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George E. Gehrels

Publications and source records attributed to George E. Gehrels.

15 recordsLinked to original sources
USGS2018

GHR1 Zircon – A new Eocene natural reference material for microbeam U-Pb geochronology and Hf isotopic analysis of zircon

We present multitechnique U-Pb geochronology and Hf isotopic data from zircon separated from rapakivi biotite granite within the Eocene Golden Horn batholith in Washington, USA. A weighted mean of twenty-five Th-corrected 206 Pb/ 238 U zircon dates produced at two independent laboratories using chemical abrasion-isotope dilution-thermal ionisation mass spectrometry (CA-ID-TIMS) is 48.106 ± 0.023 Ma (2 s analytical including tracer uncertainties, MSWD = 1.53) and is our recommended date for GHR1 zircon. Microbeam 206 Pb/ 238 U dates from laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) laboratories are reproducible and in agreement with the CA-ID-TIMS date to within < 1.5%. Solution multi-collector ICP-MS (MC-ICP-MS) measurements of Hf isotopes from chemically purified aliquots of GHR1 yield a mean 176 Hf/ 177 Hf of 0.283050 ± 17 (2 s , n = 10), corresponding to a εHf 0 of +9.3. Hafnium isotopic measurements from two LA-ICP-MS laboratories are in agreement with the solution MC-ICP-MS value. The reproducibility of 206 Pb/ 238 U and 176 Hf/ 177 Hf ratios from GHR1 zircon across a variety of measurement techniques demonstrates their homogeneity in most grains. Additionally, the effectively limitless reserves of GHR1 material from an accessible exposure suggest that GHR1 can provide a useful reference material for U-Pb geochronology of Cenozoic zircon and Hf isotopic measurements of zircon with radiogenic 176 Hf/ 177 Hf.

Geostandards and Geoanalytical Research↗
USGS2015

Detrital zircon U-Pb provenance of the Colorado River: A 5 m.y. record of incision into cover strata overlying the Colorado Plateau and adjacent regions

New detrital zircon U-Pb age distributions from 49 late Cenozoic sandstones and Holocene sands (49 samples, n = 3922) record the arrival of extra-regional early Pliocene Colorado River sediment at Grand Wash (western USA) and downstream locations ca. 5.3 Ma and the subsequent evolution of the river’s provenance signature. We define reference age distributions for the early Pliocene Colorado River (n = 559) and Holocene Colorado River (n = 601). The early Pliocene river is distinguished from the Holocene river by (1) a higher proportion of Yavapai-Mazatzal zircon derived from Rocky Mountain basement uplifts relative to Grenville zircon from Mesozoic supra crustal rocks, and (2) distinctive (∼6%) late Eocene–Oligocene (40–23 Ma) zircon reworked from Cenozoic basins and volcanic fields in the southern Rocky Mountains and/or the eastern Green River catchment. Geologic relationships and interpretation of 135 published detrital zircon age distributions throughout the Colorado River catchment provide the interpretative basis for modeling evolution of the provenance signature. Mixture modeling based upon a modified formulation of the Kolmogorov-Smirnov statistic indicate a subtle yet robust change in Colorado River provenance signature over the past 5 m.y. During this interval the contribution from Cenozoic strata decreased from ∼75% to 50% while pre-Cretaceous strata increased from ∼25% to 50%. We interpret this change to reflect progressive erosional incision into plateau cover strata. Our finding is consistent with geologic and thermochronologic studies that indicate that maximum post–10 Ma erosion of the Colorado River catchment was concentrated across the eastern Utah–western Colorado region.

Geosphere↗
USGS2014

Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir

The Pamir is the western continuation of Tibet and the site of some of the highest mountains on Earth, yet comparatively little is known about its crustal and tectonic evolution and erosional history. Both Tibet and the Pamir are characterized by similar terranes and sutures that can be correlated along strike, although the details of such correlations remain controversial. The erosional history of the Pamir with respect to Tibet is significantly different as well: Most of Tibet has been characterized by internal drainage and low erosion rates since the early Cenozoic; in contrast, the Pamir is externally drained and topographically more rugged, and it has a strongly asymmetric drainage pattern. Here, we report 700 new U-Pb and Lu-Hf isotope determinations and >300 40 Ar/ 39 Ar ages from detrital minerals derived from rivers in China draining the northeastern Pamir and >1000 apatite fission-track (AFT) ages from 12 rivers in Tajikistan and China draining the northeastern, central, and southern Pamir. U-Pb ages from rivers draining the northeastern Pamir are Mesozoic to Proterozoic and show affinity with the Songpan-Ganzi terrane of northern Tibet, whereas rivers draining the central and southern Pamir are mainly Mesozoic and show some affinity with the Qiangtang terrane of central Tibet. The ε Hf values are juvenile, between 15 and −5, for the northeastern Pamir and juvenile to moderately evolved, between 10 and −40, for the central and southern Pamir. Detrital mica 40 Ar/ 39 Ar ages for the northeastern Pamir (eastern drainages) are generally older than ages from the central and southern Pamir (western drainages), indicating younger or lower-magnitude exhumation of the northeastern Pamir compared to the central and southern Pamir. AFT data show strong Miocene–Pliocene signals at the orogen scale, indicating rapid erosion at the regional scale. Despite localized exhumation of the Mustagh-Ata and Kongur-Shan domes, average erosion rates for the northeastern Pamir are up to one order of magnitude lower than erosion rates recorded by the central and southern Pamir. Deeper exhumation of the central and southern Pamir is associated with tectonic exhumation of central Pamir domes. Deeper exhumation coincides with western and asymmetric drainages and with higher precipitation today, suggesting an orographic effect on exhumation. A younging-southward trend of cooling ages may reflect tectonic processes. Overall, cooling ages derived from the Pamir are younger than ages recorded in Tibet, indicating younger and higher magnitudes of erosion in the Pamir.

Pamir Mountains, Tibet↗
USGS2006

Detrital zircon provenance of the Late Triassic Songpan-Ganzi complex: Sedimentary record of collision of the North and South China blocks

Using detrital zircon geochronology, turbidite deposystems fed from distinct sediment sources can be distinguished within the Songpan-Ganzi complex, a collapsed Middle to Late Triassic turbidite basin of central China. A southern Songpan-Ganzi deposystem initially was sourced solely by erosion of the Qinling-Dabie orogen during early Late Triassic time, then by Qinling-Dabie orogen, North China block, and South China block sources during middle to late Late Triassic time. A northern Songpan-Ganzi system was sourced by erosion of the Qinling-Dabie orogen and the North China block throughout its deposition. These separate deposystems were later tectonically amalgamated to form one complex and then uplifted as the eastern Tibet Plateau. ?? 2006 Geological Society of America.

Geology↗
USGS2006

Preliminary integrated geologic map databases for the United States: Digital data for the geology of southeast Alaska

The growth in the use of Geographic Information Systems (GIS) has highlighted the need for digital geologic maps that have been attributed with information about age and lithology. Such maps can be conveniently used to generate derivative maps for manifold special purposes such as mineral-resource assessment, metallogenic studies, tectonic studies, and environmental research. This report is part of a series of integrated geologic map databases that cover the entire United States. Three national-scale geologic maps that portray most or all of the United States already exist; for the conterminous U.S., King and Beikman (1974a,b) compiled a map at a scale of 1:2,500,000, Beikman (1980) compiled a map for Alaska at 1:2,500,000 scale, and for the entire U.S., Reed and others (2005a,b) compiled a map at a scale of 1:5,000,000. A digital version of the King and Beikman map was published by Schruben and others (1994). Reed and Bush (2004) produced a digital version of the Reed and others (2005a) map for the conterminous U.S. The present series of maps is intended to provide the next step in increased detail. State geologic maps that range in scale from 1:100,000 to 1:1,000,000 are available for most of the country, and digital versions of these state maps are the basis of this product. The digital geologic maps presented here are in a standardized format as ARC/INFO export files and as ArcView shape files. Data tables that relate the map units to detailed lithologic and age information accompany these GIS files. The map is delivered as a set of 1:250,000-scale quadrangle files. To the best of our ability, these quadrangle files are edge-matched with respect to geology. When the maps are merged, the combined attribute tables can be used directly with the merged maps to make derivative maps.

Alaska↗
USGS2006

Constraints on the age and provenance of the Chugach accretionary complex from detrital zircons in the Sitka Graywacke near Sitka, Alaska

The Sitka Graywacke is the westernmost and youngest unit of the Chugach accretionary complex in southeastern Alaska. Using laser-ablation inductively coupled plasma mass spectroscopy, we obtained 492 detrital-zircon ages on seven typical samples of Sitka Graywacke turbidites, which were collected in a transect across much of the unit near Sitka, Alaska. Individual grains range in age from 66 to 1,802 m.y. The youngest peak ages on relative-probability plots of the western four samples (74, 72, 74, and 74 m.y., from west to east) are distinctly younger than the youngest peak ages of the eastern three samples (105, 103, and 97 m.y., from west to east). These youngest peak ages set maximum depositional ages for each sample. We suggest that these peak ages are not significantly older (within ~5 m.y.) than the depositional age of the Sitka Graywacke because the deposits accumulated in a trench along a convergent margin, where magmatic sources likely continuously introduced juvenile zircons. The differences in the youngest cluster of detrital-zircon ages between the eastern and western sample localities is likely due to both a change in provenance and a fault. The similarity of the youngest peak ages in the Sitka Graywacke to fossil ages in the Valdez Group, in Prince William Sound, implies that the western part of the Sitka Graywacke is correlative with the Valdez Group, as previously inferred. However, the eastern part of the Sitka Graywacke has youngest detrital-zircon ages older than fossil ages in the Valdez Group and younger than fossil ages in the McHugh Complex, which in south-central Alaska is the oldest part of the accretionary complex. The age distribution of zircons in the older, eastern sequence suggests sources along the British Columbia margin. The detrital-zircon ages in the younger, western sequence are similar to igneous ages from south-central Alaska to southern British Columbia. Right-lateral strike slip on various fault systems inboard of the Sitka Graywacke implies that it lay to the south when it was deposited and offscraped. Thus, although source areas as far north as the St. Elias Mountains and south-central Alaska are possible, they were most likely in coastal and interior British Columbia.

Professional Paper↗
USGS2000

Detrital zircon geochronology of the Shoo Fly Complex, northern Sierra terrane, northeastern California

U-Pb analyses have been conducted on 92 individual detrital zircon grains from 4 of the main thrust sheets of the Shoo Fly Complex . Samples from the Culbertson Lake allochthon, Duncan Peak allochthon, and Lang sequence yield mainly 1.80-2.10, 2.20-2.45, and 2.55-2.70 Ga ages, which suggests that sediments in these units originated in a cratonal region containing Paleoproterozoic and Archean igneous rocks. These ages match those of basement provinces from the northwestern Canadian shield, suggesting a provenance link with northwestern North America during early Paleozoic time. The Sierra City melange, however, has significantly different zircon ages of 551-635 and 1170-1319 Ma, with only a subordinate population of >1.8 Ga grains. These grains apparently were derived originally from an outboard Neoproterozoic-Cambrian(?) volcanic arc and from 1.0-1.7 Ga basement rocks of south- western North America . The occurrence of all three sets of ages in a sandstone that accumulated outboard of the Lang, Culbertson Lake, and Duncan Peak thrust sheets indicates that most rocks of the Shoo Fly Complex formed inboard of a volcanic arc located in proximity to the southern portion of the Cordilleran margin.

California↗
USGS2000

Detrital zircon geochronology of Neoproterozoic to Middle Cambrian miogeoclinal and platformal strata: northwest Sonora, Mexico

Eighty-five detrital zircon grains from Mesoproterozoic and/or Neoproterozoic to Middle Cambrian sedimentary strata in northwest Sonora, Mexico, have been analyzed to determine source terranes and provide limiting depositional ages of the units. The zircon suites from the Mesoproterozoic and/or Neoproterozoic El Alamo Formation and El Aguila unit yield ages between 1.06 Ga and 2.67 Ga, with predominant ages of 1.1 to 1.2 Ga. Zircons from the Lower? and Middle Cambrian Bolsa Quartzite show age groups from 525 Ma to 1.63 Ga, with a dominant population of 1.1 to 1.2 Ga grains. Grains older than 1.2 Ga in the samples were most likely derived from basement terranes and ~1.4 Ga granitic bodies of the southwest U.S. and northwest Mexico. It is also possible that the sediments were transported from the south, although source rocks of the appropriate age are not presently exposed south of the study area in northern Mexico. Three possibilities for the dominant 1.1 to 1.2 Ga grains include derivation from: (1) exposures of the Grenville belt in southern North America, (2) local 1.1-1.2 Ga granite bodies, or (3) a southern source, such as the Oaxaca terrane, that was subsequently rifted away. Sampling of additional units in the western U.S. and northern Mexico may help resolve the ambiguity surrounding the source of the 1.1 to 1.2 Ga grains.

Sonora↗
USGS1986

Geologic map of southern Prince of Wales Island, southeastern Alaska

Southern Prince of Wales Island is underlain by stratified, intrusive, and metamorphic rocks of Cretaceous through pre-Middle Ordovician age and by surficial deposits. The region was mapped originally in reconnaissance fashion by Buddington and Chapin (1929) and subsequently by W.H. Condon and I.L. Tailleur (unpublished U.S.G.S. report, 1960) primarily with the use of aerial photographs. More recently, MacKevett (I 963) studied the geology of the Bokan Mountain-Stone Rock Bay area, Herreid and others (1978) and G. Donald Eberlein, Michael Churkin Jr., and Walter Vennum (Eberlein and others, 1983; unpublished data) mapped Kassa and Klakas Inlets and regions to the north, and Thompson and others (1982) and B. Col lot (in Saint-Andre and others, 1983) studied the Bokan Mountain Granite. The inset map of Plate 1 shows where this published and unpublished mapping has been incorporated into our geologic map.

Alaska↗
USGS1984

Geologic map of southeastern Alaska

Southeastern Alaska is underlain by sedimentary, volcanic, intrusive, and metamorphic rocks of Quaternary to Cambrian, and probable Proterozoic age. These rocks have been classified on the geologic map and in this pamphlet into units that emphasize the regional distribution of lithically similar and generally coeval geologic units. The distribution, age, stratigraphic or intrusive relations, and metamorphic and structural characteristics of the map units are described below in the Description of Map Units, and the temporal relations between units are shown on the Correlation of Map Units (Plate). Figure I shows the primary sources of information used in compiling the geologic map.

Alaska↗