William Harbert

Late Neogene relative motions of the Pacific and North America Plates

In this paper a new set of finite rotations describing the relative motion of the Pacific and North America plates during the last 10 m.y., incorporating recently published studies of the Pacific‐Antarctic, Antarctic‐Africa, and Africa‐North America plate boundaries is presented. These finite rotations show that changes have occurred in Pacific‐western North America motion at 2.48 Ma and between 3.40 and 3.9 Ma, resulting in increased compression along the Pacific‐North America plate boundary. The most significant change in relative motion was of the latter age. During this change the predicted motion of the Pacific plate along the California coast changes from transform to transpressive, due to a clockwise rotation of the relative convergence vector by 12°. This timing of the onset of transpression agrees well with a variety of geologic data along the California plate boundary, including the onset of compressive deformation in onshore and offshore sedimentary basins, formation of reverse faults and anticlines (which are parallel to strike‐slip faults of the previous, more westerly directed azimuth of relative motion), a change in the orientation of the San Andreas fault, and formation of a set of new, more northerly trending strike‐slip faults. In this model this change in relative motion is caused by a change in the absolute motion of the Pacific plate, due to the detachment of a slab beneath the Fiji Basin between 3.4 and 3.9 Ma. The detachment of this slab and the resultant change in overall Pacific plate torque resulted in a noncollisional “orogeny” along the California plate boundary. This study shows that minor adjustments in the motion of large oceanic plates, such as the Pacific plate, can have profound consequences on the preserved geologic record. Copyright 1991 by the American Geophysical Union.

Relative motions between Eurasia and North America in the Bering Sea region

Knowledge of the relative motion of the North American and Eurasian plates during the late Mesozoic and Cenozoic provides insight into the observed timing and style of deformational events in the Bering Sea region. Periods of strong convergence from ≈ 70 to ≈ 50 Ma (Maastrichtian to Paleocene) are correlated with compressional deformation between the Chukotsk Peninsula and northern Alaska and movement along the Denali fault. The convergence may also be the cause of a previously proposed counterclockwise rotation of western Alaska. Transform motion between these plates from 50 to 38 Ma (Middle to Late Eocene) correlates with subsidence of the Bering Shelf, creation of a series of pull-apart basins along the Bering margin and cessation of calc-alkaline volcanism in western Alaska. Slight compressive convergence from 37 Ma to the present may be responsible for the anticlinal deformation of basin filling sediments in the Anadyr and Khatyrka basins.

Analysis of light hydrocarbons in soil gases, Lost River region, West Virginia: Relation to stratigraphy and geological structures

Analyses of 471 near-surface soil-gas samples for light hydrocarbons, C1–C4, C2L, C3L, and H2 from the Lost River gas field in Hardy County, West Virginia, reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases, which occur directly over the faulted, eastern limb of the Whip Cove anticline. Compositional changes in the soil-gases data clearly define major changes in the maturity and locations of potential source beds. Grids placed on botanically defined anomalies confirm a possible correlation between these two independent indicators. Statistical analysis shows that samples from 45 sites contain anomalously large concentrations of light hydrocarbons in the soil-gas constituents. Large concentrations, coupled with high saturate-to-olefin ratios, further confirms that this active seepage is near macroseep levels. Variations in soil-gas compositional trends separate the soil-gas data into two domains, with oilier compositions to the west and gassier compositions to the east. Although the composition of the shallow soil gases above the Lost River gas field are oilier than the reservoir gases, they occur directly over the eastern, faulted limb of the producing anticlinal structure, suggesting that the dry gases from the Oriskany reservoir are probably mixed with oilier gases from organic-rich strata among Devonian shales. The eastern anomalies are much gassier and are very similar to the Oriskany gases produced by the Lost River gas field. The eastern anomalies directly overlie near-vertical beds of Devonian and older age formations that are likely conduits for deeper, mature thermal gases.

New paleomagnetic data from the Mongol-Okhotsk collision zone, Chita region, south-central Russia: implications for Paleozoic paleogeography of the Mongol-Okhotsk ocean

We present a reconnaissance paleomagnetic study of rocks from several formations of the Chita region of south-central Russia (representative location λ=51°N, φ=116°E), within the Mongol–Okhotsk collision zone. Sections of siltstone and fine-grained sandstone, dated using fossils as Bashkirian to Moscovian epochs (USSR Carboniferous system, Middle Carboniferous), Early Permian, Late Permian and Triassic in age were sampled. The resulting collection from five stratigraphic sections is a total of 319 oriented samples from 51 sites. Generally, 5 to 10 samples per site were collected. Sample orientation was determined using both magnetic and sun-shadow azimuths. All paleomagnetic measurements were completed in the Paleomagnetic Laboratory at the University of Pittsburgh using a 3-component 2G Superconducting Rock Magnetometer (SRM) in a magnetically shielded room. Thermal demagnetization was completed using between 12 to 20 heating steps up to temperatures of 685°C. Principal component analysis of the demagnetization data was successful in isolating two characteristic remanent magnetizations. The lower unblocking temperature component, component A, fails the fold test, is always of downward directed magnetic inclination, and may record the present-day Earths (PDF) magnetic field [PDF I=69.4°, D=351.8°; component A, Ig=65.1°, Dg=356.5°, α95=6.2°, N (sites)=50]. The higher unblocking temperature magnetic component (B), was observed in the Triassic (BT, N=3), Late Permian (BLP, N=14), Early Permian (BEP, N=5) and Bashkirian to Moscovian epochs of the Late Carboniferous (BMC, N=8) sections. The B component differs significantly from component A, and is recorded by sites of both downward and upward directed magnetic inclinations in the Late Permian and Bashkirian to Moscovian epoch sections. Component BMC and BLP may represent primary remanent magnetizations. The Bashkirian to Moscovian epochs of the Late Carboniferous mean paleolatitude is λMC=19.9°±14.8° and the Late Permian mean paleolatitude is λLP=19.6°±14.5°. Both are similar to that expected from reference paleomagnetic poles from the North China block, but significantly different from paleolatitudes calculated using reference poles from the Siberian or European plates. We interpret the results of this study to suggest that the sampled sections were located near, or associated with, the North China Block during their deposition.

Integrating velocity measurements in a reservoir rock sample from the SACROC unit with an AVO proxy for subsurface supercritical CO2

One of the most promising methods proposed to mitigate global CO2 and one that is useful in enhanced oil recovery is carbon sequestration, a process in which CO2 is pressurized and injected into geologic formations. A technical challenge surrounding the geologic sequestration of CO2 is tracking the movement of the fluids pumped underground. Monitoring, verification, and accounting activities related to CO2 storage are important for assuring that any sequestered CO2 does not escape to the surface and can be considerably aided by reflection seismic-based detection methods. Through the use of lab-scale velocity measurements under in-situ conditions, combined with multiple 3D reflection seismic surveys, we hope to effectively track the movements of CO2 after injection.

Using HEM surveys to evaluate disposal of by-product water from CBNG development in the Powder River Basin, Wyoming

Production of methane from thick, extensive coal beds in the Powder River Basin of Wyoming has created water management issues. Since development began in 1997, more than 650 billion liters of water have been produced from approximately 22,000 wells. Infiltration impoundments are used widely to dispose of by-product water from coal bed natural gas (CBNG) production, but their hydrogeologic effects are poorly understood. Helicopter electromagnetic surveys (HEM) were completed in July 2003 and July 2004 to characterize the hydrogeology of an alluvial aquifer along the Powder River. The aquifer is receiving CBNG produced water discharge from infiltration impoundments. HEM data were subjected to Occam’s inversion algorithms to determine the aquifer bulk conductivity, which was then correlated towater salinity using site-specific sampling results. The HEM data provided high-resolution images of salinity levels in the aquifer, a result not attainable using traditional sampling methods. Interpretation of these i...

Stress modeling of tectonic blocks at Cape Kamchatka, Russia using principal stress proxies from high-resolution SAR: new evidence for the Komandorskiy Block

Abstract A new data set of 851 lineaments mapped from European Remote Sensing satellites 1 and 2 full resolution Synthetic Aperture Radar (SAR) data are interpreted as geological proxies for stresses resulting from plate and block collision near the Cape Kamchatka region of the Kamchatka Peninsula using a Geographical Information Systems-based analysis. Numerical Manifold Method (NMM) analysis is used to model the stress field within the Cape Kamchatka region resulting from the collision of lithospheric plates and blocks. Results of our NMM model, using different plate motion and plate configuration in the region, are compared with orientation data for the mapped set of lineaments. These data suggest that the lineaments observed in SAR cannot be fully explained by a simple two-plate model in this in the Cape Kamchatka region. As an alternative, we propose that the data can be explained by the existence of the previously proposed Komandorskiy Block. Recent Global Positioning Satellite measurements in the Aleutian Islands support our lineament-derived model and show that the near Islands/Komandorskiy Island block of the extreme western Aleutians is moving independently of the North American Plate.

Paleomagnetic data from Alaska: reliability, interpretation and terrane trajectories

Virtually the entire body of paleomagnetic data collected from southern Alaska depicts a clear decrease in paleolatitude with increasing age, strongly suggesting that southern Alaska represents a displaced terrane. In this paper, paleomagnetic studies from southern Alaska have been classified with respect to a Quality Index that is based on four criteria. These criteria are the presence of both polarities of magnetic remanence, stepwise thermal or alternating field demagnetization of specimens, principal component analysis of demagnetization data, and a successful fold test. Of the 51 studies compiled, only four from southern Alaska and one from western Canada are demonstrated to satisfy all criteria and fall therefore in the category of Group 3, (“highly reliable”). Two studies from southern Alaska satisfy three of the four criteria, lacking both polarities of characteristic remanence, and are judged to be of Group 2 (“reliable”). Two of these paleomagnetic studies constrain the accretion time of the southern Alaska terrane to the relatively stationary region of central Alaska north of the Denali fault. Four paleomagnetic studies from the southern Alaska terrane show a distinct paleolatitude anomaly when compared with their expected paleolatitudes from the North American apparent polar wander path. Using the model of Engebretson et al. (1984), a series of models are presented to best fit these highly reliable and reliable paleomagnetic studies. The model preferred in this article assumes an accretion time with North America of 50 Ma, and documents pre-50 Ma displacement of the southern Alaska terrane on the Kula plate. If the Ghost Rocks paleomagnetic magnetizations (Plumley et al., 1983) are assumed to be of earliest Tertiary age, this model fits all of the low paleolatitudes observed in southern Alaska. Models incorporating coastwise translation of the southern Alaska terrane along the western boundary of the North America plate and a 50 Ma suturing age of this terrane to North America do not satisfy the paleomagnetic constraints. Four of the reliable studies can be modeled assuming strike-slip motion of the southern Alaska terrane along the western boundary of the North America plate if a 40 Ma suturing age is assumed. In this model, however, the paleomagnetic data must be presumed to represent the highest permissible paleolatitude and the Ghost Rocks Formation paleolatitudes are problematically lower than is possible to model with coastwise translation.

Major evolutionary phases of a forearc basin of the Aleutian terrace: Relation to North Pacific tectonic events and the formation of the Aleutian subduction complex

Combined geologic and seismic reflection data from the Atka Basin region of the Aleutian forearc show that the upper 2-3 km of slightly deformed sediment filling the basin are probably of late Miocene to Holocene age. The depositional axis of the basin shifted arcward over time because of the progressive and differential rise of the basins outer ridge. Units filling the basin unconformably overlie and, along the edges of the basin, onlap beds of Oligocene age and older(?). The basal units of the basin fill are characterized by little variation in thickness, somewhat irregular internal reflectors, fault offsets, and possible wedge outs against units of Eocene(?) age. A fault with at least 500 m of vertical displacement cuts the outer high of the forearc basin and displaces beds of the basin-filling series relative to those trenchward of the trenchslope break. The Atka Basin appears to have formed in response to a combination of (1) initiation of trench-floor-filling turbidite deposition, in part derived from glacial marine sedimentation from mainland Alaska; (2) an increased rate and normal component in Pacific plate subduction beneath the central Aleutian arc beginning in early Pliocene time; and (3) formation of a broad and thick accretionary wedge that progressively uplifted the outer high of the Aleutian terrace.

Paleomagnetism of Khatyrka and Maynitsky superterranes, Koryak highlands, far eastern Russia

Abstract We have completed a paleomagnetic reconnaissance study of sedimentary and volcanic extrusive rocks collected from two major tectonic zones in northeastern Russia. Paleomagnetic sites were sampled within the fault-bounded structural units of the Khatyrka and Maynitsky superterranes and an overlap sequence of the Khatyrka superterrane. These sampling localities were chosen to allow both within-site and between-site fold tests. Stepwise thermal demagnetization within the temperature range 200–640°C showed a characteristic linear demagnetization path between thermal demagnetization steps of 400°C and 530°C. For thermal steps above 550°C, the magnetic intensity of many samples began to increase rapidly with magnetic directions, which were random between heating steps, suggesting the formation of new magnetic phases in these samples. Paleomagnetic samples collected from basalts and sediments of the Khatyrka superterrane and basalts and gabbros of the Maynitsky superterrane pass fold tests and show significant poleward motion of these superterranes since the formation of their rocks. The observed paleomagnetic paleolatitudes between 24°N or S and 32°N or S can be compared with expected paleolatitudes of 57°N to 79°N. Paleomagnetic results from sites collected from overlapping Senonian rocks pass a fold test at the 99% confidence level and give a pole position not significantly different from that expected from the apparent polar wander path for the Eurasia or North America plates, suggesting that these sedimentary units overlapping the Khatyrka superterrane were deposited along the ancient northeast margin of the Eurasian plate. The declination, in stratigraphie coordinates, shows a maximum clockwise rotation of about 20° when compared with the Eurasian APWP.