The Western San Juan Mountains: Their Geology, Ecology, and Human History

2. Paleotectonic History

CHAPTER 2 PALEOTECTONIC HISTORY DOUGLAS C. BREW To unravel the geologic story of southwestern Colorado, one must understand its paleotectonic history. The field of tectonics looks at the processes that deform the earth's crustal rocks on the global scale; paleotectonics examines such processes over the span of geologic time. The geological evolution ofa region such as the San Juan Mountains might not seem to require a mechanism of global dimensions to explain it, yet the fundamental forces that have shaped this region are of that scale. For about 1.8 billion years southwestern Colorado has been in a position on the North American continent where its crustal rocks have been affected by tectonic activity. The mountainous terrain of the region is an obvious reflection of such activity; less obvious evidence is found in the igneous, sedimentary, and metamorphic rocks of the region. Subtle details of rock composition, variations in the thickness of sedimentary rock units, and the regional distribution patterns of specific units have recorded the influence of tectonism through geologic time. The record is not perfect; nevertheless, it reveals a fascinating, complex story. To understand what drives local or regional tectonism, one must understand plate tectonics. This theory holds that the outer portion of the earth consists of several rigid plates; some underlie oceanic basins, and others encompass both oceanic and continental regions. These plates are in motion relative to each other and to the interior of the earth. 18 Paleoteaonic History • I9 Plate motion is clearly the major process shaping presentday geological activiry, and the same process appears to have characterized past geological history. To better understand how plate tectonics has shaped the San Juan region, we must first examine the basic tenets of plate tectonic theory. PLATE TECTONICS The theory of plate tectonics gives rise immediately to several questions: What is the nature of the plates? What allows and controls their motion, the directions and rates? What drives this process ? Before proceeding, we must first examine these questions. PLATES: WHAT ARE THEY? The plates are pieces ofthe outer, essentially rigid layer ofthe earth known as the lithosphere. Fitting together a bit like pieces ofa moving and ever-changing jigsaw puzzle, they are roughly 65 miles (100 km) thick under continents and slightly thinner in oceanic regions . A quick glance at the map of the modern configuration of plates (Fig. 2.1) reveals that some plates include both continental and oceanic regions (e.g., the North American plate), whereas others are largely oceanic (e.g., the Pacific plate). Fig. 2.1 The modem configuration of plates. Subduction zones indicated by heavy lines, spreading centers by stippled lines, transform boundaries by single lines. Arrows show relative directions of plate motion. 20 The Western San Juan Mountains It's quite clear that the present-day plate arrangement differs from those ofearlier geologic times. Indeed, the origin and subsequent transformation of any continent or ocean basin can be explained in terms of plate tectonic evolution. Through geologic time, continents (or even "supercontinents") have formed, have been added to, and perhaps have later fragmented; ocean basins have opened, closed, and reopened. Throughout this rearranging of the earth's geography, continental rocks have been preserved, and the total area of continents has increased, even though the form and distribution of continents have been continuously changing. It follows, then, that continental rocks should be relatively ancient. Radiometric dating confirms this; the oldest continental rocks are approximately 4 billion years old. The oldest rocks found along the San Juan Skyway are not quite this ancient, having been dated at about 1.8 billion years old (see Chapter 3). By contrast, rocks that constitute the oceanic lithosphere have been dated at only 150 to 200 million years old. The oceans clearly have been transitory features. They have opened, widened, and then closed as the plates went on their inexorable journeys. HOW DO PLATES MOVE? Answering the question of how plates are able to move is relatively easy. Underlying the lithosphere is a zone extending from about 45 to 65 miles (70-100 km) beneath the surface to a depth ofapproximately 160 miles (250 km). Long known to seismologists as the "low velocity layer," this zone, called the asthenosphere, possesses pressure-temperature conditions that render the rocks ofthe interior soft and capable of slow flow. The lithospheric plates are thus in a sense floating on this underlying "plastic" layer; it allows them both to...