Amy L. Weislogel

Detrital zircon geochronology of pre-Tertiary strata in the Tibetan-Himalayan orogen

Detrital zircon data have recently become available from many different portions of the Tibetan-Himalayan orogen. This study uses 13,441 new or existing U-Pb ages of zircon crystals from strata in the Lesser Himalayan, Greater Himalayan, and Tethyan sequences in the Himalaya, the Lhasa, Qiangtang, and Nan Shan-Qilian Shan-Altun Shan terranes in Tibet, and platformal strata of the Tarim craton to constrain changes in provenance through time. These constraints provide information about the paleogeographic and tectonic evolution of the Tibet-Himalaya region during Neoproterozoic to Mesozoic time. First-order conclusions are as follows: (1) Most ages from these crustal fragments are <1.4 Ga, which suggests formation in accretionary orogens involving little pre-mid-Proterozoic cratonal material; (2) all fragments south of the Jinsa suture evolved along the northern margin of India as part of a circum-Gondwana convergent margin system; (3) these Gondwana-margin assemblages were blanketed by glaciogenic sediment during Carboniferous-Permian time; (4) terranes north of the Jinsa suture formed along the southern margin of the Tarim-North China craton; (5) the northern (Tarim-North China) terranes and Gondwana-margin assemblages may have been juxtaposed during mid-Paleozoic time, followed by rifting that formed the Paleo-Tethys and Meso-Tethys ocean basins; (6) the abundance of Permian-Triassic arc-derived detritus in the Lhasa and Qiangtang terranes is interpreted to record their northward migration across the Paleo- and Meso-Tethys ocean basins; and (7) the arrival of India juxtaposed the Tethyan assemblage on its northern margin against the Lhasa terrane, and is the latest in a long history of collisional tectonism. Copyright 2011 by the American Geophysical Union.

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

The majority of the Songpan-Ganzi Triassic flysch sequence is believed to have derived from denudation of the Dabie and Sulu ultrahigh-pressure (UHP) metamorphic belt in eastern China (e.g., [Nie et al., 1994][1]; [Zhou and Graham, 1996][2]); however, intense debate still exists on the sources of

Detrital zircon provenance from three turbidite depocenters of the Middle–Upper Triassic Songpan-Ganzi complex, central China: Record of collisional tectonics, erosional exhumation, and sediment production

To test the idea that the voluminous upper Middle to Upper Triassic turbidite strata in the Songpan-Ganzi complex of central China archive a detrital record of Dabie ultrahigh-pressure (UHP) terrane unroofing, we report 2080 single detrital U-Pb zircon ages by sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis from 29 eastern Songpan-Ganzi complex sandstone samples. Low (<0.07) Th/U zircons, consistent with crystallization under UHP conditions, are rare in eastern Songpan-Ganzi complex zircon, and U-Pb ages of low Th/U zircons are incompatible with a Dabie terrane source. An unweighted pair group method with arithmetic mean nearest-neighbor analysis of Kolmogorov-Smirnov two-sample test results reveals that the eastern Songpan-Ganzi complex is not a single contiguous turbidite system but is instead composed of three subsidiary depocenters, each associated with distinct sediment sources. The northeastern depocenter contains zircon ages characterized by Paleozoic and bimodally distributed Precambrian zircon populations, which, together with south- to southeast-directed paleocurrent data, indicate derivation from the retro-side of the Qinling-Dabie (Q-D) collisional orogen wedge. In the central depocenter, the dominantly Paleozoic detrital zircon signature and south- to southwest-oriented paleocurrent indicators reflect a profusion of Paleozoic zircon grains. These data are inter preted to reflect an influx of material derived from erosion of Paleozoic supra-UHP rocks of the Dabie terrane in the eastern Qinling-Dabie orogen, which we speculate may have been enhanced by development of a monsoonal climate. This suggests that erosional unroofing played a significant role in the initial phase of UHP exhumation and likely influenced the petrotectonic and structural evolution of the Qinling-Dabie orogen, as evidenced by compressed Triassic isotherms/grads reported in the Huwan shear zone that bounds the Dabie terrane to the north. The central depocenter deposits reflect a later influx of bimodally distributed Precambrian zircon, signifying either a decrease in the influx of Paleozoic zircon grains due to stalled UHP exhumation and/or dilution of the same influx of Paleozoic zircons by spilling of Precambrian zircon from the northeastern depocenter into the central depocenter basin, perhaps due to infilling and bypass of sediment from the northern depocenter or due to initial collapse and constriction of the eastern Songpan-Ganzi complex basin. The southeastern depocenter of the eastern Songpan-Ganzi complex bears significant Paleozoic, Neoproterozoic, and Paleoproterozoic zircon populations derived from the South China block and Yidun arc complex, likely recording nascent uplift of the Longmenshan deformation belt due to impingement of the Yidun arc complex upon the western margin of the South China block.

U-Pb zircon ages from the southwestern Karoo Basin, South Africa - Implications for the Permian-Triassic boundary

U-Pb ages determined using sensitive high-resolution ion microprobe-reverse geometry on 205 single-grain zircons from 16 ash beds within submarine fan deposits of the Ecca Group provide the first evidence of a marine Permian-Triassic (P-T) boundary in the Karoo Basin of South Africa. These U-Pb ages provide an objective basis for correlating the deep-marine sediments of the southwest Karoo Basin with fluvial-deltaic deposits in the central and eastern parts of the basin where the P-T boundary is recorded in a diverse macrofauna. Furthermore, these new zircon ages and their correlation imply asymmetric subsidence and variable sedimentation rates across the basin.

Petrogenesis and provenance of distal volcanic tuffs from the Permian–Triassic Karoo Basin, South Africa: A window into a dissected magmatic province

We present zircon rare earth element (REE) compositions integrated with U-Pb ages of zircon and whole-rock geochemistry from 29 volcanic tuffs preserved in the Karoo Supergroup, South Africa, to investigate the history of magmatism in southern Gondwana. Whole-rock compositions suggest a subduction-driven magmatic arc source for early (before 270 Ma) to middle Permian (270–260 Ma) Karoo tuffs. After ca. 265 Ma, the magmatic source of the volcanic deposits transitioned toward intraplate shallow-sourced magmatism. Zircon U-Pb ages and REE chemistry suggest that early to middle Permian magmas were oxidizing, U- and heavy (H) REE–enriched, melts; middle Permian to Triassic zircons record HREE-depleted, more reduced magmatism. Middle Permian to Triassic tuffs contain increasingly large volumes of zircon cargo derived from assimilated crustal material; therefore magmas may have been zircon undersaturated, resulting in less zircon growth and increased inheritance in late Permian to Triassic Gondwanan volcanics. Zircon U-Pb ages and zircon REE chemistry suggest a shift from arc magmatism in the early Permian to extensional magmatism by the late Permian, which may be associated with development of a backarc magmatic system adjacent to western Antarctica that predates known extensional volcanism elsewhere in Gondwana. Opening of the Southern Ocean in the Jurassic–Cretaceous paralleled this extensional feature, which may be related to reactivation of this Permian–Triassic backarc. This study demonstrates the potential of zircon U-Pb age and REE compositions from volcanic tuffs preserved in sedimentary strata to provide a more complete record of magmatism, when the magmatic province has been largely lost to active tectonism.

U-PB zircon tuff geochronology from the Karoo Basin, South Africa: implications of zircon recycling on stratigraphic age controls

Along the >650 km long southern margin of the Karoo Basin in South Africa, we traversed four evenly spaced stratigraphic transects and collected 22 samples of volcanic, air-fall tuffs thought to be distal deposits derived from the Permian–Triassic Southern Gondwanan volcanic arc. We present 469 new U-Pb zircon ages determined by sensitive high-resolution ion microprobe reverse geometry (SHRIMP-RG) at the Stanford–USGS Microanalytical Center in order to constrain the maximum depositional ages for the southern Karoo Basin strata. Weighted means of these youngest coherent zircon populations were selected to maximize the number of analyses while minimizing the mean square weighted deviation (MSWD) to increase the robustness and decrease the influence of Pb-loss and inheritance in determining the maximum depositional age. Maximum depositional ages for the marine Ecca Group range from 250 to 274 Ma, whereas in the conformably overlying terrestrial Beaufort Group maximum depositional ages ranged from 257 to 452 Ma. Across the southern Karoo Basin, the Ecca Group tuffs produce maximum depositional ages that young upward; however, the Beaufort Group tuffs yield maximum depositional ages that are geochronologically out of sequence. Furthermore, maximum depositional ages of the Beaufort Group tuffs are consistently older than ash ages within the underlying marine strata. Our results are supported by previously published U-Pb tuff zircon geochronology in the Karoo Basin and demonstrate that the presence of out-of-sequence, older tuff ages are repeatable in Beaufort Group tuffs along the southern margin of the basin. We propose that tuffs in the Karoo Basin are correlative with tuffs in southern South America, and that the age spectra of these tuffs were influenced by magmatic crustal recycling. We use these data to highlight the complexity of U-Pb zircon datasets from tuffs, address the use of U-Pb zircon ages to provide absolute age controls, and discuss the implications of these new age controls on the Permian-Triassic Karoo strata.

Mesozoic Tectonics and Sedimentation of the Giant Polyphase Nonmarine Intraplate Ordos Basin, Western North China Block

The Mesozoic Ordos Basin is a large intracontinental basin that is characterized by a thick, undeformed stratigraphic succession over a wide region that is bounded by deformed polyphase orogenic belts on all margins. Structural and stratigraphic studies document a diverse history of Late Palaeozoic through Cretaceous shortening, extension and strike‐slip on the northern and western basin margins, and a longer‐lived contractile setting in the southern part of the basin. In response to these structural episodes, associated basins formed within, or adjacent to, the marginal mountain belts and were filled by a variety of nonmarine depositional processes. Synchronously, subsidence across the whole of the Ordos block resulted in a large, integrated basin, also filled by nonmarine systems, that spanned across distinct structural domains. These stratigraphic and structural characteristics are interpreted to suggest that mechanical contrasts between the basin interior and the marginal mountain belts fundamentally control the timing, distribution and styles of deformation and basin formation on the margins of the Ordos block. This mechanical contrast between the Ordos Basin and its margins encouraged the repeated, polyphase deformation on its margins, whereas the vast area of the Ordos block continued to subside uniformly. These characteristics are typical of many large basin systems found on relatively recently assembled continents, like southern Eurasia. Examples that may be similar in many respects to the Ordos Basin include other collisional successor basins of China, such as the Tarim, Junggar and Sichuan basins.

Mesozoic Development of Nonmarine Basins in the Northern Yidun Terrane: Deposition and Deformation in the Eastern Tibetan Plateau Prior to the India‐Asia Collision

To determine the timing and spatial patterns of syntectonic nonmarine deposition and deformation in the eastern Tibetan Plateau, we present field data, thin section petrology, and detrital U-Pb zircon geochronology from the west Ganzi basin (WGB) and east Ganzi basin (EGB) in the northern Yidun terrane. The WGB and EGB contain ~800 and ~1,300–1,600 m of nonmarine strata, respectively, deposited in alluvial environments. Along the southern margins of both the WGB and EGB and the northwestern margin of the WGB, reverse faults place Triassic Ganzi Pluton and Triassic Yidun Grouprocks on top of nonmarine strata. Growth strata in the footwalls of basin-bounding faults indicate that deposition was synchronous with fault development. The western and eastern margins of the WGB, as well as the northern margin of the EGB, exhibit uncomformable contacts with the underlying Triassic Ganzi Pluton and Triassic Yidun Grouprocks. Provenance results suggest a regionally mixed sediment source from the Yidun and Songpan-Ganzi terranes. Maximum deposition ages from detrital U-Pb zircon geochronology, as well as previously reported thermochronology and palynology results, indicate a Late Mesozoic depositional age for WGB and EGB strata. We interpret the spatial and temporal developments of the WGB and EGB to be associated with the Lhasa-Qiangtang collision.

Sediment source regions and paleotransport of the Upper Jurassic Norphlet Formation, eastern Gulf of Mexico

The Upper Jurassic Norphlet Formation is an eolian sandstone and important hydrocarbon reservoir that overlies the Louann Salt in the Gulf of Mexico (GOM). Because the sand was concentrated into dunes formed by Late Jurassic winds, determining the source areas and paleotransport direction of the sand can improve predictions of the distribution of the dune facies around the GOM. Paleo–wind-blown sediment transport into the proto-GOM was controlled by wind direction and magnitude and the extant topography of the basin and adjacent uplands. Analysis of the Norphlet Formation in the eastern GOM shows that wadis and alluvial fans controlled by the location of highs were the primary route for introducing sediment of varied provenance into the eolian erg. Eolian transport directions interpreted from dip-log analyses are south directed in southern Alabama and west to northwest directed in western Florida. Interpretations of regional, two-dimensional, prestack-depth-migrated seismic data show that erosional incision of the Middle Ground arch occurred prior to and during the time of Norphlet deposition; this as well as preexisting lows in the basement topography may have facilitated basinward sand transport of sediment that fed the Norphlet Formation erg preserved in the deep-water subsurface eastern GOM.