T51D-01 INVITED 08:00h
Double- and triple-dating of single detrital zircons with (U-Th)/He, fission-track, and U/Pb systems, and examples from modern and ancient sediments of the western U.S.
Measuring ages of multiple radioisotopic systems in single detrital crystals provides unique constraints on sedimentary provenance, depositional ages, and orogenic evolution of source terranes. We developed methods for measuring U/Pb, fission-track (ZFT), and (U-Th)/He (ZHe) ages on the same single zircon crystals (He-Pb double-dating or He-FT-Pb triple-dating), providing both crystallization and cooling (eruption or exhumation) ages through one or two distinct closure temperatures ($\sim$$240\deg$C for ZFT and $180\deg$C for ZHe). U/Pb ages are measured by laser-ablation ICP-MS in exterior pits or polished surfaces of crystals, after which He and U-Th contents of bulk crystals are measured by laser heating and dissolution/isotope dilution. Multiple replicates of quickly-cooled volcanic zircons confirm accurate ages for each technique on single crystals. We demonstrate the utility of these methods with examples from: 1) Eocene paleofluvial deposits in Oregon that indicate extensive exhumation and volcanism at 45-50 Ma in the Idaho batholith and surroundings, 2) Miocene flysch of the Olympic Peninsula, which yields surprisingly few first-cycle volcanic zircons (He age = FT age = U/Pb age), instead indicating primary derivation from the interior Pacific northwest, where magmatism and exhumation appear to have been strongly decoupled, 3) modern sediment of the Mississippi river, illustrating the dominance of volcanic detritus younger than 100 Ma, from the western U.S., and 4) modern sediment from the Missouri river, illustrating roughly subequal contributions from 1.4-1.8-Ga Belt sources and $<%100-Ma volcanics, and a pronounced crystallization-age gap between 0.5-1.0 Ga. In two settings with active margin flysches (Kamchatka and Olympics), multi-method dating shows that zircon He ages were partially reset following burial (by $\sim$60-80% and $\sim$10%, respectively), despite the fact that the apatite fission-track (AFT) systems were only partially reset. We suggest that incomplete resetting of AFT systems in detrital apatite may often be caused by variable annealing kinetics, leading to erroneously low maximum burial temperature estimates.
T51D-02 08:15h
Pre-, Syn- And Post-Rodinian Detrital Zircon Record From Sedimentary Successions, Northeast Laurentia
Dating of detrital mineral suites is a powerful tool to assist palaeogeographic and tectonic reconstructions. Late Mesoproterozoic to early Paleozoic siliciclastic rocks are widespread around the margins of Laurentia. In northeast Laurentia these can be divided into three lithotectonic groupings on the basis of their age relations with respect to the supercontinent Rodinia. Group 1 units consist of late Mesoproterozoic successions that pre-date assembly of Rodinia and include the little deformed sequences of the Stoer Group of northeast Scotland, the Composite Arc Belt of the Grenville Orogen, and the Mid-Continent Rift System. Detrital zircons from the Stoer Group are dominated by Archaean detritus with minor Palaeoproterozoic detritus and no Mesoproterozoic detritus, whereas those from the Composite Arc Belt and related units show a range of ages from latest Archean to end Mesoproterozoic. Group 2 units are of early Neoproterozoic age and occupied an intracratonic position within an assembled Rodinia, which in northeast Laurentia was bordered by Baltica to the east and probably Amazonia to the southeast. Group 2 included the early latest Mesoproterozoic to early Neoproterozoic Moine and equivalent successions of Scotland, the Torridon Group, also of Scotland, and the Krummedal and lower Eleonore Bay supergroups of Greenland. These are characterized by late Paleoproterozoic and Mesoproterozoic zircon detritus with Archean age zircons absent or rare, except for the Torridon Group where they form a significant component. Group 3 consist of late Neoproterozoic to early Paleozoic successions that accumulated during the breakup of the Rodinia supercontinent and include Neoproterozoic to Cambrian siliciclastic sequences in Newfoundland, the Dalradian sequence in Scotland, and the upper Eleonore Bay Supergroup, Tillite Group and Zebra Series in East Greenland. The detrital zircon record of the group is characterized by late Archean, late Paleoproterozoic, and Mesoproterozoic detritus, except for the lower Dalradian succession (Grampian Group) which lacks Archean detritus. The significant features of this detrital zircon record with respect to the temporal and spatial paleogeographic evolution of northeast Laurentia are the variable distribution of Mesoproterozoic detritus in Group 1 and 2 units as well as the general paucity of Archean detritus is Group 2 sequences. The absence of Mesoproterozoic detritus in the Stoer Group is consistent with its inferred depositional age of around 1.2 Ga, predating Grenville orogenesis whereas metasedimentary sequences in the Composite Arc Belt are interstratified with metavolcanic sequences and contain Mesoproterozoic detritus related to accretionary assembly of the Grenville orogen, but predate final collisional orogenesis. The strong input of Mesoproterozoic detritus in Group 2 and 3 units indicates development of the Grenville orogenic welt during assembly of Rodinian and its subsequent maintenance as a source region during development of the passive margin succession along the East Laurentia margin that faced an open Iapetus Ocean. The general absence of Archean detritus in Group 2 units could reflect the Grenville Orogen acting as a barrier to input from the cratonic interior of Laurentia or the blanketing of Archean source regions by late Mesoproterozoic strata (e.g. Stoer and Torridon groups).
T51D-03 INVITED 08:30h
Detrital Record of Phanerozoic Tectonics in Iran: Evidence From U-Pb Zircon Geochronology
Ion-microprobe U-Pb ages of 91 detrital zircon grains supplement ongoing investigations of the tectonic history of Iran, a critical region bridging the gap between the Alpine and Himalayan orogenic belts. These data improve understanding of the distribution of continental blocks during a complex history of Late Proterozoic (Pan-African) crustal growth, Paleozoic passive-margin sedimentation, early Mesozoic collision with Eurasia, and Cenozoic collision with Arabia. U-Pb analyses of detrital zircon grains from four sandstone samples (two Lower Cambrian, one uppermost Triassic-Lower Jurassic, one Neogene) collected from the Alborz mountains of northern Iran reveal a spectrum of ages ranging from 50 to 2900 Ma. Most analyses yield concordant to moderately discordant ages. The Lower Cambrian Lalun and Barut sandstones yield age distribution peaks at approximately 550-650, 1000, and 2500 Ma, consistent with a Gondwanan source area presently to the south and west in parts of Iran and the Arabian-Nubian shield (Saudi Arabia and northwestern Africa). The uppermost Triassic-Lower Jurassic Shemshak Formation exhibits a broad range of U-Pb ages, including peaks of approximately 200-260, 330, 430, 600, and 1900 Ma, requiring a Eurasian source area presently to the north and east in the Turan plate (Turkmenistan and southwestern Asia). Neogene strata display both the youngest and oldest ages (approximately 50 and 2900 Ma) of any samples, a result of substantial sedimentary recycling of older Phanerozoic cover rocks. Because the youngest zircon ages for three of the four samples are indistinguishable from their stratigraphic (depositional) ages, these data suggest rapid exhumation and help constrain the termination age of Late Proterozoic-Early Cambrian (Pan-African) orogenesis and the timing of the Iran-Eurasia collision.
T51D-04 08:45h
Interactions Between Tectonics, Sedimentation and Climate in the Intramontane B\'{o}lson de Fiambal\'{a} Basin: Southern Puna Plateau (NW Argentina)
The Puna Plateau is part of the larger Puna-Altiplano region which is the second largest Plateau on Earth. The Puna is characterized by high mean elevation at ~3700m with peaks above 6000m, internal drainage and hyper-aridity. The uplift of this important region must have influenced climate patterns, erosion, sediment- dispersal patterns and the evolution of the tectonic stress field in adjacent regions. In order to better understand the interactions between these important processes we have investigated the intramontane Bols\'{o}n de Fiambal\'{a} Basin (BF) at about 27 \deg45'S, 67 \deg45'W. The BF is located in the structural transition between the high-angle reverse fault bounded Sierra Pampeanas structural province and the southern margin of the intra-Andean Puna. The investigation of clastic sediments deposited in intramontane basins straddling the eastern and southern margin of the Puna provides fundamental information on timing and spatial patterns of plateau evolution in the orogen interior. Our sedimentologic and structural data show that sedimentation in the BF started under E-W contraction in late Miocene time in an climate characterized by an ephemeral fluvial system sourced in the west. Subsidence analysis suggests that accommodation space was provided by tectonic loading due to the eastward advance of thrust sheets. Sedimentation with more humid intervals continued during the late Miocene-Pliocene associated with further thrust-front advance and the influence of proximal western sediment sources and the formation of an established fluvial system. Probably by late Pliocene time the BF became overfilled and thick, coarse alluvial fan conglomerates sourced in the W and NE were deposited. Our analysis suggests that subsidence in this last period was mainly due to sediment loading, while tectonic loading by western thrusts apparently played a minor role. This interpretation is in line with provenance data which suggest an enlargement of the drainage system with the contribution of Puna-related rocks to the basin fill, suggesting an uplift pulse in the plateau realm. These events appear to have been roughly coeval with a change in the shortening direction from E-W to WNW-ESE to a neotectonic ENE-WSW orientation, also observed in other parts of the Argentine Andes farther north. Our new observations in the BF thus provide important information on the last phase of plateau uplift at the transition between the late Pliocene and early Pleistocene.
T51D-05 09:00h
Successive Shortening and Extensional Faulting on the Southern Margin of the Puna Plateau and Fiambala Basin, Northwest Argentina
On the southern margin of the Puna Plateau, active normal and strike-slip faults accommodate N-S extension, overprinting earlier NW-SE and NE-SW shortening. These relationships, noted elsewhere by other researchers, can be seen north of the Fiambala Basin at the intersection of the plateau, the Cordillera Principal and the northwestern Sierras Pampeanas structural provinces. Here, N-S striking thrust faults place basement over Tertiary strata and late Pliocene-Quaternary Punaschotter Conglomerate (PSC), in a NE-SW direction. Structural, geomorphic and provenance data show deformation began before and continued during deposition of the PSC, initiating intramontane basin conditions, and ended before re-incision of the Fiambala basin began. The thrust faults are superseded by a NE striking, steeply NW dipping normal fault with a minimum vertical displacement of 200 m. The fault terminates against a N-S thrust fault. Tilted ignimbrites in the hanging wall and recent, unconsolidated volcanic ashes indicate the fault is active and has been a depocenter since faulting began. This area thus experienced a kinematic shift from Plio-Quaternary NW-SE directed shortening to active N-S extension. Similar relationships are observed elsewhere on the southern Puna margin, where active E-W, NE-SW or NW-SE normal and strike-slip faults accommodate N-S extension. These faults are oblique to and often terminate against older, N-S thrust faults. This relationship may be a result of (1) delamination of the mantle lithosphere beneath the Puna Plateau, (2) the structural interaction at intersection of the plateau to the north, the Sierras Pampeanas to the south and the Cordillera Principal to the west, (3) changes in absolute plate motion, (4) N-S extensional collapse of the Puna Plateau, or (5) a combination of these factors. Regardless, the change in extension and shortening directions in this region reflects important aspects of plateau development and may herald a new stage in the evolution of the world's second largest plateau.
T51D-06 09:15h
Controls on Cenozoic Chaco Foreland Basin Development, Bolivia
The up to 200 km wide, Oligocene to Recent Chaco basin is a type example of a retroarc foreland basin system. It is bordered by the Brazilian Shield to the west and by the Subandean Zone to the east. This study combines outcrop stratigraphic, sedimentologic, paleoelevation, seismic, and well data to demonstrate the tectonic, climatic, and eustatic influence on sequence development in the Chaco basin. The Cenozoic fill of the Chaco foreland basin contains an overall coarsening- and thickening-upward stratigraphic succession up to 7.5 km thick and consists of five stratigraphic units: (1) The basal, Oligocene-Miocene-age Petaca Formation consists of dominantly calcretes and fluvial facies in arid setting; (2) The overlying, Late Miocene Yecua Formation records numerous small-scale transgressive-regressive-cycles of marginal marine facies; (3) The Late Late Miocene Tariquia Formation consists of poorly channelized, low-sinuosity anastomosed deposits in semi-arid setting (4) The overlying, Lower Pliocene Guandacay Formation composes of fluvial and distal alluvial fan facies with thin-bedded coal indicating humid paleoclimate; (5) The Late Pliocene, Emborozu Formation consists predominantly of proximal alluvial-fan-facies. Our study shows a marked upsection change in fluvial styles, fluctuations in accommodation space and sediment supply, regulated by basin subsidence. This is supported by a sourceward shift of the depositional system and shown by asymmetrical lateral and vertical variations in grain size and consistent thickness variation and facies changes. Based on our results, the Cenozoic basin development in the Subandean Zone primarily was driven by compressional tectonics and secondarily by climatic and eustatic changes. Tectonically, it formed as response to Andean lithospheric loading, erosion, and shortening by the Subandean thin-skinned fold- and thrust belt. Late Late Miocene arid-to-humid climatic shift contributed to the sedimentation style and rate, whereas global eustacy influenced the sequence only during a Late Miocene marginal-marine incursion.
T51D-07 09:30h
$^{40}$Ar/$^{39}$Ar Temporal Constraints on Eocene Uplift, Subsidence, and Paleohydrology in the Laramide Foreland, Western U. S.
Due to their sensitivity to relatively subtle changes in regional drainage patterns, Eocene lake deposits of the Green River Formation offer a unique and richly detailed record of landscape modification caused by orogenic processes in the broken foreland of the western U. S. Recently obtained $^{40}$Ar/$^{39}$Ar age determinations for 22 interbedded tephras provide excellent temporal resolution of this record, and enable inter-basin correlations at an unprecedented level of precision (approaching 2$\sigma$ uncertainties of $\pm$ k.y.). Green River Formation strata span an interval of ~8 m.y., beginning and ending with freshwater fluvial-lacustrine deposits. Two episodes of regional basin closure and evaporite deposition, each lasting $\sim$1-2 m.y., coincide with evidence for active Laramide faulting at basin margins and increased rates of sediment accumulation. Evaporite deposition therefore appears to have been principally caused by enhanced uplift of basin sills rather than increased aridity. Regional stratigraphic relations, facies types, and $^{40}$Ar/$^{39}$Ar geochronology permit deduction of the following paleodrainage history: 1) $>$$\sim$51.3 Ma: Fluvial-lacustrine deposition occurred in greater Green River, Piceance Creek and Uinta basins. The onset of lacustrine deposition is not well-dated due to a paucity of tephras. 2) $\sim$51.3-49.7 Ma: The greater Green River and Piceance Creek basins both became terminal sinks that received overflow from neighboring freshwater basins. Coarse clastic basin-marginal alluvial strata, cross-cutting fault relations, and pronounced differential subsidence in both basins indicate active uplift of the Uinta Mountains and surrounding ranges. 3) $\sim$49.7-49.1 Ma: Lake Gosiute expanded in extent, coincident with an influx of water and sediment derived from volcanic centers to the north. Episodic overflow over the eastern Uinta uplift flushed dissolved solutes southward, freshening Lake Gosiute while evaporite deposition continued in Lake Uinta. 4) $\sim$49.1-48.4 Ma: Fresh water spilled consistently from Lake Gosiute into an expanding Lake Uinta. The saline, organic-rich Mahoghany zone of the Parachute Creek Member was deposited over an interval of 0.6 $\pm$ 0.3 m.y. coincident with deposition of the freshwater upper LaClede bed of the Laney Member and alluvial Bridger Formation in the greater Green River Basin. 5) $\sim$48.4-46.3: Fluvial volcaniclastic sediments progressively filled the greater Green River and Piceance Creek basins. Alluvial and freshwater lacustrine deposition dominated both basins. Saline lake deposition continued unabated in the Uinta Basin. 6) $\sim$46.3-45.0 Ma: Lake Uinta, limited to the western Uinta Basin, became hydrologic closed as evidenced by bedded evaporite deposition. The change to evaporite deposition coincided with an increase in differential subsidence, reflecting a renewal of tectonic deformation. The up-section disappearance of west-directed volcaniclastic input into the eastern Uinta Basin at $\sim$46.3 Ma suggests that drainage diversion may also have contributed to hydrologic closure. 7) Following $\sim$45.0 Ma, Lake Uinta returned to fluvial-lacustrine deposition and was subsequently filled with alluvial deposits.
T51D-08 09:45h
Uplift of the Colorado Plateau and Rocky Mountains during the Laramide Orogeny: a Finite Element Study
The Laramide deformation, characterized by basement-cored compression and uplifts in the central and southern Rocky Mountains, has been related to flat subduction of the Farallon plate. Previous modeling has shown that the flat subduction may have peeled away the mantle lithosphere (delamination) under western US, and the associated basal shear could have driven an eastward crustal flow that thickened the crust in the Rocky Mountain region. An alternative hypothesis has emerged in recent years from paleoelevation studies that suggest the existence of an elevated Sevier Plateau in much of the western Cordillera since Late Cretaceous. It has been argued that gravitational potential energy from this plateau could have driven crustal flow to lead to uplift of the Colorado Plateau and the Rocky mountains. We have constructed a 2-D finite element method to test this hypothesis. A series of numerical experiments has been conducted to assess the relative roles of gravitational spreading, plate boundary forces, and basal shear in the Laramide orogeny. Our results indicate that tectonic compression from the plate boundary has little effects on the Laramide deformation, whereas both gravitational buoyancy force and basal shear could have played a major role in the uplift of the Colorado Plateau and the Rocky Mountains. Gravitational spreading of an elevated Sevier Plateau would have caused an eastward propagation of uplifts, whereas basal shear under much of the western Cordillera would have caused faster and earlier uplift in the Rocky Mountains than in the Colorado Plateau. These results provide a framework for further testing of the driving mechanisms of the Laramide orogeny by refined constraints on the deformation history of these regions.