Showing papers in "Geological Society of America Bulletin in 1973"

Plate Tectonics and the Evolution of the Alpine System

Abstract: It is contended that the Late Triassic to present-day gross evolution of the Alpine system in the Mediterranean region has been the result of activity along an evolving network of accreting, transform, and subducting plate boundaries between the large stable cratons of Europe and Africa. A refined assembly of the outlines of the continents around the North and central Atlantic, before the initial dispersion of Gondwanaland in Early Jurassic times, is presented. By considering geologic facies, structural fabric, and paleomagnetic criteria, the smaller continental fragments now found within the Alpine system are restored to their proposed initial positions relative to each other in the reconstruction offered. The motion of the major plate of Africa relative to Europe, commencing with the initial continental fragmentation, is documented by analysis of the sea-floor spreading history of the Atlantic Ocean, with the assumption that plate accretion there has occurred between torsionally rigid lithospheric plates. By the computerized fitting of well-defined and well-dated key pairs of symmetric magnetic anomaly lineations back together by a series of finite rotations, the relative position of North America to both Europe and Africa has been determined for the following times: 180 m.y. (Toarcian Stage, Early Jurassic); 148 m.y. (Kimmeridgian Stage, Late Jurassic); 80 m.y. (Santonian Stage, Late Cretaceous); 63 m.y. (Danian Stage, Paleocene); 53 m.y. (Ypresian Stage, Eocene); and 9 m.y. (Tortonian Stage, Miocene). From these positions, a series of rotation poles presumed to describe the stepwise motion of Africa relative to Europe were computed. The motions of the smaller intervening microplates have been inferred from the style of tectonic deformation on their borders, and these motions have been constrained to satisfy both changes in paleo-latitude with time and progressive rotations relative to the large macroplates that can be deduced from paleomagnetic measurements. The evolution of Tethys does not involve a single simple plate boundary between Europe and Africa, as has been envisioned previously, but, instead, a constantly evolving mosaic of subsiding continental margins, migrating mid-oceanic ridges, transform faults, trenches, island arcs, and marginal seas (back-arc basins). The periods of passive-continental margin development are recognized by a transgressive facies of platform carbonate rocks and thick prisms of continental-rise type sedimentation; accreting ridges by ultramafic rocks, gabbro, pillow basalt, deep-sea pelagic ooze, and abyssal red clay of the ophiolite suite; trenches by a migrating series of progressively younger linear flysch troughs whose immature mineral composition reflects nearby andesitic and metamorphic source terrains; the arcs themselves by calc-alkaline volcanism and the intrusion of silicic to intermediate plutons; the polarities of these arcs by the direction of overthrust nappe sheets and gradients in the ratio of potash to silica in the extrusives; their orientation by paired belts of high T and P and high P-T metamorphics; and finally the spreading back-arc basins by outpourings of basaltic magmas and evidence of flipping Benioff planes. A compilation of eight phases or chapters in Atlantic spreading history are outlined, which are based on the recognition of discrete differences and (or) relative motion between the continents bordering the Atlantic. All of these changes are reflected in the Tethys by reorganizations of the intervening plate boundaries and, we believe, are most explicitly recorded in the deformational history of the subducting zones. A montage of geometrically assembled plate-boundary interpretations are pictorially displayed as time-lapse frames of the evolving Alpine system. The montage begins with the Late Triassic (pre-Atlantic) setting of the Tethys 1 Ocean and extends to the present through nine phases of Tethyan history. Each phase is recognized on the basis of the age of intrusion and extrusion of basic lavas in ophiolite complexes, which mark the creation of new oceanic areas by both axial accretion in rift valleys of mid-oceanic ridges between rigid plates or by a more uncertain type of spreading in basins behind active island arcs. All the schemes presented are best estimates of the gross geometrical arrangements at discrete time intervals and should be treated as merely educated guesses. Despite the fact that we only have rigorous constraints for the relative positions of the nondeformed forelands of Europe and Africa, our models nevertheless imply that the motions of the larger plates will, by and large, dictate the general behavior of the smaller microplates through the particular styles of deformation set up along the adjoining plate boundaries. The Tethys 1 Ocean, located between Africa and Europe in Triassic times, has been almost entirely swallowed up in subduction zones of the Major Caucasus Mountains along its former northern margin and in similar zones of the Pontides and Minor Caucasus along its southern margin. The only remnants of Tethys 1 are the areas of oceanic crust in the Black and South Caspian Seas. There is considerable evidence to suggest that the Tethys 1 Ocean had an actively spreading ridge. Some tens of millions of years prior to the opening of the central North Atlantic, a branch of this ridge system entered into the Vardar Zone of eastern Greece and broke off fragments of northeast North Africa to initiate the development of the present-day Ionian and Levantine Basins of the eastern Mediterranean. Additional fragments (the Moroccan and Oranaise Meseta) were ruptured from northwest Africa following its separation from North America. The intervening Jurassic Atlas, seaway developed along an accreting plate boundary extending from the eastern Tethys to the crest of the embryonic Mid-Atlantic Ridge where it formed a migrating triple junction whose trace, we believe, follows the trend of the New England seamount chain. The western Mediterranean basins of the Alboran, Balearic, and Tyrrhenian Seas are very much younger, being initially opened in the early Miocene as a string of back-arc marginal seas behind the developing Apennine, Tel Atlas, and Rif suture zone that today marks the sites of subduction of Jurassic and Lower Cretaceous oceanic crust. The contemporary Alpine system displays a spectrum of stages in the building of mountain belts. Embryonic nappes within the Mediterranean Ridge in proximity to melange zones of the inner wall of the Hellenic Trench are, perhaps, signs of the initial deformation of sedimentary passengers on oceanic crust arriving at a subduction zone. Total closure of an ocean followed by the partial consumption of a passive continental margin leads to events such as the tectonic emplacement of crystalline basement nappes of the European “chaine calcaire” onto northwest Africa. Arc-continent collisions of this type which have then been succeeded by total destruction of marginal back-arc basins are recognizable in the Hellenides and Pontides. There are, as well, collisions that have not involved the disappearance of large oceanic areas; these are most apparent in the particular tectonic style of the Pyrenees and High Atlas Mountains.

Properties of some common igneous rocks and their melts at high temperatures

Abstract: The properties of four igneous rocks (a tholeiitic and an alkali-olivine basalt, an andesite, and a rhyolite) and a synthetic lunar sample have been determined at atmospheric pressure over a range of temperatures including their melting interval. Viscosity, density, electrical and thermal conductivity, ultrasonic wave velocities, surface tension, and vesiculation rates were measured directly; these data have been used to calculate values for thermal expansion, compressibility, ultrasonic wave attenuation, and activation-energy coefficients.

Microstructures and Preferred Orientations of Experimentally Deformed Quartzites

Abstract: An experimental study of the plastic deformation of quartzite has produced microstructures and preferred orientations similar to those found in many natural rocks, and has identified the operative orienting mechanisms in most cases. The microstructures vary widely with conditions and presumably are related to the deformation mechanisms. Below 850°C at 10−5/sec (or 650°C at 10−7/sec), no recrystallization occurs; the deformation of the original grains is very inhomogeneous and deformation lamellae of many orientations are observed. At higher temperatures or slower strain rates, grain boundary recrystallization is present; the original grains are continuously flattened with increasing strain and only basal and prismatic deformation lamellae are observed. Above 800°C at 10−7/sec, recrystallization is complete after low strain. Below 800°C at 10−5/sec (or 600°C at 10−7/sec), a maximum of c axes develops parallel to the compression direction (σ1), while at higher temperatures and slower strain rates, a small-circle girdle of c axes develops about σ1. The opening half-angle of this girdle ranges from 20° to 45° and increases with increasing temperature and decreasing strain rate. Super-imposed on both of these c axis patterns is a tendency for the poles to positive trigonal forms, and the pole to the second order prism to be aligned parallel to σ1. The preferred orientations of the c axes and the prisms are consistent with external rotations produced by the observed intragranular glide. The difference in the preferred orientations of the positive and negative forms is due to mechanical Dauphine twinning. Strong evidence exists that these same orienting mechanisms have operated in many naturally deformed rocks.

Variations in Sr, Rb, K, Na, and Initial Sr87/Sr86 in Mesozoic Granitic Rocks and Intruded Wall Rocks in Central California

Abstract: Initial Sr87/Sr86 of granitic rocks which are exposed north of the Garlock fault in California, and which represent the entire 130-m.y. time span of emplacement during the Mesozoic, ranges mainly from 0.7031 to 0.7082, with one value of 0.7094. A systematic areal variation, independent of age, exists for initial Sr87/Sr86 in these granitic rocks and is the same as the areal variation in initial Sr87/Sr86 of superjacent upper Cenozoic basalts and andesites. Two values of initial Sr87/Sr86, 0.7040 and 0.7060, mark natural separations of granitic rock data on K-Rb, K-Sr, and Rb/Sr-Rb variation diagrams, and also, when contoured, seem to represent geographic markers of paleo-geographic, geochemical, and physiographic significance. Upper Precambrian sedimentary and metamorphic rocks in California crop out only in the region where initial Sr87/Sr86 of granitic rocks is greater than 0.7060. A line of initial Sr87/Sr86 = 0.7060 is approximately coincident with the boundary between Paleozoic eugeosynclinal and miogeosynclinal rocks. Granitic rocks intruded into Paleozoic miogeosynclinal rocks have initial Sr87/Sr86 greater than 0.7060, whereas those intruded into eugeosynclinal Paleozoic rocks have initial Sr87/Sr86 less than 0.7060. The line of initial Sr87/Sr86 = 0.7040 is the eastern limit of principal exposures of ultramafic rocks, the western limit of Cretaceous granitic rocks, and is coincident with an abrupt change in “topographic expression” on the Bouguer gravity map of California. Correlation of the isotopic variations with these major crustal features suggests that there has been a sharp lateral contrast in crust-mantle chemistry across the region of study that has been fixed in position from the Precambrian to the present time. The chemical and isotopic variations observed are best explained if the parent magmas of the majority of granitic rocks investigated were derived in a region that was laterally variable in composition and in a zone of melting that intersected both upper mantle and lower crust. However, some igneous rocks, such as Jurassic volcanic rocks in wall rocks and roof pendants and some granitic rocks with high strontium concentrations and low Rb-Sr ratios, suggest that deeper sources are also involved in the total spectrum of igneous rocks in the region.

Crustose Coralline Algae: A Re-evaluation in the Geological Sciences

Abstract: The crustose coralline algae are well known in shallow tropical waters as reef frame-builders and sediment producers. Although their abundance at greater depths and in arctic waters has been previously recorded, this knowledge in recent decades has been largely ignored by geologists and marine scientists in general. Many erroneous or misleading ecological and paleoecological statements and conclusions have resulted, and we have endeavored to clarify matters through the citation of the older literature along with more recent ecological studies. A parallel tendency to “simplify” the taxonomic structure of crustose corallines has threatened to add considerable confusion to modern marine studies. We have discussed recent work on anatomy, reproduction, and taxonomy. These and classical data are summarized in the form of keys and an evolutionary tree, which are intended to provide the geologist and marine biologist with a working facility with the group. A number of quantitative ecological studies treating crustose corallines have appeared during the past decade; these results are discussed and possibilities for future ecological work indicated. The occurrence of rhodoliths (maerl, free corallines) and the factors controlling the development of these deposits are also noted.

Analysis of Axial Orientation Data, Including Till Fabrics

Abstract: An error in the significance test associated with a rotational vector procedure for analysis of axial orientation data is pointed out and explained. An alternative approach already well established in the geologic literature, which does not require that sense be assigned to the axial data, is discussed; the results of the application of both methods to 36 till fabrics are compared. It is concluded that the alternative approach is superior because it is statistically sound and more efficient with regard to computer time required; it also provides more information about the fabric than does the rotational vector procedure.

Tertiary Pacific Plate Motion Deduced from the Hawaiian-Emperor Chain

Abstract: Nearly all linear island and seamount chains on the Pacific plate are parallel to small circles generated about either a Hawaiian pole at 72° N., 83° W. or an Emperor pole at 17° N., 107° W. The rates of rotation of the Pacific plate relative to the Hawaiian melting (hot) spot are calculated from age data from the Hawaiian-Emperor chain. Extrapolation of the known age progression along the Hawaiian chain yields a 27-m.y. estimate of the age of the Hawaiian-Emperor bend; however, recent radiometric ages from Koko seamount in the southern Emperor chain indicate that the Hawaiian-Emperor bend is 42 to 44 m.y. old. The Pacific plate apparently moved slowly, if at all, relative to the Hawaiian melting spot from about 20 to 42–44 m.y. ago. The rates of rotation calculated are 1.3° per m.y. about the Hawaiian pole (0 to 20–25 m.y.), The proposed rotational motion of the Pacific plate relative to the Hawaiian melting spot can be used to predict ages of seamounts and islands in other chains if the various melting spots are fixed with respect to one another. Almost all ages from other chains are consistent with the rotational model except for two K-Ar ages from the Austral chain. The proposed rotational motion of the Pacific plate can be used to reconstruct a paleomagnetic polar path of the Pacific plate if the melting spots are fixed with respect to the spin axis. The melting-spot polar path agrees well with the Late Cretaceous and limited Neogene paleomagnetic data. However, an extension of this polar path through the Late Cretaceous based on the Line Islands appears to be inconsistent with existing paleomagnetic data. The assumption that melting spots are fixed relative to the spin axis (and therefore the equator) can be tested by comparing the equatorial belt of high sedimentation with the sediment distribution predicted by the rotational-plate motion model. Almost every Deep Sea Drilling Project (DSDP) site presently located just north of the equator has its highest sedimentation rate at the time when the melting-spot motion model predicts that it was located at the equator. An equatorial sedimentation model adapted after Winterer (1972) is combined with the melting-spot motion model to generate a predicted isopach map of the post-middle Eocene equatorial sediments. The resulting isopach map is remarkably similar to the Ewing and others (1968) map of actual isopachs determined by seismic profiling. No data at present require motion of the Hawaiian melting spot relative to the spin axis. The tectonic histories of Japan and the Aleutians appear to reflect the proposed discontinuities of Pacific plate motion.

Contemporary Compressive Stress and Seismicity in Eastern North America: An Example of Intra-Plate Tectonics

Abstract: A large region of high horizontal compressive stress is delimited in eastern North America from a combination of fault plane solutions of earthquakes, in situ stress measurements, and geologic observations. Each of these methods, including in situ stress determination by both overcoring and hydrofracturing, yields nearly identical directions for the principal stresses. The maximum compressive stress trends east to northeast over an area extending from west of the Appalachian Mountain system to the middle of the continent, and from southern Illinois to southern Ontario. In eastern North America, intra-plate earthquakes appear to occur in areas of high stress along unhealed fault zones of late Paleozoic or younger age. An example of this is the seismic belt trending from Boston to the northwest through Ottawa. This seismic zone appears to be located along a continental extension of the Kelvin seamount chain which is postulated by others to be a transform fault related to the early opening of the North Atlantic. Similarly, the 1929 Grand Banks earthquake and the Charleston, South Carolina, seismic trend appear to be located along extensions of other oceanic fracture zones. The relation between high stress and unhealed fault zones may provide a means to assess the earthquake risk within plates. The observed pattern of stresses appears to be post-Mesozoic in origin. This work supports Voight9s hypothesis that the compressive stress observed within the North American plate may be generated by the same mechanism that drives the movements of large lithospheric plates. If this is indeed the case, stress measurements may furnish one of the best clues to the driving mechanism of plate tectonics.

Archean Ultramafic Flows in Munro Township, Ontario

Abstract: A unique exposure within the Abitibi orogenic belt of northeastern Ontario provides an exceptionally clear view of early Precambrian (Archean) ultramafic volcanic rocks. Approximately 60 ultramafic flow units are exposed over a stratigraphic thickness of 125 m. Individual flow units range in thickness from 0.5 to 15 m, average about 3 m, and display remarkable strike lengths and internal structure, which suggest extreme fluidity. The upper margin of the flow units (0.1 to 1.0 m) is thicker than the lower margin (about 1 cm), and the upper margin is highly fractured, thereby providing an easily recognizable top determinator. Many flow units contain a zone of spinifex, ranging in width from zero to 5 m, which underlies and is gradational with the flow top. Spinifex is a textural term to describe an array of criss-crossing sheafs or booklets characterized by numerous closely spaced and parallel blade- or plate-like crystals of olivine. The texture is generally accepted as having formed by rapid cooling of a crystal-free ultramafic liquid in situ. The variation in size of bladed olivine crystals within a spinifex zone is always from coarse (up to 1 m long) at the bottom to fine at the top, thereby providing a primary top determination. In flow units not containing a spinifex zone the chilled and fractured flow top grades downward into a zone of moderately foliated peridotite which generally constitutes the remaining part of a flow unit. Chemically, the flow units resemble similar ultramafic rocks described from northeastern Ontario, western Australia, and the Barberton area of South Africa; one unique feature is the exceptionally high alumina content of the clinopyroxene

Johannes Walther's Law of the Correlation of Facies

Abstract: The writings of Johannes Walther (1860–1937) have been neglected in the west and his Law of the Correlation (or Succession) of Facies has been ignored or misstated in many textbooks of stratigraphy. Walther should be recognized as a pioneer stratigrapher-sedimentologist, important as both a world traveller and explorer of modern sedimentary environments (deserts, reefs, laterites), and as a theorist. His main theoretical contributions were his championing of the actualistic method for the study of fossils and sedimentary rocks and his founding of the science of comparative lithology. Comparative lithology was seen by Walther as the analogue for sedimentary rocks of comparative anatomy for fossils. It has been neglected in the West until the recent revival of the concept of facies models. Walther's Law was the key concept within comparative lithology, and was originally stated as follows: “The various deposits of the same facies areas and similarly the sum of the rocks of different facies areas are formed beside each other in space, though in cross-section we see them lying on top of each other. As with biotopes, it is a basic statement of far-reaching significance that only those facies and facies areas can be superimposed primarily which can be observed beside each other at the present time.” In Russia, Walther's writings appear to have had a greater influence than they have had in Europe and America. They have been partly responsible for the development there of “lithology” as a branch of the geological sciences separate from stratigraphy or petrology.

Origin of Metalliferous Sediments from the Pacific Ocean

Abstract: Sediments from near the basement of a number of Deep Sea Drilling Project (DSDP) sites, from the Bauer Deep, and from the East Pacific Rise have unusually high transition metal-to-aluminum ratios. Similarities in the chemical, isotopic, and mineralogical compositions of these deposits point to a common origin. All the sediments studied have rare-earth-element (REE) patterns strongly resembling the pattern of sea water, implying either that the REE's were coprecipitated with ferromanganese hydroxyoxides (hydroxyoxides denote a mixture of unspecified hydrated oxides and hydroxides), or that they are incorporated in small concentrations of phosphatic fish debris found in all samples. Oxygen isotopic data indicate that the metalliferous sediments are in isotopic equilibrium with sea water and are composed of varying mixtures of two end-member phases with different oxygen isotopic compositions: an iron-manganese hydroxyoxide and an iron-rich montmorillonite. A low-temperature origin for the sediments is supported by mineralogical analyses by x-ray diffraction which show that goethite, iron-rich montmorillonite, and various manganese hydroxyoxides are the dominant phases present. Sr87/Sr86 ratios for the DSDP sediments are indistinguishable from the Sr87/Sr86 ratio in modern sea water. Since these sediments were formed 30 to 90 m.y. ago, when sea water had a lower Sr87/Sr86 value, the strontium in the poorly crystalline hydroxyoxides must be exchanging with interstitial water in open contact with sea water. In contrast, uranium isotopic data indicate that the metalliferous sediments have formed a closed system for this element. The sulfur isotopic compositions suggest that sea-water sulfur dominates these sediments with little or no contribution of magmatic or bacteriologically reduced sulfur. In contrast, ratios of lead isotopes in the metalliferous deposits resemble values for oceanic tholeiite basalt, but are quite different from ratios found in authigenic marine manganese nodules. Thus, lead in the metalliferous sediments appears to be of magmatic origin. The combined mineralogical, isotopic, and chemical data for these sediments suggest that they formed from hydrothermal solutions generated by the interaction of sea water with newly formed basalt crust at mid-ocean ridges. The crystallization of solid phases took place at low temperatures and was strongly influenced by sea water, which was the source for some of the elements found in the sediments.

River channel change with time: an example

Abstract: Monumented channel cross sections were resurveyed over a period of 20 yrs (1953 to 1972) to determine the amount and kind of change of channel area and position on a 3.7-sq-mi basin, Watts Branch near Rockville, Maryland. For the first 12 yrs, the channel progressively but slowly became smaller as urbanization of the basin gradually proceeded. After 1966, a threshold of change apparently was passed and, probably as a result of an increased rate of land alteration upstream, large amounts of sediment were deposited within the channel and overbank. The number of floods exceeding channel capacity increased dramatically from an average of two to more than ten per year. Simultaneously, the channel area began to increase. Despite the trend toward increasing cross-sectional area, the net result after 20 yrs was a channel smaller by 20 percent than it had been originally. Urbanization did not alter the rate of channel migration.

Two Plinian Pumice-Fall Deposits from Somma-Vesuvius, Italy

Abstract: Two of the major pumice-fall deposits of Somma-Vesuvius are described. One, the Pompei Pumice, resulted from the classic eruption of A.D. 79, which is taken as the type example of a Plinian eruption. The Pompei Pumice is dispersed south and southeast of the volcano and has been traced to a distance of 72 km. The other, the Avellino Pumice, is probably a thousand years older. It is dispersed to the east and northeast and has been traced to a distance of 50 km. The two deposits are remarkably similar in character, each consisting of a lower part of white pumice and an upper part of mafic gray pumice having a similar grain-size distribution. The distinction between the two deposits is based on (a) the differing proportions of felsic crystals, mafic crystals, and lithic fragments in selected sieve grades; (b) the abundance of nepheline crystals in the Avellino Pumice and their absence from the Pompei; and (c) the greater abundance of crystals in the Avellino Pumice and of lithic debris in the Pompei. The change in composition of the erupting magmas was accompanied by an increase in the vigor of each eruption, as shown by an increase in the density and size of the pumice, in the content and size of lithic fragments, and in the area of dispersal. This is correlated with an increasing depth of origin in a compositionally zoned magma chamber. Granulometric analyses of some ninety samples show that the deposits are well sorted, the sorting improving slightly with increasing distance from the source. Maps showing the median terminal fall velocity, based on the grain-size data, enable limiting values to be derived for the height of the eruptive column and the wind strength. The volume of each deposit has been estimated from the isopach maps, namely, 2.6 km 3 for the Pompei Pumice and 2.1 km 2 for the Avellino Pumice.

Diffusion Flow Laws in Metamorphic Rocks

Abstract: Rocks which deform by pressure solution obey a diffusion flow law with a linear viscous or Newtonian stress to strain-rate relation. Undeformed relics of original grains preserved within newly grown crystals at grain boundaries under tension and presolved surfaces, together with accumulation of inert particles at grain boundaries under compression, are diagnostic evidence of a diffusion flow law. At a given stress, strain-rate is inversely proportional to the grain size to a power of two or three. A geologically useful plot has inverse temperature versus the logarithm of grain size as coordinates. Such a graph is separated into fields by three boundaries which meet at a triple point; within each field, either lattice diffusion, grain-boundary diffusion, or a dislocation flow law is predominant. It may be possible to calibrate this graph from naturally deformed rocks. Photomicrographs of isoclinally folded greenschist-grade quartzites and rhyolitic flows from the South Mountain–Blue Ridge area in Maryland demonstrate a diffusive mass transfer deformation mechanism, but estimates of effective diffusion coefficient compared to currently available laboratory diffusion data are insufficient to identify the diffusion path with certainty. However, the comparatively low ratio of metamorphic temperature to melting temperature and the physical nature of grain boundaries in metamorphic rocks, particularly concentrations of low-density impurities at grain boundaries, suggest the grain-boundary diffusion flow law.

Pahoehoe Flows from the 1969–1971 Mauna Ulu Eruption, Kilauea Volcano, Hawaii

Abstract: Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement. Three types of chemically similar pahoehoe flows were observed to form during the 1969–1971 Mauna Ulu eruption. (1) A cavernous type called shelly pahoehoe, characterized by fragile gas cavities, small tubes, and buckled fragments of surface crust, was deposited when gas-charged lava welled out of the source fissure with little or no accompanying fountaining. (2) A comparatively smooth-surfaced, dense type, characterized by surface channels and only a few large cavities, formed from voluminous flows of partly degassed fallout away from the foot of lava fountains more than 100 m high. (3) A relatively dense type, characterized by hummocky surfaces with abundant low tumuli and overlapping pahoehoe toes and lobes, formed when largely degassed lava issued from tubes after flowing underground for several kilometers or more. Shelly pahoehoe is rarely found in the geologic record, but the other two types occur commonly. These three types of pahoehoe, which are completely intergradational, can be related qualitatively to the relative gas content and mode of flowage of the lava. The present surface of Kilauea is underlain mostly by hummocky, tube-fed pahoehoe.

Theoretical Analysis of Forced Convective Heat Transfer in Regional Ground-Water Flow

Abstract: Among the numerous processes that will cause anomalous temperature distributions in geologic basins is the spatial redistribution of heat by moving ground water. This problem is examined by solving the energy equation for the simultaneous transport of water by hydraulic gradients and heat by forced convection. The factors that affect the temperature distribution in a given basin include the intrinsic properties of the medium and contained fluid—namely, the thermal diffusivity of the solid-fluid complex and the hydraulic conductivity, the water-table configuration, and the ratio of basin depth to basin length. The severity of an anomalous geothermal gradient or temperature measurement depends primarily on the relative magnitude of the ratio of hydraulic conductivity to thermal diffusivity, and on the geometry of the flow field. A dimensionless group may be formulated from these parameters, and provides a relative measure of the simultaneous transport of heat by the bulk motion of the fluid to that by pure conduction. Solutions to the equation itself indicate that convective heat losses in ground-water recharge areas are balanced by convective heat gains in discharge areas. The geothermal gradient accordingly increases with increasing depth in recharge areas, decreases with increasing depth in discharge areas, and is a manifestation of pure conduction at the hinge line separating areas of recharge and discharge.

Stable Isotope and Magnesium Geochemistry of Recent Planktonic Foraminifera from the South Pacific

Abstract: O 18 /O 16 , C 13 /C 12 , and magnesium analyses were performed on a large number of Recent planktonic Foraminifera from South Pacific Ocean sediments. Results show that oxygen isotopic temperatures of Foraminifera tests may be used to locate ocean currents and to define the orientation of large crustal plates relative to the earth9s rotational poles. Selective solution effects may cause isotopic temperatures of some species to become progressively colder with increasing water depth of the sediments from which they are taken. Where this is not taken into account, erroneous conclusions may result from the comparison of isotopic temperatures of samples from different locations. Depths at which Foraminifera secrete their tests appear to be determined by density and ultimately by osmotic equilibration with surrounding sea water. Susceptibility of Foraminifera tests to selective solution after death increases with magnesium content. Carbon isotope ratios correlate crudely with both temperature and salinity. The C 13 /C 12 ratio of dissolved or particulate carbon in the oceans is probably the most important factor in determining the C 13 /C 12 ratio of the test.

Crustal and Upper Mantle Structure of the Central Aleutian Arc

Abstract: Seven reflection profiles over the central Aleutian trench and terrace are presented. Undeformed trench turbidites terminate abruptly against the inner wall which is acoustically opaque for the next 30 km. The southern half of the Aleutian terrace is an uplifted anticlinal arch, Hawley ridge. Acoustically visible sediments thicken to at least 2 km in the northern part of the terrace. A sonobuoy refraction profile near the terrace axis indicates 5 to 8 km of sediment over an oceaniclike crust (6.4 km per sec). Incorporation of this data with previous near-bottom geophysical and dredge work suggests that thrusted and complexly deformed sediments outcrop between the trench and terrace. Two alternative two-dimensional gravity models extending 200 km south and 350 km north of the trench are presented which take into account both the crustal and upper mantle structure. The fundamental ambiguity between the two models is the dip of the Benioff zone beneath the Aleutian terrace (between 5° and 20°). Low-density (2.2 to 2.4 g per cc) deformed sediments appear to make up the first 30 to 70 km north of the trench axis. The total thickness of sediments near the terrace axis appears closer to 7 to 8 km according to the gravity data. The density contrast between the descending lithosphere and surrounding aesthenosphere appears to be near +0.05 g per cc for an 80-km-thick plate, but a low-density zone in the crust and mantle beneath the volcanic chain is also necessary to fit the observed gravity. Assuming that the Pacific crust inverts into ecologite at a depth of 30 km produced reasonable results, but this estimate is difficult to confirm with present data. The effects of modifying some of these boundaries and density contrasts are calculated and discussed.

Crystallization and Fractionation Trends in the System Andesite-H2O-CO2-O2 at Pressures to 10 Kb

Abstract: Phase relations of a Mount Hood andesite, which has the composition of an average orogenic andesite, have been determined as a function of O 2 fugacity at 1 atm and of H 2 O fugacity to pressures of 10 kb, at O 2 fugacities of the quartz-fayalite-magnetite (QFM) buffer. All runs contained either a H 2 O or H 2 O–CO 2 fluid phase; melts in runs with a H 2 O–CO 2 fluid phase were H 2 O undersaturated. The H 2 O contents of the melts and H 2 O fugacities were calculated from NaAlSi 3 O 8 –H 2 O thermo-dynamic data on the assumption of ideal mixing in the system H 2 O–CO 2 . One-atmosphere runs show that melting relations of silicates are little affected by f o 2 but that both ilmenite- and magnetite-out temperatures are raised by higher f o 2 . Ilmenite precipitates at higher temperature than magnetite. In these runs and in all runs at high pressure with H 2 O and H 2 O–CO 2 fluid phases, oxides were not stable at temperatures of the silicate liquidus. Oxides might be stable on the silicate liquidus if f o 2 rose two or more log units above the Ni–NiO (NNO) buffer. However, calculations indicate that in natural magmas, those processes which might change f o 2 —crystal-liquid equilibria or exchange of H 2 , or H 2 and H 2 O with the wall rocks—cannot raise f o 2 by that magnitude. Because differentiation of basalt melts to andesite must involve iron-rich oxide phase subtraction, such fractionation models appear unreasonable. For the Mount Hood andesite composition, plagioclase is the liquidus phase under H 2 O–saturated conditions to 5 kb and under H 2 O–undersaturated conditions at 10 kb when the H 2 O content of the melt is less than 4.7 wt percent. For higher H 2 O contents, either orthopyroxene or, at H 2 O saturation at pressure greater than 8 kb, amphibole assumes the liquidus. In all cases, clinopyroxene crystallizes at lower temperature than orthopyroxene. Melting curves in the H 2 O–under-saturated region may be contoured either as percent H 2 O in melt or as P e H 2 O ; in either case, the topology of the various silicate melting curves is different from the case of H 2 O–saturated melting. Therefore, melting relations determined at H 2 O–saturated conditions cannot be used successfully to predict melting relations in the H 2 O–undersaturated region. Amphibole melting relations were studied isobarically at 5 kb as a function of temperature and fluid-phase composition. Amphibole has a maximum stability temperature of 940 ± 15°C for fluid compositions of 100 to 44 mole percent H 2 O; for fluids containing more CO 2 than 56 percent (or, equivalently, less than 4.4 wt percent H 2 O in melt), the melting temperature is lower. The same relations would be seen if CO 2 were not present and the melt were H 2 O undersaturated. These rather low melting temperatures, relative to other silicate phases, preclude andesite generation by basalt fractionation involving amphibole at pressures less than 10 kb.

Garlock Fault: An Intracontinental Transform Structure, Southern California

Abstract: The northeast- to east-striking Garlock fault of southern California is a major strike-slip fault with a left-lateral displacement of at least 48 to 64 km. It is also an important physiographic boundary since it separates along its length the Tehachapi–Sierra Nevada and Basin and Range provinces of pronounced topography to the north from the Mojave Desert block of more subdued topography to the south. Previous authors have considered the 260-km-long fault to be terminated at its western and eastern ends by the northwest-striking San Andreas and Death Valley fault zones, respectively. We interpret the Garlock fault as an intracontinental transform structure which separates a northern crustal block distended by late Cenozoic basin and range faulting from a southern, Mojave block much less affected by dilational tectonics. Earlier ideas that the Garlock fault terminates eastward at the Death Valley fault zone appear to us to be in error, although right-lateral offsetting of the Garlock along that zone by about 8 km is necessary. Displacement along the Garlock fault must increase westward from its eastern terminus, a point of zero offset now buried beneath alluvial deposits in Kingston Wash to the east of the Death Valley fault zone. Much of the displacement on the Garlock fault due to east-west components of basin and range faulting appears to have been derived from block faulting in the area between Death Valley and the Nopah Range. Westward displacement of the crustal block north of the Garlock by extensional tectonics within it totals 48 to 60 km in the Spangler Hills–Slate Range area and probably continues to increase westward at least as far as the eastern frontal fault of the Sierra Nevada. Westward shifting of the northern block of the Garlock has probably contributed to the westward bending or deflection of the San Andreas fault where the two faults meet. Many earlier workers have considered that the left-lateral Garlock fault is conjugate to the right-lateral San Andreas fault in a regional strain pattern of north-south shortening and east-west extension, the latter expressed in part as an eastward displacement of the Mojave block away from the junction of the San Andreas and Garlock faults. In contrast, we regard the origin of the Garlock fault as being directly related to the extensional origin of the Basin and Range province in areas north of the Garlock. Recent models for development of that province related to intracontinental spreading east of an east-dipping subduction zone along the Cenozoic margin of western North America may best account for the differential east-west extension which has occurred in the crustal blocks to the north and south of the Garlock fault. Other possible examples of intracontinental transform faults in the southwestern Cordillera with geometries similar to that of the Garlock fault include the left-lateral Santa Cruz–Sierra Madre fault zone along the southern margin of the western Transverse Ranges, and the right-lateral Las Vegas shear zone and Agua Blanca fault of Baja California.

Potassium-Argon Ages and Paleomagnetism of the Waianae and Koolau Volcanic Series, Oahu, Hawaii

Abstract: Paleomagnetic and potassium-argon measurements on 786 oriented cores from 99 volcanic units at 18 sites in the Waianae and Koolau Ranges, Oahu, when combined with data from previous studies, show that the sub-aerial Waianae Volcano was active only from about 3.6 to 2.4 m.y. ago and the subaerial Koolau Volcano from about 2.6 to 1.8 m.y. ago. There is some evidence that Waianae Volcano was still active when Koolau Volcano emerged from the sea. The predominantly tholeiitic lower and middle members of the Waianae Volcanic Series are approximately contemporaneous and were extruded during the late Gilbert and early Gauss geomagnetic polarity epochs. They were followed within less than 0.2 m.y. by the alkalic lavas of the upper member, which were probably extruded largely during the later part of the Gauss normal polarity epoch. The Koolau Volcanic Series was extruded entirely during the early Matuyama reversed polarity epoch. Data from three thick stratigraphic sections in the Waianae and Koolau Volcanic Series indicate that stacks of lava flows as much as 470 m thick can be formed in less than 0.25 m.y. and that the maximum average period between superimposed lava flows is on the order of 103 yrs. Additional data on the hawaiite flow that led to the discovery of the Kaena reversed event indicate that this reversed flow is 2.85 ± 0.05 m.y. old. Angular dispersion of virtual geomagnetic poles (VGP) in the Hawaiian Islands appears to have decreased during the past 5 m.y. This may be caused by a decrease in dipole wobble, a decrease in the nondipole component of the Earth9 magnetic field, or the accumulated effects of weathering, tectonism, and geomorphic processes in older rocks. The mean Waianae and Koolau VGPs are slightly on the side of the Earth9s rotation axis away from Oahu. This supports, but does not prove, the hypothesis that the axial dipole is displaced slightly northward from the Earth9s center. Three VGP “excursions” were recorded in sections of lava in the Waianae and Koolau ranges. During these excursions, the VGPs appear to have traveled away from or toward the geographic axis along great circle paths, suggesting they may be related to the dipole rather than the nondipole field and may record aborted reversals in polarity or rapid and infrequent dipole tilts.

Glacio-Eustatic Control of Late Ordovician–Early Silurian Platform Sedimentation and Faunal Changes

Abstract: Late Ordovician–Early Silurian stratigraphic sequences across the world's platforms bear evidence for onlapping relations in the early part of the Silurian. In general, Late Ordovician marine benthic faunal communities and faunas found in platform rocks differ from those characteristic of the early part of the Silurian. Planktonic graptolites are also conspicuously different in Early Silurian strata from those in Late Ordovician strata. The impressive evidence for Late Ordovician–earliest Silurian continental glaciation in Africa and significant portions of South America and evidence for only restricted orogenic activity in the Late Ordovician-Early Silurian interval suggests that glaciation was the principal agent behind the changes seen among faunas as well as in the stratigraphic record. Shallowing of marine waters across the Late Ordovician platforms was probably related to “locking up” oceanic waters in the glaciers during glaciation, and onlapping relations took place as the glaciers melted and sea level rose. Restriction of platform marine environments and shallowing as well as cooling of the oceans were primary environmental factors behind the changes in marine faunas. Llandovery deglaciation coincides with a time interval during which animal communities were widely spread areally.

Interpretative Synthesis of Metamorphism in the Alps

Abstract: Compilation maps are presented showing major tectonic features, selected lithologies, and zones of progressive metamorphism in the eastern half of the western Alps (scale 1:400,000) and the eastern Alps (scale 1:800,000). Two principal complexes are distinguished on them: (1) the Caledonian and Hercynian metamorphosed terrane in the Southern Alps + Austroalpine sheets, overlain by deformed but largely unrecrystallized uppermost Paleozoic and younger platform-type sedimentary rocks; and (2), the tectonically lower Sesia-Lanzo + Lepontine-Pennine + Helvetic realms, a sequence of Hercynian and pre-Hercynian plutonic igneous + metamorphic rocks and a younger, chiefly Mesozoic cover sequence consisting of shelf, slope, and deep-sea sediments + ophiolites, incompletely to pervasively recrystallized during Alpine metamorphism. 1. The pre-Mesozoic metamorphism involved several cycles in both complexes, but it has not been possible to distinguish these on the maps. Judging from the mineral para-geneses, recrystallization events seem to have taken place under moderate to very high temperatures at low to moderately high pressures. 2. Three contrasting but intergradational, temporally overlapping episodes of Alpine metamorphism are recognized in the Sesia-Lanzo + Lepontine-Pennine + Helvetic terrane: (a) an early, high-pressure, low-temperature event syntectonic with nappe formation, which produced eclogites + albite amphibolites, glaucophane schists, and allied greenschists, with lower grade, more recently metamorphosed sections lying externally (that is, toward the European foreland) relative to the older, progressively higher grade, more internal, imbricated sections lying to the south and east; (b) a middle syntectonic to post-tectonic stage characterized by more “normal” physical conditions, resulting in the partial or complete conversion of the products of event (a) to greenschist (prasinite) and low-rank amphibolite facies metamorphic rocks; and (c), a late, and in most cases syntectonic to post-tectonic recrystallization involving moderately high temperatures and pressures, which locally obliterated the products of both (a) and (b). Of the recrystallization continuum, event (a) is best preserved in the Franco-Italian Alps, and in Switzerland in the cantons of Wallis and Graubunden, (b) is nearly ubiquitous in the Sesia-Lanzo + Pennine + Helvetic complex, and (c) is confined to the Lepontine gneiss area of the Italian and Swiss Alps, and to the central gneiss domes of the Tauern Fenster, central Austria. The timing of Alpine metamorphism evidently varied laterally along and across strike of the belt. For instance, in Austria, event (a) may have begun in Late Cretaceous (?) time, whereas it probably commenced during Paleocene-Eocene time in the western Alps. Moreover, “early” Alpine, low-grade zeolitization occurred in the external parts of the Helvetic realm probably during Oligocene time— nearly contemporaneously with the more internal “late” Alpine higher grade Lepontine recrystallization of event (c). The contact between (1) the Southern Alps + Austroalpine nappes on the one hand, and the structurally lower (2) Sesia-Lanzo + Lepontine-Pennine + Helvetic realms on the other, juxtaposes rocks of markedly contrasting petrologic and tectonic histories. This zone, here referred to as the Alpine Suture, is postulated to represent the crustal expression of a Late Mesozoic-early Tertiary convergent lithospheric plate junction. The early Alpine high-pressure paragenesis appears to reflect subduction and shuffling of the more northerly terrane beneath the stable lithospheric dab capped by the Southern Alps + Austroalpine sheet. If so, the observed blueschist-type metamorphic zoning probably was generated by progressively greater depths of underflow and a consequent depression of the isotherms. A variable rate or time at which the complex was exhumed locally could account for the later establishment of a more normal thermal regime, and thus, succeeding higher temperature mineral assemblages as displayed in the Lepontine gneiss area and the Tauern gneiss domes.

Mantle Convection and Volcanic Periodicity in the Pacific; Evidence from Hawaii

Abstract: The thermal-feedback theory of mantle melting proposed by Shaw in 1969 is found to be quantitatively consistent with data pertaining to the evolution of the Hawaiian Ridge. Applicable rate factors are estimated from relations between lava volumes and position along the ridge given in this paper and the radio-metric age distributions given by Jackson and others in 1972. Rate curves derived from these data provide a new method of age extrapolation or interpolation; results indicate that previous methods used to estimate the age of the Hawaiian-Emperor Bend are in error. No definite age is established, but calculations suggest an age greater than 50 m.y. Much more extensive radiometric data are required to define kinematic relations between the Hawaiian Ridge and Emperor Seamount chain. It appears to be firmly established from the work of Jackson and others and from the present study that the evolution of the Hawaiian Ridge has been episodic, with episodes of several different time scales. Average growth rates of the entire ridge system are divided into two regimes with a discontinuity at a position roughly 1,000 km northwest of Kilauea; the estimated age of this discontinuity is about 10 m.y. Other episodes relate to the durations of eruptive sequences along individual or contiguous lines of volcanoes within the en echelon set of locus lines defined by Jackson and others. The latest of these episodes, beginning about 6 m.y. ago, is marked by accelerating volume rates of eruption and accelerating rates of ridge propagation; this episode appears to be approaching a culminating stage represented by the present activity of Kilauea Volcano. The calculated rate of eruption of Kilauea (0.11 km 3 per yr) is virtually identical with a rate independently estimated by Swanson in 1972 using different data. Calculated durations for older locus lines are generally greater than 6 m.y., but major time overlaps occur that are not adequately understood. Episodic behavior of shorter durations also exists relative to growth of individual shields or to synchronous activity on neighboring shields (for example, Mauna Loa and Kilauea). Some of these shorter term effects are partly explained in terms of isostatic factors acting on the lithosphere and asthenosphere. The longer episodes are explained in terms of variations of melting rates in the asthenosphere, governed by viscous heating produced by the interaction of lithosphere translation and both vertical and horizontal shear flows in the subjacent mantle. Accelerations of eruption and propagation rates are explained by melting instabilities in the upper zones of the asthenosphere as a result of thermal feedback. During the latest melting episode, shear stresses in the asthenosphere derived from the rate data as interpreted by the thermal feedback model are in the range 100 to 200 bars; apparent viscosities range from 2 × 10 21 to 4 × 10 20 poise, decreasing with increasing melting rate. In general, a thermomechanical model is shown to be consistent with the idea that oceanic melting spots can be fixed relative to the deep mantle, although this invariance is not completely established. The thermal plume model of Morgan is not definitely ruled out but does not seem to be required for internally consistent interpretations of oceanic chains of volcanism. It is concluded that motion vectors of the Pacific plate cannot be inferred directly from rates of propagation of volcanic chains, because these rates reflect local, not average, relative velocities of lithosphere versus mantle flow. During growth of the Hawaiian Ridge, propagation speeds calculated on the basis of rate data for the southeastern Hawaiian Islands ranged from less than 1 cm per yr near the Hawaiian-Emperor Bend to nearly 30 cm per yr at the present ridge front.

Species Diversity: Patterns in Modern and Miocene Foraminifera of the Eastern Margin of North America

Abstract: Patterns of foraminiferal species diversity were examined along the eastern margin of North America by utilizing the number of species, S, the information function, H(S), and species equitability, E. The 350 modern samples we studied extended from the Arctic to the Gulf of Mexico at depths ranging from a few meters to more than 5,000 m. In addition, 29 samples from Miocene strata of the Atlantic Coastal Plain and continental shelf were studied. Modern species diversity as measured by S and H(S) generally increases as depth increases and latitude decreases. Some notable exceptions occur, however, which are difficult to explain. For example, species diversity in the Arctic depth interval of 0 to 100 m is as high or higher than that found immediately south of Nova Scotia, in the Gulf of Maine, on Browns and Georges Banks, and even off the Gulf of Mexico deltas. At the moderate depth interval of 100 to 1,000 m, however, the entire margin north of Browns and Georges Banks has lower diversities than that to the south. The highest diversity by far in this depth interval occurs in the northeastern Gulf of Mexico. At the depth interval greater than 1,000 m, the more southern areas studied generally have a higher species diversity than the more northern Cape Cod to Maryland area. An exception to this is the northwestern Gulf of Mexico; this area is also an exception in that species diversity is significantly lower in the deeper waters than in the shallower waters in the same area. The measure of species equitability, E, showed no clear pattern with depth or latitude. This may be so because no simple pattern of species proportions exists or because the sampling was inadequate to measure it. Samples from the Miocene strata show a striking resemblance in species diversity to modern samples at similar depths and latitudes. Our observations indicate that species diversity and equitability have not increased during the last 15 × 106 yrs. The fossil and modern data indicate that each environment has its own carrying capacity and that this capacity is reached rather quickly. Although time and environmental stability are undoubtedly important in determining species diversity, as presently defined they are inadequate to explain all observed patterns. Long-term observations in various environments will be required to determine the relative importance of variables that affect species diversity.

Lead Isotope Systematics and Uranium Depletion in the Granite Mountains, Wyoming

Abstract: Isotopic composition and concentration of lead in whole rock and microcline and concentration of uranium and thorium in whole-rock samples of granite from the Granite Mountains, Wyoming, have been determined. The lead isotopic composition in the whole rocks was found to be highly radiogenic with a range in Pb 206 /Pb 204 of 19.58 to 42.27; the corresponding range in microclines is 15.39 to 22.44. A Pb 206 /Pb 204 versus Pb 207 /Pb 204 plot of the whole-rock data yields an apparent isochron age of 2,790 ± 80 m.y. as the time of crystallization of the granite. Chemically determined values of U 238 /Pb 204 in the whole rocks lie between 3.3 and 18.4 and are too low to account for the amount of radiogenic lead observed. A material balance of lead, thorium, and uranium components indicates that an average of approximately 75 percent of the amount of uranium required to produce the radiogenic lead was removed from the rocks, whereas, on the average, there was no apparent loss of thorium. Loss of uranium from the granite is demonstrated to extend at least to a depth of 165 ft in a drill core. The average uranium loss from the samples analyzed represents about 20 g uranium per 1,000 kg of rock that apparently was removed during the Cenozoic and that probably constitutes the major source of uranium now in ore deposits in central Wyoming basins. The lead isotopic composition of the microclines indicates that lead was mobilized within the granite and was isolated in the feldspar during a thermal event about 1,640 + 120 m.y. ago. However, there is no evidence that the whole rocks themselves became open systems at that time. Whole-rock and microcline isochrons intersect at Pb 206 /Pb 204 and Pb 207 /Pb 204 of 13.77 and 14.86, respectively, indicating a characteristic U 238 /Pb 204 of 8.96 in the source region of the granite magma.

Regional Geochronology of Chile South of 50° Latitude

Abstract: In southern Chile the oldest known rocks are metamorphosed. Gneiss from the basement rocks of the Magellan Basin at the Atlantic entrance to the Strait of Magellan has been dated by Rb-Sr total-rock analyses at 306 ± 156 m.y. (λβ = 1.47 × 10 −11 yr −1 ), with an initial Sr 87 Sr 86 ratio of 0.7112 ± 0.0033. Biotite from a sample of gneiss has been dated as Permian age by the Rb-Sr and K-Ar methods; this implies that the basement rocks of the Magellan Basin have been involved in one or more Paleozoic geologic events. Paraschist from the basement complex along Chile9s Pacific margin indicated Paleozoic to Mesozoic total-rock Rb-Sr ages, based on an assumed initial Sr 87 Sr 86 ratio of 0.710; minerals separated from the schists gave late Mesozoic Rb-Sr and K-Ar dates. Volcanic rocks that overlie the basement rocks and are generally accepted as being of Late Jurassic or Early Cretaceous age based on stratigraphic position gave total-rock Rb-Sr and K-Ar dates of Late Cretaceous to Tertiary age; these dates are considered to represent the time of final closure of their isotopic systems, perhaps associated with deformation in the region. The plutonic igneous rocks that constitute the southern Andes Patagonian batholithic complex range in age from Jurassic to Tertiary. Late Jurassic to Early Cretaceous (155 to 120 m.y.), Late Cretaceous (100 to 75 m.y.), and middle to late Tertiary (50 to 10 m.y.) phases of magmatic activity have been recognized. In the Cordillera Darwin region of the Beagle Canal, minerals separated from plutonic rocks of the Cordillera Darwin suite and from the metamorphic rocks it intrudes gave Rb-Sr and K-Ar dates of Late Cretaceous to early Tertiary age.

Plate-Edge Deformation and Crustal Growth, Gulf of California Structural Province

Abstract: Fracture zones in the Gulf of California are charted from data of reflection profiling and bathymetry. Positions of active spreading centers are located through magnetic anomalies at the mouth, positions of deep troughs in the central gulf, and locations of oceanic ridge-type earthquake swarms in the northern gulf and Salton Trough. Assuming the 6-cm-per-yr spreading rate determined by earlier magnetic anomaly studies has been constant and that spreading has been symmetrical and has not involved transpeninsular faulting, areas of new and old crust within the province are delineated. With the same assumptions, translation of Pacific Plate terrain 240 km back along the 306° azimuth of fracture zones shows the initial Pacific-North American Plate boundary and results in geography clearly requiring a protogulf prior to the current episode of plate separation. Interpretations of reflection profiles and faunal studies of dredged rocks imply that sediments of the central protogulf were deposited in depths significantly greater than 1,000 m and that, in early stages of rifting, they may have been uplifted and subsided as much as 1,200 m. Poorly developed, semi-coherent stratification of postrifting new crustal area is contrasted to well-developed strata of the protogulf areas. This contrast, plus the general absence of ponded turbidites and recording of normal growth-faults at a trailing plate-edge trough, lead to the hypothesis of “clastic compensation.” As continental, or intermediate, crusts are pulled apart by plate motions in the presence of high or moderate clastic sediment sources, crustal growth and resulting structure are controlled by the balance between separation and the flow of sediment into the gaps. Partial isostatic compensation is achieved by supraplate injection of elastics and subplate rising of mantle-derived basalts. Very high supplies of elastics, as in the northern Gulf of California province fed by the Colorado River, preclude the formation of oceanic-type crust as a result of plate separation and lead instead to the formation of intermediate crust, typical neither of oceanic nor continental realms.