Introduction to special issues: Lithospheric structure and evolution of the Rocky Mountains (Parts I and II)

Two issues (Parts I and II) of Rocky Mountain Geology (RMG) are an attempt to summarize the structure and evolution of the continental lithosphere in a Rocky Mountain transect from Wyoming to New Mexico (Fig. 1). After decades of geologic work, our understanding of the complex history of this region is still incomplete. As in many other regions, firstorder questions remain about the deep structure, processes of formation, and evolution of the continental lithosphere. Fundamental new insights are most likely to come through integration of a broad range of data, involving close collaboration among geologists, geochemists, and geophysicists. The Continental Dynamics program of the National Science Foundation has provided funding recently for a collaborative effort in the Rocky Mountains that involves many of the contributors to these theme issues of RMG. Plans for coordinated activities include acquisition of about 600 km of seismic reflection and refraction data (in 1999), passive seismic studies, xenolith studies, and a range of geologic and geochronologic studies. We hope that the papers in the two issues of RMG will promote discussion of the best design for our upcoming seismic experiments. In the longer term, we hope that the planned geologic and geophysical experiments will catalyze a new generation of collaborative research in the Rocky Mountains. The goal of our research is to understand the growth, stabilization, and reactivation of the lithosphere of southwestern North America. This area represents the juncture of two globally unique tectonic regimes, each of fundamental importance for understanding continental tectonics. Together they represent an unparalleled field laboratory for studies of continental lithosphere. First, a 1200 km-wide, juvenile, Proterozoic orogenic belt records a globally unique episode (between 1.8 and 1.6 Ga) of rapid accretion of continental materials from mantle sources and their assembly to southern Laurentia. One aspect of our studies, therefore, is to evaluate the processes of formation of new continental lithosphere by investigating the deep structure and evolution of major Precambrian province boundaries (Fig. 1). Secondly, the present high elevations of the regional orogenic plateau in the Southwest and in the Southern Rocky Mountains–Rio Grande rift region is the manifestation of Phanerozoic and ongoing modification of Proterozoic lithosphere. Thus another major aspect of our work is to better document present lithospheric structure, how it evolved though time, and the processes of tectonism that affected the interiors of continents. Although our goals go beyond a single hypothesis, our main testable hypothesis is that lithospheric structure, produced during assembly of the southwestern United States, profoundly influenced the physical and chemical modification of the continental lithosphere during all subsequent tectonism (see paper by Karlstrom and Humphreys). Influence of that ancient assembly continues in the form of ongoing reorganization of small-scale asthenospheric convection and its lithospheric tectonic and magmatic response. Such long-term influences are suggested by the northeast-trending embayments of hot “upwelling” mantle along proposed Proterozoic province boundaries in the Southwest (see cover) and an apparent correspondence of mantle anisotropy and velocity provinces with Precambrian crustal provinces. The Rocky Mountains–Colorado Plateau region is especially important for understanding mantle–crust interactions, because it marks a broad transition from old, cold, unmodified mantle tectosphere of the midcontinent to hot, young, modified mantle of the Basin and Range province (Lerner-Lam et al.). Thus, this is an area in which processes of mantle modification are ongoing and perhaps more readily deciphered. Overall, our investigation is designed to understand interrelationships between lithospheric evolution (using geologic studies), present lithosphere structure (using geophysics), and continental dynamic processes (using modeling). Papers in the two issues discuss several testable aspects of our hypothesis. For example, we postulate that different mantle provinces exist beneath different crustal provinces (Snelson et al.; Keller et al.), and that a profound and long-lived difference between Archean and Proterozoic lithosphere is involved (Chamberlain; Smithson and Boyd; Lester