Proterozoic Sedimentary Basins of India

The influence of addition elements on the early resistance changes observed during electromigration testing of Al metal lines

New, high-resolution lithofacies data from hanging-wall Miocene synrift (Rudeis Formation) exposures of the eastern Suez Rift margin, Egypt, reveal a submarine slope depositional system dominated by coarse-grained (pebble), heterogeneous, lenticular beds, formed by coalescing turbidity currents, slumps, and debris flows deposited on deforming submarine substrate. Flows include prerift clasts and contemporaneous shallow-marine fossil fragments originating from an uplifted eastern hinterland. Multiple terrestrial drainages debouched onto faulted offshore slopes or fed small fan deltas on narrow shelves ( 360 m Rudeis Formation is divided into stratigraphic units R1 and R2, which exhibit upward-coarsening and unordered vertical motifs. Ongoing faulting influenced synrift deposition by controlling the locus of subsidence and gravity base level. Mesoscale faults became inactive during Rudeis Formation times, with strain localized on the large rift border fault system, leading to a wider basin with time. We compare 16 subaqueous rift-margin basin fills from various tectonic and geographic settings and show they generally represent proximal gravity-flow deposits dominated by nongraded beds. We find little commonality in vertical grain-size trends, highlighting the diversity of stratal architectures. Most basin fills show an inverse relationship between maximum clast size and shelf width. We propose a new model to capture the spectrum of sedimentary responses to rifting within rift-margin basins, varying as a function of shelf width, slope gradient, maximum grain size, and textural maturity. Rudeis Formation strata at north Wadi Baba represent a particularly coarse-grained end member, deposited on steep slopes, with a narrow shelf.

The presence of undesirable hydrogen-related impurities and the resulting stress instability in chemical vapor deposited silicon dioxide films are important issues. In this work, the bonding nature and stress behavior of relatively low-temperature deposited silicon dioxide films deposited at high rates were investigated. Films were deposited at 1000 Å/min and at a substrate temperature in the 250–350 °C range. A considerable change in stress was observed in these films upon annealing in the 250–400 °C temperature range. Both as-deposited and annealed films were then stored in a cleanroom environment for long periods of time, and their stress was monitored intermittently. In parallel, Fourier transform infrared studies were performed on an identical set of as-deposited and annealed films to investigate changes in the bonding nature of the films during aging. Thus, film stress and their bonding nature were studied concurrently over an extended period of time. Si–H and silanol (Si–OH) were identified as impurities responsible for the observed stress instability of the deposited films. Initial concentrations of these impurities have been observed to vary depending on the deposition conditions. Also, depending on the concentrations of these impurities, both reversible and irreversible bond reconstruction were observed in the films upon annealing. Concomittantly, reversible and irreversible changes in stress were observed in annealed films, the amount of change depending on the impurity type and content. Impurities responsible for reversible and irreversible bond reconstruction were identified. Good correlation between film stress and bonding was observed.

Paleoproterozoic continental crust generation events at 2.15 and 2.08 Ga in the basement of the southern Brasília Orogen, SE Brazil

PreviousNext No AccessSEG Technical Program Expanded Abstracts 1989Deep structure and tectonic setting of sedimentary basins in Western BotswanaAuthors: Jeremy HallJames A. WrightBrian H. HoffeJeremy HallMemorial University of Newfoundland, Canada, James A. WrightMemorial University of Newfoundland, Canada, and Brian H. HoffeMemorial University of Newfoundland, Canadahttps://doi.org/10.1190/1.1889854 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Permalink: https://doi.org/10.1190/1.1889854FiguresReferencesRelatedDetails SEG Technical Program Expanded Abstracts 1989ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 1989 Pages: 1375 publication data© 1989 Copyright © 1989 Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 10 Feb 2005 CITATION INFORMATION Jeremy Hall, James A. Wright, and Brian H. Hoffe, (1989), "Deep structure and tectonic setting of sedimentary basins in Western Botswana," SEG Technical Program Expanded Abstracts : 112-114. https://doi.org/10.1190/1.1889854 Plain-Language Summary PDF DownloadLoading ...

In situ zircon U-Pb dating and whole-rock geochemistry of metasedimentary rocks from South Liaohe Group, Jiao-Liao-Ji orogenic belt: Constraints on the depositional and metamorphic ages, and implications for tectonic setting

The Nutzotin basin of eastern Alaska consists of Upper Jurassic through Lower Cretaceous siliciclastic sedimentary and volcanic rocks that depositionally overlie the inboard margin of Wrangellia, an accreted oceanic plateau. We present igneous geochronologic data from volcanic rocks and detrital geochronologic and paleontological data from nonmarine sedimentary strata that provide constraints on the timing of deposition and sediment provenance. We also report geochronologic data from a dike injected into the Totschunda fault zone, which provides constraints on the timing of intra–suture zone basinal deformation. The Beaver Lake formation is an important sedimentary succession in the northwestern Cordillera because it provides an exceptionally rare stratigraphic record of the transition from marine to nonmarine depositional conditions along the inboard margin of the Insular terranes during mid-Cretaceous time. Conglomerate, volcanic-lithic sandstone, and carbonaceous mudstone/shale accumulated in fluvial channel-bar complexes and vegetated overbank areas, as evidenced by lithofacies data, the terrestrial nature of recovered kerogen and palynomorph assemblages, and terrestrial macrofossil remains of ferns and conifers. Sediment was eroded mainly from proximal sources of upper Jurassic to lower Cretaceous igneous rocks, given the dominance of detrital zircon and amphibole grains of that age, plus conglomerate with chiefly volcanic and plutonic clasts. Deposition was occurring by ca. 117 Ma and ceased by ca. 98 Ma, judging from palynomorphs, the youngest detrital ages, and ages of crosscutting intrusions and underlying lavas of the Chisana Formation. Following deposition, the basin fill was deformed, partly eroded, and displaced laterally by dextral displacement along the Totschunda fault, which bisects the Nutzotin basin. The Totschunda fault initiated by ca. 114 Ma, as constrained by the injection of an alkali feldspar syenite dike into the Totschunda fault zone.These results support previous interpretations that upper Jurassic to lower Cretaceous strata in the Nutzotin basin accumulated along the inboard margin of Wrangellia in a marine basin that was deformed during mid-Cretaceous time. The shift to terrestrial sedimentation overlapped with crustal-scale intrabasinal deformation of Wrangellia, based on previous studies along the Lost Creek fault and our new data from the Totschunda fault. Together, the geologic evidence for shortening and terrestrial deposition is interpreted to reflect accretion/suturing of the Insular terranes against inboard terranes. Our results also constrain the age of previously reported dinosaur footprints to ca. 117 Ma to ca. 98 Ma, which represent the only dinosaur fossils reported from eastern Alaska.

Sedimentologic, stratigraphic, compositional, and structural data from four elongate basins ( 30 km long) in the Nangqian-Yushu region of east-central Tibet (near the headwaters of the Mekong and Yangtze Rivers) indicate nonmarine sedimentation synchronous with Paleocene–Eocene northeast-southwest shortening. Sedimentation in the Nangqian, Niuguoda, Xialaxiu, and Shanglaxiu basins involved (1) mud and carbonate accumulation in offshore to nearshore lacustrine environments and (2) gravel and sand deposition in fan-delta to alluvial-fan environments localized near basin margins. Growth strata in both fine- and coarse-grained deposits, primarily in upper levels of basin fill, provide evidence for sedimentation contemporaneous with motion on fold-thrust structures. Provenance data collected from 25 measured stratigraphic sections, including >1500 paleocurrent measurements and >150 conglomerate compositional analyses, show that each basin was fed sediment from several directions by proximal source areas composed of Carboniferous–Triassic rocks. The source-area proximity and a stratigraphic variability among the basins suggest that each basin evolved independently and was filled by sediment derived from relatively small drainage networks (<103 km2). Age control for basin fill is based on Paleogene fossils, 38–37 Ma 40Ar/39Ar ages from volcanic rocks interbedded with uppermost strata of the Nangqian basin, and 51–49 Ma 40Ar/39Ar ages from igneous rocks that intrude and unconformably overlie strata of the Shanglaxiu basin. Strata containing middle Cretaceous palynomorph and ostracod assemblages are present only locally in the lowermost part of the Nangqian basin. Although the tectonic setting for Cretaceous sedimentation is unclear, early Tertiary basin development was controlled by thin-skinned fold-thrust deformation. We interpret the narrow widths of Paleocene–Eocene basins to be a result of thrust spacing, which in turn may have been controlled by the depth to the decollement (∼5 km deep according to our balanced cross section) from which imbricate thrusts ramped up through the Carboniferous–Triassic section. Sedimentologic and provenance evidence for internal drainage, limited unroofing, and relatively low average sediment-accumulation rates in these syncontractional, plateau-interior basins indicates generally small drainage systems, short main-stem rivers, shallow regional slopes, and limited denudation in east-central Tibet during early Tertiary time. Such geomorphic conditions, which are similar to the modern low-relief interior of the Tibetan plateau, suggest that the deeply incised Mekong and Yangtze Rivers of eastern Tibet were not established until after the termination of Paleogene basin development in the region.

ABSTRACTThe Longmen Shan Foreland Basin developed as a flexural foredeep during the Late Triassic Indosinian orogeny, spanning the time period c. 227–206 Ma. The basin fill can be divided into three tectonostratigraphic units overlying a basal megasequence boundary, and is superimposed on the Palaeozoic–Middle Triassic (Anisian) carbonate‐dominated margin of the South China Block. The remains of the load system responsible for flexure of the South China foreland can be seen in the Songpan‐Ganzi Fold Belt and Longmen Shan Thrust Belt. Early in its history the Longmen Shan Foreland Basin extended well beyond its present northwestern boundary along the trace of the Pengguan Fault, to at least the palinspastically restored position of the Beichuan Fault.The basal boundary of the foreland basin megasequence is a good candidate for a flexural forebulge unconformity, passing from conformity close to the present trace of the Beichuan Fault to a karstified surface towards the southeast. The overlying tectonostratigraphic unit shows establishment and drowning of a distal margin carbonate ramp and sponge build‐up, deepening into offshore marine muds, followed by progradation of marginal marine siliciclastics, collectively reminiscent of the Alpine underfilled trinity of Sinclair (1997). Tectonostratigraphic unit 2 is marked by the severing of the basin's oceanic connection, a major lake flooding and the gradual establishment of major deltaic‐paralic systems that prograded from the eroding Longmen Shan orogen. The third tectonostratigraphic unit is typified by coarse, proximal conglomerates, commonly truncating underlying rocks, which fine upwards into lacustrine shales.The foreland basin stratigraphy has been further investigated using a simple analytical model based on the deflection by supracrustal loads of a continuous elastic plate overlying a fluid substratum. Load configurations have been partly informed by field geology and constrained by maximum elevations and topographic profiles of present‐day mountain belts. The closest match between model predictions and stratigraphic observations is for a relatively rigid plate with flexural rigidity on the order of 5 × 1023 to 5 × 1024 N m (equivalent elastic thickness of c. 43–54 km). The orogenic load system initially (c. 227–220 Ma) advanced rapidly (&gt;15 mm yr−1) towards the South China Block in the Carnian, associated with the rapid closure of the Songpan‐Ganzi ocean, before slowing to &lt; 5 mm yr−1 during the sedimentation of the upper two tectonostratigraphic units (c. 220–206 Ma).

Triassic strata in northeastern Mexico distributed over an area of ∼120,000 km 2 have received relatively little attention in paleogeographic and tectonic reconstructions of western equatorial Pangea. Triassic marine sequences of the Zacatecas Formation in western San Luis Potosi and Zacatecas constitute facies corresponding to different sections of a submarine fan system previously described as the Potosi fan and were deposited on the paleo-Pacific margin of Pangea. New geochronologic data and field observations permit revision of the stratigraphy from Late Triassic to Early Jurassic fluvial deposits of the Huizachal Group that crop out in Nuevo Leon and Tamaulipas and indicate a link between the lower part of this succession (Triassic strata) and the Potosi fan. This work proposes and defines the El Alamar formation, which represents the only Triassic strata of the Huizachal Group (lower part of the Late Triassic–Early Jurassic La Boca Formation), and is interpreted as a continental succession that records a major fluvial system draining equatorial Pangea and flowing west into the Potosi fan. Petrographic and geochemical studies indicate that both Triassic successions, the Zacatecas Formation (marine) and El Alamar formation (continental) have continental block and recycled orogenic provenances. U-Pb detrital zircon geochronology by laser ablation–multicollector–inductively coupled plasma–mass spectrometry show three main zircon age populations: (1) Grenvillian (1300–900 Ma); (2) Pan-African (700–500 Ma); and (3) dominant Permian–Triassic (280–240 Ma). The presence of these zircons displays evidence of sources in Grenvillian Oaxaquia block, Pan-African terrains such as Yucatan and southeastern Texas, and a prominent contribution from the Permian–Triassic east Mexico magmatic arc. Notable is the absence of detrital contributions from the southwestern North American craton. The geochronological data thus argue against proposed southeastward displacement of Triassic successions and their basement to their current position as proposed by the Mojave-Sonora megashear hypothesis. We propose that continental Triassic strata were deposited in eastern Mexico before the opening of the Gulf of Mexico basin, and are thus autochthonous and transported detritus toward the ancient Pacific margin into the Potosi fan, which is also autochthonous.

Studies of geology and hydrology in the Basin and Range Province, Southwestern United States, for isolation of high-level radioactive waste - Basis of characterization and evaluation

The Xigaze forearc basin records the evolution of the southern Lhasa terrane convergent margin, largely affected by Neo-Tethyan subduction processes, prior to the Paleocene Tethyan Himalaya–Eurasia collision. New geologic mapping and U-Pb detrital-zircon geochronologic data from the Lazi region, 340 km southeast of Lhasa, show that forearc basin sedimentation began ca. 110 Ma conformably atop the Yarlung–Tsangpo ophiolitic melange. By this time, the arc–trench system along the southern margin of the Lhasa terrane (Eurasia) consisted of an accretionary complex, overlying ophiolitic melange, the Xigaze forearc basin, and the Gangdese magmatic arc. There is no geological evidence in the Lazi region for more than one subduction zone between the southern Lhasa terrane margin and India. Sedimentological facies analysis from Albian to Santonian clastic and carbonate sedimentary rocks preserved in the Xigaze forearc basin indicate deep-marine sedimentation characterized by hemipelagic carbonate and volcanogenic sediment-gravity-flow deposits. Sandstone modal petrographic and U-Pb detrital-zircon geochronologic data reveal Asian continental margin, Gangdese magmatic arc, and central to northern Lhasa terrane provenance. During this time, basin fill was deposited in cycles of high and low sediment flux characterized by alternating successions of clastic turbidite inner- to outer-fan deposits and hemipelagic limestone and marlstone sequences. Along-strike differences in the timing of initial forearc basin sedimentation are likely the result of intra-basin topography and/or diachronous development of ophiolitic forearc basement.

Suture zones located across the Tibetan region clearly demarcate the rift-and-drift and continental accretion history of the region. However, the intraplate responses to these marginal plate-tectonic events are rarely quantified. Our understanding of the Paleo-Tethyan orogenic system, which involved ocean opening and closing events to grow the central Asian continent, depends on the tectonic architecture and histories of major late Paleozoic–early Mesozoic orogenic belts. These opening and collision events were associated with coupled intracontinental deformation, which has been difficult to resolve due to subsequent overprinting deformation. The late Paleozoic–early Mesozoic Zongwulong Shan–Qinghai Nanshan belt in northern Tibet separates the Qilian and North Qaidam regions and is composed of Carboniferous–Triassic sedimentary materials and mantle-derived magmatic rocks. The tectonic setting and evolutional history of this belt provide important insight into the paleogeographic and tectonic relationships of the Paleo-Tethyan orogenic system located ~200 km to the south. In this study, we integrated new and previous geological observations, detailed structural mapping, and zircon U-Pb geochronology data from the Zongwulong Shan–Qinghai Nanshan to document a complete tectonic inversion cycle from intraplate rifting to intracontinental shortening associated with the opening and closing of the Paleo-Tethyan Ocean. Carboniferous–Permian strata in the Zongwulong Shan were deposited in an intracontinental rift basin and sourced from both the north and the south. At the end of the Early–Middle Triassic, foreland molasse strata were deposited in the southern part of the Zongwulong Shan during tectonic inversion in the western part of the tectonic belt following the onset of regional contraction deformation. The Zongwulong Shan–Qinghai Nanshan system has experienced polyphase deformation since the late Paleozoic, including: (1) early Carboniferous intracontinental extension and (2) Early–Middle Triassic tectonic inversion involving reactivation of older normal faults as thrusts and folding of pre- and synrift strata. We interpret that the Zongwulong Shan–Qinghai Nanshan initiated as a Carboniferous–Early Triassic intracontinental rift basin related to the opening of the Paleo-Tethyan Ocean to the south, and it was then inverted during the Early–Middle Triassic closing of the Paleo-Tethyan Ocean. This work emphasizes that pre-Cenozoic intraplate structures related to the opening and closing of ocean basins in the Tethyan realm may be underappreciated across Tibet.

Fluvial networks of the Iberian Peninsula: a chronological framework

The tectonic setting of a transcontinental belt of 1500–1300 Ma granitic intrusions that extends from southern California to Labrador is controversial; however, the granites are conventionally considered to be “anorogenic.” Detailed field, microstructural, and geochronologic data from the 1425 Ma Beer Bottle Pass pluton, southern Nevada, indicate that major mylonite zones recording dextral‐contractile strains were probably active during and/or shortly after pluton emplacement and suggest that the anorogenic interpretation for this pluton requires reevaluation. Mylonite zones up to 100 m thick strike northeast, dip moderately northwest, and contain a consistent west plunging elongation lineation. Mylonites occur along 15% of the exposed granite‐wall rock contact and extend into both the pluton and the wall rock. Mesoscopic and microscopic kinematic indicators record an oblique, dextral/reverse (pluton side down) movement sense. Synkinematic mineral assemblages of hornblende and biotite and dynamic recrystallization of feldspars suggest that deformation occurred minimally under amphibolite facies conditions. A K/Ar biotite date of 1399±32 Ma, obtained from a sample of mylonitic granite, suggests that deformation took place during or soon following pluton crystallization. We reject forcible emplacement of the Beer Bottle Pass pluton as a mechanism for formation of the mylonite zones because (1) rocks near the granite‐wall rock contact are largely unstrained, (2) the mylonite zones conform only locally to the pluton‐wall rock contact, (3) mylonite zones strike at high angles to, and truncate, the intrusive pluton‐wall rock contact, (4) the pluton‐side‐down shear sense is more compatible with a uniform sense simple shear zone than a forcibly intruding pluton, and (5) fabrics indicative of noncoaxial deformation dominate over flattening fabrics. We suggest that the Beer Bottle Pass pluton is fundamentally synkinematic with respect to either (1) a local, contractile deformational event or (2) regional strains produced by distant plate tectonic processes operative between 1500 and 1300 Ma as suggested by Nyman and Karlstrom (1994). The timing and kinematics of shear zones associated with other circa 1400 Ma plutons in the western United States (e.g., Graubard and Mattinson, 1990; Nyman et al., 1994; Kirby et al., this issue) support the latter interpretation. Our kinematic data are more consistent with a contractile or transpressive tectonic setting at circa 1400 Ma (Nyman and Karlstrom, 1994; Nyman et al, 1994) than with models involving regional extension (e.g., Windley, 1993).