Granite Mountains Research Articles

Benthic ecology of a spring‐fed river of interior Alaska

SUMMARY1. A massive aquifer between the Gerstle, Tanana and Delta rivers in interior Alaska receives water from them and from smaller streams that flow from the Granite Mountains in the Alaska Range, Groundwater from the aquifer intersects the surface in a mid‐sized (20m3s−1× 10%) spring‐fed stream, Clearwater Creek.2. Mean annual air temperature is about ‐2.6°C. However, even in winter when air temperature often reaches —40°C, the stream does not form a complete ice cover. Water temperature ranges from 0 to 7.8°C. Specific conductance and the concentrations of major ions vary little throughout the year, and summed ionic salinity exceeds 250 mg1‐−1.3. Benthic algal standing crop (as chlorophyll a) was at least an order of magnitude higher than that in a nearby surface‐water stream, the upper Chena River, Standing crop peaked in spring and autumn (about 20mgm−2) and averaged about half this value, although biomass of an early spring bloom of Hydrurus foetid us was underestimated.4. Algal standing crop was inversely related to the concentrations of inorganic nitrogen and orrhophosphate‐phosphorus in the water column. The ratio of total nitrogen to total phosphorus (as mass concentrations) was always about 30. Measurements of primary production made in Clearwater Creek were among the highest reported for streams in subarctic Alaska.5. Macroinvertebrate diversity in Clearwater Creek was low. Numbers of ‘morpho‐species’ in monthly Surber samples (0.09m2) averaged nine, and ranged from three to fourteen. However, benthos and drift densities were similar to those reported from other Alaskan streams. In early spring and autumn, drifting macroinvertebrates were primarily Ephemeroptera, Plecoptera and Trichoptera, but in summer, Diptera dominated the drift. The low diversity of macroinvertebrates is hypothesized to be a consequence of the small annual range in water temperature and the relatively constant discharge of Clearwater Creek.

Silicic Magma Chambers as Traps for Basaltic Magmas: The Cadillac Mountain Intrusive Complex, Mount Desert Island, Maine

The Cadillac Mountain intrusive complex, located on Mount Desert Island, Maine, provides a superb record of episodic invasion of a floored silicic magma chamber by many pulses of basaltic magma. The complex consists of three units: the Cadillac Mountain granite (CMG), the Somesville granite (SG), and gabbro-diorite (G-D). Basaltic magmas were emplaced before, during, and after the emplacement and crystallization of the granitic plutons, and the chambers of silicic magma acted as traps for basaltic magma. Basaltic magma emplaced into the surrounding country rocks crystallized as homogeneous diabase and gabbro, while basaltic magma emplaced within the perimeter of the granite displays abundant evidence for commingling with silicic magma. The G-D unit (about 1.5 km thick) consists of interlayered gabbroic, dioritic, and granitic rocks. Although thin (<1 m thick) gabbroic layers in the G-D unit are typically chilled on all margins, thicker layers of gabbro commonly form the lower parts of macrorhythmic units that may grade upward from basally chilled gabbro through hybrid dioritic rocks to granite. These macrorhythmic units provide a cumulate record of the temporary stratification of mafic and silicic magmas in the CMG chamber. Widespread, small mafic enclaves occur at all levels of the CMG and were probably generated along turbulently stirred double-diffusive boundaries between mafic and silicic magma. Heat from basaltic infusions promoted convection in the overlying silicic magma, and this convection distributed enclaves thoughout the CMG chamber. Scarce, finegrained enclaves of hybrid, intermediate rock in the CMG may represent disrupted portions of a hybrid magma that evolved near the base of the chamber due to repeated infusions of basaltic magma. Some zones of transgressive granophyre in the CMG probably represent highly silicic liquids that evolved within the CMG magma chamber. The Somesville granite may represent a mixture of magma that evolved within the CMG chamber and injections of new silicic magma from below.

Lead isotopic evidence for mixed sources of Proterozoic granites and pegmatites, Black Hills, South Dakota, USA

The lead isotopic compositions of K-feldspars separated from the ca. 1700 Ma Harney Peak Granite complex and spatially associated granitic pegmatites indicate that these rocks were derived from at least two sources. It has been reported previously that the core of the Harney Peak Granite complex is dominated by relatively lower/ gd 18O (avg. 11.5 %.) granites, whereas higher / gd 18O (avg. 13.2%.) granites occur around the periphery of the complex. The higher δ 18O granites and one simple pegmatite have low values of 207Pb 204Pb for their 206Pb 204Pb Thus, they likely were derived from a source with a short crustal residence time. This source may have been the pelitic schists into which the Harney Peak Granite complex and pegmatites were intruded. Feldspars from granites with lower / gd 18O values have significantly higher 207Pb 204Pb for their 206Pb 204Pb . The data define a linear array with a slope equivalent to an age of ca. 2.6 Ga with t 2 defined to be 1.7 Ga. Such a slope could represent a mixing array or a secondary isochron for the source. These low δ 18 O granites could have been derived from a source with a high U/ Pb and with a crustal residence beginning before the Proterozoic. The source (s) of these granites may have been a sediment derived from late Archean continental crust. The highly evolved Tin Mountain pegmatite has lead isotopic systematics intermediate between those of the two granite groups, suggesting either a mixed source or contamination. Two late Archean granites, the Little Elk Granite and the Bear Mountain Granite, had precursors with high U Pb and low Th U histories. The Th U history of the Bear Mountain Granite is too low for this rock to have been an important component of the source of the Proterozoic granites. However, crustal rocks with lead isotopic compositions similar to those of the Little Elk Granite were an important source of lead for some of the Proterozoic granitic rocks.

Glacial dispersal of rock debris in central Gaspésie, Quebec, Canada

The lithological content of tills in central Gaspésie is evaluated by pebble counting of 231 samples collected in excavation pits and containing 200 pebbles each. The results are used here to establish the pattern of debris dispersal and to infer the glacial history of the area. The dispersal pattern is characterized by well-defined southeasterly (160–170°) and northeasterly (40–60°) trending trains. Half-distance values of glacial transport along the trains range from 5 to 9 km for both directions, suggesting ice flow events of considerable magnitude. The volume of material in the trains represents 1–6 m of glacial erosion of the bedrock. Glacial cirques and short U-shaped valleys, about 100–200 m deep, are incised into the McGerrigle Mountains granite pluton as well as the adjacent metabasalt. The corresponding trains are aligned with these erosional features, indicating that their clast content was derived from those features during an early Alpine Glacier Phase. The southeasterly trending dispersal trains are associated with an invasion of central Gaspésie by the Laurentide Ice Sheet during the Early Wisconsinan, whereas the northeasterly trending trains are associated with a local centre of outflow over Gaspésie during the Late Wisconsinan.

Crustal and uppermost mantle structure under southern California

The travel times of Pg and Pn arrivals are used together as a new approach to estimate the station delays, lateral velocity variations in the upper crust and upper mantle, crustal thickness variations, and Pn anisotropy in the upper mantle under southern California. The main advantage of this approach is that crustal effects are estimated directly from the data and therefore do not contaminate the results from Pn arrivals. Significant negative station delays are found in the Colorado desert, San Jacinto Mountains, San Gabriel Mountains, central Mojave desert and from the southern Ventura Basin to Santa Cruz Island in addition to the positive delays associated with well-known sedimentary basins. Lateral velocity variations in the upper crust exceed a few percent only in the Coso ranges. These variations show no clear association with surface faulting; they also show little resemblance to the upper mantle velocity variations, which have fast velocity under the Mojave and near the offshore area and slow velocity in most other areas. The southern California Moho shows large variations. Large crustal thinning occurs near the Salton Sea, northwest of the White Wolf fault, in the Coso Range near San Diego and offshore, while the San Bernardino Mountains, western Granite Mountains, and eastern Mojave (south of the Ludlow fault) are underlain by roots. A significant amount (3%) of Pn anisotropy is present in the uppermost mantle with a fast direction N 75° ± 4°W. This could be explained very well by averaging the Pn anisotropy estimates east and west of the San Andreas.

Subalpine and alpine vegetation of granite ranges in western Nelson, New Zealand

Abstract Twenty two vegetation units, derived subjectively from qualitative field notes, are described from several glaciated granite mountains and ranges in western Nelson: Lead Hills in the north, Herbert Range, Kendall Ridge, and Scarlett Range north of Little Wanganui Saddle, the adjacent western part of Allen Range south of the saddle, and Glasgow Range on the western side of the Mokihinui River. Their area sums to about 190 km2. The ranges all have high rainfall (4800-6400 mm), high cloud cover, and very infertile granite soils. The main cover is Nothofagus menziesii forming the timberline, scrub dominated by Olearia colensoi and Dracophyllum uniflorum, tussocklands of Chionochloa pallens ssp. pallens, C. rubra ssp. occulta, and C. australis, and Oreobolus pectinatus — Donatia novae-zelandiae cushion-herbfield. Vegetation units characteristic of more fertile soils in other parts of western Nelson are uncommon, e.g., Chionochloa flavescens ssp. lupeola tussockland and Hebe topiaria scrub. Calcicoles are rare on granite and are confined to dry rock overhangs, e.g., Asplenium lyallii. Several conspicuous species are not found on all ranges. Some differences are because Kendall Ridge has a more extensive alpine zone and several species that are rare or absent from other granite ranges are found there, e.g., Chionochloa oreophila and Raoulia subulata. Other differences are possibly related to the position of the ranges with respect to the prevailing moisture-laden westerlies, e.g., the absence of Celmisia spectabilis from the Glasgow Range. However, for several conspicuous species there is no ready explanation for their distribution patterns, e.g., Aciphylla hookeri and a large unnamed Gentiana taxon. Anomalous distribution patterns of Nothofagus forest on the Glasgow Range that do not appear to be related to geology or fire history factors require investigation.

The distribution of vascular plant species and guilds in space and time along a desert gradient

Abstract. We studied distribution patterns of vascular plant species and environmental variables for three years along a permanent transect traversing a closed‐drainage watershed in the northern Chihuahuan Desert of south‐central New Mexico, USA. The transect extended for 2.7 km from a basin floor playa (1310 m elevation, fine‐textured soil), across a piedmont slope, and onto the base of a granitic mountain (1410 m elevation, coarse‐textured soil). The gradients in elevation and soils across our transect, along with variable seasonal rainfall, downslope redistribution of water and organic matter, and soil texture‐related variation in infiltration, water holding capacity, and moisture release characteristics, interact to generate a complex spatial and temporal gradient of available soil water and nitrogen. We grouped plant species into guilds according to growth form and photo synthetic pathway type. These guilds are spatially and temporally differentiated along the transect such that particular groups utilize particular seasonal phases or spatial regions of the gradient. We identified six distinct plant communities along the transect. C4 perennial grasses dominated the mesic/high nitrogen portion of the gradient, which occurred at the highest (upper piedmont grassland, dominated by Bouteloua eriopoda) and lowest (playa, dominated by Panicum obtusum) elevations along the transect. C3 shrubs were dominant in the xeric/low nitrogen portion of the gradient located near the middle of the transect (bajada shrubland, dominated by Larrea tridentata). C3 shrubs also dominated a narrow zone of vegetation located adjacent to the playa (playa fringe, dominated by Prosopisglandulosa). C4 perennial grasses, C3 subshrubs, and C3 and C4 perennial forbs and annuals were co‐dominant in the intermediate locations along the gradient, which occurred below (mixed basin slopes) and above (lower piedmont grassland) the bajada shrubland. Life‐form distribution patterns at the small scale of our study reflect some of the patterning that occurs at larger scales in response to climate gradients. The distributions of some species and guilds along the transect are apparently modified by competitive interactions.

Basin-wide Architecture of Sandstone Reservoirs in the Fort Union Formation, Wind River Basin, Wyoming

Architecture of hydrocarbon-bearing sandstone reservoirs of the Paleocene Fort Union Formation in the Wind River basin, Wyoming, was studied using lithofacies, grain size, bounding surfaces, sedimentary structures, internal organization, and geometry. Two principal groups of reservoirs, both erosionally based and fining upward, consist of either conglomeratic sandstone or sandstone lithofacies. Two types of architecture were recognized in conglomeratic sandstone reservoirs: (1) heterogeneous, multistacked, lenticular and (2) homogeneous, multiscoured, wedge-sheet bodies. Three types of architecture were recognized in sandstone reservoirs: (3) heterogeneous, multistacked, elongate; (4) homogeneous, multilateral, lenticular; and (5) homogeneous, ribbon-lensoid bodies. Conglomeratic sandstone reservoirs in the southern and southwestern parts of the basin suggest deposition in gravel-bedload fluvial systems influenced by provenance uplift of the Granite and southern Wind River mountains. Type 2 reservoirs represent deposits of eastward-flowing braided streams aggrading an alluvial valley in response to base level rise. Thus, to determine basin-wide architecture of reservoirs requires understanding the interplay between base level conditions, basin subsidence, and provenance uplift. These interrelated factors, in turn, control differences in hierarchies of fluvial systems throughout the basin.

Middle Pennsylvanian Strike-Slip Faulting on the Eastern Edge of the Llano Uplift, Central Texas

ABSTRACT Pre-Strawn rocks of the Llano Uplift, Central Texas, are cut by a pervasive system of NE-trending faults whose movement more-or-less coincides with the culmination of Ouachita orogeny in the area. Map patterns indicate that major faults are near-vertical, which can be explained best by strike-slip faulting. Right-lateral displacements probably dominate near Marble Falls, Texas. The complex fault slice centered on Marble Falls, near the eastern edge of the Paleozoic outcrop, is more than 30 km long but only 2-8 km wide. Structure within this block exhibits a syncline and an anticline having axes at about N60W that is, dips are NE in the southwestern part of the block, SW for 5 or 6 km NE of Marble Falls, and thence NE toward the unconformable Cretaceous cover. The eastern bounding fault at Mormon Mills has an apparent vertical displacement of 500 meters or so (Marble Falls Limestone and Smithwick Shale against Tanyard Dolomite). The western boundary fault has an apparent vertical displacement of more than 1 km at Marble Falls (Smithwick Shale and Marble Falls Limestone against Town Mountain Granite) but to the N splinters into several left-stepping blocks. Some splinter faults die out northward within a few hundred meters to zero displacement, and the western bounding fault of the dominant splinter block some 10 km N of Marble Falls has only 100-200 meters apparent vertical displacement (Tanyard Dolomite against San Saba Dolomite). Much of the differential displacement clearly was accommodated by folding of strata within the fault block, although cross-faults are abundant. Outcrop and quarry exposures E of U. S. 281, near Sudduth Siding, show that the Threadgill Member of the Tanyard Formation and the overlying Staendebach Dolomite, near the crest of the broad antiform, are shattered by fractures in sets that trend N20W, N80E, N40-50E, and N30-40W. The N20W and N80E sets are most prominent on 1940 and current aerial photographs; these sets bound the Brownlee Ranch Dolomite Quarry (Dow Chemical USA) on the west and north. Randomly-oriented blocks of chert-bearing Staendebach Dolomite were dropped into breccia zones along NE-SW and E-W fracture sets within Threadgill Dolomite. No younger material was structurally incorporated into the breccia. Where fractures are well-exposed in solid rock between breccia zones, little vertical displacement could be discerned across most individual fractures, but nearly-horizontal to shallowly-dipping slickensides and mullions are common. Some fractures of the N20W, N40-50W, and N40-50E sets display "flower structures" within the quarry on a scale that cuts dolomite into vertically-bounded blocks. In pasture outcrops these features can be mistaken easily for vertically-dipping beds 5 cm to 1 m thick. We believe that the N20W/N80E and the N40-50E/N40-50W fracture sets are pairs that formed at different times under different stress conditions, but could not determine unequivocal temporal relationships. Saddle dolomite druse in single to multiple layers coats fracture faces of all sets and no vertical or steeply-dipping stylolites were found, although horizontal stylolites are abundant. The gross morphology of the Ouachita belt indicates first-order westward movement of the orogen in Central Texas. We suggest that as the orogen impacted the generally NW-SE trend of the Central Texas craton, extensive strike-slip faults sliced the craton into blocks, producing gentle folds within blocks during the process. Blocks also were extensively fractured internally, probably during more than one horizontally-compressive stress event. Along the eastern exposed margin of the Central Texas craton, horizontal compression ceased before fractures could be closed and modified by pressure-solution into vertical stylolites. End_of_Record - Last_Page 11--------

A Comparison of Tectonic and Eustatic Control on Maastrichtian Depositional Sequences, South-Central Wyoming: ABSTRACT

Timing and geometry of Maastrichtian third-order sequences and systems tracts of south-central Wyoming were mostly controlled by rapid basin subsidence resulting from thrust loading of foreland basin crust. The basement of the Red Desert/Washakie basin was affected by thrusting in the Sevier belt to the west and by thrusting along the edge of the Wind River Mountains and Granite Mountains. A transgressive systems tract consists of the youngest Ericson Formation above a regional unconformity, backstepping Almond Formation nearshore and nonmarine facies, and shelfal mudstones of the lower Lewis Shale. Regionally, the onset of transgression is younger from east to west, beginning at 71.5 Ma in eastern Wyoming and at 70.5-71 Ma in south-central Wyoming. The transgression culminated in a condensed shale section evident on gamma-ray logs that occurs throughout the basin. Above the gamma-ray zone, the highstand systems tract of the middle and upper Lewis Shale, Fox Hills Sandstone, and Lance Formation consists of an aggradational/progradational interval overlain by a dominantly progradational clastic unit. Depositional environments consist of delta, slope, and submarine fan in the Lewis Shale, shoreface and shallow marine in the Fox Hills Sandstone, and nonmarine facies in the Lance Formation. Tectonic control of sequences is indicated bymore » two lines of evidence. (1) The timing of the transgressive, regressive, and condensed section facies does not correlate with the inferred global eustatic curve. The Lewis Shale condensed section occurs at 70.5 Ma and is older than the closest global condensed section at 69.5 Ma. (2) Subsidence, estimated by the amount of delta aggradation of the Lewis, totals 700 m roughly 0.5 Ma and greatly exceeds the 40 m of eustatic rise hypothesized to have occurred at about the same time.« less

Tectonic Controls on Late Cenozoic Strike-Slip Faulting, Volcanism, and Landscape Development in the Mojave Desert, California: ABSTRACT

Recent studies of the late Cenozoic tectonics of the Mojave Desert Block suggest that strain is regionally heterogeneous and has been partitioned into six domains that are separated by major strike-slip faults and extensional zones. Tectonic rotation of these domains as well as their internal deformation by strike-slip faulting have occurred as the result of broadly distributed regional right shear. Sixty-five kilometers of total right slip is reckoned to have occurred along faults of the southern half of the province (between the Helendale and Granite Mountains faults). The broad network of faults of the Mojave, along with kinematically and temporally similar strike-slip faults of the Death Valley region (Furnace Creek and Southern Death Valley fault zones), constitute a regional, through going zone of right shear named the Eastern California shear zone (ECSZ). This zone of intracontinental shear also likely includes the Walker Lane belt of western Nevada. Because of its physical connection to faults the southern portion of the San Andreas fault system, the ECSZ must have also accommodated a portion of Pacific-North American transform motion. In addition to imparting the strong NW structural grain to the region, the tectonic regime has had a profound effect on Neogene paleogeography andmore » has apparently facilitated local magmatism. Faulting and block rotations have created a series of structurally controlled basins and uplifts of many geometries. Extension in several basins has also been accompanied by young basaltic magmatism. These include surface flows such as at Mt. Pisgah (shield volcano) and subsurface dike emplacement beneath Troy Lake.« less

Uplift, unbuckling, and collapse: Flexural history and isostasy of the Wind River Range and Granite Mountains, Wyoming

Two‐dimensional gravity and flexural models constrain the structure and isostatic compensation of the Wind River Range and Granite Mountains, Wyoming. These ranges are asymmetric, basement‐cored Laramide uplifts. Their large positive isostatic gravity anomalies imply that the topography of the uplifts is not locally compensated. For the Wind River Range, gravity data are consistent with thrusting along a listric fault which flattens and soles out at 30 km depth with no detectable offset on the Moho. We model the large Bouguer gravity high over the range with high‐density lower crustal rocks emplaced over lower‐density middle crustal rocks along the Wind River thrust. The high positive gravity anomaly over the Granite Mountains can be modeled as an extension of the South Pass greenstone belt from the southern Wind River Range eastward beneath the Emigrant Trail thrust. A crustal root, as required in local (Airy) compensation, for either range requires unreasonably high‐density material in the upper crust to obtain the observed anomaly. Combined gravity and flexural models show that a flexural rigidity at least 2.55×1022 N m (maximum deflection of 2 km of the Moho) is required to satisfy the gravity data. Our results are consistent with models of uplift due to horizontal compression and support of the resulting topography by lithospheric strength. An otherwise puzzling episode of Oligocene uplift of the Wind River Range can be explained as “unbuckling” (decrease of deflection of lithosphere) accompanying release of Laramide compressional stresses. Flexurally supported uplifts will collapse if the lithosphere is weakened. A number of Laramide uplifts have collapsed, primarily east‐west trending structures such as the Granite Mountains, the southern end of the Wind River Range, and the Uinta Mountains. We propose that collapse has occurred along zones of established weakness in the crust in response to north‐south extension.

Mineralogy of the Granite Mountain Syenite

Within the last few years, several new occurrences of rare minerals have been documented from the syenite pegmatites of Granite Mountain.

The Mojave Extensional Belt of southern California

It is proposed that the regional pattern of early Miocene low‐ and high‐angle normal faulting and dike swarm emplacement in the western and central Mojave Desert is the result of regional extension developed within a roughly east trending zone, named here the “Mojave Extensional Belt”. This zone of extension, although now disrupted by late Cenozoic right‐slip faulting, can be traced from the intersection of the San Andreas and Garlock faults to the eastern Mojave Desert near Bristol Lake and the Granite Mountains fault. Restoration of post‐Oligocene movement along the San Andreas fault system places the now buried western part of the Mojave Extensional Belt opposite a similar age‐extended area of west central California. Surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the Mojave Extensional Belt was low‐angle, normal sense, simple shear (detachment faulting). Strain within the Mojave Extensional Belt is partitioned between four domains (Edwards, Waterman, Daggett, and Bullion terranes) that each consist of one or more half‐grabens. Each half‐graben is composed of tilted, normal fault‐bounded blocks that lie above a rooted, low‐angle, brittle‐ductile normal sense shear zone. Tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping (∼80°) planar to slightly curviplanar normal faults. At least one generation of high‐angle faults postdates the formation of the detachments. Differential extension between the domains was accommodated by strike‐slip faults (transfer zones). Similarly, the lack of early Miocene extensional elements north of the western segment of the Garlock fault suggests that the Garlock may have been active in the early Miocene and served as the northern transfer boundary of the Mojave Extensional Belt. The Mojave Extensional Belt initially began to open about 22 m.y. ago, with the major phase of extension occurring between ∼22 and 20 Ma. Local, high‐angle, normal faults cut older detachments and were active between ∼20 and 17 Ma. Kinematic indicators suggest that the western part of the Mojave Extensional Belt opened in a NE–SW direction, whereas the eastern portion extended in an ENE–WSW direction (present‐day reference frame). Rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the Mojave Extensional Belt was originally oriented ∼N–S and was rotated as much as 50° (clockwise) between ∼20 and 18 Ma (Early Miocene).

A magnetotelluric profile across the western boundary of the Salton Trough in northern Baja California, Mexico

Eighteen magnetotelluric (MT) stations were occupied in an east-west profile across the northern part of the Baja California Peninsula immediately south of the international border between Mexico and the U.S.A. The profile stretches across the Peninsular Ranges Batholith into the Salton Trough, a region of active crustal spreading. Data were collected in the frequency range of 0.001–25 Hz using the remote reference technique. We have conducted a preliminary modeling of the data by fitting both the transverse electric and the transverse magnetic apparent resistivity curves using a two-dimensional (2-D) forward modeling algorithm. Across the batholith, the geoelectric model exhibits surface resistivity > 1000 Ω m, decreasing by an order of magnitude with depth. However, a very high conductivity has been modeled between depths of 12 and 32 km in an isolated block. This anomaly may be due to a zone of entrapped water. In the Laguna Salada Basin on the western margin of the Salton Trough, the near-surface resistivities are of the order of tens of Ω m. In the center of the basin, at depths < 2.5 km, the resistivities fall to very low levels, probably owing to sediments saturated with saline water and with a high content of clays. These sediments are underlain by a resistive basement (500 Ω m) followed at depth by an 80 Ω m conductor at near 25-km depth. The deep conductor shallows towards the east, reflecting a thinning of the crust under the Mexicali Valley. The sounding measured closest to an intertrough granitic mountain range, the Sierra de los Cucapa, shows a complicated structure. The reduced thickness of the resistive crust beneath the Cucapa range may be geologically significant; for example, the MT data may be indicating that the Cucapa range is near the leading edge of a thrust sheet or probably shows the presence of three-dimensional (3-D) effects due to its finite length in the north-south direction. The model for the Mexicali Valley near the center of the Salton Trough presents very low near-surface resistivities, some near 1 Ω m. In general, one sees approximately a horizontal interface between more and less conductive sediments at ∼ 2-km depth. This contact deepens toward the center of the valley. The two layers correspond to clay-rich unconsolidated sediments saturated with saline water in the upper zone and consolidated sediments with limited pore space below. A systematic anisotropy present in all the soundings could be accounted for by the 3-D geometry of the Salton Trough which may enhance the electric field parallel to the structure.

花こう岩山地を供給源とする堆積物の異なる環境における粒度組成

花こう岩山地を供給源とする堆積物の異なる環境における粒度組成

Mid‐Cretaceous paleomagnetic reference field for North America: Restudy of 100 Ma intrusive rocks from Arkansas

We have obtained new paleomagnetic data from three middle Cretaceous alkalic intrusive complexes in central Arkansas: Magnet Cove and Potash Sulfur Springs intrusions (about 100 Ma, 20 sites), and Granite Mountain stock (about 88 Ma, 6 sites). Out of 254 specimens measured, 147 from 20 sites were accepted. The remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region. The geologic evidence indicates that the intrusions have not been tilted since emplacement. The directions of magnetization calculated with respect to present‐day horizontal yield a mean paleopole at 192.5°E, 74.1°N (N = 20, K = 34, A95 = 5.7°). This does not differ at 95% confidence from that determined previously by Scharon and Hsu, but the error is diminished by half. The angular dispersion (Sd) of site virtual geomagnetic poles (VGPs) is 13.6°, similar to the value of Sd (about 15°) predicted by current paleosecular variation models. The revised Arkansas paleopole is combined with four other cratonic North American paleopoles of mid‐Cretaceous age to yield a revised middle Cretaceous reference pole at 196°E, 71°N, A95 = 4.9°. The mean of nine paleopoles derived from overprints in rock units from the craton and craton margin, which range in age from Proterozoic to Jura‐Cretaceous, is 202°E, 68°N, A95 = 8.2°. This is not different from the reference paleopole, indicating that overprinting was very widespread across North America in mid‐Cretaceous time.

Geology and uranium-thorium mineral deposits of the Bokan Mountain Granite Complex, southeastern Alaska

The Bokan Mountain (Kendrick Bay) uranium-thorium deposits are associated with a Late Jurassic peralkaline granite ring-dike complex. The uranium-thorium deposits form pipe-like bodies along contacts, or occur as pods in en echelon northwest-striking shear zones. The host intrusive is a multiphased body consisting mainly of riebeckite granite porphyry and subordinately of pegmatite-aplite, aegirine granite porphyry, and eight other lesser rock types; the ore is closely associated with an albitized aegirine syenite. The ore consists of thorium and rare earth-rich rock containing uranothorite, uraninite, and generally less than 2% of sulfide species. About 100,000 tons (89,000 t) of ore have been mined at an average grade of about 1% U 3O 8 and nearly 3% ThO 2, mainly from the Ross-Adams pipe. The pipe itself lies along the contact of aegirine syenite and aegirine granite porphyry; it is as much as 24 m across and was mined along strike for over 300 m. The ore in shear zones occurs in lenses as much as 3 m thick and 30 m in strike length. Wallrock alteration within and adjacent to orebodies consists of pervasive hydrothermal albite and lesser amounts of chlorite, fluorite, calcite, quartz, sericite, and tourmaline. Hematite is present in the outer-distal parts of the ore zones. The minerals produced to date occur within the outer annular zone of the Bokan Granite Complex in which aegirine is present as the major alkali ferro-magnesian mineral. Ore emplacement occurred during the first subsidence event in association with devolatilization of the magma chamber. The pod-like ore zones appear to have formed during regional faulting synchronous with magma crystallization and subsequent hydrothermal events. Subsequent magma crystallization yielded riebeckite granites that were depleted in U and Th. Filling temperatures in ore stage fluid inclusions within quartz range from 320 to 331°C with pressure corrections increasing the temperature of formation to 420°C or higher, depending on dissolved gas content. Sulfur isotopic analyses on pyrite, galena, sphalerite and pyrrhotite indicate disequilibrium, but δ 34S H 2S is estimated at 7.6‰. Carbon and oxygen isotopic analyses on ore stage calcite yield δ 18O H 2O of 6.8 to 8.1‰ and δ 13C ΣC of −4.3 to −7.0‰. The oxygen and carbon isotopic data support a magmatic origin for the calcite and associated thorium-uranium deposits.

Analysis of Sediment Transport in a Torrent in a Weathered Granitic Mountain

Analysis of Sediment Transport in a Torrent in a Weathered Granitic Mountain

Rb-Sr isotopic determinations and the timing of Tertiary central complex magmatism in NE Ireland

Summary Full Rb-Sr isotopic data and whole-rock regressions are presented for each of the Mourne Mountains granites (G1–G5 inclusive) and a set of granophyres/microgranites from the Slieve Gullion central complex. The former were intruded at approximately 56 Ma (G1–G4 inclusive) and later (G5), and may all be younger than the major intrusions of the Slieve Gullion and Carlingford central complexes. The initial Sr isotope ratios are &gt;0.7060 for all five Mourne granites, the Slieve Gullion granophyres/microgranites analyzed and the rhyolitic rocks of the Tardree area (Co. Antrim), and crustal involvement is indicated in their petrogenesis. In the E Mourne Centre the first (the least silicic and most Sr-rich) granite has a significantly higher initial ratio (0.7129 ± 2) than G2 (0.7109 ± 3) and G3 (0.7104 ± 6), and the isotopic data provide further support for the recent revision to the mapping of the first and second granites. Preliminary Rb-Sr isotopic data for a comprehensive suite of Tertiary rhyolites from NE Ireland, some separated by &gt;50 km, define a linear array whose explanation is considered.