Discovery of Major Basement‐Cored Uplifts in the Northern Galiuro Mountains, Southeastern Arizona: Implications for Regional Laramide Deformation Style and Structural Evolution

Abstract

The Laramide orogeny is poorly understood in southeastern Arizona, largely because of complex structural overprinting by mid-Cenozoic extension that occurred over large areas. This study integrates new geological mapping with previous work, combined with structural reconstructions and forward modeling, to determine the primary structural style, timing, evolution, and kinematics of Laramide shortening in the northern Galiuro Mountains. Cenozoic normal faulting in the study area is minor and has only resulted in up to 13°of eastward tilting, as indicated by the gentle dips of synextensional strata. Detailed mapping has revealed newly identified reverse fault systems measuring at least 50 km in combined strike length. Each major fault strikes north-northwest, dips moderately to the west, places older rocks on younger, and has related fault-propagation folds. Once restored to their original orientation, reverse faults range in dip from 38°to 47°. These moderate dips of faults combined with related folds, the significant degree of basement involvement, and cover sequence lacking obvious penetrative deformation indicate that these faults are thick-skinned, basement-cored uplifts. Forward modeling and Cenozoic erosion surfaces suggest regionally extensive Laramide-age tilting to the west-southwest and gentle folding, possibly caused by a regional-scale reverse fault underlying the study area. These results are consistent with the interpretation that Laramide shortening in southeastern Arizona was primarily characterized by thick-skinned tectonics. Kinematic indicators, folded basement rocks, north-northwest strikes of reverse faults, and lack of evidence for basin inversion suggest that preexisting basement faults and fabrics had little or no effect on the subsequent structural evolution.Plain Language Summary The Late Cretaceous to early Eocene Laramide orogeny was a period of crustal shortening in the North American Cordillera that involved two different styles of reverse faulting. One style involves low-angle thrusts that typically slip parallel to bedding planes in layered rocks, whereas the other style involves faults that cut across bedding at moderate angles and continue downward through underlying crystalline basement rock. In southeastern Arizona, the style of Laramide shortening is debated and not well understood, in part because most of the region has undergone subsequent Cenozoic extension that has significantly rotated, dismembered, and buried most faults formed during Laramide crustal shortening. This study examines a newly discovered set of Laramide reverse faults that extend for more than 50 km along strike and that have only been affected by minor extension. Results from field mapping and structural modeling indicate that these faults are basement-involved, moderate-angle reverse faults. Because the upper crustal architecture across the region is largely consistent, the region as a whole may be characterized by moderate-angle reverse faults. Thus, nearby Laramide faults that have been previously interpr...

Cenozoic structural evolution of the Catalina metamorphic core complex and reassembly of Laramide reverse faults, southeastern Arizona, USA

“…1). However, recent studies in highly extended locales have shown that previously identified thin-skinned thrusts are basement-cored uplifts with moderate-angle reverse faults Seedorff, 2017, 2020), which is consistent with the geology of nearby areas that have undergone minor extension (Davis, 1979;Favorito and Seedorff, 2018). This suggests that interpretations of thin-skinned thrusts elsewhere in the region (Fig.…”

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

“…The southern MCCs, of which the Coyote Mountains are a part, are unique compared to the northern ones in the fact that they are located within an area characterized by Laramide shortening of the craton (e.g., Coney, 1980; Coney & Harms, 1984; Spencer & Reynolds, 1990). The exact style of Laramide deformation as well as the timing and magnitude of shortening remains poorly constrained, owing to the subsequent widespread extensional tectonics, including both MCC and Basin and Range extension that overprinted most of these structures (see Favorito & Seedorff, 2018, and references therein). However, recent work examining the geochemistry of continental‐arc rocks by Chapman et al (2020) suggests that the crust of the southern U.S. Cordillera (western and southern AZ, northern Sonora) was 57 ± 12 km thick during the Laramide orogeny.…”

“…As mentioned above, the southern MCCs are in an area characterized by Laramide shortening. As a result, contractional structures are widespread (e.g., Favorito & Seedorff, 2018; Spencer et al, 2019) and include the Baboquivari thrust (Haxel et al, 1984; Wright & Haxel, 1982). Evidence for reactivation of an earlier structure is not present here (Davis et al, 1987; Gardulski, 1980; Haxel et al, 1984), and as far as we can tell, the microstructures of the Coyote Mountains detachment shear zone are entirely extensional.…”

Geochemical evidence for an orogenic plateau in the southern U.S. and northern Mexican Cordillera during the Laramide orogeny

“…Aptian-Albian marine limestone in southern Arizona suggests that the region was at or below sea level during mid-Cretaceous time (Dickinson and Lawton, 2001) and only transitioned to a contractional regime at the start of the Laramide orogeny (Chapman et al, 2018). Structural studies in the area indicate that deformation primarily occurred by folding or slip associated with high-angle reverse faults, which only accumulated a few to several tens of kilometers of horizontal shortening (Davis, 1979;Krantz et al, 1989;Clinkscales and Lawton, 2017;Favorito and Seedorff, 2018). Conversely, parts of the thin-skinned Sevier thrust belt in the central United States Cordillera accommodated ≥ 300 km of horizontal shortening (DeCelles and Coogan, 2006).…”

“…However, subsequent research has demonstrated that many of the thrust faults used to support a thrust belt interpretation are lowangle normal faults or other types of geologic contacts such as unconformities (Dickinson, 1984;Krantz et al, 1989;Clinkscales and Lawton, 2017). As a result, most researchers now believe that the Sevier thrust belt terminates in the Mojave region of southern California (DeCelles, 2004;Yonkee and Weil, 2015) and that shortening in southern Arizona and northern Sonora was temporally restricted to the Laramide orogeny and predominantly occurred along high-angle reverse faults, in part reactivating Late Jurassic to Early Cretaceous rift-related structures (Davis, 1979;Krantz et al, 1989;Lawton, 2000;Favorito and Seedorff, 2018;Fitz-Díaz et al, 2018).…”