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Geologic Structure and History of the Sierra Nevada

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Geologic Structure and History of the Sierra Nevada UMR Journal -- V. H. McNutt Colloquium Series Volume 1 Article 8 April 1968 Geologic Structure and History of the Sierra Nevada Paul C. Bateman Follow this and additional works at: https://scholarsmine.mst.edu/umr-journal Part of the Geology Commons Recommended Citation Bateman, Paul C. (1968) "Geologic Structure and History of the Sierra Nevada," UMR Journal -- V. H. McNutt Colloquium Series: Vol. 1 , Article 8. Available at: https://scholarsmine.mst.edu/umr-journal/vol1/iss1/8 This Article - Journal is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in UMR Journal -- V. H. McNutt Colloquium Series by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. UMR Journal, No. 1 (April 1968) 121 Geologic Structure and History of the Sierra Nevada P a u l C. B a te m a n * ABSTRACT The Sierra Nevada is a huge block of the earth’s crust that has broken free on the east and has been tilted westward. It is composed chiefly of Mesozoic granitic rocks and Paleozoic and Mesozoic metamor­ phosed sedimentary and volcanic roclra. The granitic rocks constitute the Sierra Nevada batholith, which is part of a more or less continuous belt of plutonic rocks that extends northward from Baja California through the Sierra Nevada at a small angle to the axis of the range and into western Nevada. The batho­ lith is localized in the axial region of a complexly faulted ssmclinorium. It is composed chiefly of rocks that range in composition f j:om quartz diorite to alaskite but includes scattered smaller and darker masses of mafic igneous rock and remnants of metamorphic rocks. The granitic rocks are in discrete plutons that range in outcrop area from less than a square mile to 500 square miles or more. Isotopic dates indi­ cate three widely separated episodes of magmatism at 183 to 210 m.y. ago, 124 to 136 m.y. ago, and 80 to 90 m.y. ago. Other magmatic episodes doubtless have occurred. During and following the emplacement of the granitic rocks, the Sierra Nevada region was uplifted and eroded to great depths. Following a period of virtual standstill during most of the Eocene and the Oligocene, the range began to tilt westward, and during the Pliocene the east side was uplifted by tilting to its present great height. Faulting along the east side of the range generally lagged behind westward tilting. As a result of uplift, the rivers that drain the west slope were deeply incised. During the Pleisto­ cene, the range was repeatedly glaciated. Glaciers sharpened ridges and peaks and widened and deepened stream valleys, producing much of the spectacular scenery of the range. The root beneath the high Sierra Nevada extends to a depth of more than 50 km and probably originated during the Mesozoic when the synclinorium was formed and the granitic rocks emplaced. Granitic magmas are pictured to have formed repeatedly during the Mesozoic as a result of depression and thickening of the relatively fusible and radioactive upper crust. INTRODUCTION But the Sierra Nevada is more than a physi­ The Sierra Nevada is a lofty and beautiful cal and climatic barrier; until recently it has mountain range in eastern California. It is 60 been a remarkably effective barrier to geologic to 80 miles wide, and it extends for 400 miles, thought. Its towering eastern escarpment has from the Mojave Desert on the south to the been a boundary for thinking about problems Cascade Range on the north (Fig. 1). The in the Great Basin; and geologists working range is assymetric. It has a long gentle wes­ along the Pacific slope, even in the foothills of tern slope and a high steep eastern escarp­ the Sierra Nevada itself, have seldom looked ment. The highest peaks are along the eastern ejastward for correlations. The early literature edge. Mount Whitney, in the southern part of the Sierra Nevada is replete with the names of the range, attains a height of 14,495 feet. of geologic “greats” who were lured into at­ The “High Sierra”, a spectacular span of the tacking some of its vexing problems— names crestal region that extends north from Mount such as J. D. Whitney, Clarence King, Joseph Whitney for about a hundred miles into Yo- LeCbnte, A. C. “ Andy” Lawson, Adolph Knopf, semite National Park, is a glaciated region Waldemar Lindgren, F. L. Ransome, W. H. characterized by numerous lakes and a pro­ Turner, and Francois Matthes. But in spite of cession of magnificent 13,000- and 14,000-foot the attentions of these able geologists, we are peaks. still only on the threshold of understanding The Sierra Nevada is an obvious physio­ the unique role the Sierra Nevada has played graphic barrier and is well known as a cli­ in the geologic history of the West. It is be­ matic barrier. Polar-front cyclones sweep in coming evident, however, that the Sierra Ne­ from the Pacific, expand adiabatically as they vada should be thought of not as a barrier pass over the range, and are cooled to well but as a connecting link. below their dewpoint, causing heavy precipita­ The relation of the Sierra Nevada to the tion. Eastward, the descending air is warmed provinces on both sides during the Cenozoic adiabatically and can hold more moisture than is obvious. On the one hand, it is a tilted fault it contains. Hence the Great Basin on the east block, differing from many Nevada ranges is arid. only in its greater size, and that is part of the ♦Chief, Branch of Field Geochemistry and Petrology, U.S. Geological Survey, Menlo Park, California. Publication authorized by the Director, U.S. Geological Survey. 122 Paul C. Bateman EXPLAHATI ON C' V Cran i t i c rocks Includes d io r ite and gabbro U 11 ranaf i c rocks Largely serpentine O / o tsMorphoseci sodiaonti and volcanic rocks Und ifferentiated •ataaorphic rocks scdiaentary and volcanic rocks Metaaorphosod sediaentary and Igneous rocks Fig. 1. Geologic map of the Sierra Nevada. general problem of Basin and Range structure. uation. The secrets of its Mesozoic and Paleo­ On the other hand, it slopes westward beneath zoic history are locked in its granitic and met­ the Great Valley of California, and the Ceno­ amorphosed sedimentary and volcanic rocks. zoic sedimentary rocks there have been de­ These rocks do not give up their story easily, posited and deformed on its downslope contin­ and it is only recently that enough work has UMR Journal, No. 1 (April 1968) Geologic Structure and History of the Sierra Nevada 123 been done to decipher its broad outline. I will of the batholith which here constitute the emphasize the pre-Cenozoic structures and White and Inyo Mountains. The region in events, because most of my work and that of which the U.S. Gelogical Survey has concen­ my colleagues with the U.S. Geological Survey trated its most recent studies is well situated has been on the bedrock geology and because for comparing and relating the rocks and it is in this area that the most significant re­ structures on the two sides of the batholith, cent advances have been made (Bateman and because the southern end of the western meta­ others, 1963). During the last 20 years, the morphic belt and the northern end of the area U.S. (Geological Survey has concentrated its of good exposures on the east side of the bath­ efforts between the 37th and 38th parallels, and olith overlap here (Fig. 2). most of' what I have to say pertains specifi­ cally to that belt, but studies made elsewhere THE FRAMEWORK ROCKS in the Sierra Nevada make me confident that The Paleozoic rocks are miogeosynclinal in this belt is representative of the range as a the east and eugeosynclinal in the western whole. metamorphic belt. In the southern Inyo Moun­ tains, limestone, dolomite, quartzite, and shale GENERAL GEOLOGIC RELATIONS are common. The strata generally coarsen The Sierra Nevada is a huge block of the northward, and in the Mamoth Lakes region earth’s crust, composed of plutonic and meta­ the Ordovician rocks include abundant chert morphic rocks of Paleozoic and Mesozoic age, and slate. The Paleozoic rocks of the western that has broken free on the east along the metamorphic belt are typically eugeosynclinal. Sierra Nevada fault system and has been The Mesozoic rocks are eugeosynclinal on tilted westward. It is overlapped on the west both sides of the batholith. Their eastern limit by Upper Cretaceous and Cenozoic sedimen­ follows approximately the east edge of the tary rocks of the Great Valley and on the batholith. Most of them are either volcanic or north by Cenozoic volcanic sheets extending sedimentary rocks derived from volcanic south from the. Cascade Range. A blanket of rocks; graded beds, some several feet thick, volcanic material caps large areas in the are common. northern part of the range. Most of the south­ The belt of metamorphic remnants that ex­ ern half of the Sierra Nevada and the eastern tends south from the western metamorphic part of the northern half are composed of plu­ belt contains schist, conspicuously crossbedded tonic (chiefly granitic) rocks of Mesozoic age. quartzite, marble, and locally a little tuff. Tri­ These rocks constitute the Sierra Nevada assic fossils have been reported from the Min­ batholith, which is part of a more or less con­ eral King pendant (Durrell, 1940, p.
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