The mountainous, western third of the U.S. is a classic Cordilleran orogenic system and chiefly formed because of subduction of oceanic plates beneath North America. Cordilleran orogenic systems comprise multiple components, including a magmatic arc, orogenic plateau, retroarc thrust belt, and foreland basin.  Modern examples of these components are observed in the northwest U.S. and ancient remnants of these components are recognized in the central U.S.  However, it is unclear whether these components existed in the southwest U.S., which suggests that the tectonic and geodynamic processes that occurred this region remain unresolved.  The goal of this research project was to evaluate whether an orogenic plateau and retroarc thrust belt were present in the southern U.S. Cordillera during Late Cretaceous to Paleogene time (approximately 80 to 40 million years ago).  Understanding and identifying these components has important implications for the distribution of natural resources in the southwest U.S.

 We undertook a multidisciplinary study of the southwest U.S., focused on southern Arizona, using a combination of methods including geochemistry, petrology, geochronology, thermochronology, and structural geology.  The project outcomes have advanced our knowledge of this region in a number of significant ways. Rare earth element ratios of Late Cretaceous to Paleogene age continental arc rocks indicate that the crust was relatively thick (ca. 55-60 km), supporting the hypothesis that an orogenic plateau with thick crust was present in the region during this time.  A belt of high-silica anatectic granites, formed by crustal melting, was also recognized in this region and determined to be Paleocene to Eocene in age (60 to 40 million years old).  Detailed studies of these rocks indicate that maximum temperatures in the deep crust reached up to ~850 °C, the first temperature estimates for this region.  High temperatures and evidence for crustal melting also support the hypothesis that an orogenic plateau was present in the study area.  Previous studies have linked regions with thickened crust to high Cu (copper) concentrations and the formation of Cu-porphyries, which may help explain the abundance of Cu mines in southern Arizona.

 Another question we sought to answer in this project is how the crust in southern Arizona was thickened.   Prior to this study, the leading paradigm for southern Arizona was that contractional deformation during Late Cretaceous to Paleogene time was accommodated by relatively high-angle reverse faults.  Relatively low-angle faults, including thrust faults, are required to stack rocks up and significantly thicken the crust.  We recognized several thrust faults in the region and reinterpreted several faults as thrust faults, consistent with the evidence for thick crust. 

 The recognition of thrust faults supports another outcome of the project – the interpretation of the southern U.S. and northern Mexican Cordillera as an orogenic wedge.  Orogenic wedges have thick crust in the hinterland (interior of the mountainous region) and taper toward the foreland. Orogenic wedges have several characteristics including deeper exposed structural levels toward the hinterland, higher metamorphic grade rocks exposed in the hinterland, and decreasing fault displacement toward the foreland.  The southern U.S. and norther Mexican Cordillera display all these characteristics, although they have remained difficult to recognize because of subsequent large-scale extensional deformation and normal faulting during late Cenozoic time (since approximately 25 million years ago) that has dismembered the landscape into a series of discontinuous mountain ranges and created the southern Basin and Range physiographic province. 

 The southern U.S. Cordillera has traditionally been considered to be part of the Laramide orogen and much of the mountainous terrain in southern Arizona was thought to be analogous to the mountains in the central Rocky Mountain region (e.g., Wyoming, Colorado) that were produced by the Laramide orogeny. The new research suggests that the tectonic history of the southwest U.S. may have more in common with northwest Mexico (e.g., Sonora, Chihuahua) and the geodynamic events that constructed the Cordillera there.  It also strengthens the metallogenic associations between the southern U.S. and northern Mexico and provides insight into how natural resources associated with the Cordillera are distributed in North America.

 The results of this project have produced eight peer-reviewed publications in scientific journals, new geologic maps, and generated new geochemical, isotopic, geochronologic, and thermochronlogic data.  Results have been presented at a variety of professional and scientific conferences and disseminated to the public through a series of field trips and popular news articles. The key outcomes are freely accessible to academic and industry researchers and provide a foundation for future research. The research was conducted at a publicly funded university and supported several graduate and undergraduate students. The project provided training and educational opportunities for students, including underrepresented domestic minority, female, and physically disabled students.  All of the students involved in this project are continuing their education or have entered the workforce in STEM-related fields.

Last Modified: 12/15/2024
Modified by: James B Chapman