. These observations suggest that Miocene extension in the area was much less than previously hypothesized and that the “upper plate” was not translated more than a few km during the inferred episode of extension.

Although we are unsure of the exact cause of the Miocene extension in the central Mojave, our preferred interpretation is that the extension was related to emplacement of the Waterman Hills Granite and associated dike swarms and not large-magnitude extension. This study supports recent hypotheses that some metamorphic core complexes in the Basin and Range Province previously interpreted to have formed by large-magnitude extension may instead be the result of minor extension coupled with intrusion of plutons (e.g., Gans and Gentry, 2016; Anderson, 2017).