James Gilluly

Plate Tectonics and Magmatic Evolution

The validity of the general idea of plate tectonics is accepted; the magmas evolved along the spreading ridges are thought to be largely tholeiitic basalt, although alkalic olivine basalt and ultramafic rocks of several kinds have also been dredged from them. The ultramafics may be residual from the partial melting of pyrolite while the tholeiite was being formed at shallower depths, or they may possibly be fragments of the mantle raised by the injection of sills. Bouvet and Jan Mayen Islands, both on the crest of the Mid-Atlantic Ridge, are largely composed of alkali basalt with very minor differentiates of trachyte and even rhyolite that may be readily accounted for by differentiation at a high level in the volcanic edifice. Iceland, though, has so much granite and rhyolite widely distributed that it seems likely, as suggested by several students, that its basement is sialic. The volcanic islands tend to be more alkalic the farther they are from the ridges; perhaps they rose from deeper sources in areas of low heat flow and are not related to plate margins. If the African Rifts are incipient plate margins, it is noteworthy that the magmas associated with them are wholly different from the tholeiites of the oceanic ridges. They are among the most highly alkaline of any rocks known. The magmatic activity at the subduction zones, where the plates are being destroyed, is very different. There are three varieties of these plate junctions: continental against oceanic, oceanic against oceanic, and continental against continental. In both the junctions involving oceanic crust the material being consumed includes a variable thickness of sediment, underlain by 5 or 6 km of tholeiitic basalt overlying the downgoing mantle. These rocks are much less refractory than the pyrolite of the mantle and must surely compose a large part of material parental to the magmas formed along the subduction zones, the andesites, granodiorites, and granites. There is nowhere the tremendous volume of intermediate rocks that would have had to be formed if these voluminous magmas had been products of crystallization differentiation from a basaltic magma. The presently most active of the continent-continent junctions is along the Himalayas where India is underthrusting the continent of Asia; here there is no evidence of magmatism except along the transcurrent faults at either end of the main range. But there are large volcanic and plutonic masses that have no obvious relation to the plate boundaries active in Mesozoic and Cenozoic time. The Eogene volcanics of the San Juans and the Neogene volcanics of the Yellowstone are more than 1,500 km from any obvious subduction zone, and these regions of magmatic activity seem no more closely related to subduction zones than are the Tertiary igneous rocks of West Texas, the Cretaceous tuffs and plutons of Arkansas, the Cretaceous intrusives of the Monteregian Hills, and the minor Tertiary intrusives of Virginia.

Sedimentary Volumes and their Significance

Sedimentary volumes are of prime interest in many fields of geology: as measures of erosional rates, of geochemical balance, and recently, with the virtual demonstration of continental drift, as measures of movement of the continental and oceanic plates. The Basement Map of the United States, published by the U.S. Geological Survey in 1968, provides a partial basis for an improved estimate of the volume of Phanerozoic rock in the center, minous United States. The map requires correction for this purpose, because all metamorphic rocks of whatever age have been classed as basement. We have, therefore, attempted to allow for the metamorphic rocks of Phanerozoic age. We have made estimates of volumes for areas not controlled by contours on this map and have used such offshore data as we have been able to assemble from the literature in order to extend our estimates to include offshore sediments reasonably attributable to erosion from the area of the contiguous United States. Our results are as follows: We consider this estimate to be within 10 percent of the true volume. Of it, we estimate about 3.2 ×10 6 km 3 to be volcanic rock, not representing erosion of pre-existing rock. The remaining 56.8 × 10 6 km 3 , rounded to 57 × 10 6 km 3 , we consider products of continental denudation. This volume is so large, representing, as it does, only 5.3 percent of the continental surface of the earth and only a sixth of recognizable geologic time, that it appears to invalidate schemes of geochemical balance such as those of Clarke, Goldschmidt, and others. These students assume that the salt in the sea is a measure of the amount of some “average igneous rock” that has been eroded during the whole of geologic time to produce some “average sedimentary rock.” Instead, our result points strongly toward the hypothesis of Livingstone, Gregor, Earth, and others that the oceanic salt is merely the cyclic salt not yet returned to the continents in a continuing cycle. Assuming that this volume was derived from erosion of the contiguous United States—an assumption that we recognize as invalid in detail, though not seriously in error—we obtain an ostensible average rate of Phanerozoic erosion of about 10 m/ m.y., about a sixth of the present rate. But inasmuch as present erosion is attacking a surface that exposes about 76 percent sedimentary rocks and only 24 percent igneous, most of its product is recycled rather than first-cycle sediment. An analysis of the broad features of the paleo-geography of the country indicates that a similar disproportion between first-cycle and recycled sediment has been characteristic of nearly all the Phanerozoic. The ostensible erosion rate is therefore spurious, and it is likely that the average erosion rate durin g the Phanerozoic was more than half that of the present, and perhaps was nearly or quite equal to it. The great disparity in volumes of sediment offshore in the Atlantic and Pacific—in a ratio of more than 5 to 1—is consonant with expectations if the continent has been moving westward and over-riding the Pacific Basin on a Benioff fault system activated at the beginning of the Mesozoic, though now dormant.

Geological Perspective and the Completeness of the Geologic Record

The completeness of the geologic record obviously diminishes with the passage of time, not simply because younger rocks come to bury the older, but also because the younger have been largely derived by the cannibalizing of the older. This fact has been all too often ignored in generalizations about geologic history and the record has been treated as though all parts of the column are susceptible of equally fine discrimination. Many generalizations that burden the geologic literature—that orogeny has been speeding up with time, that erosion rates show a secular increase, and many other deductions as to trends in geologic processes—lose much, if not all, of their credibility when account is taken of this fact. Accurate data are not available to permit quantitative evaluation of the loss in the record with increasing geologic age. It seems highly unlikely, too, that the rate of loss has been uniform through time. But map data provide a crude approach to such an evaluation and plainly show the prevailing tendency. As with optical perspective, it is clear that a feature distant in time must be larger than one close at hand if it is to appear as distinctly. Perhaps it is more accurate to say that our chances of reading the geologic record in similar detail diminish with the passage of time. In this study I have measured on the geologic maps of North and South America the areas occupied by rocks of the several categories distinguished on them and have compared the areas of exposure of each category with the time span during which it was formed. The results are presented in tables and graphs. These data give a rough measure of the loss of detail of the record and raise some questions as to apparent secular variation in such processes as volcanism and plutonism.

Atlantic Sediments, Erosion Rates, and the Evolution of the Continental Shelf: Some Speculations

The volume of Triassic and younger sediment on and offshore from the Atlantic coast between Virginia and Nova Scotia can be estimated from the isopach map by Drake, Ewing, and Sutton (1959) of Atlantic coastal and offshore sediments. This volume is compared with that which would have been derived from the probable source area at present rates of erosion. It is found that the average rate of erosion in Triassic and later time was probably not less than three fourths and perhaps equal to the present rate. The arrangement of the sedimentary troughs identified by Drake, Ewing, and Sutton suggests that the Continental Shelf was formed in part by isostatic sinking of the offshore crust beneath the sedimentary load supplied by the rivers. But the presence of a nearly continuous median ridge in the sedimentary basin suggests that isostatic sinking is not alone responsible for the downwarp and in fact could not suffice to bring it about. The density of the sediment must be less than two thirds that of the subcrustal material displaced. If the basement had been originally horizontal or sloping uniformly seaward, the basin landward of the median ridge must have sunk by a mechanism other than isostatic depression beneath an additional 5000 feet of sediment, for the seafloor is roughly at the same depth over both ridge and inner basin. This differential depression of the basement, as well as the sinking of a former land area to form such basins, requires thinning of the crust; it is here suggested that subcrustal erosion by currents beneath the M discontinuity caused the thinning. Possibly the median ridge and the basins are due partly to instabilities produced by such currents, although they seem too large to be attributed to drag.

SUBSIDENCE IN THE LONG BEACH HARBOR AREA, CALIFORNIA

Surveys and other observations in the area of Long Beach Harbor, California, indicate a general subsidence over a large area. Over the near-by plain to the north and east of the harbor, this subsidence averages a few tenths of a foot over a period of about 20 years. In this larger field of subsidence there is a localized area within which the depression of the surface amounts to several feet, reaching a maximum of more than 4 feet in the region of the Inner Harbor. This area of maximum subsidence, when contoured according to available level data, coincides remarkably with the productive area of the Wilmington oil field. It is also highly significant that the subsidence, as indicated by tide-gauge records, first became notable in 1937, shortly after the beginning of the development of the field. The following possible causes of the subsidence have been considered: Tectonic—earthquakes, horizontal and vertical movements in the earth9s crust, faults, tilting. Decline of pressure in water sands, lowering of the water table. Increased loads due to structures and fill deposited on the surface. Oil-field operations, including removal of oil from underground, decline of pressure in the oil reservoirs, with resulting increased load on them, and elastic shortening and plastic deformation expected therefrom. The possible effect of each of these factors is discussed, together with data on the mechanical properties of the oil sands. Conclusions reached are: (1) A very small part of the subsidence—perhaps a few tenths of a foot—may possibly be due to tectonic movements but is not certainly so. It is equally likely to be due to decline of water pressure in aquifers underlying the whole region or, in part, to loss of reservoir pressure due to flow of water in the oil sands to neighboring oil fields. (2) The excessive subsidence localized within the Wilmington oil field is primarily the result of oil-field operations. Reasons for excluding tectonic factors from responsibility are summarized. There is no correlation between the amount of oil production from a given part of the field and the amount of subsidence there, when areas as large as a “production block” (about half a square mile) are considered. There is some correlation between subsidence and production in areas of intensive exploitation, but this is believed to be indirect and dependent upon the influence of pressure decline on both processes. On the other hand, there is a very close agreement between the relative subsidence of the various parts of the field and the pressure decline, thickness of oil sand affected, and mechanical properties of the oil sands. This correlation is so close as to constitute conclusive evidence of a cause and effect relation between pressure decline and subsidence. An accurate forecast of the amount of ultimate subsidence is impossible. In part it is dependent on the rate of production of oil and gas from the field, for the pressure drop in the reservoir sands may be expected, over a long time, to parallel this production rate. Although the current rate of subsidence is higher than the rate of pressure decline, this will probably eventually slacken, so that it may require 8 or 10 years for the subsidence of another 4 feet, and the ultimate subsidence to be expected if the production of the oil field proceeds to depletion is estimated at about 10 feet. This amount will not, however, be reached over the entire field, but is expected to be limited approximately to the present area of maximum subsidence. From this area the subsidence will diminish in all directions, so that, though the area affected by considerable sinking will eventually be somewhat larger than that now involved, the amount of subsidence will not be more than about two to three times that which has already taken place. In other words, the ultimate isobase of 1 foot subsidence may be expected to be near the present isobase of .50 foot subsidence. These estimates of ultimate subsidence are predicated upon continued exploitation of the oil field and will not be realized should production be substantially curtailed or stopped. On the other hand, should additional oil zones be discovered and produced, the above estimate would have to be increased because of contraction of the additional oil sands.

Physiography of the Ajo region, Arizona

INTRODUCTION Ajo lies in the Papago country of southwestern Arizona, in the Sonoran desert section1 of the Basin and Range province (Fig. 1). The region, which displays a wide variety of topographic forms, offers unusual opportunity for study to those interested in the physiography of arid regions because the United States Geological Survey has completed topographic maps, one of the Ajo quadrangle on the scale of 1: 48,000, and one of a few square miles in the mining area near Ajo on the scale of 1: 12,000. The Ajo quadrangle includes the small mountain masses of the Little Ajo Mountains, Childs Mountain, and part of the Batamote Mountains. Much of the area is occupied by desert plains, of which the largest is the Valley of the Ajo. Altitudes range from 1,150 to 3,200 feet. Local relief is moderate and only exceptionally reaches 1,000 feet in a square mile. The climate . . .

Oceanic Sediment Volumes and Continental Drift

The volume of sediment off the Atlantic Coast of the United States is at least six times as great as that off the Pacific Coast. This disparity is readily accounted for if the continent is drifting westward and has overrun large volumes of sediment on a former Benioff zone. Such an overrunning is also consonant with many features of the geology of the western United States.

Basin Range Faulting Along the Oquirrh Range, Utah

Introduction LOCATION OF THE AREA STUDIED The Oquirrh Range is in west-central Utah, about 15 miles west of the Wasatch Range, from which it is separated by the nearly level Jordan Valley. Its north end, which is about 20 miles from Salt Lake City, is washed by Great Salt Lake, from which the range extends southward for about 25 miles. Owing to the southeasterly trend of the Wasatch Range, the two ranges diverge somewhat toward the south, and between them stands another range, the Lake Mountains. The southern half of the range, which is the part of it studied in detail by the writer, is included in the Stockton and Fairfield quadrangles of the U. S. Geological Survey Topographic Atlas. PREVIOUS WORK Basin Range faulting along the west front of the Oquirrh Range was long ago recognized by Gilbert, 2 who described the fault at Ophir Creek and assumed that it . . .

Possible Desert-Basin Integration in Utah

The geographic and hypsometric relations of Rush Valley and Tooele Valley, Utah, are described. The forms of the valley floor and peculiar features of Rush Lake and of the alluvial fans of Rush Valley are interpreted as evidences of the probable integration of the drainage of this valley with that of Tooele Valley, the first stage leading toward the coalescence of the basins. Two examples of capture, in the restricted sense, in other parts of the Basin and Range province are cited. The conclusion seems justified that this stage of the arid cycle can now be established inductively and need rest no longer on a purely deductive basis.

Chronology of intrusion, volcanism, and ore deposition at Bingham, Utah

Volcanic rocks like those of the Bingham area are intruded by a dioritic intrusive porphyry, so the generalization that all the volcanic rocks are younger than the intrusive rocks does not necessarily hold away from the Bingham area (For original paper see ibid., v. 63, p. 612-621, 1968.)