The geologic history of the American Southwest is both fascinating and important, not least because it is openly revealed to both the professional Earth scientist and casual observer alike. The exposures in this arid to semi-arid region are generally superb. This book, then, is intended to present a systematic and comprehensive picture of the geology of the Southwest since the formation of its earliest rocks in subduction zones, through the formation and fragmentation of at least two and possibly additional supercontinents. It will supplement other books, including the popular road guides, presenting more detailed pictures of the geology of the region.

But the real importance of geological studies of this region lies in their broad application to other areas of the Earth. Geological paradigms developed in the Southwest have global import. Thus, a secondary purpose is to highlight the numerous concepts that have grown out of study and research in the Southwest and West and that have bearing elsewhere in the world. Since the geographical and topographical surveys of the middle and latter part of the nineteenth century, the geology of the West has strongly influenced the development of the geological sciences. Examples derived from the American Southwest have been used in the professional training of geologists for over a hundred years. In part, this book will illustrate how concepts derived from study of modern rocks feed back into an understanding of rocks from earlier periods of Earth's history, and vice versa.

Setting

By the beginning of the Mesozoic, Pangea was nearly fully assembled. Only a few microcontinents, such as North and South China, remained yet to collide with the main Pangean landmass, which they did by the end of the Triassic (Rees, 2002). Stretching nearly from pole to pole, Pangea drifted slowly northward across the equator (Fig. 6.1). The western margin of the supercontinent and of present North America continued to be a collisional boundary during the Mesozoic era, with the Cordilleran miogeocline present along the continental margin. Great tracts of new continental crust – ‘exotic terranes’ – were accreted to the continent during the Mesozoic. Together with magmatic arcs and sediments of the adjoining trenches, these terranes formed what are now the States of California, Oregon, and Washington, and much of Nevada and Arizona. The Early Triassic also saw the beginning of the end of Pangea, as great continental blocks were carved from Pangea to form new plates. The demise of Pangea was not sudden, but rather was drawn out over the next 150 Myr, not to be completed until the early Tertiary (c. 45 Ma). However, the disassembly of Pangea did not affect the western part of Pangea, including the present Southwest.

The edge of the craton extended through central Utah and southern Nevada. To the west, in central Nevada, lay terrane accreted to the continent during the Antler orogeny, and the miogeocline. On the continental margin, shallow-marine sediments alternated with terrigenous deposits as the shoreline fluctuated.

The Southwest evokes images of dusty desert landscapes beset with narrow mountain ranges, of the vast and colorful expanses of the Colorado Plateau and land of the Diné, of Monument Valley, and perhaps of the Spanish and Mexican cultural heritage. Terrains range from barren, seemingly lifeless, deserts to verdant, forested mountains, and vegetation zones from Sonoran to Alpine. Its varied landscapes have challenged explorers and settlers; beckoned artists and adventurers. They may elicit wonderment and awe but can be haunting, even intimidating. The beauty of the Southwest is often stark, typically subtle.

The Southwest of the United States is a region that defies precise geographic definition, eschewing neatly defined physiographic subdivisions. Most usages of the term ‘Southwest’ include the arid and semiarid region stretching from west Texas across New Mexico and Arizona to southern California (Fig. I.1; see also Plate 1). This vast expanse of desert, the heart of the Southwest, comprises dominantly the Basin and Range and Colorado Plateau physiographic provinces. The Basin and Range province, typifying the southern parts of New Mexico and Arizona and northern Mexico, is that region characterized by a distinctive physiography of narrow mountain ranges separated by broad, sediment-filled desert basins. In contrast, the Plateau is that region of northern New Mexico, northern Arizona, western Colorado, and much of Utah characterized by broad plateaus, deeply incised canyons, and mainly flat-lying sedimentary strata.

Introduction

The latest chapter in the geological development of the Southwest encompasses the most recent 35 Myr or so of the Earth's history. For perspective, recall that 35 Myr is less than one percent of the age of the Earth. Despite this short period of time, it is the time about which geologists know the most. The present chapter is defined principally by events following cessation of widespread crustal shortening in the western Cordillera. Extension and vertical crustal adjustments replaced shortening.

Despite the fact that the Earth's tectonic plates were nearly in their present relative positions in the Oligocene (Fig. 8.1), a profound change in plate geometry occurred during the latest 35 Myr along the western margin of North America. The Farallon plate, whose subduction beneath the North American plate for more than 140 Myr created the series of nearly continuous orogenies in western North America, was gradually and nearly completely consumed. Only fragments of the original Farallon plate, all carrying different names, remain along the margins of North and South America. Disappearance of the Farallon plate had major implications for the tectonics in the interior of the plate, dramatically affecting the geology and landscape of the Southwest. The change in plate geometry and relative plate motions resulted in massive magmatism and a different kind of orogeny, an ‘extensional orogeny,’ which radically reshaped the landscape.

Introduction: supercontinent Rodinia

From a variety of evidence, including the global pattern of Proterozoic rocks, geologists now recognize that Mesoproterozoic to Neoproterozoic Laurentia was actually part of a supercontinent, i.e. a ‘continent’ comprising continental-scale crustal blocks assembled separately and brought together by plate-tectonic processes. The Southwest and all of present North America lay interior to this vast supercontinent. The present chapter describes the demise of this supercontinent by rifting, and the transition from rifting to drifting to form a passive margin. Part of the evidence that a continent once larger than the present North America existed is the abrupt westward termination of southwest-trending Paleoproterozoic to Mesoproterozoic age provinces (Fig. 1.3). Much of the evidence is provided by Neoproterozoic strata, which are interpreted to indicate a west-facing passive margin with no sign of a continent to the west. Because Mesoproterozoic rocks of the western USA and Canada (including those of the Grand Canyon Supergroup) are interpreted as intracratonic in their setting, i.e. deposited within the interior of a continent, formation of a passive margin implies that such a continent was subsequently rifted apart. Some contention exists concerning priorities for naming the Mesoproterozoic to Neoproterozoic supercontinent (Young, 1995), but ‘Rodinia’ seems to be the most accepted. In addition, the timing of breakup is not as well determined as geologists might like. Very possibly a second supercontinent – Pannotia – formed as the pieces of Rodinia recollided in a different configuration.

Mesoproterozoic and Neoproterozoic stratified rocks

Following the formation of the crust in the Southwest, completed during the Grenville orogeny 1.2–1.0 Ga, an enormous expanse of time was to pass before the widespread, relatively continuous and well-preserved rocks of the Phanerozoic Era were deposited. The present chapter covers part of that interval of time, from roughly 1.35 Ga to about 780 Ma, a length of time approximately equal to the entire Phanerozoic.

This chapter represents a considerable overlap in time with the previous chapter, but focuses on a completely different group of rocks. The rocks covered in the present chapter consist almost entirely of stratified rocks, mostly sedimentary but in part igneous, which were laid down on the previously formed crust. They rode high upon that crust and therefore escaped the great crushing stresses and high temperatures that accompanied the processes of initial crustal formation. They are somewhat arbitrarily distinguished from the little-metamorphosed stratified rocks of the previous chapter by having been deposited in intracratonic basins far from the continental margins. The beginning of this chapter at about 1.35 Ga is the earliest time for which such stratified rocks are preserved; the cutoff at 780 Ma, although it may appear to be arbitrary, signifies a major change in the tectonic events of the Southwest.

The overlap in time with the previous chapter comes about because, as discussed there, the formation of the crust in the Southwest occurred over an interval of 700 Myr, from assembly of the Yavapai province at roughly 1.7 Ga to the Grenville at about 1 Ga.