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Geological Origin of the Baja California Peninsula and Gulf Of

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Geological Origin of the Baja California Peninsula and Gulf Of A BRIEF GEOLOGIC HISTORY OF NORTHWESTERN MEXICO © Richard C. Brusca Vers. 28 April 2016 (for references, see “A Bibliography for the Gulf of California” at http://rickbrusca.com/http___www.rickbrusca.com_index.html/Research.html) NOTE: This document is periodically updated as new information becomes available (see date stamp above). Photos by the author, unless otherwise indicated. This essay constitutes the draft of a chapter for the planned book, A Natural History of the Sea of Cortez, by R. Brusca; comments on this draft chapter will be appreciated and should be sent to [email protected]. SECTIONS Geological Origin of the Baja California Peninsula and Gulf of California The Colorado River The Upper Gulf of California The Sierra Pinacate and Gran Desierto de Altar Rocks that Tell Stories Endnotes Glossary of Common Geological Terms Geological Origin of the Baja California American Plate, powered by the same Peninsula and Gulf of California spreading center and tectonic forces. But Although the Gulf of California (aka, “the plate dynamics are complicated, and the Gulf,” the Sea of Cortez) is a young sea, just North American Plate was simultaneously 7 million years old (mya), the present-day moving slowly westward driven by the topography of its basin and adjacent land opening of the Mid-Atlantic Ridge. As a masses (including the Baja California result, the East Pacific Rise itself was slowly Peninsula) evolved through a series of approaching North America, following along grand geological events that began many “behind” the Farallon Plate (see figure). millions of years ago, long before the At first, the Farallon Plate’s angle of peninsula itself formed, and that stretch subduction was moderate, about 45 across all of western North America. degrees, which is typical of submerging Beginning in the latest Paleozoic or early plates. This deep dive engendered Mesozoic, the Farallon Plate, an eastward- mountain-building uplift and volcanism along moving seafloor lithospheric plate, began the western margin of North America due to subducting under the western edge of the compresisonal forces, crumpling its surface North American Plate. The Farallon Plate into mountains like a wrinkled rug pushed was spreading eastward from an active from one edge. But evidence suggests that, seafloor spreading center called the East for a time (~70-40 mya), the subduction Pacific Rise, or East Pacific Spreading angle of the Farallon changed and the plate Ridge (Endnote 1). To the west of the East began moving under North America at a Pacific Rise, the gigantic Pacific Plate was very shallow angle. During this shallow moving westward relative to the North subduction phase (“flat-slab subduction”), 1 Mountain Building in the Southwest. © Arizona-Sonora Desert Museum. Concept development by R. Brusca and W. Moore; graphic by Linda M. Brewer. 2 the plate pushed nearly 1000 miles (1500 -building event, known as the Laramide km) under North American before switching Orogeny, extended from southern Canada back to a steeper angle of subduction. to northern Mexico, and as far east as the During this subduction process, much of the Black Hills of South Dakota, and included initial uplift of the Rocky Mountains and the Laramie Mountains of Wyoming (for Sierra Madre Occi-dental (of Mexico) took which it was originally named). place. The Rockies formed mostly by During that long period of moderate and compression and uplift; the Sierra Madre shallow subduction, the oceanic rocks of the mostly by volcanic activity. Farallon Plate were carried deep enough to Mountain building associated with the melt and create magma. Some of this Farallon subduction seems to have taken magma crystallized below the surface to place in a series of pulses and intervening form intrusive batholiths (commonly granitic quiescent periods. This great mountain- in compositon), including those that are now exposed in the Sierra Nevada and Peninsular Ranges in California and along the length of the Baja California Peninsula. Much of it, however, erupted at the surface as andesitic volcanoes, probably looking very much like present-day Mt. St. Helens in the Cascade Range. The Sierra Madre Occidental experienced massive and widespread volcanism. Today, the Rocky Mountains and Sierra Madre Occidental together comprise North America’s Western Cordillera (also known as the North American Cordillera), a 4500 mil-long mountain chain. The only gap in this long chain of mountains is in southeastern Arizona/northeastern Sonora—the so-called Cordilleran Gap, studded by ~65 isolated ranges known as Sky Islands. (Endnote 2). In addition to initiating the formation of igneous rocks by subduction, the Farallon Plate and other smaller plates moving eastward in the Pacific Basin carried on their backs numerous volcanic oceanic islands Geologic Time Table (Designed by R. Brusca and L. Brewer) and archipelagos, both submerged and 3 emergent. When these oceanic islands and west of the Basin and Range Province seamounts encountered the trench at consists of a heterogenous patchwork of America’s continental margin, they were these allochthonous terranes, accreted onto scraped off as the plates slowly dived the edge of the North American Craton (i.e., beneath North and South America. These what is left of the ancient continent of accreted allochthonous, or “exotic,” terranes Laurentia) from the Paleozoic to as recently whose origins were long a geological as Late Miocene. However, many of these mystery, stretch from Alaska to South terranes are hidden beneath yougner America. They not only augmented and deposited or erupted rocks. Although no built-out the continental landmass of the allochthonous terranes have been positively Americas, they also created many of the identified in Arizona or Sonora, much of the coastal mountain sections that we recognize Baja California Peninsula and western today. In fact, nearly all of North America The Colorado Plateau, Mogollon Rim, Lower Colorado River Basin, and the Upper Gulf of California (NASA) 4 Mexico probably consist of these ancient California was still attached to western North relocated land masses. America. The subduction of the Farallon Plate beneath the North American Plate generated volcanism in the Sierra Madre Occidental, notably in two episodes,100-45 mya and 34-23 mya, and in what is now eastern Baja California and westernmost Sonora from 24 to 10 or 8 mya. As the Farallon Plate steadily subducted and Barrancas del Cobre (Copper Canyon), Sierra Madre Occidental (Photo by Jim Welden) Tectonic map of the Gulf of California. Basins correspond to spreading centers bounded by active disappeared beneath North America, the transform faults forming the San Andreas-Gulf of California Fault System. (From Suarez-Vidal et al., final phase of subduction resulted in a in Dauphin & Simoneit eds, 1991, The Gulf and continental volcanic arc forming ~24-15 Peninsular Province of the Californias. AAPG Mem. 47) mya. The rocks generated from this From the Mesozoic to the late Oligocne, volcanic arc now constitute some of the the west coast of North America was uplifted mountains of the easternmost Baja bordered by an unbroken subduction zone— California Peninsula (e.g., much of the the Farallon Trench—with an associted Sierra La Giganta in Baja California Sur), but volcnic arc (similar to the Andes in its are best seen in the mountain ranges in nature). Also prior to the late Miocene, Baja southwestern Sonora. The granitic 5 batholiths of the modern Sierra Nevada ~27 mya, the southern California section range (California) also represent the eroded between 13 and 22 mya, and the Gulf of core of this ancient cordillera, whereas the California segment about 11 mya. Today, chaotic coastal San Franciscan formations the Mendocino Triple Junction marks the are the remains of various islands, northern limit of the San Andreas Fault seamonts, sediments and metamorphic system, and the Rivera Triple Junction rocks accreted to the North American Plate marks the southern limit. as the Farallon Plate was subducted. This first contact of the East Pacific Rise Throughout the Cretaceous and into the with the North American Plate divided the Cenozoic, the Farallon Plate grew narrower Farallon Plate into northern and southern as it was consumed by the trench. By the parts. The remnant microplates of the Cretaceous, a wedge, or segment of the Farallon Plate, to the north and south, Pacific Plate and East Pacific Rise had continue to move slowly against and under begun pushing eastward into the heart of the the North American Plate today. The Farallon Plate, sliding along the Mendocino northern remnants of the Farallon Plate are and Murray Fracture Zones in the north and the Juan de Fuca, Explorer, and Gorda south. This section of the Pacific Plate (and Plates (currently subducting beneath its leading segment of East Pacific Rise) northern California, Canada, and Alaska). was moving eastward faster than the The southern fragments are the Rivera Plate Farallon Plate was, thus forming a (a microplate along the coast of Nayarit and prominent eastward projection and slowly Jalisco), Cocos Plate (currently subducting dividing the Farallon in half. Several under southern Mexico and Central microplates calved off the Farallon Plate as America), and the Nazca Plate (currently the leading edge of the Pacific Plate forced subducting under South America). its way through it, and as the Farallon itself As the East Pacific Rise subducted moved toward the subducting trench. Some beneath western Mexico, portions of North of these smaller calved plate pieces still America’s crust gradually became attached exist, and others were subducted long ago. to the Pacific Plate. The attachment began The central region of the Farallon Plate ~12 mya in northwestern Mexico and was disappeared as the leading segment of East probably fully completed ~7 mya. The Pacific Rise met the North American Plate, attachment of a small fragment of crust from ~25-30 mya in central California, and ~11-15 one plate (in this case, the North American mya in northern Mexico.
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