BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l . 4 7, pp. I69 M 712 November 30.1936

THE SIERRA NEVADA IN THE LIGHT OF ISOSTASY BY ANDREW C. LAWSON CONTENTS Page Introduction...... 1691 Post-Jurassic orogeny...... 1693 The Sierra Nevada batholith...... 1695 The Great Valley...... 1698 Isostatic relations...... 1699 General statement...... 1699 Case I—Batholith melt limited to sial...... 1700 General description...... 1700 Two uplifted valley floors...... 1701 Uplift III...... 1702 Variations in thickness and position of the batholith...... 1703 Cause of Uplift III...... 1705 Isostatic balance of the Great Valley...... 1706 Case II—Basalt incorporated in batholith melt...... 1709 Consequences of thickening of the batholith...... 1711 Mount Dana section...... 1712

INTRODUCTION It is plain, from the folding of their constituent sedimentary rocks, that high mountain ranges represent local concentrations of mass, which, in every case, must act as a load imposed upon the earth’s crust in the region of the uplift. This added load may be supported in one of two possible ways. If the crust be strong enough, it may support the load by virtue of rigidity; if it be not strong enough, the range must be supported by flotation of its relatively light superficial rocks in the heavier rocks below. If the rigidity of the crust were adequate to support the load without depression—i.e., if the concentration of mass were limited to up­ ward protrusion—then the added mass of the range would affect measure­ ments of the force of gravity, so that the excess mass could be easily detected and positively proved to exist. The force of gravity has been measured on many mountain ranges, and no excess mass comparable to that of the range has been found.1 We are not at liberty, therefore, to 1 Unfortunately, we have only one determination of force of gravity in the Sierra Nevada. With its vast interest in isostasy, the United States Coast and Geodetic Survey might have been expected to give a few determinations of gravity distributed over the length and breadth of the range. ( 1691)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1692 A. C. LAWSON----SIERRA NEVADA AND ISOSTASY entertain the idea that great mountain ranges are supported by the rigid­ ity of the crust. The rejection of that idea compels us to adopt the alternative possibility of isostatic support. Isostasy as applied to great mountain ranges is theoretically simple. The upward protrusion of rock, in a zone of crustal deformation, to form a mountain range, is accompanied pari passu by a much larger downward protrusion into the heavier material of the subcrust. Support of the range is, then, just as simple a matter mechanically as that of an iceberg in the sea; with, of course, a great difference in the viscosities of the supporting media. To make way for such a downward protuberance, the rock of the sima must flow away into neighboring regions; and, in doing so, it will tend to flow toward regions that are being relieved of load by erosion. Thus, as a geosynclinal sea on the margin of a continent accommodates a thick deposit of sediments by subsidence, the subcrustal flow will be toward the continental area whence the sediments are derived. The denudation of the continental margin and the negative load thereby created are prime conditions of the subsidence of the geosyncline, no less than the positive load of sediments accumulating in it. The nega­ tive load of the continental margin draws away some of the sima from beneath the geosyncline and so provides for subsidence. But the specific volume of the sediments deposited on the subsiding area is always greater than that of the sima withdrawn; so that, while they contribute by their positive load to the subsidence, they tend to fill the trough. For pro­ tracted subsidence and deep fill, the substitution of dunite for basalt appears to be a necessary part of the mechanism. The effect of flowage in solid rocks under deforming stress is a familiar phenomenon to geolo­ gists, and there can be no objection to the mechanism thus invoked to maintain balance when equilibrium is disturbed by shift of load or by crustal deformation. When, however, we come to particular cases, the attempt to analyze the operation of this mechanism and to evaluate the forces engaged be­ comes a complicated matter. Detailed data are inadequate and modify­ ing factors are elusive; so that the results of the analysis are, at best, tentative. But tentative as they are, these results have their value for geology, for the discussions that lead to them clarify and enlarge our notions of the kind of thing that happens in orogenesis. Without being either exact or precise, it is possible to unravel an interesting history. We lack precision in our knowledge of the densities of rocks as they lie in the earth in large masses; the thickness of the layers of diverse den­ sity, which make up the stratiform crust, is inferential and inexact. The thermal gradient is inconstant and uncertain; the effect of high tempera­ tures upon the density of several common types of rock has yet to be

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 INTRODUCTION 1693 measured. With these handicaps to sure deduction well in mind, it is proposed in this paper to consider the application of the principle of isostasy to the tectonics of the Sierra Nevada, in an effort to elucidate the evolution of the range. The purpose is not to prove anything or to contribute any new idea to geological science, but merely to illustrate the application of the principle of isostasy, in perhaps its most naive form, to the problem of uplift and depression of the earth’s crust, a prob­ lem with which geologists have long been conversant, but which still awaits solution. POST-JURASSIC OROGENY The region occupied by the Sierra Nevada was, until late Jurassic time, a Mesozoic sea floor. The sea extended eastward across northern Nevada and Oregon to the 117th meridian, which is approximately the boundary between Oregon and Idaho. By way of southern Nevada and Arizona, it may have been continuous with the extensive Mesozoic sea of the Rocky Mountain region of the United States. The conditions of sedimentation in the Califomia-Nevada portion of this sea are somewhat difficult to understand. Lithologically, the Jurassic rocks of the Sierra Nevada are prevailingly, laminated argillites, significant of deposition on a sea floor remote from high-grade streams, as might be supposed from the extent of the sea and the absence of evidence of landward transgres­ sion. From the extent of the sea, also, and from the almost complete absence of basal conglomerates, it is fair to infer that the sea floor was a submerged continental surface of low relief, analogous to the shallow seas off the southeast coast of Asia today. There were probably volcanic islands in the western Mesozoic sea, but they have not yet been recog­ nized as features of the paleogeography of the time. Whether the west­ ern side of the sea was open to the Pacific, or was shut off from that ocean by a land mass that contributed sediment to the sea floor, is un­ known. In the descriptive text of the folios of the Geologic Atlas of the United States, conglomerates are mentioned as occurring at several localities, interstratified with the argillites of the Mariposa formation. Some of these conglomerates are doubtless correctly named; but the one with which the writer is most familiar, at Colfax,2 is certainly not a conglom­ erate in the ordinary sense of the term. It has been described as a breccia, by Moody,3 who, however, left its genesis and significance an open question. Since the publication of Moody’s paper, the writer made an excavation at the base of the Mariposa formation at Colfax and 3 Geological Atlas, Colfax Folio No. 00, p. 3. 3 Clarence L. Moody: The breccias of the Mariposa formation in the vicinity of Colfax, California, Univ. Calif. Dept. Geol., Bull., vol. 10, no. 21 (1917) p. 383-420.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1694 A. C. LAWSON----SIERKA NEVADA AND ISOSTASY found the breccia resting on a smoothly polished, rounded, and fluted surface of radiolarian chert. The character of the breccia and of the surface upon which it rests, together constitute a proof of glaciation comparable to that on the Vaal River near Kimberly, or that at Hallett’s Cove near Adelaide, South Australia. So, if other conglomerates of the Mariposa formation are similar to those at Colfax—and some of them appear to be—then, their presence is not significant of high-grade streams discharging into the Mariposa sea. The evidence from the argillites, that torrential streams were far removed from the basin of deposition, remains unchallenged. The accumulation of thick deposits of marine sediments, interleaved with lavas and tillites, was interrupted by a mountain-making move­ ment, which sharply folded the Jurassic and earlier formations. As a result of this concentration of rock mass in the zone of yielding to hori­ zontal compression, the zone was uplifted as an incipient mountain range. In accordance with the principle of isostasy, the uplift was supported, and caused, by the flotation of a concomitant and much larger down­ ward protrusion of the crushed zone into the heavy rock of the sima. When the uplift was well advanced, and widespread dynamic meta­ morphism had been induced in the rocks of the zone by their deforma­ tion,4 a granitic magma of vast dimensions welled up from the depths and invaded the zone, partly by melting, partly by stopping, and partly by injection. On the periphery of the granite, thermal metamorphism was imposed upon the encasing rocks, in addition to the already existing dynamic metamorphism. The invasion of the granite, in some cases, still further deformed the rocks of the zone, thrusting them aside and causing their foliation to adapt itself to the contours and profiles of the intrusive masses; thus exhibiting still a fourth method of upward in­ vasion.5 The appearance of granite in the core of mountain ranges, intrusive into the rocks of the flanks, is a common phenomenon, which may prob­ ably be explained by the depression of the crushed zone into regions of high temperature, where it melts. The resulting magma is, thus, a part of the excess mass that determines the existence of the range by flotation in the heavy sima. It is lighter than the unfused rocks of the crushed zone and so rises through them by stoping and by injection. It is, for * “The cleavage of the schists. . .existed before the granitic irruption” [Waldemar Lindgren: Granitic rocks of California, Am. Jour. Sci., 4th ser., vol. 3 (1897) p. 304]. BH. W. Turner: Further contribution to the geology of the Sierra Nevada, U. S. Geol. Surv., 17th Ann. Rept., pt. I (1896) p. 554; Description of the Bidwell Bar quadrangle, U. S. Geol. Surv., Geol. Atlas, Bidwell Bar folio, no. 43 (1898). H. W. Turner and F. L. Ransome: Description of Sonora quadrangle, U. S. Geol. Surv., Geol. Atlas, Sonora folio, no. 41 (1897). Waldemar Lindgren: Description of the Colfax quadrangle, U. S. Geol. Surv., Geol Atlas, Colfax folio, no 66 (1900).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 POST-JURASSIC OROGENY 1695 the most part, confined to a chamber, which it makes for itself in the crushed zone, and the outer flanks of the range are part of its encasing shell. But, as fusion proceeds, the increase of volume creates an enor­ mous pressure on the chamber walls, which tends to lift the roof or burst the chamber. This is probably the force that causes the migration of the magma into the surrounding country, to form satellitic bodies such as dikes, sills, and laccoliths. If the melt begins at the bottom of the crushed zone, the burden to be lifted by the expansive force would, at first, be the same in the column under the new mountain range and in that under the immediately adjoining region, for at that depth the col­ umns balance. But as the melt rises in the crushed zone, particularly as it rises above the top of the sima, the zone of deformation, culminating in the new range, is much heavier, and the expansive force is expended in lifting the region of less resistance on the flanks of the range, in broadening the batholithic chamber, and in injecting magma into its walls. But we are not here concerned with the enlargement of the cham­ ber nor with the escape of magma from it. The batholith exposed in the Sierra Nevada represents the freezing of a melt after enlargement of the chamber, and after escape of magma from it. For the same reason, we are not concerned with the effects of hydrostatic pressure of the melt, nor with the vapor tension developed in it, both of which forces were merely additive to much greater stress of expansion. THE SIERRA NEVADA BATHOLITH The Sierra Nevada has great individuality and geomorphic simplicity as a mountain range. It forms the western border of the Great Basin, presenting to the east a high, bold front; its western flank is a long, rela­ tively gentle slope of less than 2 degrees, which passes down under the Upper Cretaceous, Tertiary, and Quaternary formations of the Great Valley of California. It is recognized by geologists as a tilted block of the earth’s crust, the largest and the finest of the Basin Ranges. Its eastern front is regarded as the expression of a somewhat degraded mul­ tiple fault scarp; the intersection of this scarp with the tilted surface forms the crest of the range. This individuality and simplicity per­ tains, however, only to the geomorphic form of the range. Its internal structure and the sequence of events that imposed that structure upon it are complicated. In its extent and configuration, the Sierra Nevada has only slight relation to the greater range that was called into being at the time of the mid-Mesozoic Revolution. The structure and meta­ morphism of its original rocks, and the granite that invaded these, are, however, a direct inheritance from the vast antecedent range. The geographic limits of the Sierra Nevada as we know it today were deter­

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1696 A. C. LAWSON— SIERRA NEVADA AND ISOSTASY mined by the tilting of an orogenic block in post-Tertiary time. The tilted block, although it is now the largest of the Basin Ranges, is only a small strip of the region embraced in the upheaval at the close of the Jurassic; and the mountains then formed had suffered deep denudation before the tilting was inaugurated. The granite, which is the dominating rock in the mass of the Sierra Nevada, extends in large, but isolated, ex­ posures across the State of Nevada as far as the 117th meridian, which is also the eastern limit of the Jura-Trias basin of deposition. It is separated from the granite, of the same age, of central Idaho, by a mantle of Tertiary volcanics and continental deposits. Beneath that mantle, the granite of the Sierra Nevada and that of central Idaho may be in direct continuity. Westward, the granite is continuously exposed from the southern Sierra Nevada, around the south end of the Great Valley, into the Coast Ranges. How much of the granite of the Coast Ranges is to be identified with that of the Sierra Nevada is, however, an unsettled question. The width of the granite, assuming it to be continuous at shallow depths, is, thus, at least 230 miles, or three times the width of the Sierra Nevada. If it be continuous with that of central Idaho, then the width of the granite is doubled. In length, the batholith, of which the granite of the Sierra Nevada is a part, is either co-extensive with the west coast of North America, from southern Alaska to southern Mexico, or it is one of an elongated system of batholiths having that extent, and contemporaneous in origin. It will be apparent from the foregoing that, when the writer refers to the Sierra Nevada batholith, he does not think of it as co-extensive with the Sierra Nevada Range, but as a much larger mass, out of which the range has been segregated as a fault block. It is of interest to determine, if possible in the Sierra Nevada, the position of the top of the magma chamber at the time when the melt began to freeze. The plutonic rock resulting from its consolidation now occupies Mount Whitney and many other high peaks of the range. But the crest of the latter, as will appear in the sequel, has been raised to its present altitude by isostatic uplift. We do not know with certainty how much granite was removed from the top of the range after the roof of the batholith had been worn away; but there is ground for the belief that the present summit of the range is approximately the top of the batholith. Reid, discussing this question, says: “The irruptive contact . . . though locally irregular, yet shows in its larger form nearly a plain, with occasional blocks sunk into the granodiorite. At present most of the fragments of the old roof of the batholith are more nearly horizontal than vertical, some quite so. . . . It is probable that the original contact surface lay nearly horizontal in this area, with, of course, sharp local modifications.”8 6 John A. Reid: The geomorphogeny of the Sierra Nevada northeast of Lake Tahoe, Univ. Calif. Dept. Geol., Bull., vol. 6 (1911) p. 97.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 THE SIERRA NEVADA BATHOLITH 1697 A flat-lying remnant of the roof of the batholith, resting on granite, occupies the upper part of Mount Dana, the summit of which has an altitude of 13,050 feet. In the Kern River region, the tables of the Sum­ mit Upland, such as the top of Mount Whitney, Table Mountain, Sheep Mountain (Mount Langley), and Cirque Peak, are probably exhumed flat areas of the top of the batholith, a view which the writer7 expressed in 1904, but to which Knopf 8 took exception in 1918. The evidence cited by Knopf, against accepting these tables as representing the top of the batholith exhumed by erosion under structural control, consists chiefly in the existence of roof pendants in the same region, sunk deeply into the granite. It had not occurred to the writer that roof pendants excluded flat-topped areas of the batholith. It seemed, rather, that the pendants were peripheral enclosures of flat-topped domes of granite. The con­ figuration of the upper surface of the Sierra Nevada batholith includes both the sharply V-shaped troughs, which contain the pendants, and the flat or gently undulating areas, which, on their borders, plunge steeply to accommodate them. Flat surfaces of a coherent resistant rock exer­ cise, in general, an extraordinary control on erosional degradation, and, therefore, it seems probable that the tables of the Summit Upland do, to say the least, strongly reflect flat areas of the top of the batholith. Knopf regards them as the higher parts of the same erosional surface as is represented by the Subsummit Plateau. There is no objection to the view that the tables of the Summit Upland were evolved as surface features in the same erosional cycle as was the Subsummit Plateau, but it does seem clear that the tables emerged in that cycle, under structural control. Nor is there objection to the view that the top of the batholith, if viewed devoid of cover, is characterized by sharp relief. But this relief does not exclude flat-topped domes and ridges. It still seems to be a fair hypothesis that the Summit Upland represents original flat areas on the surface of the Sierra Nevada batholith. The writer has never entertained, nor expressed, the view that the Summit Upland represented the whole of the surface of the batholith, and he has always been cogni­ zant of the sharp relief of that surface. The Table Mountain at Oro- ville has exceedingly sharp relief, yet its top is as flat as a baseball field. Sharp relief and tables are not mutually exclusive. The slope of the top of the batholith, as here recognized and as modi­ fied by erosion, is downward to the north from Mount Whitney. The summits of Mount Whitney, Mount Dana (granite surface), Pyramid Peak, and Buck Mountain are nearly on a straight line, 284 miles long, 7Andrew C. Lawson: The geomorphogeny of the upper Kern basin, Univ. Calif. Dept. Geol., Bull., vol. 3, no. 15 (1904). 8Adolph Knopf: A geological reconnaissance of the Inyo Range and the eastern slope of the southern Sierra Nevada, California, U. S. Geol. Surv., Prof. Pap. 110 (1918).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1698 A. C. LAWSON— SIERRA NEVADA AND ISOSTASY and lie on an even gradient of about 25 feet to the mile. With the de­ cline of the top of the batholith to the north, along the crest of the range, beyond the canyon of the San Joaquin River, larger and larger remnants of the roof have been preserved. This northward down slope of the top of the batholith may possibly have been an original feature; but it is more probable that differential uplift has been a large factor in the tilt. If we take the line from Mount Whitney to Buck Mountain, N. 34° W., as the axis of the range, a section at right angles to this axis, passing through Nevada City, shows about equal areal proportions of batholith and roof at the surface. The axial gradient of the surface on this section has an altitude of about 2 kilometers, and the difference be­ tween this and the altitude of Mount Whitney is about 2.4 kilometers. The larger proportion of roof left is significant of a smaller erosional removal and, to some extent, of a smaller consequent rise of the batholith; though, of course it is impossible to evaluate the quantitative relations. THE GREAT VALLEY The west slope of the Sierra Nevada is fairly uniform away from the summit region. It is recognized by geologists generally, as a tilted por­ tion of a flatter surface, which, under conditions of partial aggradation, endured throughout Tertiary time. The great canyons that dissect the slope are due to differential uplift. The upward tilt to the east appears to be associated with a down tilt, or sag, to the west, under the Great Valley. But the surface of the bedrock complex, as it passes beneath the present floor of the Valley, supports thick deposits of marine sedi­ ments of Tertiary age, which, in turn, pass under a thick alluvial deposit. The depression of the surface of the bedrock complex to receive these sediments, therefore, long antedates the post-Tertiary uplift of the same surface to form the western slope of the Sierra Nevada. The Great Valley has the structural features of a geosynclinal trough bounded on either side by a fault zone, the trace of which is, for the most part, obscured by later contributions to the sediments of the valley fill. The thickness of the fill is very great, as determined from wells and from seismic and gravimetric soundings by oil companies operating in the southern part of the valley. A well of the Pure Oil Company, drilled at a point about 21 miles south of Merced, penetrated valley fill to a thick­ ness of 8387 feet and entered quartz diorite at a depth of 8244 feet below sea level; and, 3 miles west of this well, seismic sounding showed a depth of 8850 feet for the surface of the bedrock complex, the dip of that sur­ face being 3° SW. F. E. Vaughan, who has been conducting gravi­ metric observations for the Shell Oil Company, has kindly placed at the disposal of the writer, important data as to the depth and configuration

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 THE GEEAT VALLEY 1699 of the surface of the bedrock complex under the valley. For this infor­ mation, the writer desires to express his grateful appreciation to the Shell Oil Company and to Mr. Vaughan. According to these data: “At various points from Porterville southward, bore holes have traced the surface of the Basement westward until depths exceeding 1200 meters below sea level were reached. Seismometric surveys map this surface readily because of the fact that seismic waves are transmitted through the Basement rocks at a much higher velocity than through the overlying sediments. Moreover, several zones can be recognized in the sediments, the seismic velocities ranging from 1.7 to 3 km. per second. In the latitude of Hanford the Basement surface has been followed from the eastern margin of the valley to a point 6 km. west of the town, at which point it lies at a depth of 3 km. below sea level. The strike of this surface is approximately N. 20° W. and the dip is 5° S.W. This general attitude obtains southeastward to the vicinity of McFarland.” Vaughan states, further, that the syncline thus indicated can be fol­ lowed for more than 110 kilometers, approximately parallel to the Kettle- man Hills and Lost Hills anticlinal axis, and at a distance therefrom of 5.5 to 9 kilometers. He says: “On the basis of the observations west of Hanford and the evidence of continuity of depth to the southwest, I find that the depth of the Basement at a point on the axis of the major syncline immediately to the northeast of the Kettleman Hills is not less than 7 km., nor likely more than 8.5 km. These estimates apply along the syncline northwestward at least 30 miles beyond this point and southeastward to the region just north of the Elk Hills.” For the purposes of the present discussion, the mean thickness of the fill resting on the eroded surface of the batholith, on a line transverse to the axis of the range, through Mount Whitney, has been taken, in the light of the foregoing estimates, to be about 4 kilometers. ISOSTATIC RELATIONS GENERAL STATEMENT In attempting to analyze the isostatic relations of the Sierra Nevada, both now and in the past, we may begin with the mid-Mesozoic Revolu­ tion. The essential features of that movement were, as already stated: first, an acute compressive deformation of a broad zone of the earth’s crust, resulting in a concentration of mass. The increased mass was ac­ commodated by protrusion, both upward and downward, until the mass of sima displaced by the downward protrusion equalled the excess mass of the zone of deformation, so that the latter was supported by flotation. The depth to which crushing extended in the deformative process is un­ certain. The controlling factor, in answer to this question, appears to be relative viscosity. If the crushing and mashing together of the rocks extended down to the bottom of the sial, and then passed into rock

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1700 A. C. LAWSON— SIERRA NEVADA AND ISOSTASY flowage, the underlying basalt would flow away to accommodate the downward protrusion of the sial. The excess load due to horizontal com­ pression would float, first, in the basalt, and later, in the full develop­ ment of the orogenic process, the downward protuberance would pass through the basalt and be immersed in the dunite. If, however, the basalt participated in the crushing and mashing, as distinguished from flowage, then the column of the crushed zone would consist of both sial and basalt, and would have a larger mean density than if composed only of sial. It would be accommodated by flowage of the dunite, and would float in the latter. In this uncertainty as to the depth of crushing, two cases may be considered. In the first, the assumption is that the crushed zone, and the batholith developed from it by melting, extend down to the bottom of the sial. In the second, it is assumed that the crushed zone and the consequent batholith include both sial and basalt, extend­ ing down to the bottom of the latter. Other possibilities, of course, pre­ sent themselves, such as: the development of the batholith from the upper part of the sial only, or its inclusion of the whole of the sial and only part of the basalt. These, however, will not be considered here; for their examination would not modify in principle the results arrived at in the two cases selected for discussion. CASE I—BATHOLITH MELT LIMITED TO SIAL General description.—Here, the column of crushed rock involved in the orogenic collapse of the crust, comprising both upward and down­ ward protuberances, consists of sial having a mean specific gravity of 2.72. The granodiorite and related rocks of the Sierra Nevada batholith are considered to be the result of melting and subsequent freezing of the crushed and thickened sial; they, also, have a mean specific gravity of 2.72. In estimating the height of the column of granodiorite—i.e., the thick­ ness of the batholith, which thus includes the excess load supported isostatically in the present Sierra Nevada—attention must first be given to the surface relief. In the secondary block lying between the eastern front of the range and the Kern River rift, the relief ranges through 9,000 feet. The mean horizontal plane surface equivalent to this relief was found by constructing a series of parallel transverse profiles, deter­ mining the mean heights of these, and taking the mean of the means. In making the profiles, the eastern limit was taken at contour 5,500 feet, which is the western border of the Alabama Hills, and the approximate trace of a bounding fault of the mountain front. The western limit of the profiles was taken at the Kern River. The altitude of the mean surface thus found is 3.08 kilometers. The batholith having this alti­

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 ISOSTATIC RELATIONS 1701 tude of mean surface must protrude down through the basalt into the dunite. Let x be the depth of immersion in the latter, then, equating with the Pacific column, 2.72(3.08 + 46.4 + x) = 5 + 41.4 X 3.05 + 3.3a: whence x = 5.71 km. Its thickness at present is, therefore, 3.08 + 46.4 + 5.71 = 55.19 km. and its bottom is 55.19 — 3.08 = 52.11 km. below sea level. It is, of course, not intended to imply that this block, the surface of which has a mean altitude of 3.08 kilometers, attains balance independently of the rest of the range. The range as a whole maintains its balance by im­ mersion in heavier rock below, and the block selected for discussion moves up or down, with shift of load, as a part of that whole. Two uplifted valley floors.—As set forth by the writer in an earlier paper,9 and further discussed by Knopf,10 the summit region of the south­ ern Sierra Nevada presents two geomorphic features that testify clearly to its uplift in two stages well separated in time. These are the old valley floors, now terraces, named the Subsummit Plateau and the Chagoopa Plateau. These broad valley floors could not have been evolved at their present altitude, but only when they stood, in turn, close to the base level of erosion. The general attitude of the Subsummit Plateau is about 11,500 feet above sea level, “with a hypsometric range in 20 miles of 2,000 feet,” 11 which may be in part due to the tilt of the uplift. The surface of the Chagoopa Plateau has today a southerly slope, ranging in altitude from 10,500 feet in its northern part to 8,500 feet in Toowa Valley, and 7,000 feet in Little Kern Plateau. This variation in altitude is doubtless, in part, an expression of original drainage slope, but is probably chiefly due, also, to tilt of later uplift. The mean alti­ tude of the old valley floor, or floors, may be taken at 8,500 feet. The mean value for the hypsometric difference between this level and the Sub­ summit Plateau is, thus, about 3,000 feet, an interval of 500 feet greater than that adopted in a paper on the Upper Kern,12 published in 1904. The Chagoopa Plateau is traversed by the canyon of the Kern, which has a depth of about 2,500 feet. In this canyon, the river is still vigorously corrading vertically. The uplift registered by this dissection is about 7,500 feet. 8 Andrew C. Lawson: The geomorphogeny of the upper Kern basin, Univ. Calif. Dept. Geol., Bull., vol. 4 (1904). 10 Adolph Knopf: A geologic reconnaissance of the Inyo Range and the eastern, slope of the Southern Sierra Nevada, California, U. S. Geol. Surv., Prof. Pap. 110 (1918). 11 Op. cit., p. 83. 13 There was no contoured map of the region then in existence.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1702 A. C. LAWSON----SIERRA NEVADA AND ISOSTASY These two old valley floors represent two distinct cycles of erosion, which, for convenience in the following discussion, will be designated Cycle I and Cycle II. The uplift due to erosional levitation in Cycle I will be called Uplift I, and that due to the same cause in Cycle II will be called Uplift II. But these two uplifts together do not account for the present altitude of the Sierra Nevada range, and a third is recognized and called Uplift III. Uplift I is measured by the hypsometric difference be­ tween the two valley floors, and amounts to 3,000 feet, or .91 kilometer. The sum of Uplifts II and III is the difference between the altitude of Chagoopa Plateau and that of the base level of erosion to which it was cut. At no time in either of these cycles was the region more than 300 kilo­ meters from the ocean. We may, therefore, assume, without serious error, that the local base level of erosion had an altitude of .3 kilometer above sea level. It is the difference between this figure and the altitude of Chagoopa Plateau that gives 7,500 feet, or 2.29 kilometers, as the sum of Uplifts II and III. There is good reason for the belief that, in the region between the Kern River rift and the eastern front of the range, the batho­ lith had been denuded of its roof, except for an occasional roof pendant, before the inauguration of Cycle II. In Cycle I, considerable remnants of the roof probably remained to be removed in the course of the cycle. Uplift III.—It is possible to estimate the quantity of rock removed from the region between the Subsummit Plateau and the Chagoopa Pla­ teau in Cycle II, and this amounts to the equivalent of a layer, uniformly .4 kilometer thick, over the entire region, after making due allowance for the erosional effect on the upland above the Subsummit Plateau in the same cycle. Another way of stating the erosional effect of the cycle is that the mean thickness of the batholith was reduced .4 kilometer. Now, for a batholith of specific gravity 2.72, floating in basalt of specific gravity 3.05, to loose a load, .4 kilometer thick, from its surface means that the net lowering of the surface was .04 kilometer, and that the rise of the column was .36 kilometer. Thus, of the total 2.29 kilometers up­ lift of the Chagoopa Plateau, erosional levitation accounts for .36 kilo­ meter, and the balance, 2.29 — .36 = 1.93 kilometers, was due to some other cause. Here, then, we may specify quantitatively the three uplifts recognized in the later history of the Sierra Nevada: Uplift I was .91 kilo­ meter, measured directly as the hypsometric difference between the Sub­ summit and the Chagoopa Plateau, and ascribed to erosional levitation in Cycle I. Uplift II was .36 kilometer, measured indirectly as the result of erosional levitation in Cycle II. Uplift III was 1.93 kilometers, a resi­ due of uplift that cannot be ascribed to erosional levitation.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 ISOSTATIC RELATIONS 1703 Variations in thickness and 'position of the batholith.—Starting with the present mean surface of 3.08 kilometers, and using the figures just given for uplifts I, II, and III, it is possible to trace the fluctuations of the mean surface and the vertical movements of the batholith. At the end of Cycle II, and before Uplift III, the batholith, as a whole, and its mean erosional surface must have stood 1.93 kilometers lower than at present—that is, the mean surface had an altitude of 3.08 — 1.93 = 1.15 kilometers. For a column of specific gravity 2.72, floating in basalt to this height above sea level, the bottom is 40.5 kilometers below sea level. For, if x be this depth, by equating it with the Pacific Oceanic column,13 we have the fol­ lowing relation: 2.72 (a; + 1.15) = 5 + 3.05 (x — 5) whence x = 40.5 km. At this stage, the end of Cycle II before Uplift III, the batholith in this part of the range had a thickness of 40.5 + 1.15 = 41.65 kilometers. In Cycle II, a layer was removed by erosion having a mean thickness of about .4 kilometer, whereby, to maintain isostatic balance, the mean surface of the batholith was lowered .04 kilometer, and the column as a whole was raised .36 kilometer. Therefore, at the end of Cycle I, the mean surface stood at 1.15 -f- .04 = 1.19 kilometers, the bottom of the batholith was at 40.5 -f .36 = 40.86 kilometers below sea level, and its thickness was 42.05 kilometers. Due to erosional levitation in Cycle I, the batholith had been raised .91 kilometer, its surface had been lowered .11 kilometer, and the mean thick­ ness of the layer removed was 1.02 kilometers. This thickness of the layer removed by erosion in Cycle I (1.02 kilometers), in order to induce a rise of the batholith of .91 kilometer, is about twice as great as, accord­ ing to a careful estimate, that of the layer removed below the original mean surface immediately under the roof. It is, therefore, necessary to recog­ nize that the erosional removal in this cycle included considerable rem­ nants of the roof. Of the layer removed in Cycle I, having a mean thick­ ness of 1.02 kilometers, about half, or .52 kilometer, is considered to be rock of the batholith and the other half, .5 kilometer, roof rock. The net thickness of the batholith at the beginning of Cycle I was, thus, 42.05 + .52 = 42.57 kilometers; its top stood at 1.19 -f .11 — .5 = .8 kilometer above sea level, and its bottom at 40.86 + .91 = 41.77 kilometers below sea level. It will be understood, of course, that, prior to Cycle I, when the roof was much thicker, the surface of the batholith was depressed by its roof load, far below sea level. Thus, if the roof had been 10 kilometers thick when the batholith froze, we may take 2.67 as the specific gravity of the 18Andrew C. Lawson: Insular arcs, foredeeps, and geosynclinal seas of the Asiatic Coast, Geol. Soc. Am., Bull., vol. 43 (1932) p. 372.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1704 A. C. LAWSON— SIERRA NEVADA AND ISOSTASY upper 9.5 kilometers and 2.72 as that of the lower 0.5 kilometer, consider­ ing the latter as a metamorphic aureole. Then, at this stage, if x be the depth of immersion of the batholith in dunite, we have: 2.67 X 9.5 + 2.72(0.5 + 42.57) = 5 + 41.4 X 3.05 + 3.3x whence x = 3.4 km. The bottom of the batholith was 46.4 + 3.4 = 49.8 kilometers below sea level, and its thickness was 42.57 kilometers. The mean surface of the roof stood at 42.57 + 10 — 49.8 = 2.77 kilometers above sea level, and the top of the batholith at 10 — 2.77 = 7.23 kilometers below sea level. Some such deep position as this, the depth being dependent upon the thickness of the roof, represents the original emplacement of the bath­ olith, if we ignore volume changes due to cooling. It may here be observed that the erosional removal of the greater part of the roof might reasonably be included in Cycle I ; but, for convenience in discussion, Cycle I is arbitrarily made to include only the erosion that, by levitation, induced the rise of the batholith 0.91 kilometer, as measured by the hypsometric difference between the Subsummit Plateau and the Chagoopa Plateau. It was the removal of the roof, until its mean thickness was only 0.5 kilometer, that brought the hypsometric locus of the uncut Subsummit Plateau up to the altitude coincident with that of the base level of erosion at the beginning of Cycle I. Thus, by a somewhat devious route, we have arrived at quantitative expressions for the thickness and position of the batholith under the summit region of the present Sierra Nevada, at several different stages of its history, and for its relation to the sima at those stages. These quantities are summarized in Table 1, in the first column of which an arbitrary thickness of roof is taken for illustrative purposes. In these calculations, it is assumed that the normal depth of the bottom of the basalt, which is also an important equipotential surface of the dunite, is 46.4 kilometers below sea level, as in the adopted standard Pacific column. On this basis, it appears, from Column A, that the bath­ olith, with its original thick roof, floats in dunite, and that there is no basalt in the vertical column. The basalt had been pushed aside by the downward protuberance of the orginal zone of deformation, as it sank into the dunite. By the time the roof had been removed, except for 0.5 kilometer at the beginning of Cycle I, Column B shows that the batholith had emerged from the dunite, and that the basalt had flowed back between its bottom and the top of the dunite, to a thickness of 4.63 kilometers. Owing to the continued rise of the batholith by ero­ sional levitation, the basalt thickened to 5.54 kilometers at the end of Cycle I, and to 5.9 kilometers at the end of Cycle II. After Uplift II,

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 ISOSTATIC RELATIONS 1705 there was no erosional levitation to cause further appreciable rise of the batholith; but between that uplift and the present, occurred the most important uplift of all, amounting to 1.93 kilometers. Cause of Uplift III.—This uplift may be explained, from an arith­ metical point of view, in one of two ways. Either, the batholith was T able 1.—Stages of uplift under Case I

A B C D E After After Early Beginning Uplift I— After Uplift I l l - stage of Beginning Uplift II— Thickening Batholith before Cycle I of End of of erosion Cycle II Cycle II batholith (km.) (km.) (km.) (km.) (km.)

Roof...... ca. 10 .5 0 0 0 Uplift...... 8.03 .91 .36 1.93 (rise of mean surface) Mean altitude of top. -7.23 .8 1.19 1.15 3.08 Thickness...... 42.57 42.57 42.05 41.65 55.19 Depth of bottom__ 49.80 41.77 40.86 40.50 52.11 Thickness of basalt.. 0 4.63 5.54 5.90 0 thickened at the expense of the basalt, so that its bottom passed through the latter into the dunite and caused it to float higher; or, the basalt may have been thickened at the expensé of the dunite, and its immersion in this rock floated the batholith higher. How could the batholith be thickened? Manifestly, only by a recurrence of compressive stress such as gave rise to the range in the mid-Mesozoic Revolution. Owing to the massive character of the batholith and the absence of stratified structure, the compression, when it became effective for deformation, would produce not folding, but thrusting and telescoping. This would involve the shov­ ing of one part over, or under, the opposing part, and so thicken the mass. The result of the thickening would be the same as in the original concentration of mass at the genesis of the range. Both upward and downward protuberances in the zone of deformation would be accen­ tuated. To lift the surface 1.93 kilometers to the present altitude of 3.08 kilometers above sea level, the downward protuberance would extend through the basalt and 5.71 kilometers into the underlying dunite. For, if x be the depth of immersion in the dunite, then— 2.72(3.08 + 46.4 + x) = 5 + 3.05 X 41.4 + 3.3x whence x — 5.71 km.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1706 A. C. LAWSON----SIERRA NEVADA AND ISOSTASY The thickness of the batholith becomes 3.08 + 46.4 -j- 5.71 = 55.19 kilo­ meters, and the increase in thickness, 55.19 — 41.65 = 13.54 kilometers. Under these conditions, the column comprising the Sierra Nevada portion of the batholith would be in balance with the rest of the crust, with its mean surface at 3.08 kilometers above sea level. If, on the other hand, we seek to explain Uplift III as a result of thickening of the basalt, the batholith itself maintaining a constant thickness of 41.65 kilometers, we may compute the required thickness of the basalt from the following equation, in which x is the quantity sought: 2.72 X 41.65 + 3.05a; = 5 + 41.4 X 3.05 + 3.3(41.65 - 3.08 + x — 46.4) whence x — 31.43 km. The column, thus consisting of 41.65 kilometers of batholith floating in basalt, which, in turn, is sunk 31.43 — 7.83 = 23.6 kilometers in dunite, would, if it could exist, balance the rest of the crust at the depth of 46.4 -f 23.6 = 70 kilometers below sea level. But it is extremely doubtful if such a combination is possible. We have concluded that Uplifts I and II were due to erosional levitation, and that, as the batholith rose, basalt flowed in beneath it to maintain the balance of the column. The basalt is then in the zone of flowage. The compressive stress, which must be invoked to thicken the basalt, would apply also to the underlying dunite. The basalt floats normally at a depth of 46.4 kilometers, upon an equi- potential surface of the dunite, and under this condition of equality of Stress could not descend deeper into it. This means that the basalt could not be thickened to float the batholith higher. Thus, under the fundamental assumption of Case I—the melt that gave rise to the batholith did not extend below the bottom of the sial— the only way of accounting for Uplift III is by the thickening of the batholith; and there appears to be no way in which this could have been accomplished other than by thrusting and telescoping, in a rejuvenation of the compressive stress of orogenesis. Isostatic balance oj the Great Valley.—In the Great Valley, the surface of the batholith passes down to the west, under a great thickness of Tertiary marine sediments and later alluvium. Seismic sounding by oil geologists shows that these deposits have a thickness of several kilo­ meters, 8 kilometers as a maximum. In the line of section normal to the axis of the Sierra Nevada through Mount Whitney, the mean thick­ ness is about 4 kilometers, and the mean specific gravity of the sedi­ mentary formations in the Valley is taken at 2.4. The surface upon which the marine Tertiary beds rest appears to have been that of a peneplain. If this peneplain had been cut as a plain exactly at sea

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 ISOSTATIC RELATIONS 1707 level, the thickness of the batholith would probably have been 31 kilo­ meters. Its general altitude, however, even in the past stages of its development, was doubtless somewhat above sea level; so we may con­ sider the thickness of the batholith to be 32 kilometers, which would give a surface 0.1 kilometer above sea level. This surface, then, was depressed beneath a transgressing sea in middle Tertiary time and re­ ceived a load of sediments about 4 kilometers thick. These deposits, as they accumulated, must have contributed to the depression of the subsiding surface, but it is doubtful if their load initiated the downward movement; and it certainly cannot have caused the whole depression. In accordance with the principle of isostasy, a great local depression of the earth’s surface can take place only by reason of the substitution of heavier for lighter, far below the surface. When the peneplaned sur­ face of the batholith, upon which the Tertiary sediments now rest in the Great Valley, stood 0.1 kilometer above sea level, the bottom of the batholith was 31.9 kilometers below sea level; and, to balance this, at the top of the dunite, there intervened, between the latter and the batholith, 14.5 kilometers of basalt. If the basalt were withdrawn, wholly or in part, and its place taken by dunite, rising from below, the batholith would sink, the column still remaining in balance, and its surface would become the floor of a basin in which sediments might accumulate. Let us suppose, for example, that the whole of the basalt were withdrawn, so that the batholith rested directly on the dunite, and its depressed surface were covered with sediments of specific gravity 2.4, to a height of 0.15 kilometer above sea level, this being the present alti­ tude of the valley floor. Let x be the depth of the top of the batholith below sea level. Then— 2.4(* + 0.15) + 2.72 X 32 + 3.3(46.4 — 32 — *) = 5 + 3.05 X 4.14 whence x = 4.05 km. The thickness of the sediments, under this supposition, is then— 4.05 + 0.15 = 4.2kilometers; and the bottom of the batholith is 32 + 4.05 = 36.05 kilometers below sea level. The substitution of dunite for basalt of greater thickness would then be the mechanism that activated the depression. Dunite, to the thickness of— 46.4 — 36.05 = 10.35 kilometers, and 4.2 kilometers of sediments have taken the place of 14.5 kilometers of basalt, leaving the column comprising these in balance with the rest of the crust. As the basalt floats in dunite, there is no theoretical objection to the rise of the latter above the 46.4 kilometer level, if the basalt be removed to make way

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1708 A. C. LAWSON----SIERRA NEVADA AND ISOSTASY for it. In view of the general isostatic movements of the sial, assumed to be floating in basalt, the latter may be regarded as a relatively mobile layer. But it is not necessary to invoke this mobility of the basalt. The substitution of dunite for the latter, 10.35 for 14.5 kilometers, supple­ mented by 4.2 kilometers of valley fill, might just as well have been effected by a zone of low-angle thrust, which crossed and displaced the boundary between the basalt and the dunite, along the line of the present valley. This mechanical replacement of basalt by dunite would produce the valley sag, quite as effectively as the flowage of the basalt away from the valley zone and the rise of the dunite to take its place. But whether the heavier rock replaced the lighter by flowage or by thrusting, the event occurred in Tertiary time, long before post-Tertiary uplift of the Sierra Nevada of 1.93 kilometers. Thus, the event is probably to be correlated with the orogeny of the Coast Ranges, rather than with that of the Sierra Nevada. But another possible mechanism must be mentioned, if only for the purpose of rejecting it. The substitution of dunite for basalt might have been effected by a sundering of the crust, its pulling apart by drag of a current in the dunite, and the rise of the latter in a wide fissure, so produced. This suggestion will present no difficulty to the advocates of continental migration, and has been adopted by the writer,14 as the best explanation of the origin of oceanic deeps. It seems highly improbable, however, that this mechanism has operated to cause the sinking of the Great Valley. Such a fissure as would permit dunite to rise and displace basalt over any considerable fraction of the width of the valley would necessarily also sunder the batholith and so unroof the dunite. So profound a rupture and sundering of the crust along the axis of the Great Valley can scarcely be thought of without the implication of manifestations of vulcanism. But there has been no volcanic activity in the valley, south of Marysville Buttes, since early Tertiary time. Moreover, even if no vulcanism were engendered, the unroofing of the dunite necessitates the inference that the crustal column under the valley consists only of dunite and Tertiary and later sediments. We may ascertain the relative amounts of dunite and sediments in such a column to make it balance at the depth of 46.4 kilometers from the following equation: Let x be the thickness of the dunite above depth of 46.4 kilometers. Then— 2.4(46.4 + 0.15 — a;) -f 3.3a; = 5 + 41.4 X 3.05 whence x = 21.72 km. 14 Andrew C. Lawson: Insidar arcs, foredeeps, and geosynclinal seas of the Asiatic Coast, Geol. Soc. Am., Bull., vol. 43 (1932) p. 372.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 ISOSTATIC RELATIONS 1709 The thickness of the sediments is, thus, 46.4 + 0.15 — 21.72 = 24.83 kilometers, which is three times greater than the maximum value found by seismic sounding. Another objection is that the folding of the marine Tertiary strata, as displayed in the oil fields of the south end of the valley, is inconsistent with the tensile stress that the notion of a rupture implies. The suggestion of rise of the dunite in a wide fissure, as the mechanism that induced the subsidence of the valley, is, therefore, rejected as inconsistent with: (1) the absence of vulcanism, (2) the known thickness of the Tertiary sediments, and (3) the folded condi­ tion of the latter. If the Great Valley be regarded as a graben, or dropped block, that implies both a tensional stress and a deficiency of mass, which calls for compensation in depth, and, therefore, a rise of the region embracing the graben. As the folded Tertiary indicates com­ pressive stress; and as the evidence from sedimentation indicates not uplift, but continuous depression since mid-Tertiary time, the hypothesis of a graben seems to be negatived. CASE II—BASALT INCORPORATED IN BATHOLITH MELT In Case I, it has been assumed that the crushing in the zone of deformation, and the subsequent melting of the crushed rock to make the magma of the batholith, extended down only to the bottom of the sial. But we are just as free to assume that the crushed zone included the basalt, and even extended down into the dunite, and that the melt, which, on freezing, became the batholith, was equally deep. This is the condition which it is proposed to discuss as Case II. For the sake of simplicity in discussion, it will be assumed that the crushing and fusion extended only to the bottom of the basalt; it being recognized that the melt may have gone deeper and included some of the dunite. The com­ bined sial and basalt of the crushed zone, as well as the resulting batho­ lith, are considered to have a mean specific gravity of 2.83. The argu­ ment is the same as in Case I, but will be stated more summarily. The mean surface of the summit region in the Mount Whitney section has an altitude of 3.08 kilometers. For a batholith of specific gravity 2.83 to float this high, it would be immersed 18.63 kilometers in the dunite. Its thickness would be 68.11 kilometers, and its bottom would be 65.03 kilometers below sea level. As before, the present altitude is the aggregate effect of uplifts I, II, and III. Uplift I is 0.91 kilometer, the hypsometric difference between the Chagoopa Plateau and the Sub­ summit Plateau, and was caused by the erosional removal in Cycle I of a layer, 1.08 kilometer thick, of specific gravity 2.72, consisting of 0.5 kilometer of residual roof and 0.58 kilometer of batholith. After the uplift, the thickness of the batholith was 54.75 kilometers, its bottom was 53.56 kilometers below sea level, immersed 7.16 kilometers in dunite,

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1710 A. C. LAWSON— SIERRA NEVADA AND ISOSTASY and the altitude of its mean surface in the region under consideration was 1.19 kilometer. Uplift II was caused by the erosional removal of a layer, 0.4 kilometer thick, of specific gravity 2.72, from the top of the batholith. This reduced its thickness to 54.35 kilometers, raised its bottom 0.33 kilometer to 53.23 kilometers below sea level, and lowered the mean surface 0.07 kilometer to 1.12 kilometer above sea level . But after this came Uplift III of 3.08 — 1.12 = 1.96 kilometers. To explain this, we must invoke the thickening of the batholith, for, in this case, there is no basalt intervening between its bottom and the dunite.

T able 2.—Stages of uplift under Case II

A B C D E Early Uplift III stage— Beginning End End to Batholith before of Cycle I— Cycle II— present erosion Cycle I Uplift I Uplift II condition (km.) (km.) (km.) (km.) (km.)

Roof...... ca. 10. .5 0 0 0 Uplift...... 91 .33 1.96 Mean altitude of top. -6.84 .86 1.19 1.12 3.08 Thickness...... 55.33 55.33 54.75 54.35 68.11 Depth of bottom__ 62.17 54.47 53.56 53.23 65.03 Depth of immersion in dunite...... 15.77 8.07 7.16 6.83 18.63

As, after Uplift II, the batholith had a thickness of 54.35 kilometers and, after Uplift III, must have had a thickness of 68.11 kilometers, in order to float to a height of 3.08 kilometers, the increment is 68.11 — 54.35 = 13.76 kilometers. As in Case I, this thickening can be ascribed only to thrusting and telescoping of the batholith. The fluc­ tuations of the hypsometric position and thickness of the batholith, under the assumption of Case II, are summarized in Table 2. The results appear to be as acceptable as those obtained in Case I. The thickening of the batholith by thrusting, necessary to give the present mean altitude of 3.08 kilometers, is, in the one case, 13.54 kilometers and, in the other, 13.76 kilometers. When we turn our consideration again to the Great Valley, however, we are limited, in attempting to explain the Tertiary subsidence, to the substitution of dunite for the rock of the batholith; and this was more probably effected by thrusting than by flowage. That is to say, on a low-angle thrust crossing the

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 ISOSTATIC RELATIONS 1711 boundary of the batholith and the underlying dunite, in the line of the present valley, a wedge of the latter may have displaced a mass of the batholith sufficiently thick to have induced the sinking of the column in which it occurred. As an alternative, we might, of course, postulate a sundering of the batholith itself, along the axis of the valley, and a welling up of the dunite into the fissure. The objections to this hypothesis have been set forth in the discussion of Case I; they apply with equal force here. Thus, to summarize: in Case I, in which the batholith melt is limited to the sial, two alternatives were considered; and, of these, one—that Uplift III was due to thickening of the basalt—was rejected. The other alternative—that Uplift III is a result of a thickening of batholith itself, by thrusting and telescoping—remains as the only acceptable conclusion. In Case II, in which the batholith melt includes both sial and basalt, the same conclusion is reached, the difference in the thickening of the batholith to produce high flotation being only 0.22 kilometer in the two cases. The Tertiary subsidence of the Great Valley is also best explained by thrusting. The thrust in this case, however, substituted heavier rock for lighter. CONSEQUENCES OF THICKENING OF THE BATHOLITH But what are the consequences of this conclusion? One is, if we accept it, that we must abandon the notion that the eastern front of the range is the scarp of a normal fault. The thrust that thickened the batholith must have had an emergence at the surface, and the only place available for the trace of that emergence is the base of the eastern front. That front is, then, the degraded edge of a thrust block. The normal faults apparent in Owens Valley, Carson Valley, and elsewhere along the base of the range, become the manifestations of minor movements of adjustment, which are necessary concomitants of thrusting on a large scale. A further consequence is that Owens Valley ceases to be a graben, in the sense that it is due to the infall of a block or wedge of the crust. Its graben-like profile is to be interpreted as due to the rise of the edge of the thrust block on the thrust plane and its relative approach to the western front of a pre-existing range—the Inyo Range; it may be reasonably inferred that the block, in its relative movement eastward, has overridden a belt of fragments fallen from its edge. If the thrust surface be a curve, other than a circle, convex upward, relative move­ ment upon it would tend to lift the upper block from the lower, and so cause its collapse. This may be the cause of the break recognized in the rift of the Kern Canyon, which originated before the cutting of the canyon, and determined its course.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/11/1691/3430546/BUL47_11-1691.pdf by guest on 02 October 2021 1712 A. C. LAWSON----SIERRA NEVADA AND ISOSTASY MOUNT DANA SECTION Discussion of the isostatic relations of the Mount Whitney section, might be duplicated on the basis of those afforded by the Mount Dana section, 105 miles farther north. At Mount Dana, a flat-lying remnant of the roof of the batholith is the summit of the range, the altitude of which is 13,050 feet. The Mount Dana Plateau, a notable feature of the profile of the range, and probably also the terrace on the west side of Kuna Crest, correspond to the Subsummit Plateau, with a mean alti­ tude of 11,600 feet. Tuolumne Meadows corresponds to the Chagoopa Plateau and has a mean altitude of 8,700 feet. The hypsometric interval between these two old valley floors is, thus, 2,900 feet, or .88 kilometer, and this figure is the measure of Uplift I, as both floors were presumably cut at base level of erosion. Tuolumne Meadows has the same geo- morphogenic relation to Tuolumne Canyon that Chagoopa Plateau has to the canyon of the Kern. Its uplift is the difference between its present mean altitude, 8,700 feet, and that of the local base level of erosion at which it was cut, here probably nearer .2 kilometer than .3 kilometer. That uplift, thus measured, is 2.45 kilometers. Careful estimate of the mean thickness of the erosional removal between contours 11,600 and 8,700, shows that the mean layer removed could not have exceeded .4 kilometer. Levitation by this amount would account for only .35 kilo­ meter of the uplift, leaving 2.45 — .35 = 2.1 kilometers to be explained. The uplift of .35 kilometer corresponds to, and is the same as, Uplift II in the Mount Whitney section; and 2.1 kilometers is the measure of Up­ lift III, being somewhat larger than at Mount Whitney. As before, the only way of accounting for Uplift III, consistently with the principles of isostasy, is by a thickening of the batholith; and the only way in which this could be accomplished is by horizontal compres­ sion, culminating in telescoping or thrusting. We must, thus, regard the front of the range, overlooking Mono Lake, as the edge of a moun­ tain block uplifted by thrusting, and interpret the evidence of normal faulting as a secondary effect, due to adjustments necessitated by the thrust.

U n iv e r sit y op C a lifo r n ia , B erk eley , C a l if. M a n u sc r ipt received by t h e S ecretary of t h e S ociety, S eptem ber 15, 1936. P aper delivered at t h e T ercentenary C o nference of A r ts and S c ie n c es at H arvard U n iv e r s it y , S eptem ber, 1936. A ccepted by t h e C om m ittee on P ublications, 1936.

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