Exhumation of the Coyote Mountains Metamorphic Core Complex (Arizona): Implications for Orogenic Collapse of the Southern North American Cordillera

Abstract

A microstructural and thermochronometric analysis of the Coyote Mountains detachment shear zone provides new insight into the collapse of the southern North American Cordillera. The Coyote Mountains is a metamorphic core complex that makes up the northern end of the Baboquivari Mountains in southern Arizona. The Baboquivari Mountains records several episodes of crustal shortening and thickening and regional metamorphism, including the Late Cretaceous‐early Paleogene Laramide orogeny which is locally expressed by the Baboquivari thrust fault. Thrusting and shortening were accompanied by magmatic activity recorded by intrusion of Paleocene muscovite‐biotite‐garnet peraluminous granites such as the ~58 Ma Pan Tak Granite, interpreted as anatectic melts representing the culmination of the Laramide orogeny. Following Laramide crustal shortening, the northern end of the Baboquivari Mountains was exhumed along a top‐to‐the‐north detachment shear zone, which resulted in the formation of the Coyote Mountains metamorphic core complex. Structural and microstructural analysis show that the detachment shear zone evolved under a strong component of noncoaxial (simple shear) deformation, at deformation conditions of ~450 ± 50°C, under a differential stress of ~60 MPa, and a strain rate of 1.5 × 10−11 to 5.0 × 10−13 s−1 at depth of ~11–14 km. Detailed 40Ar/39Ar geochronology of biotite and muscovite, in the context of the deformation conditions determined by quartz microstructures, suggests that the mylonitization associated with the formation of the Coyote Mountains metamorphic core complex started at ~29 Ma (early Oligocene). Apatite fission track ages indicate that the footwall of the Coyote Mountains metamorphic core complex experienced rapid exhumation to the upper crust by ~24 Ma. The fact that mylonitization and rapid extensional exhumation postdates Laramide thickening by ~30 Myr indicates that crustal thickness alone was insufficient to initiate extensional tectonic and required an additional driving force. The timing of mylonitization and rapid exhumation documented here and in other MCCs are consistent with the hypothesis that slab rollback and the effect of a slab window trailing the Mendocino Triple Junction have been critical in driving the development of the MCCs of the southwest.

Late Oligocene–Early Miocene Detachment Faulting in Crystalline Basement, Mecca Hills, CA, Documented With Zircon (U–Th)/He Date‐eU‐Visual Radiation Damage Patterns

“…Identification of core complex-like large magnitude extension is compatible with previously documented detachment faults in seismic reflection profiles offshore of the Los Angeles Basin (Crouch & Suppe, 1993) and similar low-temperature Cenozoic thermal histories between the Transverse Ranges and the Basin and Range sensu stricto (Blythe et al, 2000). Upper crustal extension documented in the Orocopia Mountains detachment fault system is also coeval with the timing of extension in the Catalina-Rincon metamorphic core complex (MCC) (Fayon et al, 2000;Jepson et al, 2021) and in the Coyote Mountains MCC (Gottardi et al, 2020). Prior compilation of geo-and thermochronometry data from MCCs throughout the southwestern Cordillera was used to infer that eastern MCCs exhumed earlier (∼29-23 Ma) than the western MCCs of the Colorado River extensional corridor (∼22-15 Ma; Gottardi et al, 2020 and references therein).…”

“…Prior compilation of geo-and thermochronometry data from MCCs throughout the southwestern Cordillera was used to infer that eastern MCCs exhumed earlier (∼29-23 Ma) than the western MCCs of the Colorado River extensional corridor (∼22-15 Ma; Gottardi et al, 2020 and references therein). This pattern was attributed to the westward propagation of volcanic activity and migration of the Mendocino Triple Junction together with dynamics of the Farallon slab (Gottardi et al, 2020;Ingersoll et al, 2014;McQuarrie & Oskin, 2010;Putirka & Platt, 2012). If development of the Orocopia Mountains detachment fault system mirrors the large magnitude extension that characterizes MCCs, then this implies that the upper crustal thermal and tectonic effects of the evolving plate boundary are more widespread in the southwestern Cordillera at ∼24 Ma.…”

Where did the Arizona‐Plano Go? Protracted Thinning Via Upper‐ to Lower‐Crustal Processes

“…In southern Arizona, the onset of MCC detachment faulting varies between ca. 30 and 20 Ma (e.g., Gottardi et al., 2020). The onset of detachment faulting gets younger northward from northern Mexico (from 35 to 20 Ma) central Arizona and southward from south‐eastern California (from 24 to 20 Ma) to central Arizona (Gottardi et al., 2020; Howlett et al., 2021, and references therein).…”

Magmatism and Extension in the Anaconda Metamorphic Core Complex of Western Montana and Relation to Regional Tectonics

“…Data from several MCCs in northern Mexico (e.g., Sierra Mazatan, Aconchi) are not included in our compilation because they exist south of the area that experienced the ignimbrite flare-up. That said, recent studies from this region confirm a younger-to-the-northwest trend for the onset of MCC exhumation (Gottardi et al, 2018(Gottardi et al, , 2020Wong & Gans, 2008;Wong et al, 2010).…”