Geology topics

W.D. Sharp

Publications and source records attributed to W.D. Sharp.

5 recordsLinked to original sources

USGS2011

Faulted terrace risers place new constraints on the late Quaternary slip rate for the central Altyn Tagh fault, northwest Tibet

The active, left-lateral Altyn Tagh fault defines the northwestern margin of the Tibetan Plateau in western China. To clarify late Quaternary temporal and spatial variations in slip rate along the central portion of this fault system (85°–90°E), we have more than doubled the number of dated offset markers along the central Altyn Tagh fault. In particular, we determined offset-age relations for seven left-laterally faulted terrace risers at three sites (Kelutelage, Yukuang, and Keke Qiapu) spanning a 140-km-long fault reach by integrating surficial geologic mapping, topographic surveys (total station and tripod–light detection and ranging [T-LiDAR]), and geochronology (radiocarbon dating of organic samples, 230 Th/U dating of pedogenic carbonate coatings on buried clasts, and terrestrial cosmogenic radionuclide exposure age dating applied to quartz-rich gravels). At Kelutelage, which is the westernmost site (37.72°N, 86.67°E), two faulted terrace risers are offset 58 ± 3 m and 48 ± 4 m, and formed at 6.2–6.1 ka and 5.9–3.7 ka, respectively. At the Yukuang site (38.00°N, 87.87°E), four faulted terrace risers are offset 92 ± 12 m, 68 ± 6 m, 55 ± 13 m, and 59 ± 9 m and formed at 24.2–9.5 ka, 6.4–5.0 ka, 5.1–3.9 ka, and 24.2–6.4 ka, respectively. At the easternmost site, Keke Qiapu (38.08°N, 88.12°E), a faulted terrace riser is offset 33 ± 6 m and has an age of 17.1–2.2 ka. The displacement-age relationships derived from these markers can be satisfied by an approximately uniform slip rate of 8–12 mm/yr. However, additional analysis is required to test how much temporal variability in slip rate is permitted by this data set.

Central Altyn Tagh fault↗

USGS2011

The chronology for the d18O record from Devils Hole, Nevada, extended into the Mid-Holocene

This report presents the numeric values for the chronology of the paleoclimatically relevant mid-to-late Pleistocene record of the ratios of stable oxygen isotope (delta18O) in vein calcite from Devils Hole, Nev., which recently had been extended into the mid-Holocene. Dating was obtained using 230Th-234U-238U thermal ionization mass spectrometry. Devils Hole is a subaqueous cave of tectonic origin, which developed in the discharge zone of a regional aquifer in south-central Nevada. The primary groundwater recharge source area is the Spring Mountains, the highest mountain range in southern Nevada [altitude 3,630 meters (m)], approximately 80 kilometers to the east of the cavern. The walls of the open fault zone comprising the cave system are coated with dense vein calcite precipitated from the through-flowing groundwater. The calcite, up to 40 centimeters (cm) thick, contains a continuous record of the sequential variation of the composition of stable oxygen isotopes in the ground water over time. The vein calcite has also proven to be a suitable material for precise uranium-series dating via thermal ionization mass spectrometry utilizing the 230Th-234U-238U decay clock. Earlier work has presented data from the Devils Hole core DH-11, a 36-cm-long core of vein calcite recovered from a depth of about 30 m below the water table (about 45 m beneath the ground surface). The DH-11 core provided a continuous record of isotopic oxygen variation from 567,700 to 59,800 years before present. Recent work has extended this record up to 4,500 years before present, into the mid-Holocene epoch.

Open-File Report↗

USGS2008

Ages and inferred causes of late Pleistocene glaciations on Mauna Kea, Hawai'i

Glacial landforms on Mauna Kea, Hawai'i, show that the summit area of the volcano was covered intermittently by ice caps during the Late Pleistocene. Cosmogen 36Cl dating of terminal moraines and other glacial landforms indicates that the last two ice caps, called Older Makanaka and Younger Makanaka, retreated from their maximum positions approximately 23ka and 13ka, respectively. The margins and equilibrium line altitudes of these ice caps on the remote, tropical Pacific island were nearly identical, which would seem to imply the same mechanism for ice growth. But modelling of glacier mass balance, combined with palaeotemperature proxy data from the subtropical North Pacific, suggests that the causes of the two glacial expansions may have been different. Older Makanaka airatop Mauna Kea was likely wetter than today and cold, whereas Younger Makanaka times were slightly warmer but significantly wetter than the previous glaciation. The modelled increase in precipitation rates atop Mauna Kea during the Late Pleistocene is consistent with that near sea level inferred from pollen data, which suggests that the additional precipitation was due to more frequent and/ or intense tropical storms associated with eastward-moving cold fronts. These conditions were similar to modern La Ni??a (weak ENSO) conditions, but persisted for millennia rather than years. Increased precipitation rates and the resulting steeper temperature lapse rates created glacial conditions atop Mauna Kea in the absence of sufficient cooling at sea level, suggesting that if similar correlations existed elsewhere in the tropics, the precipitation-dependent lapse rates could reconcile the apparent difference between glacial-time cooling of the tropics at low and high altitudes. Copyright ?? 2008 John Wiley & Sons, Ltd.

Journal of Quaternary Science↗

USGS2002

Mesozoic thermal history and timing of structural events for the Yukon-Tanana Upland, east-central Alaska: 40Ar/39Ar data from metamorphic and plutonic rocks

We present new 40 Ar/ 39 Ar ages for hornblende, muscovite, and biotite from metamorphic and plutonic rocks from the YukonTanana Upland, Alaska. Integration of our data with published 40 Ar/ 39 Ar, kinematic, and metamorphic pressure ( P ) and temperature ( T ) data confirms and refines the complex interaction of metamorphism and tectonism proposed for the region. The oldest metamorphic episode(s) postdates Middle Permian magmatism and predates the intrusion of Late Triassic (215212 Ma) granitoids into the Fortymile River assemblage (Taylor Mountain assemblage of previous papers). In the eastern Eagle quadrangle, rapid and widespread Early Jurassic cooling is indicated by ~188186 Ma 40 Ar/ 39 Ar plateau ages for hornblende from plutons that intrude the Fortymile River assemblage, and for metamorphic minerals from the Fortymile River assemblage and the structurally underlying Nasina assemblage. We interpret these Early Jurassic ages to represent cooling resulting from northwest-directed contraction that emplaced the Fortymile River assemblage onto the Nasina assemblage to the north as well as the Lake George assemblage to the south. This cooling was the final stage of a continuum of subduction-related contraction that produced crustal thickening, intermediate- to high- P metamorphism within both the Fortymile River assemblage and the structurally underlying Lake George assemblage, and Late Triassic and Early Jurassic plutonism in the Fortymile River and Nasina assemblages. Although a few metamorphic samples from the Lake George assemblage yield Jurassic 40 Ar/ 39 Ar cooling ages, most yield Early Cretaceous 40 Ar/ 39 Ar ages: hornblende ~135115 Ma, and muscovite and biotite ~110108 Ma. We interpret the Early Cretaceous metamorphic cooling, in most areas, to have resulted from regional extension and exhumation of the lower plate, previously tectonically thickened during Early Jurassic and older convergence.

Alaska↗

USGS1996

The 40Ar/39Ar and K/Ar dating of lavas from the Hilo 1-km core hole, Hawaii Scientific Drilling Project

Mauna Kea lava flows cored in the Hilo hole range in age from <200 ka to about 400 ka based on 40 Ar/ 39 Ar incremental heating and K‐Ar analyses of 16 groundmass samples and one coexisting plagioclase. The lavas, all subaerially deposited, include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic, transitional tholeiitic, and tholeiitic basalts. The lower section has yielded predominantly complex, discordant 40 Ar/ 39 Ar age spectra that result from mobility of 40 Ar and perhaps K, the presence of excess 40 Ar, and redistribution of 39 Ar by recoil. Comparison of K‐Ar ages with 40 Ar/ 39 Ar integrated ages indicates that some of these samples have also lost 39 Ar. Nevertheless, two plateau ages of 391 ± 40 and 400 ± 26 ka from deep in the hole, combined with data from the upper section, show that the tholeiitic section accumulated at an average rate of about 7 to 8 m/kyr and has a mean recurrence interval of 0.5 kyr/flow unit. Samples from the upper section yield relatively precise 40 Ar/ 39 Ar plateau and isotope correlation ages of 326 ± 23, 241 ± 5, 232 ± 4, and 199 ± 9 ka for depths of −415.7 m to −299.2 m. Within their uncertainty, these ages define a linear relationship with depth, with an average accumulation rate of 0.9 m/kyr and an average recurrence interval of 4.8 kyr/flow unit. The top of the Mauna Kea sequence at −280 m must be older than the plateau age of 132 ± 32 ka, obtained for the basal Mauna Loa flow in the corehole. The upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to Mauna Kea as it rode the Pacific plate away from its magma source, the Hawaiian mantle plume. The age‐depth relation in the core hole may be used to test and refine models that relate the growth of Mauna Kea to the thermal and compositional structure of the mantle plume.

Journal of Geophysical Research B: Solid Earth↗