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UMR Journal -- V. H. McNutt Colloquium Series

Volume 1 Article 8

April 1968

Geologic Structure and History of the Sierra

Paul C. Bateman

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Recommended Citation Bateman, Paul C. (1968) "Geologic Structure and History of the ," 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 that has broken free on the east and has been tilted westward. It is composed chiefly of Mesozoic granitic rocks and and Mesozoic metamor­ phosed sedimentary and volcanic roclra. The granitic rocks constitute the Sierra Nevada , which is part of a more or less continuous belt of plutonic rocks that extends northward from Baja through the Sierra Nevada at a small angle to the axis of the range and into . The batho­ lith is localized in the axial of a complexly faulted ssmclinorium. It is composed chiefly of rocks that range in composition f j:om quartz to alaskite but includes scattered smaller and darker masses of mafic 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. 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 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 in . 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 on the south to the been a boundary for thinking about problems on the north (Fig. 1). The in the ; and 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. , 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, , Joseph Whitney for about a hundred miles into Yo- LeCbnte, A. C. “ Andy” Lawson, Adolph Knopf, semite , is a glaciated region , 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 adiabatically and can hold more moisture than is obvious. On the one hand, it is a tilted 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 , 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 . 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 , 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. 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 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, , 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. 17; Chris­ tinuous belt of plutonic rocks that extends tensen, 1963, p. 163-164), and Pentaerinus from Baja, California, northward through the ^p., reported to be of or age, and the Mojave Desert, has been collected along the through the Sierra Nevada at an acute angle (Moore and Dodge, 1962). These rocks are un­ to the range, and into western Nevada. It may like rocks of known Jurassic age in the Sierra continue at depth beneath the volcanic rocks Nevada but may be Triassic. However, they of the Plains and connect with resemble Paleozoic rocks of some of the roof the Idaho batholith. pendants of the Nevada, and The was emplaced it is possible that only the more easterly strata into strongly deformed but weakly metamor­ are Triassic, and that the more westerly strata phosed sedimentary and volcanic rocks of Pa­ may be Paleozoic. If so, a Paleozoic facies leozoic and early Mesozoic age, which can be change must take place along a line that referred to as the “framework” rocks. In the crosses the Sierra southward from near northern half of the range, the batholith is Bishop to near Fresno. This line may, in fact, flanked on the west by the western metamor­ mark the southwestward continuation of the phic belt, which is the site of the famed mid-Paleozoic Antler of Ne­ Mother Lode. Farther south, scattered rem­ vada, which Ralph Roberts has described. nants of are found within Typically, the framework rocks are tightly the batholith, especially in the western foot­ folded, and many of those that have not been hills and along the crest in the east-central hornf elsed by the granitic magmas are cleaved. Sierra Nevada. The batholith extends east­ Steeply dipping structures— ^beds, cleavage, ward to the east edge of the range, but in the fold axes, and lineations— are common. East southern half one can look eastward across of the Sierra Nevada, folds are generally open to the wall rocks on the east side and rarely overturned, whereas, within the

UMR Journal, No. 1 (April 1968) 124 Paul C. Bateman

Fig. 2. Geologic map across the central Sierra Nevada showing the distribution of the plutonic and metamorphic rocks.

Sierra Nevada the strata characteristically inner belt of Triassic (?) and Jurassic strata stand on edge. Steep minor folds, and other entirely within the Sierra Nevada. In the lineations indicate repeated epochs of folding Mount Morrison pendant are more than 30,000 in many places. The transition from the gen­ feet of Paleozoic sedimentary strata standing tler dipping structures of the Great Basin to on edge, with tops predominantly to the west the steeper dipping structures of the Sierra (Rinehart and Ross, 1964, p. 1). Fossiliferous Nevada takes place along a narrow zone that Ordovician strata lie along the east side, and coincides approximately with the east side of Pennsylvanian and Permian strata lie along the batholith. the west side. In the adjacent The major structure in the metamorphic pendant are another 30,000 feet or sp of vol­ rocks of the central Sierra Nevada is believed canic and volcanic-derived strata of Mesozoic to be a complexly faulted synclinorium. This age, also on edge and with tops to the west, synclinorium is not readily apparent in the although a few large folds are present on the patterns of geologic maps, chiefly because only west side of the pendant (Huber and Rinehart, fragments of the framework rocks remain and 1965; Peck, 1964). because strike faults of large displacement in­ On the west side of the batholith, bedding terrupt the sequence of strata in the western tops are predominantly to the east, but older metamorphic belt. On the east side of the strata have been brought east of younger batholith is an outer discontinuous belt of strata along two strike faults of large dis­ Precambrian outcrops which extend from the placement. According to Lorin Clark (1964), White Mountains southeastward to Death Val­ who has studied the stratigraphy and struc­ ley, a middle belt of Paleozoic strata in the ture of the western metamorphic belt most re­ White and Inyo Mountains and in roof pen­ cently, the strata in the middle and more dants in the eastern Sierra Nevada, and an westerly of the three blocks are Upper Juras­

UMR Journal, No. 1 (April 1968) Geologic Structure and History of the Sierra Nevada 125 sic, and most of the strata in the more east­ overlap of the time of regional deformation erly block are Paleozoic, probably upper Pale­ with that of emplacement. ozoic in the area shown in Figure 2. The Paleozoic strata are assigned to the Calaveras THE BATHOLITH Form ation. The batholith is composed chiefly of quartz­ Unconformities within and between the bearing granitic rocks ranging in composition Paleozoic and Mesozoic units indicate repeated from quartz diorite to alaskite but includes movement since middle Paleozoic time. The scattered smaller masses of darker and older geometry of the structures of the framework plutonic rocks and remnants of metamorphic rocks also indicates repeated deformations. rocks. Rocks in the compositional range of Minor folds with steeply dipping axes are and predomin­ common and represent either refolding of ate and are about equally abundant. In gen­ earlier folds with subhorizontal axes or else eral, the plutonic rocks in the western part folds that were formed in strata that had been of the batholith are more mafic than those in previously so folded as to have steep dips. the eastern part. Some contain two or more axial The granitic rocks are in discrete masses surfaces of systematically different orienta­ or plutons, which generally are in sharp con­ tions. tact with one another or are separated by The Sierra Nevada lies within the Cordil­ thin septa of metamorphic or mafic igneous leran mobile belt, and its rocks reflect part of rocks. Individual plutons vary greatly in size; the deformation that has taken place there their outcrop areas range from less than a since mid-Paleozoic time. Probably the faulted square mile to more than 500 square miles. synclinorium began to take form in Permian The limits of many of the large plutons have or Triassic time, and intermittent distur­ not yet been delineated. On the whole, the bances occurred through the Jurassic. The batholith appears to consist of a few large very severe disturbance that took place near plutons and a great many small ones which the close of the Jurassic and caused the prin­ are grouped between the Jarge plutons. All of cipal folds in the Upper Jurassic strata of the the large plutons, and some of the small ones, western metamorphic belt is referred to as are elongate in a northwesterly direction, par­ the Nevadan , but both earlier and allel with the long direction of the batholith, later disturbances are known to have occurred. but many other small plutons are elongate in Unconformities in the Taylorsville region at other directions or are rounded or irregularly the north end of the western metamorphic belt shaped. The rocks in different plutons gen­ indicate disturbances between the Silurian erally can be distinguished by their appear­ and Mississippian and at the end of the Perm­ ance. Although chemical and petrographic ian, Triassic, and Jurassic (McMath, 1966). studies are very helpful, the different plutons In the eastern and central parts of the range, are identified and their boundaries are mapped folds of two periods of deformation antedate in the field. Where plutons meet, it is usually a third set that appears to have been formed possible to determine which one is older by during the Late Jurassic . In means of inclusions, dikes, truncated struc­ the western metamorphic belt, Clark (1964, tures, and the like. However, some intrusions p. 56-57) has recognized a stage of deforma­ meet along plane surfaces that parallel the tion that probably occurred after the principal internal structures of both. Relative ages can­ folding of the Upper Jurassic strata in the not be determined from these frustrating con­ Nevadan orogeny. This deformation is char­ tacts. acterized by the development of slip cleavage, The major plutons in the western part of steeply dipping minor folds, and steeply dip­ the batholith are generally older than those ping lineations. Clark believes the large faults along the crest of the range, and in the Yo- in the western metamorphic belt formed dur­ semite region Calkins (1930) has mapped two ing this deformation, probably by strike-slip series of granitic formations in which the movement. The presence of ultramaflc rocks, plutons are successively younger toward the especially serpentine, along these faults sug­ east. Nevertheless, the pattern of intrusion is gests deep penetration, possibly penetration more complicated than a simple west-to-east into the upper mantle. sequence of emplacement. The stratified rocks were everywhere de­ Isotopic dates of plutons in the central formed before the adjacent plutonic rocks were Sierra Nevada indicate three widely separated emplaced, but some plutons were later de­ epochs of plutonism at 183 m.y. and probably formed during later deformations, indicating no more than 210 m.y. (Late Triassic or Early

UMR Journal, No. 1 (April 1968) 126 Paul C. Bateman

Jurassic), 124 to 136 m.y. (Late Jurassic), is believed to have settled down and 80 to 90 m.y. (early Late Cretaceous) ward around the magma, thus providing room (Kistler and others, 1965). In addition, one for its continued upward migration. Conceiv­ large plutonic formation in the western half ably, some plutons may have become entirely of the batholith (granodiorite of “Dinkey detached from their source region and can be Creek” type) is at least 115 m.y. but may not underlain by country rock, but no field evi­ be as old as Late Jurassic. The ages of many dence for downward bottoming of plutons has mapped plutons have not been established in been found. terms of meaningful isotopic dates, and some The relative importance of several processes plutons may have been emplaced in epochs of in the emplacement of the plutons has been magmatism that have not yet been recognized. only incompletely evaluated. Wall-rock de­ Although it is possible that the isotopic dates form ation indicates that rising magma are simply points along a period of continuous squeezed the wall and roof rocks aside and magmatism, it is difficult to conceive of a par­ upward. Stoping appears to have been im­ ent magma continuing to exist for more than portant locally, but there is little evidence in 100 m.y. It seems more likely that magma was support of stoping as the principal mechanism generated at intervals within this time span. of emplacement. Processes of granitization The isotopic dates, together with intrusive and assimilation have operated on a small relations observed in the field, indicate that scale where the wall and roof rocks were am­ plutons of similar ages are distributed not phibolites or other mafic rock, but these pro­ haphazardly but in geographic belts. The plu­ cesses are of possible quantitative importance tons of Late Triassic or Early Jurassic age only in terranes of mafic volcanic rocks (Bate­ lie along the east side of the batholith, and man, 1965, p. 118-123). Melting and assimila­ they may be closely related to isolated plutons tion of politic rocks has not been proved, but of about the same age span farther east and very likely has taken place and may have been southeast in the Inyo and Argus Mountains of considerable importance. (Ross, 1965, p. 046-048; Hall and MacKevett, Broad chemical and mineralogical changes 1962, p. 30-31). Plutons of known Late Juras­ take place across the batholith. In general, the sic age in the central Sierra Nevada are con­ granitic rocks are more mafic toward the west fined to the western metamorphic belt, but it and more silicic toward the east, but this is a is likely that some plutons in the west side of gross trend and some silicic plutons occur in the batholith are also of that age. The Late the western half of the range and some mafic Cretaceous plutons constitute a belt which plutons are within the eastern half. The simple averages about 25 miles in width and extends explanation that the more rocks in the along and just west of the . This eastern half of the range are differentiates of belt is interrupted south of Yosemite by a the more mafic rocks in the western half does cross septum of metamorphic and pre-Late not hold for several reasons: 1) lengthy time Cretaceous plutonic rocks that provides a gaps probably exist between the different age window into an earlier period in the develop­ groups; 2) large granodiorite and quartz ment of the batholith. diorite plutons in the western half of the There is abundant evidence that the granitic range are accompanied by younger and more plutons rose from below as melts, shouldering felsic plutons, which are older than the large the wall rocks aside and pushing them upward granodiorite and quartz monzonite plutons in (Bateman, 1965, p. 114-118). This includes the east side of the range; and 3) the limited such relations as sharp contacts of plutons analytic data now available indicate sys- with wall rocks and with one another, dikes K O and inclusions along contacts between plutons tematically lower q ratios in the Jurassic from which the relative ages of the plutons granitic rocks of the western part of the range can be determined with consistent results, than in either the Upper Triassic or the Lower finer grain size in apophyses and in the mar­ Jurassic or Cretaceous rocks farther east gins of some plutons, wall-rock geometry that (Moore, 1959). suggests dislocation by the emplacement of plutons, and dilated walls of aschistic dikes. CENOZOIC HISTORY The plutons are pictured as having moved up­ During and following the emplacement of ward from a deeper source region, much like the Mesozoic granitic rocks, the Sierra Nevada salt domes, because molten granitic magma region was uplifted and eroded to great has a significantly lower density than rock of depths. Much of this took place by the the same composition. As the plutons rose, the time the last granitic rocks were intruded, and

UMR Journal, No. 1 (April 1968) Geologic Structure and History of the Sierra Nevada 127

it was largely completed for much of the Si­ their to depths of 2,000 to 4,000 feet erra Nevada by the end of early Eocene time below the base of the Tertiary channels. In (55 m.y. ago). The time span from early the northern part of the range, the remnants Eocene to late Oligocene (25-30 m.y. ago) was of the Tertiary channels are preserved on flat- one of virtual standstill (Bateman and Wahr- topped interstream ridges. Most of the haftig, 1966). It was during this time that cutting appears to have preceded the earliest the -bearing gravels accumulated in the recognizable glaciation on the west slope. channels that drained the northern Sierra Faulting along the eastern boundary of the Nevada. The Sierra may have been 3,000 to Sierra Nevada probably began about 10 m.y. 5,000 feet high near its crest, and mountains ago or perhaps a little later (Bateman and of resistant greenstone in the western foot­ Wahrhaftig, 1966). In the northern Sierra hills were probably 1,500 to 2,000 feet high. Nevada, most of the faulting appears to have A period of volcanic activity began in the been completed by 2 m.y. ago, but in the northern Sierra Nevada in middle Oligocene southern part of the range much of it took time with the eruption of predominantly place later. In general, faulting along the east rhyolitic tuffs and ash flows. Rhyolitic erup­ side of the range probably lagged behind the tions continued until late Miocene time, and westward tilting of the range. In the Owens were followed, in late Miocene to late Pliocene Valley segment, at least, the main faulting time, by eruptions of andesitic mudflows. The appears to be much later and may represent total thickness of the rhyolitic rocks is more collapse of the Owens Valley block in the crest than 400 feet in a few places. The andesitic of a broad arch. The Sierra Nevada consti­ rocks are much thicker and range from 3,000 tutes the west flank of this arch, and the feet along the crest of the range to about 500 desert ranges as far east as feet in the Great Valley west of the Sierra constitute the faulted east flank (Fig. 3). Nevada. Granitic mountains along the range In common with all other alpine and crest and erosion-resistant greenstone ridges , the Sierra Nevada was glaciated sev­ in the western foothills rose above the vol­ eral times during the Pleistocene. The two canic plain. Generally no more than 30 to 40 earliest glaciations, the Sherwin and McGee, flows can be identified in a single section, in­ between 2.5 and 0.7 m.y. old, are the oldest dicating that volcanic eruptions. probably oc­ glaciations stratigraphically related to radio­ curred in any one place only a few times every metrically dated materials in the world. Four million years. Thus the lavas, tuffs, and mud­ to six glaciations have been recognized on the flows do not suggest an environment any more east side of the Sierra Nevada, the last two cataclysmic, on the average, than exists in the or three corresponding to the Wisconsin. On Cascade Range today. the west side, direct evidence indicates only The volcanic cover was extensive only in the four glaciations, but indirect evidence in the northern Sierra Nevada. South of Yosemite, Great Valley indicates four major glacial per­ scattered volcanic deposits, mostly basalt or iods, of which the Wisconsin— corresponding trachybasalt, were erupted. These have iso­ to three glaciations— is the last (Bateman and topic ages that cluster around 9.5 m.y. and 2 Wahrhaftig, 1966). Moraines of the two oldest to 4 m.y. (Dalrymple, 1963, 1964). glaciations on the east side, the McGee and At some time in the Pliocene, probably in Sherwin, have lost their topographic form. middle or late Pliocene, the northern Sierra These older glaciations seem to have taken Nevada—and probably the southern Sierra place before much of the faulting along the Nevada as well—was strongly uplifted. and east side of the range took place, whereas, the tilted to the west. Thick clastic deposits in the well-preserved moraines of the younger glacia­ Great Valley suggest that uplift actually be­ tions (, Tahoe, , and Ti­ gan in the Miocene. In response to the major oga) extend from the existing canyon mouths uplift, the rivers on the west slope incised onto the basin floors.

UMR Journal, No. 1 (April 1968) 128 Paul C. Bateman

LATE PALEOZOIC

UTE JURASSIC 0

EARLY UTE CRETACEOUS 0 T®

PRESENT DAY Ot O

0 10 2 0 30 40 SO M 70 80 Km Fig. 4. Cross sections illustrating a model for magma generation and emplacement in the Sierra Nevada.

UMR Journal, No. 1 (April 1968) Geologic Structure and History of the Sierra Nevada 129

The glaciations ended about 9,500 years Experimental studies show that at atmo­ ago, and the post-glacial climate has been spheric pressure sialic rocks begin to melt marked by a period of relative warmth, fol­ fractionally at 960 °C, but that an increase in lowed by a period in the last 2,000 to 3,000 water-vapor pressure causes the melting tem­ years during which the climate cooled enough perature to drop spectacularly (Tuttle and to allow for the formation of small glaciers Bowen, 1958). At pressures of about 1 kilobar at the base of north- or northeast-facing cir­ and in the presence of enough water to satu­ que walls. rate any amount of magma that may be generated, melting begins at temperatures CRUSTAL STRUCTURE between 600° and 700 °C, the temperature Interest in the crustal structure beneath varying inversely with the pressure. If cer­ the Sierra Nevada began in 1936 when Lawson tain other substances, such as fluorine or published a paper titled:. “The Sierra Nevada chlorine, are present, the temperature at in the Light of Isostasy”. In a comment on which melting begins is lowered further. The Lawson’s paper, Byerly (1938) inferred a first melt is composed chiefly of normative root beneath the Sierra Nevada from delay quartz, orthoclase, and albite, in proportions in the arrival time at stations east of the that are different at different pressures of range in response to waves that water vapor; this melt probably is saturated originated west and northwest of the range. or nearly saturated with water. To form more Eaton (1963) has made seismic refraction calcic magma, more of the source rock must measurements across the northern part of the be melted, which requires higher tempera­ range that indicate this root may extend to a tures. Because the amount of water soluble in depth of about 45 km near , and magma may be quite high— about 17 percent Eaton and Healy (1963) have made similar by weight at 10 kilobars—and because the measurements across the high central part source rock is unlikely to contain more than that indicate the root there extends to depths a few percent of water at most, probably only of at least 50 kni. The bottom cross section of the first melts to form and the highly frac­ Figure 4 is. drawn to fit the specifications of tionated last ones to crystallize are likely to Eaton and Healy, which are that the crust be water saturated. Probably most granitic beneath the high central Sierra Nevada is at magmas contain only about 2 percent of water. least 50 km thick, about twice as thick as the A temperature of more than 1,000 °C would crust west of the range and 15 to 20 km be required to produce a melt of granodiorite thicker than the crust beneath the Basin and or quartz diorite composition with normative Range province to the east. The section is a plagioclase of about An^^ composition and 2 composite. The western part is drawn across percent water. However, it is not likely that a Yosemite, and the eastern part is a few miles parent magma was ever a complete melt, for farther south across Owens Valley. It is bet­ rarely does the An content of plagioclase ter thought of as a model than as a geologic crystals exceed 50 percent, even in the least cross section. The thickness of the basalt differentiated of the granitic rocks. Plagioclase layer east of the Sierra root zone is taken to of AUj-o composition would crystallize from be about 7 km, in accordance with the results melt containing normative plagioclase of An^^; of seismic work farther north near Fallon, composition. Such a melt, assuming only 2 Nevada, (Eaton, 1963, p. 5803). Within the percent of water, could be generated at a tem­ root, the basalt layer is shown to be thickened perature of about 900 °C. by the addition of refractory substances left The depth at which sialic rocks can be ex­ behind when the granitic magmas rose to pected to melt to granitic magma is difficult to higher levels. evaluate because our present knowledge of the The localization of the batholith in the axial generation and distribution of heat in the region of a synclinorium of great size and earth is still in a primitive state. In stable depth compels serious consideration of the parts of the crust where all the heat is carried hypothesis that the granitic magmas were to the surface by conduction, temperatures of generated by the melting of sialic rocks of 600° to 700 °C may be attained at depths of 30 the upper crust as a result of their being de­ to 50 km, but a temperature of 900 °C may not pressed into deeper regions of high tempera­ be reached at depths twice as great. However, ture: The upper three sections of Figure 4 the situation in a downfolding synclinorium illustrate a possible mode of formation of the is not ordinary because the crustal rock is existing crustal structure following this hy­ greatly thickened in the downfold. Sialic rocks pothesis. generally produce more heat by radioactive

UMR Journal, No. 1 (April 1968) 130 Paul C. Bateman decay than basaltic rocks, and the heat pro­ recognized in the metamorphic rocks could duced in a downfolded sialic layer thickened well be temporally related to the Late Triassic to about 45 km can result in temperatures in or Early Jurassic plutonism, and deformation the neighborhood of 900 °C in the lower part in the western metamorphic belt that occurred of the downfold over a period of 200 million after the Nevadan orogeny could have taken years or so (A. H. Lachenbruch, oral com­ place at the time of the early Late Cretaceous munication, 1965). These considerations sug­ plutonism, but these suggested relations have gest that a magma zone could very well form not been established. at depths of 30 to 50 km in a thickened and Northwest and west of the Sierra Nevada, depressed segment of the crust. in the and northern Coast The evidence for several epochs of pluto­ Ranges, Irwin (1964) has recognized two nism have already been discussed, and Figure periods of deformation. He identifies the ear­ 4 shows the three most firmly established lier as the Late Jurassic Nevadan orogeny epochs. The Late Triassic or Early Jurassic and the later as a Late Cretaceous “Coast magmas and the early Late Cretaceous mag­ Range orogeny”, because of its “ . . . im­ mas are shown to have formed entirely in the portance in development of the pre-Tertiary crust, because the least differentiated major structure of the Coast Ranges”. East of the plutons of these groups are no more mafic Sierra Nevada, in western and southern Ne­ than granodiorite and because Hurley and vada, several periods of compressive deforma­ others (1965, p. 172) have determined the ini­ tion since Late Mississippian time are gener­ tial SrS’^/Sr®® ratio of the granitic rocks in ally recognized, and some of them appear to the central Sierra Nevada to have been in the have occurred at about the same time as the range of 0.7073 ±0.0010. From this ratio they plutonic episodes of the Sierra Nevada. deduced that if the granitic magmas origi­ Volcanic rocks of Early Jurassic age have nated from a mixture of sialic and basaltic ma­ been identified in the eastern Sierra Nevada, terials, the ratio was one-third basalt and two- and ones of Late Jurassic age have been identi­ thirds sial. This proportion of basalt could fied in the western metamorphic belt, but most have been introduced into the melted rocks volcanic sequences are unfossiliferous, and either by at the time of deposition their precise ages are unknown. Consequently, of some of the melted rocks, by later intrusion, little can be said about the temporal relations or by mixing at the interface between the of volcanic episodes to plutonic or tectonic basalt and the overlying sial. On the other episodes, except that they span most and per­ hand, the Upper Jurassic granitic rocks along haps all of the known Mesozoic plutonic and the west side of the Sierra Nevada are gener­ tectonic episodes. ally more mafic than the ones farther east, and Difficulty with the hypothesis illustrated in their formation would have required a larger Figure 4 arises, because it places the date of proportion of basalt. Hence, the magma cham­ formation of the present root of the Sierra ber in which the parent magma was formed is Nevada in early Late Cretaceous time, when shown to include the basalt layer. the Late Cretaceous magmas were formed, and According to this hypothesis, episodes of because uplift since then apparently has not volcanism and plutonism should be approxi­ been continuous. If the uplift during the Cre­ mately contemporaneous and should be closely taceous and Tertiary was caused by isostatic related temporally to deformational episodes adjustment in response to the formation of in which the crust was depressed and thick­ the root, as seems a reasonable expectation, ened. In fact, one test for the hypothesis is uplift should have been more or less con­ whether episodes of approximately contempo­ tinuous, though at a diminishing rate, until raneous volcanism, plutonism, and deformation equilibrium was restored. However, during a can be demonstrated. At present, the data are period of at least 30 m.y. in the early Ter­ permissive only. The only established temporal tiary, the Sierra Nevada appears to have relation between tectonism and plutonism is risen very little, and uplift was renewed dur­ that of the well-known Late Jurassic Nevadan ing the late Tertiary. At the present, I have orogeny to Late Jurassic plutonism. One of no explanation to offer for these puzzling re­ the older periods of deformation that has been lations.

REFERENCES CITED Bateman, P. C., 1965, Geology and tungsten mineralization of the Bishop district, California, vnth a section on Gravity study of Owens Valley, by L. C. Pakiser and M . F. Kane, and a section on Seismic profile, by L. C. Pakiser: U.S. Geol. Survey Prof. Paper 470, 208 p.

UMR Journal, No. 1 (April 1968) Geologic Structure and History of the Sierra Nevada 131

------, Clark, L. D., Huber, N. K., Moore, J. G., & Rinehart, C. D., 1963, The Sierra Nevada batho­ lith—a synthesis of recent work across the central part: U.S. Geol. Survey Prof. Paper 414-D, p. D1-D46. & Wahrhaftig, C., 1966, Geology of the Sierra Nevada, in Bailey, E. H., ed.. Geology of : California Div. Mines and Geology Bull. 190. Byerly, Perry, 1938, The Sierra Nevada in the light of isostasy: Geol. Soc. America Bull., v. 48, supp., p. 2025-2031. Calkins, F. C., 1930, The granitic rocks of the Yosemite region in Matthes, F. E., Geologic history of the Yose­ mite Valley: U.S. Geol. Survey Prof. Paper 160, p. 120-129. Christensen, M. N., 1963, Structure of metamorphic rocks at King, California: California Univ. Pub. Geol. Sci., V . 42, no. 4, p. 159-198. Clark, L. D-., 1964, Stratigraphy and structure of part of the western Sierra Nevada metamorphic belt, California; U. S. Geol. Survey Prof. Paper 410, 70 p. Dalrymple, G. B., 1963, Potassium-argon dates of some Cenozoic volcanic rocks of the Sierra Nevada, California: Geol. Soc. America Bull., v. 74, no. 4, p. 379-390. ------, 1964, Cenozoic chronology of the Sierra Nevada, California: California Univ. Pub. Geol. Sci., V. 47, 41 p. Durrell, Cordell, 1940, Metamorphism in the southern Sierra Nevada northeast of Visalia, California: California Univ., Dept. Geol. Sci. Bull., v. 25, no. 1, p. 117. Eaton, J. P., 1963, Crustal structure from , California, to Eureka, Nevada, from seismic-refraction measurements: Jour. Geophys. Research, v. 68, no. 20, p. 5789-5806. ______, & Healy, J. H., 1963, The root of the Sierra Nevada as determined from seismic evidence (abs.): Internat. Union Geodesy and Geophysics, 13th General Assembly, Berkeley, Calif., 1963, Abs. Papers, V . 3, art. B28, p. iii-46. Hall, W. E., & MacKevett, E. M., Jr., 1962, Geology and ore deposits of the Darwin quadrangle, Inyo County, California: U.S. Geol. Survey Prof. Paper 368, 87 p. Huber, N. K., & Rinehart, C. D., 1965, Geologic map of the Devils Postpile quadrangle. Sierra Nevada, California: U.S. Geol. Survey Quad. Map GQ-437. Hurley, P. M., Bateman, P. C., Fairbairn, H. W., & Pinson, W. H., Jr., 1965, Investigation of initial Sr*VSr*® ratios in the Sierra Nevada plutonic province: Geol. Soc. America Bull., v. 76, no. 2, p. 165-174. Irwin, W. P., 1964, Late Mesozoic in the ultramaflc belts of northwestern California and southwestern Oregon: U.S. Geol. Survey Prof. Paper 501-C, p. C1-C9. Kistler, R. W., Bateman, P. C., & Brannock, W. W., 1965, Isotopic ages of from granitic rocks of the central Sierra Nevada and Inyo Mountains, California: Geol. Soc. America Bull., v. 76, no. 2, p. 155-164. Lawson, A. C., 1936, The Sierra Nevada in the light of isostasy: Geol. Soc. America Bull., v. 47, no. 11, p. 1691-1712, McMath, V. E., 1966, Geology of the Taylorville area, northern Sierra Nevada in Bailey, E. H., ed.. Geology of northern California: California Div. Mines and Geology, Bull. 190. Moore, J. G., 1959, The quartz diorite boundary line in the western : Jour. Geology, v. 67, no. 2, p. 198-210. ______, & Dodge, F. C., 1962, Mesozoic age of metamorphic rocks in the Kings River area, southern Sierra Nevada, California: U.S. Geol. Survey Prof. Paper 450-B, art. 7, p. B19-B21. Peck, D. L., 1964, Preliminary geologic map of the Merced Peak quadrangle, California: U.S. Geol. Survey Mineral Inv., Field Studies Map MF-281, scale 1:48,000. Rinehart, C. D., & Ross, D. C., 1964, Geology and mineral deposits of the Mount Morrison quadrangle. Sierra Nevada, California, with a section on A gravity study of Long Valley, by L. C. Pakiser: U.S. Geol. Survey Prof. Paper 385, 106 p. Ross, D. C., 1965, Geology of the Independence quadrangle, Inyo County, California: U.S. Geol. Survey Bull. 1181-0, 64 p. . Tuttle, O. F., & Bowen, N. L., 1958, Origin of in the light of experimental studies in the system NaAlSi308-KAlSi308-Si02-H20: Geol. Soc. America Mem. 74, 153 p.

M a n u s c r i p t r e c e i v e d , J a n u a r y 1967

UMR Journal, No. 1 (April 1968)