David G. Howell

Sedimentary masses and concepts about tectonic processes at underthrust ocean margins

Tectonic processes associated with subduction of oceanic crust, but unrelated to the collision of thick crustal masses or microplates, are presumed by many geologists to significantly affect the formation and deformation of large sedimentary bodies at underthrust ocean margins. More geologists are familiar with the concept of subduction accretion , which describes the tectonic attachment of rock and sediment masses to the margin9s bedrock framework, than with other noncollision processes—for example, sediment subduction, subduction erosion , and subduction kneading . These are equally important processes controlling the geologic evolution of underthrust margins, and any one of them may predominate at a given place. In our opinion, no single subduction-related tectonic process is the dominant or typical one that forges the geologic framework of modern underthrust ocean margins. It is likely, therefore, that the rock records of ancient underthrust margins are preserved in a multitude of structural and stratigraphic forms.

Detrital zircon reference for Cambrian to Triassic miogeoclinal strata of western North America

U-Pb analyses of 656 single zircon grains from Cambrian to Triassic miogeoclinal strata provide a latitudinal and temporal reference for the ages of grains that accumulated along the western margin of North America. Comparisons between this detrital zircon reference and the ages of grains in potentially displaced terranes outboard (west) of the miogeocline should help establish when the terranes first arrived in sedimentary proximity to western North America. North-south variations in the ages of grains in Cambrian and Devonian to Triassic strata, which reflect the north-south changes in the age of cratonal rocks near the margin, should also help place constraints on a terranes paleolatitude during these time periods. The technique cannot be used to determine paleolatitude during Ordovician time, however, because miogeoclinal strata from northern Canada to northern Mexico are dominated by grains shed from the Peace River arch (northwestern Canada).

Mesozoic accretion of exotic terranes along the New Zealand segment of Gondwanaland

Permian to middle Cretaceous strata in New Zealand can be genetically grouped into at least four tectonostratigraphic terranes. Each terrane is lithostratigraphically distinct, yet paleogeographic analyses indicate that each terrane is only part of a once larger geologic entity. The present distribution of terranes is inferred to represent accretionary processes along the margin of Gondwanaland. Accretion seems to have occurred intermittently until middle Cretaceous time, and accretion was immediately supplemented by rifting that broke New Zealand away from the Australia and Antarctica part of Gondwanaland. The terranes have been severed by Tertiary slip on the Alpine fault.

Rejuvenation of the Kuqa Foreland Basin, Northern Flank of the Tarim Basin, Northwest China

The Kuqa depression along the northern flank of the Tarim basin is filled with a thick sequence of Neogene and Quaternary coarse elastic continental sediments. This structural depression is part of a large foreland basin that lies south of the Tianshan—an orogenic belt of intracontinental convergence resulting from the northward propagation of stress following the collision of India with the southern margin of Eurasia.

Eocene Conglomerate Sedimentology and Basin Analysis, San Diego and the Southern California Borderland

ABSTRACT Eocene conglomeratic strata in the San Diego area were deposited as a narrow, west-trending progradational system that changed facies from fluvial channel, to alluvial fan, to coastal plain-fan delta, to paralic, to shelf and subsea channels. Continuing this system westward are the large Eocene subsea fan deposits that include inner, middle, and outer subfan and basin plain facies of the southern California borderland. In all environments, 81 to 96 percent of the conglomerate clasts are distinctive, well-rounded, siliceous metavolcanic stones (Poway clasts) that reach 60 cm in size and average 6 cm; 2 to 13 percent are quartzite; and 0 to 12 percent are locally derived (Peninsular Ranges) crystalline basement (granitic and volcanic) rocks. The composition, shape, and average size of t e metavolcanic and quartzite clasts are constant in all environments, while the less resistant granitic clasts show the effects of progressive abrasion by a decrease in size and abundance with increased transport distances. Three major sedimentologic trends characterize the Eocene conglomerate. (1) Conglomerate uniformly decreases, and finer grained rocks increase, in abundance and thickness from east to west. (2) The nonmarine and shallow marine conglomerate facies each contain disorganized and organized beds with no grading. Organized conglomerates are framework supported, and have imbricated clasts that dip up-current. The long axes of the clasts are oriented perpendicular or parallel to the paleoflow or are distributed randomly. Some nonmarine sandstone beds contain caliche horizons. (3) The subsea fan conglomerate is both disorganized and organized, and the beds are graded, or inversely graded. The clasts are matrix-supported, imbricated, and dip up-current. Long axes of clasts are oriented both par llel to and perpendicular to paleocurrent trends, and angular mudstone rip-up clasts and reworked marine fossils are present.

Thermal Maturity Patterns in Alaska: Implications for Tectonic Evolution and Hydrocarbon Potential

Nearly 10,000 vitrinite reflectance and conodont color alteration index determinations from sedimentary rocks in Alaska were used to produce a thermal maturity map of rocks exposed at the surface and to evaluate subsurface thermal maturity relations in the Colville and Cook Inlet basins. Rocks exposed at the surface of the Tertiary interior basins and in the Aleutian forearc and backarc basins uniformly are of very low thermal maturity, indicating that these basins are at or near maximum burial, have seen little uplift and exhumation, and are probably thermally immature with respect to hydrocarbon generation. In contrast, many sedimentary basins show elevated levels of thermal maturity at the surface, with the highest values at basin margins. This geometry suggests a patt rn of greater uplift along basin margins, possibly reflecting isostatic readjustments as crustal loads are removed by erosion. We investigated thermal maturity relations in three sedimentary basins (Colville, Cook Inlet, and Kandik) in more detail. Thermal maturity patterns in the Colville basin are broadly asymmetric, indicating systematic differential uplift ranging from a minimum of no uplift in the north (Point Thomson area) to 9-13 km of uplift and exhumation in the central Brooks Range; even greater uplift further to the south is indicated by the presence of greenschist facies and higher grade metamorphic rocks. This pattern may reflect the deflexing of the lithosphere subsequent to the principal episode(s) of crustal convergence and thickening. These patterns further suggest a similar thermal history for the proximal Colville basin and the northern foothills belt, suggesting the possibility of hydrocar on accumulations in the foothills. Thermal maturity isograds within the Brooks Range cut major thrust faults, indicating that maximum burial postdated the principal phases of thrusting. In contrast, isograds in the foothills belt to the north are warped broadly by local structure, indicating continued north-south shortening subsequent to maximum burial. Such deformation could have remobilized hydrocarbons in early traps. A broad southward extension of thermally immature rocks in the central portions of the foothills belt suggests relatively young east-west shortening (parallel to the strike of the orogene), a feature that to date has not been included in regional tectonic syntheses. Alternatively, the thermal maturity pattern could be explained by tectonically unrelated episodes of uplif in the eastern and western parts of the Brooks Range. In the Cook Inlet basin, vitrinite reflectance isograds also are indicative of relatively greater uplift at the basin margins than at the basin center, which appears to be presently at its maximum burial depth. Uplift in the Cook Inlet basin may reflect compression along the faults bounding the basin. Relatively high thermal maturity along the western margin of the basin also may reflect magmatic heat sources from the Alaska Peninsula-Aleutian volcanic arc. The Seldovia arch, a major structural feature of the End_Page 1874------------------------------ basin, does not appear to deform vitrinite reflectance isograds, implying that deformation on that structure ceased prior to maximum burial. In the Kandik basin, a thermal maturity anomaly (thermally mature younger rocks in fault contact with thermally immature older rocks) provides clues to the nature and timing of east-west thrusting. Mesozoic foreland basin deposits associated with thrusting buried Paleozoic rocks of the easternmost part of this fold-and-thrust belt to relatively shallow depths, driving potential hydrocarbon source rocks into the oil-generation window. The western foreland basin deposits were overridden by advancing thrusts, and tectonically buried to as deep as 10 km. These disparate thermal domains are juxtaposed along the Glenn Creek fault, which may represent a terrane boundary in east-central Alaska.

A Budget for Continental Growth and Denudation

Oceanic crustal material on a global scale is re-created every 110 million years. From the data presented it is inferred that potential sialic material is formed at a rate of about 1.35 cubic kilometers per year, including hemipelagic volcanic sediments that accumulate at a rate of about 0.05 cubic kilometer per year. It is estimated that the influx of 1.65 cubic kilometers per year of terrigenous and biogenic sediment is deposited on the deep ocean, and this represents continental denudation. Because all this material is brought into a subduction zone, continental accretion rates, which could include all this material, may be as high as 3.0 cubic kilometers per year with a potential net growth for continents of 1.35 cubic kilometers per year.

Late Cretaceous trench-slope basins of central California

The Cambria slab of the central California coast consists of about 4,000 m of massive, thick-bedded, unmetamorphosed Upper Cretaceous sandstone characteristic of deposition within channel facies of a submarine fan system. Sedimentary petrology indicates a provenance of uplifted sedimentary-metasedimentary and plutonic terranes. In addition, a glaucophane schist clast and large red and green chert clasts suggest partial derivation from local Franciscan assemblage sources. A trench-slope basin model best characterizes the sedimentary and tectonic environment of the Cambria and related slabs. Deposition occurred on an unstable accretionary basement, followed by tectonic incorporation of blocks of the Cambria slab into the Franciscan melange.

Paleomagnetic evidence that the central block of Salinia (California) is not a far‐traveled terrane

New paleomagnetic results from Late Cretaceous (75–85 m.y.) red beds on the central block of Salinia indicate that Salinia was located within 6° (in latitude) of its current cratonal North American position during the Late Cretaceous (after correction for Neogene San Andreas Fault transport). The red beds formed as alluvial-fan overbank deposits with hematite cement deposited directly on Salinian granites in the La Panza Range. Paleomagnetic analysis shows two components of magnetization in the red beds, a low-blocking-temperature present-day overprint residing in goethite and a high-blocking-temperature (>600°) component residing in hematite. The hematite magnetization is a chemical remanent magnetization which formed soon after deposition during pedogenesis. The bedding-corrected hematite remanence contains a magnetic polarity stratigraphy with antipodal normal and reversed directions. Twenty-three Class I sites (α95 < 20°) have an average hematite direction with inclination =54.4° and declination = 18.2° (α95 = 6.1°) after structural correction. These paleomagnetic data suggest that Salinia resided at about 35°N latitude during the Late Cretaceous, within 6° of its current location adjacent to cratonal North America. By contrast, a summary of paleomagnetic data from the Peninsular Ranges terrane and the Sur-Obispo terrane, which are currently outboard of Salinia, shows northward transport of these terranes of 12° to 22° relative to their current locations in North America since the Cretaceous. The offsets increase systematically away from the craton with the most outboard Sur-Obispo terrane (which is composed of accretionary prism and distal forearc material) showing the largest degree of northward translation.

Kinematic evolution of the junction of the San Andreas, Garlock, and Big Pine faults, California

If the San Andreas fault with about 300 km of right slip, the Carlock fault with about 60 km of left slip, and the Big Pine fault with about 15 km of left slip are considered to have been contemporaneously active, a space problem at their high-angle junctions becomes apparent. Large crustal masses converge in the area of the junctions as a result of the simultaneous large displacements on the faults. We present here a model in which an early straight north-northwest–trending San Andreas deforms to its present bent configuration in response to a westward displacement of crust north of the Garlock fault. During this deformation, the crust north of the Garlock in the vicinity of the junction undergoes north-south shortening, while the fault junction migrates along the trace of the San Andreas fault to the southeast relative to its original position. As a result of this migration, the Mojave area is displaced to the east relative to the original junction position. We suggest a similar history in mirror image for the Big Pine fault and the areas of crust adjacent to it.