Showing papers in "Geological Society of America Bulletin in 1999"

Tectonic setting, origin, and obduction of the Oman ophiolite

Abstract: The Semail ophiolite in the Oman Mountains is the world9s largest and best preserved thrust sheet of oceanic crust and upper mantle (>10 000 km 2 , ∼550 km long, ∼150 km wide); it was emplaced onto the Arabian continental margin during Late Cretaceous time. The ophiolite originated 96–94 Ma at a spreading center above a northeast-dipping subduction zone associated with initiation of immature island-arc tholeiitic lavas (Lasail arc) at the highest levels of the ophiolite. Simultaneous underthrusting of Triassic (and Jurassic[?]) mid-oceanic-ridge basalt and alkalic volcanic rocks beneath >12 km of upper mantle depleted harzburgites produced garnet + clinopyroxene amphibolites formed at temperatures of ∼850 °C, dated as 95–93 Ma. Subduction cannot have been initiated at a mid-oceanic ridge, otherwise the protolith of the amphibolites in the metamorphic sole would be the same age and composition as the ophiolite volcanic rocks above. In the northern part of the Oman Mountains in the Bani Hamid area, United Arab Emirates, ∼870 m of granulite facies rocks (enstatite + spinel ± diopside quartzites, garnet + diopside + wollastonite calc-silicate marbles, clinopyroxene-bearing amphibolites) were formed at temperatures similar to those of the garnet + diopside amphibolites of the Oman sole, 800–850 °C, but at slightly higher pressures, as much as 9 kbar. They are interpreted as deeper level metamorphosed continental margin sedimentary rocks exhumed by out-of-sequence thrusting placing granulites over mantle sequence harzburgites during the later stages of obduction. Subduction of the Arabian continental crust beneath the obducting Semail ophiolite to ∼78–90 km depth has been proven by thermobarometry of the As Sifah eclogites (to 20–23 kbar) in the eastern sector. In the United Arab Emirates the subducted continental crust began to partially melt, producing unusual biotite ± muscovite ± garnet ± tourmaline ± cordierite ± andalusite–bearing granites that intrude the uppermost mantle sequence harzburgites and lowermost crustal sequence cumulate gabbros of the ophiolite. We suggest that the entire leading (northeast) edge of the Arabian plate was subducted beneath the ophiolite during the final stages of obduction leading to eclogitization of the crustal rocks. Higher temperatures and pressures in the United Arab Emirates sector, possibly due to a thicker or double-thickness ophiolite section, led to blueschist, amphibolite, and granulite facies conditions in the metamorphic sole, and crustal melting in the subophiolite basement produced leucocratic granites that intruded up as dikes through the obducted ophiolite. A model for ophiolite obduction is presented, which accounts for all the structural and metamorphic conditions reported from the Oman Mountains.

Origin of 3.45 Ga coniform stromatolites in Warrawoona Group, Western Australia

Abstract: A new occurrence of conical and branched pseudocolumnar stromatolites in Archean dolostones in the Pilbara region, Australia, contributes significant new morphologic information on such structures. These remains are interpreted as probably representing, in part, microbially mediated accretionary growth surfaces in an Archean hypersaline depositional basin. The structures comprise laterally linked pseudocolumns of centimeter width and decimeter height, with first-order conical laminae of as much as 15 cm of synoptic relief and apical angles of 30°–80°. The conical laminae are modified by a second-order, centimeter-scale, low-amplitude primary corrugate lamination, with crests and troughs occasionally stacked to form satellitic, obliquely directed pseudocolumns; bedding surfaces exhibit a preferred direction of elongation of the cones, an orientation that is orthogonal (and unrelated) to the trend of younger folding; the microstructure is secondary. The stromatolites are better preserved than those previously known from chert in the Warrawoona succession. The remains exhibit certain distinct morphologic attributes corresponding to those in younger stromatolites, such as displayed by Thyssagetes and Jacutophyton , whose biogenicity is generally accepted (although difficult to demonstrate conclusively); the conical Warrawoona forms may represent the oldest known precursor of these taxa.

Late Pleistocene–Holocene retreat of the West Antarctic Ice-Sheet system in the Ross Sea: Part 1—Geophysical results

Abstract: In the 1994, 1995, and 1998 austral field seasons, geophysical research efforts in the Ross Sea focused on acquiring a high-resolution database designed to permit (1) reconstruction of the maximum extent and configuration of the ice sheet during the Last Glacial Maximum (i.e., oxygen isotope stage 2); (2) reconstruction of conditions at the base of the ice sheet; and (3) assessment of the relative retreat history of the ice sheet following the Last Glacial Maximum. Five seismic facies are distinguished on the basis of external geometry, relationships among features, bounding surface amplitude, intensity of internal acoustic signature, and geometry of internal reflectors. Seismic facies 1 is a transparent draping unit correlated to diatomaceous mud, interpreted as being deposited under open-marine conditions. Seismic facies 2 and 3 are comprised of units displaying subdued massive internal signatures with smooth lower bounding surfaces and hummocky upper surfaces characterized by glacial lineations. These units are interpreted to be grounding-zone proximal deposits overridden by the expanded ice sheet. Seismic facies 2 may be a deforming-till unit. Seismic facies 4a is characterized by an internally massive to chaotic signature, an erosional, often flat, lower surface, and a hummocky upper surface displaying glacial lineations. This unit is interpreted to be till and is divided into deposits associated with the most recent glacial expansion (4a) and with an earlier (pre–Last Glacial Maximum) expansion (4b). Seismic facies 5 is an acoustically laminated, ponded, and draping deposit interpreted to contain proglacial and sub–ice-shelf materials deposited continuously since the last interglacial period. The associations of these units provide context for the interpretation of the ice-edge maximum position, conditions at the base of the ice sheet, and the relative retreat history of the region. There is compelling evidence for a much-expanded ice sheet in the Ross Sea during the Last Glacial Maximum. In the western Ross Sea, the maximum grounding position is marked by an isolated grounding-zone wedge, and placed in the vicinity of Coulman Island, approximately 150 km from the continental-shelf edge. In the central Ross Sea, the maximum grounding position is close to the continental-shelf break, based primarily on the presence of an extensive 60-m-thick sheet-like deposit with a fluted upper surface. Streaming ice may have occupied bathymetric lows on the continental shelf, as suggested by (1) the configuration of bathymetry; (2) the presence of glacially eroded troughs; (3) the concentration of sediment (interpreted to be deforming till) within the middle to outer shelf reaches of the troughs; and (4) the fluted nature of the upper surface. Absolute rates of streaming ice flow relative to inter-ice-stream areas are not implied. The western Ross Sea continental-shelf deposits record the retreat history of ice derived predominantly from the East Antarctic Ice Sheet and glaciers of the Transantarctic Mountains. Ice flow on the continental shelf is interpreted to have remained fixed in position, restricted by the walls of the troughs. On the inner shelf of the western Ross Sea, the ice flowed over and eroded lithified sedimentary strata. Slower-moving ice occupied flat-topped banks. A single grounding-zone wedge occurs on the western Ross Sea central shelf; no substantial deposits are observed on the inner continental shelf. This lack of grounding-zone features reflects a restricted sediment supply and a relatively steady, rapid retreat of the ice sheet. During retreat, ice flow acted independently in each major trough and on the bank tops. Ice retreated from Victoria Land Basin–Drygalski Trough before it retreated from JOIDES Basin. Ice remained on the bank tops, shedding material into the abandoned troughs. Grounded ice in the central Ross Sea was derived predominantly from an expanded West Antarctic Ice Sheet. The ice sheet remained grounded on the continental-shelf edge after retreat of ice from the western Ross Sea. Expanded ice eroded the inner shelf and deposited sheets of till on the central and outer shelf. Ice-stream drainage shifted laterally, as recorded by rounded, laterally accreting ridges that separate bathymetric troughs. The central Ross Sea is the repository for larger volumes of sediment, derived from ice flow across basins of relatively thick, unlithified sedimentary deposits. Two till sheets mark grounding-zone positions in the central Ross Sea, reflecting the higher sediment supply and a stepped deglaciation. Ice remained grounded on the Pennell Bank to the west; a series of moraines marks the retreat of ice across the bank. Mega-scale glacial lineations and other streamlined, subglacial geomorphic features occur across the continental shelf, primarily within the troughs. The lineations substantiate the maximum reconstruction and support the interpretation of a deforming substrate beneath the outer reaches of the expanded ice sheet. This deforming substrate may have contributed to the onset of deglaciation. Features associated with meltwater are rare or absent, suggesting that basal meltwater played a minor role in retreat of the ice sheet.

Late Pleistocene–Holocene retreat of the West Antarctic Ice-Sheet system in the Ross Sea: Part 2—Sedimentologic and stratigraphic signature

Abstract: Sedimentologic, geotechnical, geochemical, and accelerator mass spectrometer (AMS) radiocarbon data from two marine geologic cruises in the Ross Sea have allowed us to constrain facies relationships and temporal changes in the West Antarctic Ice Sheet. The selection of core sites was facilitated by the use of multibeam bottom imagery and a good quality (CHIRP) subbottom reflection system. A complex but consistent succession of facies documents a number of environments from subglacial to open marine. Massive, mud-rich diamictons have low water contents, contain (in places) a calcareous microfossil assemblage, show minimal textural variation, and contain low and uniform total organic carbon values. This reflects a subglacial setting. This unit passes upward into a stratified, thin, granulated facies consisting of pelletized, sandy, muddy gravel that is loosely compacted and contains a variable water content and concentrated horizons of pebble-sized clasts. This facies reflects the lift-off zone or thin water film between the basal debris and sea floor. Overlying this unit are silty clays that contain a well-sorted, very fine-grained sand component. There are no coarse grains within this unit. This facies is, in part, laminated and reflects deposition beneath an ice shelf, near and away from the grounding-line zone. The ice-shelf facies passes upward into a siliceous mud and ooze unit that represents deposition in an open-marine setting. A sandy, volcaniclastic-rich subfacies marks the transition from ice shelf to open-marine environments found at the calving line. The 86 AMS radiocarbon dates on organic matter provide an accurate chronology for 19 cores. Ice-shelf conditions were established in the outer Drygalski Trough by 11 ± 0.25 ka and perhaps earlier. This transition took place in the JOIDES Basin by 10–8 ka. The calving front of the Ross Ice Shelf passed over the Drygalski Trough at 74°S by 9.5 ± 0.25 ka. The timing of deglaciation in the Ross Sea calls into question current models for the contribution of Antarctic glacial ice to Holocene sea-level rise and suggests that recession was relatively gradual and more closely aligned with Northern Hemisphere deglaciation and its associated eustatic pulse.

Debris-flow deposition: Effects of pore-fluid pressure and friction concentrated at flow margins

Abstract: Measurements of pore-fluid pressure and total bed-normal stress at the base of several ~10 m 3 experimental debris flows provide new insight into the process of debris-flow deposition. Pore-fluid pressures nearly sufficient to cause liquefaction were developed and maintained during flow mobilization and acceleration, persisted in debris-flow interiors during flow deceleration and deposition, and dissipated significantly only during postdepositional sediment consolidation. In contrast, leading edges of debris flows exhibited little or no positive pore-fluid pressure. Deposition therefore resulted from grain-contact friction and bed friction concentrated at flow margins. This finding contradicts models that invoke widespread decay of excess pore-fluid pressure, uniform viscoplastic yield strength, or pervasive grain-collision stresses to explain debris-flow deposition. Furthermore, the finding demonstrates that deposit thickness cannot be used to infer the strength of flowing debris.

Structure and petrology of Tauride ophiolites and mafic dike intrusions (Turkey): Implications for the Neotethyan ocean

Abstract: Cretaceous Neotethyan ophiolites occur in four east-west–trending subparallel zones within the Tauride tectonic belt in southern Turkey The ophiolites of the Inner, Intermediate, and Outer zones tectonically overlie the Mesozoic platform carbonates of the Tauride belt and are commonly underlain by a Cenomanian ophiolitic melange These ophiolites consist mainly of tectonized mantle rocks, mafic-ultramafic cumulates, and gabbros, and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence Metamorphic soles that are several hundred meters thick occur as thrust-faulted slices beneath these ophiolites and show well-developed metamorphic field gradients Ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit Dike rocks are made of subalkalic basalt to andesite typical of evolved island-arc tholeiites; they display large compositional variations, with SiO 2 content between 50 and 60 wt% and MgO between 8 and 4 wt%, and contain higher Ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates The majority of the analyzed dike rocks show a slight depletion in light rare earth elements (REE) with low La/SmN ratios and are depleted in both high-field strength (HFS) and heavy REEs, while enriched in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalt (MORB) These characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by LILE- and light REE-enriched fluids Geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window, in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field, or in a transitional zone between the garnet and spinel stability fields at about 60 km depth This asthenospheric window was probably created during subduction of a Neotethyan ridge system; magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere The new 40 Ar/ 39 Ar hornblende dates of 92–90 Ma and 90–91 Ma from the metamorphic soles and dike swarms, respectively, show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the Inner Tauride seaway These data suggest that the Tauride ophiolites within the three zones to the north originated from the same root zone situated north of the Tauride carbonate platform, and that they constitute remnants of a single ophiolitic nappe sheet derived from the Inner Tauride seaway within the Neotethyan ocean

Carboniferous isotope stratigraphies of North America: Implications for Carboniferous paleoceanography and Mississippian glaciation

Abstract: We present detailed isotope stratigraphies for Carboniferous time based on brachiopod shell calcite from the midcontinent region of North America. Evidence for shell calcite preservation includes (1) preservation of shell microstructure, (2) lack of cathodoluminescence, (3) low Si, Al, Fe, and Mn contents, (4) Na, Sr, and S contents comparable to those of modern brachiopod shells, and (5) δ13C and δ18O values higher than those of associated cements and matrix. The Carboniferous δ13C record for North America is characterized by three isotopic stages. The earliest stage, C1, follows a 2.0‰ increase in Kinderhookian time (early Tournaisian), from 1.5‰ to 3.5‰, and includes a brief and perhaps local late Kinderhookian excursion to 5.4‰. The δ13C values remain stable at 3.5‰ to 4‰ during stage C1, then decrease about 1‰ near the Meramecian-Chesterian boundary (Visean) to 2‰–3‰ (stage C2). Stage C2 ends with a 1‰–2‰ increase (C2-C3 transition) between middle Chesterian and early Morrowan time (Serpukhovian-Bashkirian). Stage C3 values remain mostly between 3‰ and 4.5‰ upsection to Virgilian strata (Gzhelian). Increases in δ13C probably reflect global increases in sedimentary organic carbon burial and suggest that rho CO2 declines in the earliest and middle Carboniferous strata. The middle Carboniferous δ13C shift of B. Popp, T. Anderson, and P. Sandberg, an ∼3‰ increase in European sections, occurs in North America (C2-C3 transition) but is limited to ∼1.5‰. This 1.5‰ increase was probably caused by increased organic carbon burial, whereas the additional ∼1.5‰ shift in European sections likely reflects changes in ocean circulation patterns associated with the closing of the Equatorial seaway. Based on the timing of the δ13C divergence between North America and Europe, the isolation of the Paleotethys began in late Chesterian time (Serpukhovian). The δ18O record can also be separated into the three stages. There is a 3‰ increase during Kinderhookian-Osagean time (Tournaisian), corresponding to the Devonian to Carboniferous transition to stage C1, a 3‰ decrease during Meramecian–early Chesterian time (Visean; C1-C2 transition), then a 2‰ increase in late Chesterian–early Morrowan time (Serpukhovian-Bashkirian; C2-C3 transition). The δ18O values then fluctuate between −1‰ and −3‰ (C3 stage) upsection to the Virgilian strata (Gzhelian). If global, the 2‰ to 3‰ δ18O shifts are compelling evidence for cooling and glaciation in Early Mississippian time, warming and deglaciation in Late Mississippian time, and a return to cool, glacial conditions in earliest Pennsylvanian time. The general correlation between δ13C and δ18O shifts suggests that cooling is associated with drawdown of atmospheric CO2.

Tertiary deformation history of southeastern and southwestern Tibet during the Indo-Asian collision

Abstract: Geologic mapping and geochronological analysis in southwest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary crustal shortening along the classic Indus-Tsangpo suture in the Yalu River valley. The older event occurred between ca. 30 and 24 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused extensive denudation of the Gangdese batholith in its hanging wall and underthrusting of the Xigaze forearc strata in its footwall. Examination of timing of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initiation and development of north-south shortening in the eastern Kunlun Shan of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tagh fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the Eurasian plate and the Indian and Pacific plates. The younger thrusting event along the Yalu River valley occurred between 19 and 10 Ma along the south-dipping Great Counter thrust system, equivalent to the locally named Renbu-Zedong thrust in southeastern Tibet, the Backthrust system in south- central Tibet, and the South Kailas thrust in southwest Tibet. The coeval development of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal weakening of the north Himalayan and south Tibetan crust, due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya.

Controls on the geometry of fold-thrust belt salients

Abstract: Salients, convex-to-the-foreland curves in the map traces of orogens, typically involve fold-thrust belts formed along the foreland margin of orogens. An examination of the map-view shape, degree of asymmetry, dimensions, and structural trend-line patterns (i.e., the variation in fold trend and fault strike along the length of the belt) of 20 fold-thrust belt salients worldwide illustrates relationships between salient geometry and the geologic setting in which the salient formed. In order to understand these relationships, we used simple sandbox models to simulate the formation of three of the several types of salients. Specifically, we examined indenter-controlled salients (the geometry of which reflects interaction of a fold-thrust belt with a hinterland indenter), basin-controlled salients (the geometry of which reflects lateral variations in the thickness of predeformational basin fill), and detachment-controlled salients (the geometry of which reflects lateral variations in the strength of a detachment surface). Our models demonstrate that thrusts in these three types of salients initiate with a curved shape, and that rocks within thrust sheets on the limbs of salients can undergo a vertical-axis rotation in response to simple shear, without requiring major rotation of the fault trace. Furthermore, trend-line patterns distinguish basin-controlled from indenter-controlled salients. Specifically, structural trend lines in basin-controlled salients converge at the end points of the curve, while in indenter-controlled salients, they diverge at the end points of the curve. The asymmetry of a salient correlates with the asymmetry of the predeformational basin and/or of the indenter, and with the convergence direction. Thus, our results explain geometric characteristics of individual salients, and address controversies concerning paleomagnetic data in salients. They also may help prioritize petroleum-exploration targets in salients (foreland-basin oil fields concentrate at the apex of basin-controlled salients) because the geologic factors that generate the salient also contribute to localizing oil fields.

Postapocalyptic greenhouse paleoclimate revealed by earliest Triassic paleosols in the Sydney Basin, Australia

Abstract: The Permian-Triassic boundary in the Sydney Basin of Australia is coincident with a pronounced decrease in δ 13 C isotopic values of organic carbon, the last coals anywhere in the world for all of the Early Triassic (ca. 6 m.y.) time, and extirpation of the Glossopteris flora at the top of the Illawarra and Newcastle Coal Measures. Coal-bearing paleosols of the latest Permian represent extensive swamplands of the seasonally deciduous Glossopteris flora in a humid cold temperate lowland southwest of an Andean-style volcanic arc. Stone-rolls (large ribs of floor rock up into the coal) within some of the uppermost coals of the Permian coal measures can be interpreted as string bogs of a kind now found in cold climates at latitudes of 68°–70°, which is compatible with a paleomagnetically estimated paleolatitude of 65°–85°S for the Sydney Basin. Paleolatitude was not much different for earliest Triassic time, but paleosols of that age include Inceptisols and Entisols showing substantial chemical and textural weathering, more like soils now forming at latitudes of 40°–58° than those within polar circles. This anomalous high-latitude warmth set in at the Permian-Triassic boundary. Sedimentation rates increased at the boundary marked by geochemically unusual acidification of clay and a dramatic carbon isotopic excursion. These changes in environments and ecosystems can be explained by soil erosion following deforestation implied by the plant extinctions and abundant fungal remains at the Permian-Triassic boundary. Evidence from paleosols can now be added to that from paleontological and isotopic studies showing that disruption of the carbon cycle at the Permian-Triassic boundary resulted in a CO 2 or CH 4 post-apocalyptic greenhouse paleoclimate.

Paleomagnetic polarity reversals in Marinoan (ca. 600 Ma) glacial deposits of Australia: Implications for the duration of low-latitude glaciation in Neoproterozoic time

Abstract: A paleomagnetic investigation of Marinoan glacial and preglacial deposits in Australia was conducted to reevaluate Australia’s paleogeographic position at the time of glaciation (ca. 610‐575 Ma). The paleomagnetic results from the Elatina Formation of the central Flinders Ranges yield the first positive regionalscale fold test (significant at the 99% level), as well as at least three magnetic polarity intervals. Stratigraphic discontinuities typical of glacial successions prevent the application of a magnetic polarity stratigraphy to regional correlation, but the positive fold test and multiple reversals confirm the previous low paleolatitude interpretation of these rocks (mean D = 214.9°, I = ‐14.7°, α 95 = 12.7°, paleolatitude = 7.5°). The underlying preglacial Yaltipena Formation also carries low magnetic inclinations (mean D = 204.0°, I = ‐16.4°, α 95 = 11.0°, paleolatitude = 8.4°), suggesting that Australia was located at low paleolatitude at the onset of glaciation. The number of magnetic polarity intervals present within the Elatina Formation and the Elatina’s lithostratigraphic relationship to other Marinoan glacial deposits suggest that glaciation persisted at low latitudes in Australia for a minimum of several hundreds of thousands to millions of years.

Sediment-transporting flows in headwater streams

Abstract: The equilibrium alluvial stream channel has a geometry that allows it to pass the water and sediment supplied from the watershed. At the same time, the equilibrium alluvial channel is built and maintained by the flows and sediment delivered to it. A prerequisite for understanding the creation of the equilibrium channel is an understanding of the sediment conveyance and competence of the flows the channel receives. This study describes the bed-load transport regime as it is linked to hydrology and geomorphology in 23 headwater gravel-bed streams in snowmelt-dominated parts of central and northern Idaho. At sites, drainage areas range from 1.29 to 381 km 2 , stream gradients range from 0.0042 to 0.0747, and median bed surface particle sizes range from 4 to 207 mm. Stream architecture includes riffle-pool, planar, and step-pool beds. The bed load is much finer than the surface and subsurface material, suggesting selective transport of the finer sizes. Nonetheless, the majority of the load is sand at all flow discharges. Progressively coarser sediment was collected as flow discharge increased, and painted rock experiments documented the transport of coarser particles at higher discharges. The supply of sediment to the streams appears limited, as indicated by observed clockwise hysteresis in bed-load transport rates during each spring snowmelt and by the coarse surface armor observed at sites. Flows above bankfull discharge move 37% of the bed load, whereas flows between mean annual discharge and bankfull move 57% of the bed load. The bed-load effective discharge has a recurrence interval that averages 1.4 yr and the magnitude of effective discharge averages 80% of bankfull discharge. The recurrence interval of bankfull discharge averages 2.0 yr. The ratio of effective discharge to bankfull discharge is independent of basin size, grain size, and gradient, although the ratio increases with the relative magnitude of large infrequent events.

Synorogenic extension: Quantitative constraints on the age and displacement of the Zanskar shear zone (northwest Himalaya)

Abstract: Structural, thermobarometric, and geochronological data place limits on the age and tectonic displacement along the Zanskar shear zone, a major north-dipping synorogenic extensional structure separating the high-grade metamorphic sequence of the High Himalayan Crystalline Sequence from the overlying low-grade sedimentary rocks of the Tethyan Himalaya. A complete Barrovian metamorphic succession, from kyanite to biotite zone mineral assemblages, occurs within the 1-km-thick Zanskar shear zone. Thermobarometric data indicate a difference in equilibration depths of 12 ± 3 km between the lower kyanite zone and the garnet zone, which is interpreted as a minimum estimate for the finite vertical displacement accommodated by the Zanskar shear zone. For the present-day dip of the structure (20°), a simple geometrical model shows that a net slip of 35 ± 9 km is required to regroup these samples to the same structural level. Because the kyanite to garnet zone rocks represent only part of the Zanskar shear zone, and because its original dip may have been less than the present-day dip, these estimates for the finite displacement represent minimum values. Field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the Zanskar shear zone occurred as a continuous process starting at the Barrovian metamorphic peak and lasting throughout the subsequent extension-induced exhumation. Geochronological dating of various leucogranitic plutons and dikes in the Zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19.8 Ma and that extension most likely initiated shortly before 22.2 Ma.

Variation of Cenozoic extension and volcanism across the southern Sierra Madre Occidental volcanic province, Mexico

Abstract: The middle to late Cenozoic tectonic-magmatic evolution of the Sierra Madre Occidental volcanic province south of the Tropic of Cancer is summarized and analyzed for the first time, based on new geologic and structural work and published information. In the eastern part of the study region (Mesa central physiographic province) silicic volcanism occurred in a short-lived episode culminating at ca. 30 Ma and was followed by crustal-scale extension between 30 and 27 Ma. In the western part of the study area (Sierra Madre Occidental physiographic province) a voluminous episode of ignimbrite volcanism at 24‐21 Ma was succeeded by east-west extension that produced regularly spaced grabens affecting only the upper crust. In the westernmost part of the study region, an andesitic to rhyolitic arc, formed between 17 and 12 Ma, was affected by crustal-scale, north-northwest‐trending, extensional faulting, leading to the formation of the Gulf of California. In the Mesa central the maximum extension was oriented approximately east-west and amounted to ~20%. In the eastern Sierra Madre Occidental physiographic province extension was only 8% and oriented approximately east-west. We observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late Oligocene, early Miocene, and late Miocene time. Comparison with the offshore tectonics indicates that the first two tectonic-magmatic pulses coincide with periods of fast spreading at the Pacific-Farallon boundary, south of the Shirley fracture zone. We propose that increases in the spreading rate are related to periods of high subduction rate, which in turn correspond to episodes of retreating subduction. A retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge, producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate. Boundary conditions (i.e., plate tectonics) ultimately determined timing, magnitude, and orientation of extension, whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism.

A comparison of fluvial megafans in the Cordilleran (Upper Cretaceous) and modern Himalayan foreland basin systems

Abstract: The Campanian–Maastrichtian Hams Fork Conglomerate Member of the Evanston Formation in northeastern Utah and southwestern Wyoming consists of a widespread (>10 000 km 2 ) boulder to pebble, quartzitic conglomerate that was deposited by east-southeastward–flowing, gravelly braided rivers on top of the frontal part of the Sevier fold-thrust belt and in the adjacent foredeep of the Cordilleran foreland basin. In northeastern Utah the conglomerate was deposited in a lobate fan-shaped body, up to 122 m thick, that trends southeastward away from its principal source terrane in the southern end of the Willard thrust sheet. The Willard sheet contains thick Proterozoic quartzite units that produced highly durable clasts capable of surviving long-distance fluvial transport. Although the main source of sediment for the Hams Fork Conglomerate was the Willard sheet, the active front of the thrust belt lay 40–50 km to the east along the Absaroka thrust system. Displacement along the Absaroka system uplifted and topographically rejuvenated the Willard sheet, and antecedent drainages carried detritus from hinterland source terranes into the proximal foreland basin. Although topographic ridges associated with fault-propagation anticlines along frontal thrusts locally influenced transport directions, they provided relatively little sediment to the Hams Fork Conglomerate. Lithofacies, paleocurrent, and isopach data indicate that the Hams Fork Conglomerate was deposited in fluvial megafans and stream-dominated alluvial fans, similar in scale and processes to megafans and alluvial fans in southern Nepal and northern India that are forming along the proximal side of the Himalayan foreland basin system. The Himalayan fluvial megafans have areas of 10 3 –10 4 km 2 , slopes of 0.05°–0.18°, and are deposited by large transverse rivers that are antecedent to frontal Himalayan structures and topography. The main fluvial channels on the upper parts of the megafans are anastomosed and braided at bankfull stage but commonly have braided thalwegs at low-flow stage. Downstream, these channels become predominantly braided and meandering and ultimately merge with the axial Ganges trunk river system. Stream-dominated alluvial fans in the Himalayan foreland basin system fringe the topographic front of the fold-thrust belt in the intermegafan areas. These fans have areas of ∼10 2 km 2 and slopes of ∼0.5°. The proximal parts of both types of fans are dominated by extremely coarse (boulder-cobble) bedload that is in transit mainly during the monsoon. The prevalence of fluvial megafans in the modern and Miocene Himalayan foreland and in the Upper Cretaceous–lower Tertiary stratigraphic record of the Cordilleran foreland suggests that these types of deposits may be the volumetrically largest gravel accumulations in nonmarine foreland basin systems.

Rapid Miocene slip on the Snake Range–Deep Creek Range fault system, east-central Nevada

Abstract: New fission-track data together with 1:24 000-scale geologic mapping and analysis of Tertiary sedimentary deposits provide better constraints on the time and nature of motion along the Snake Range decollement, a classic Basin and Range metamorphic core complex detachment fault in east-central Nevada. Here, the fission-track method provides a particularly effective tool for dating faulting where bracketing or crosscutting relations are not available. These new data suggest that the Snake Range decollement forms part of a more extensive, 150-km-long north-south–trending fault system, the Snake Range–Deep Creek Range fault system. This fault system extends along the eastern flank of the northern and southern Snake Range, Kern Mountains, and Deep Creek Range, and accommodated at least 12–15 km of rapid slip in the Miocene, ca. 17 Ma. This component of motion is distinctly younger (by about 15–20 m.y.) than an earlier episode of slip and extension across the region bracketed stratigraphically and geochronologically as late Eocene–early Oligocene age. Apatite fission-track ages (n = 57) in most parts of the Snake Range and adjacent ranges cluster at 17 Ma, indicating rapid cooling from >125 to 310 to <50 °C. Formation of at least part of the pervasive mylonitic fabrics in the northern Snake Range may have occurred during this Miocene time interval, very late rather than early in the extensional history of the region. Coarse fanglomerate and rock-avalanche deposits in flanking Tertiary basins provide additional evidence for major tectonism at this time. Comparison of the timing of events in the northern Snake Range to that along strike of the fault system indicates that Miocene slip along the low-angle northern Snake Range decollement and exhumation of extensive footwall mylonites were coeval with more typical Basin and Range high-angle rotational faulting in the Deep Creek Range and Kern Mountains to the north and in the southern Snake Range to the south. This suggests that the two styles of faulting (low-angle detachment and high-angle rotational) can occur simultaneously along the length of a single normal fault system. Data from the northern Snake Range also underscore the importance of a vertical component of uplift of the range in Miocene time, leading to the present domal geometry of the northern Snake Range decollement. When considered together with footwall deformational fabrics, the new data are most simply explained as the consequence of higher local geothermal gradients and a shallower brittle-ductile transition zone along the northern Snake Range part of the fault system. It can be speculated that the Snake Range metamorphic core complex represents the top of a stretching welt of hotter, deeper level crust that rose during extension. This rising welt may have been localized by the presence of previously thickened crust beneath the region and could have been triggered by increased regional magmatism and heating accompanying rapid extension in Miocene time.

Evolution of the Minle and Chaoshui Basins, China: Implications for Mesozoic strike-slip basin formation in Central Asia

Abstract: The Minle and Chaoshui Basins of northern China are pull-apart basins that originated during Jurassic to Early Cretaceous strike-slip and extensional deformation in the western North China block (Alashan), and the region to its southwest, known as the Hexi Corridor. The basins are separated by the Longshou Shan, which has been a positive topographic feature since Jurassic time. Lower Cretaceous successions in the Minle and Chaoshui Basins display similar rift-related, fining-upward motifs. These comprise alluvial conglomerates and sandstones that pass upward into lacustrine mudrocks, followed by a final fine-grained alluvial phase with extensive paleosol development. The upper part of each succession includes a second lacustrine interval, which is related either to a short-lived humid climatic period, or downstream damming of the exit for the fluvial systems of both basins. North-south–trending normal faults and rare synsedimentary rotation and slump structures affect the basin fills. The Minle and Chaoshui Basins are only two of a series of Late Jurassic–Early Cretaceous strike-slip–related basins in this region that nucleated on three basement blocks, Tarim, Qilian Shan, and Alashan. All of the basins are characterized by a nonmarine, alluvial-lacustrine fill, controlled by a combination of strike-slip and normal faults. From an analysis of the distribution, orientation, and motion sense of subsurface and exposed faults we conclude that three types of strike-slip–related basins are present: pull-apart basins (Minle, Huahai-Jinta, Chaoshui), transtensional basins (Southwest Badanjilin, northern Wuwei), and a third type that developed between divergent strike-slip faults of opposing motion senses. We term this type “extrusion fault-wedge basins” (Jiuquan, Bayanhaote, western Wuwei). The cause of Late Jurassic to Early Cretaceous strike-slip and extension in this region is interpreted to be the Lhasa block–Asia collision. Compression arising from this event is speculated to have caused eastward extrusion of the crust of the Hexi Corridor and Alashan. Deformation was accommodated by a 25° counterclockwise rotation of the Alashan with respect to the remainder of the North China block; thrusting occurred in the intervening Helan Shan. The extension in the Hexi Corridor occurred at the same time as vast foreland basins formed to the west (Tarim) and east (Ordos), and while extension affected much of eastern Mongolia and northeastern China.

Basin analysis of the Jurassic–Lower Cretaceous southwest Tarim basin, northwest China

Abstract: The Jurassic through Cretaceous southwest Tarim basin, northwest China, contains more than 6 km of fluvial and lacustrine strata deposited in a foreland setting during the successive collisions with Eurasia of the Changtang block during Late Triassic–Early Jurassic time and with the mega-Lhasa block during Late Jurassic–Early Cretaceous time. This tectonism is chronologically linked with the creation of a narrow lower Middle Jurassic transtensional basin with thick sedimentary infill, succeeded by a broader Upper Jurassic–Lower Cretaceous compressional(?) basin with thinner sedimentary infill. The older basin formed between a north-northwest–striking dextral fault on the eastern side of the southwest Tarim basin in the Tian Shan and a postulated strike-slip or normal fault on the western margin of the basin along the Kunlun Shan. The former fault is now the Talas-Ferghana fault; the latter may be a predecessor to the Main Pamir thrust. Subsidence analysis of the thickest sedimentary section suggests thermal subsidence, interpreted as the result of transtension between the two basin-bounding faults. The younger basin extends farther east and west and does not preserve evidence of activity along the Talas-Ferghana fault. The change in basin style between these two episodes of basin development likely reflects either a small counterclockwise rotation of basin-bounding structures during the first episode or a small clockwise rotation of the maximum compressive stress between the two episodes.

Late Quaternary loess in northeastern Colorado: Part I—Age and paleoclimatic significance

Abstract: Loess in eastern Colorado covers an estimated 14 000 km 2 , and is the westernmost part of the North American midcontinent loess province. Stratigraphic studies indicate there were two periods of loess deposition in eastern Colorado during late Quaternary time. The first period spanned ca. 20 000 to 12 000 14 C yr B.P. (ca. 20‐14 ka) and correlates reasonably well with the culmination and retreat of Pinedale glaciers in the Colorado Front Range during the last glacial maximum. The second period of loess deposition occurred between ca. 11000 and 9000 14 C yr B.P. This interval may be Holocene or may correlate with a hypothesized Younger Dryas glacial advance in the Colorado Front Range. Sedimentologic, mineralogic, and geochemical data indicate that as many as three sources could have supplied loess in eastern Colorado. These sources include glaciogenic silt (derived from the Colorado Front Range) and two bedrock sources, volcaniclastic silt from the White River Group, and clays from the Pierre Shale. The sediment sources imply a generally westerly paleowind during the last glacial maximum. New carbon isotope data, combined with published faunal data, indicate that the loess was probably deposited on a cool steppe, implying a last glacial maximum July temperature depression, relative to the present, of at least 5‐6 °C. Overall, loess deposition in eastern Colorado occurred mostly toward the end of the last glacial maximum, under cooler and drier conditions, with generally westerly winds from more than one source.

Ice-sheet numerical modeling and marine geophysical measurements of glacier-derived sedimentation on the Eurasian Arctic continental margins

Abstract: Long-range side-scan sonar images of the Barents Sea continental margin have been analyzed in conjunction with results from previous geophysical investigations to determine a qualitative model for sedimentation over the Bear Island and Storfjorden trough mouth fans. These data indicate that gravity-driven debris flows are major processes in the downslope transport of glacial material, delivered to the shelf break when ice sheets advanced across the continental shelf. During late Weichselian time, ∼4000 km 3 of sediments were deposited over the Bear Island fan (280 000 km 2 ) while ∼700 km 3 of sediments were deposited over the Storfjorden fan (40 000 km 2 ). A numerical ice-sheet model, including sediment deformation and transport beneath ice streams, reconstructs the glacial conditions required to transport large volumes of sediment to the late Weichselian Eurasian continental margin. Model results indicate that glaciation of the Eurasian High Arctic occurred after 28 ka, and that ice streams within bathymetric troughs were active by ca. 25 ka. The maximum ice-sheet thickness over the Barents Sea was about 1400 m; there was a secondary dome −1 (0.13 cm yr −1 averaged over the fan); the rate was 6 cm yr −1 (equivalent to 0.6 cm yr −1 over the fan) over the Storfjorden trough mouth. The modeled sediment volume at the continental margin of the Bear Island and Storfjorden troughs agrees well with the volumes of late Weichselian sediment inferred from seismic records from these large prograding submarine fans. Sensitivity experiments show that adjustments to model environmental inputs do not significantly affect the results.

Distinguishing subglacial till and glacial marine diamictons in the western Ross Sea, Antarctica: Implications for a last glacial maximum grounding line

Abstract: Analyses of lithology, stratigraphy, and tephra from marine sediment cores collected from the western Ross Sea during cruises Eltanin 32 and 52 and Deep Freeze 80 and 87 indicate that subglacial till does not extend to the continental shelf edge. Subglacial till occurs as the lowest unit in most cores landward (south) of approximately 74°S, while seaward of approximately 74°S, the lowest diamicton units are glacial marine diamictons. Glacial marine diamictons are distinguished from subglacial tills by the presence of higher and more variable total organic carbon content downcore, distinct tephra layers, stratification, higher diatom and foraminifera abundances, higher sand content, and radiocarbon dates in chronological order downcore. Sand-sized tephra layers from two cores on the outer continental shelf are interpreted as single eruptive events, one likely to have been derived from the Mount Melbourne volcano and the other from the Pleiades volcano. Radiocarbon dates from sediment above and below the tephra layer in one of these cores (Df87-32) show that deposition indicative of open-water conditions occurred between 22 and 26 ka in the western Ross Sea.

Paleoclimatology and provenance of the glaciogenic Gowganda Formation (Paleoproterozoic), Ontario, Canada: A chemostratigraphic approach

Abstract: The Gowganda Formation (dated as about 2.3 Ga) is widely (but not universally) interpreted as glaciogenic, and our chemostratigraphic study supports such an interpretation. The bulk compositions of diamictite matrix materials and associated argillites are considered to have resulted from the mixture of two detrital components; one is glacial flour, similar in composition to the unweathered Archean upper crust, whereas the other is chemically weathered detritus resembling average Proterozoic shale in composition. The first component has a chemical index of alteration (CIA) value of ∼50, reflecting the abundance of feldspar and dearth of aluminous clay minerals. The second component has a CIA value of ∼70 and was produced primarily in weathering profiles by chemical weathering, typical of temperate climatic regimes. Diamictite matrix materials (92 samples) have an average CIA value of 57. The argillites have higher values; the average is 62 (97 samples). The lower CIA values of the diamictites reflect a higher proportion of glacial flour, suggesting a stronger influence of frigid climatic conditions than that which prevailed during deposition of the argillites. Systematic trends in CIA values are developed across diamictite-argillite boundaries and within a thick argillite between two major diamictite units. These values reflect climatic variations, with maximum amelioration during deposition of the middle section of the argillite. Rare earth element data and Th/Sc and Ti/Al ratios from the diamictite matrix materials all suggest a provenance that included a large supracrustal component (∼45%), together with lesser amounts of tonalite-trondhjemite-granite and granite. The proportion of supracrustal materials such as volcanic rocks and shales may have been exaggerated by selective comminution during glacial transport. The matrix materials of the Gowganda diamictites are severely depleted in Ca and proportionally enriched in Na, relative to the source. The anorthitic component of plagioclase has been selectively replaced by an albitic component during diagenesis and metamorphism. A chemostratigraphic approach provides high-resolution data for reconstruction of paleoclimatic conditions and valuable information concerning provenance and metasomatic processes.

Channel narrowing by vertical accretion along the Green River near Green River, Utah

Abstract: The Green River is the longest tributary of the Colorado River. Near the town of Green River, Utah, the Green River narrowed in two discrete phases during the twentieth century. The first phase of narrowing decreased average width by about 5% and occurred between about 1930 and 1940, when the magnitude of 2-yr flood, mean annual discharge, and effective discharge decreased by about 30%, 28%, and 37%, respectively. During this first phase of narrowing, saltcedar ( Tamarisk spp.), an invading non-native tree, began to establish itself in the study area, but botanists of that time did not describe the tree as abundant. Channel width was stable in the 1940s and 1950s even though saltcedar were becoming already abundant on the river9s banks. Further narrowing of an additional 14% occurred after 1959. This latest period of narrowing began following three successive years when the magnitude of floods was less than the present 1.5-yr recurrence flood and when saltcedar were already abundant along the river. The deposits that comprise the banks of the narrowing Green River are composed of the suspended load of the river, and these alluvial deposits are characterized by horizontal layers, which indicate that they formed by vertical accretion. A mechanism is proposed to explain the coarsening-upward sequence of beds found in these vertically accreted deposits. These changes in the channel of the Green River are based on analysis of more than 2600 discharge measurements made by the U.S. Geological Survey, resurvey of an abandoned measurement site, matches of historical ground-level photography, and analysis of historical aerial photography within a geographic information system. We have developed analytical techniques that permit analysis of width data from U.S. Geological Survey discharge measurements where gaging cross sections have adjustable beds and banks. These techniques allow the spatially rich but temporally poor data from aerial photographs to be supplemented with gaging station data, which add great detail about the timing and actual processes of channel narrowing that cannot be determined from aerial photographs alone. Such an analytical strategy provides a more complete record of historical channel adjustment than can be obtained by other means.

U/Pb dating of detrital zircons: Implications for the provenance record of Gondwana margin terranes

Abstract: SHRIMP U/Pb age data for more than 300 detrital zircons from late Mesozoic samples of the Torlesse and Waipapa arc-trench terranes in New Zealand range from ca 100 Ma (Early Cretaceous) to 3140 Ma (Archean) More than 65% of the analyzed zircon grains are Permian or Mesozoic age The remaining detritus is largely of Paleozoic age with progressively smaller amounts of Proterozoic and Archean debris Cathodoluminescence imaging indicates that the younger grains are exclusively of igneous origin, whereas the older grains show evidence for a more complex history including metamorphic overprints and inherited cores The youngest zircon grains in most of the samples approximate the age of deposition of the rock units, suggesting input into the depositional basins from contemporaneous igneous activity The overall age profile of the detrital zircons is consistent with sediment accumulation adjacent to the Gondwana margin rather than in exotic blocks accreted to the margin The bulk of the detritus is derived from a late Paleozoic to Mesozoic Gondwana margin, Andean-style magmatic arc Elements of this arc extend from Marie Byrd Land in Antarctica, through New Zealand (Median tectonic zone) to New England in eastern Australia Paleozoic and older grains form a minor but significant component of all samples and have an age signature indicative of derivation from the Paleozoic and Neoproterozoic fold belts of East Australia and Antarctica (Gondwana) A characteristic feature of the older grains is ages in the range 500–650 and 1000–1200 Ma, which is also a feature of the zircon age spectrum for early Paleozoic graywackes from the Lachlan-Tuhua fold belt, suggesting derivation from these sedimentary rocks or from the same original source rocks

Structural and lithological controls on large Quaternary rock avalanches (sturzstroms) in arid northwestern Argentina

Abstract: Landsat thematic mapper (TM) analysis, aerial photograph interpretation, and field studies of the semiarid Puna Plateau, adjacent Eastern Cordillera, and Sierras Pampeanas of Argentina (lat 24°–28°S) have revealed the existence of at least 55 rock-avalanche deposits with volumes larger than 10 6 m 3 that formed by the collapse of entire mountain fronts. The spatial distribution of landslide deposits is not random, but it clusters along mountain fronts bounded by active faults. Inspection in the field reveals five principal controls on the distribution of these events. The source area of the rock avalanches has two topographic constraints: (1) vertical relief contrasts between the breakaway zone and the mountain front must exceed a threshold of 400 m, and (2) the slope inclinations must be steeper than 20°. Rock avalanches are restricted to three types of lithology: granites, low-grade metamorphic rocks, and coarse clastic sediments. Structural controls are very important. Rock avalanches are controlled by planar structures such as bedding planes, exfoliation joints, minor faults, and cleavage that all dip toward the valley. In addition, major slide clusters occur along mountain fronts that underwent Quaternary reverse-fault reactivation of former transfer faults with strike-slip kinematics. The trigger mechanism for the majority of these landslides is interpreted to be seismic, although the ages of some major slides are about 30 ka, and they may correspond to a more humid interval in southern South America.

The dimensions and topographic setting of Antarctic subglacial lakes and implications for large-scale water storage beneath continental ice sheets

Abstract: Radio-echo sounding (RES) at 60 MHz has penetrated to the ice-sheet bed on about 400 000 km of flight track covering more than 50% of the 13.5 x 106 km2 Antarctic ice sheet. About 70 subglacial lakes have been identified by characteristic strong, mirror-like, and very flat RES reflectors. Most are located in the ice-sheet interior, where numerical modeling predicts basal melting, and 33% are within 100 km of the ice crest. Mean ice thickness above the lakes is about 3000 m. About 75% of lakes have radio-echo lengths of 30 km. The largest is Vostok Lake, which is 230 km long, 14 000 km2 in area, and has a water volume of about 2000 km3. More than 60% of lakes have maximum local bedrock elevations of <400 m and bed gradients of <0.1 adjacent to their margins. Extrapolating the observed side-slope topography implies water depths from tens to several hundred meters in a few cases. About 20 000 km2 of subglacial lakes are inferred from RES, which can be doubled for the entire ice sheet to yield an estimated total area of about 40 000 km2, excluding Vostok Lake. Assuming average lake depths between 50 and 250 m, and including the 2000 km3 of water stored within Vostok Lake, we calculate an envelope of about 4000 to 12 000 km3 for the likely volume of water stored in modern Antarctic subglacial lakes, equivalent to 10–35 mm of global sea-level rise. In these volume calculations it is assumed that no subglacial lakes the size of Lake Vostok remain to be discovered and that RES length measurements along single transects are representative of overall lake dimensions. Our estimates of the total water volume of Antarctic subglacial lakes, which may be a suitable analogue for full glacial Quaternary ice sheets, do not necessarily conflict with the possible significance of large meltwater floods during the deglaciation of former midlatitude ice sheets.

Active tectonics of the Seattle fault and central Puget Sound, Washington—Implications for earthquake hazards

Abstract: We use an extensive network of marine high-resolution and conventional industry seismic-reflection data to constrain the location, shallow structure, and displacement rates of the Seattle fault zone and crosscutting high-angle faults in the Puget Lowland of western Washington. Analysis of seismic profiles extending 50 km across the Puget Lowland from Lake Washington to Hood Canal indicates that the west-trending Seattle fault comprises a broad (4–6 km) zone of three or more south-dipping reverse faults. Quaternary sediment has been folded and faulted along all faults in the zone but is clearly most pronounced along fault A, the northernmost fault, which forms the boundary between the Seattle uplift and Seattle basin. Analysis of growth strata deposited across fault A indicate minimum Quaternary slip rates of about 0.6 mm/yr. Slip rates across the entire zone are estimated to be 0.7–1.1 mm/yr. The Seattle fault is cut into two main segments by an active, north-trending, high-angle, strike-slip fault zone with cumulative dextral displacement of about 2.4 km. Faults in this zone truncate and warp reflections in Tertiary and Quaternary strata and locally coincide with bathymetric lineaments. Cumulative slip rates on these faults may exceed 0.2 mm/yr. Assuming no other crosscutting faults, this north-trending fault zone divides the Seattle fault into 30–40-km-long western and eastern segments. Although this geometry could limit the area ruptured in some Seattle fault earthquakes, a large event ca. A.D. 900 appears to have involved both segments. Regional seismic-hazard assessments must (1) incorporate new information on fault length, geometry, and displacement rates on the Seattle fault, and (2) consider the hazard presented by the previously unrecognized, north-trending fault zone.

Application of a critical wedge taper model to the Tertiary transpressional fold-thrust belt on Spitsbergen, Svalbard

Abstract: The Tertiary opening of the North Atlantic Ocean involved major and long-lived overall dextral transpression between the Svalbard and Greenland plates. On Spitsbergen, this tectonic event is manifest as a 100–200-km-wide contractional fold-thrust belt in the form of an east-pinching prism. This belt can be subdivided into (1) a western, basement-involved hinterland province that reveals more complex deformation, including thrust, transcurrent, and normal faulting, and (2) an eastern thin-skinned fold-thrust belt with structures oriented subparallel (north-northwest–south-southeast) to the transform plate boundary. The time-space distribution and interaction of different structural styles of Tertiary deformation evident on Spitsbergen support a model with linked, long-term and short-term (episodic) dynamic growth of a composite contractional and transcurrent fold-thrust wedge. The growth of a narrow, high-taper (critical-supercritical) contractional wedge occurred during northward-directed crustal shortening (stage 1) in an oblique, dextral transcurrent setting. Crustal thickening in the form of thrust uplift and basin inversion and strike-slip duplexing during the main contractional event (stages 2 and 3) created an unstable, supercritical wedge of basement and cover rocks in the hinterland. At the same time, a broader and more homogeneous frontal part of the wedge developed eastward by in-sequence imbrication in order to reduce the taper angle. Local erosion and lateral wedge extrusion (stages 3 and 4) modified the oversteepened hinterland wedge to a critical taper angle. Continued tectonic activity in the hinterland caused renewed internal imbrication of the frontal wedge, where deformation was accommodated by tear faulting and out-of-sequence thrusting (stage 4). Adjustment toward a stable taper geometry included local extension (stage 5) and erosion and sedimentation. In a transpressional fold-thrust belt, as on Spitsbergen, out-of-plane (orogen oblique to parallel) transport in the hinterland may cause local and lateral supercritical and subcritical wedge tapers. Hinterland geometries could trigger adjustments in a frontal thrust wedge in a decoupled situation, and/or orogen oblique or parallel motions in a coupled situation. Changing kinematics may thus be expected along strike in such an orogen.

Fluid inclusion and stable isotope analyses of veins from the central Appalachian Valley and Ridge province: Implications for regional synorogenic hydrologic structure and fluid migration

Abstract: Fluid inclusion microthermometric analyses and O and C stable isotopic analyses of vein minerals are used to determine the chemistry and trapping conditions of fluids present in the central Appalachian fold-and-thrust belt during the late Paleozoic Alleghanian orogeny. The upper Paleozoic rock section contains three regional hydrostratigraphic systems based on fluid chemistry and temperature. The Ordovician Trenton Formation through the Devonian Helderberg Group was a regional aquitard and was dominated by high-salinity, CH 4 -saturated, in situ fluids. The Devonian Oriskany Formation through the lower portion of the Chemung Formation was a regional aquifer system and underwent an influx of warm migrating fluids. The upper portion of the Devonian Chemung through Pocono Formations was also a regional aquifer, but it was dominated by an influx of meteoric water that mixed with in situ fluids. The migrating fluid was a warm (160 to >220 °C) CH 4 -saturated NaCl-CaCl 2 brine that was stratigraphically restricted to the Oriskany Formation through the lower portion of the Chemung Formation, although there is evidence for infiltration into lower stratigraphic units. Two separate fluid migration events are recorded in the rocks. The first event is either late synfolding to postfolding, and the second event is postfolding. Approximately 2‐4 km of overburden were removed by erosion between the two migration events. The source of the warm migrating fluids is still unknown. However, the most likely source would be fluids that were tectonically driven through the fold-and-thrust belt by large-scale, out of sequence thrusting in the hinterland. The migrating fluids were transported far into the foreland where they may have been directly responsible for (1) flushing of hydrocarbons from the upper Paleozoic of the Valley and Ridge into the Plateau province and (2) elevated thermal maturation indicators in the Valley and Ridge and Plateau provinces.

Chronostratigraphy of the Miocene-Pliocene Sagantole Formation, Middle Awash Valley, Afar rift, Ethiopia

Abstract: The Sagantole Formation comprises more than 200 m of lacustrine, alluvial, and volcaniclastic sediments, plus compositionally bimodal tephras and basaltic lavas, exposed in a domelike horst named the Central Awash Complex in the southwestern Afar rift of Ethiopia. The Sagantole Formation is widely known for abundant vertebrate faunas, including the 4.4 Ma primitive hominid Ardipithecus ramidus . New lithostratigraphic data are used to subdivide the Sagantole Formation into the Kuseralee, Gawto, Haradaso, Aramis, Beidareem, Adgantole, and Belohdelie Members, in ascending order. The members are defined on the basis of lithologic differences and laterally continuous bounding tephras. 40 Ar/ 39 Ar dating of 12 intercalated volcanic units firmly establishes the age of the Sagantole Formation to be 5.6 to 3.9 Ma, significantly older than previous proposals based on erroneous correlations. Magnetostratigraphic data reveal eight paleomagnetic polarity zones, which can be correlated unambiguously with the Thvera, Sidufjall, Nunivak, and Cochiti Subchrons of the Gilbert Chron. Thus, by reference to the geomagnetic polarity time scale, seven additional chronological datums can be placed in the Sagantole Formation. With a total of 19 such datums, the age resolution anywhere in the Sagantole Formation is better than ±100 k.y., making this the best-dated Miocene–Pliocene succession in Africa.