T23C-1530
Isotopic tracers of Nile sediment sources
We analyzed Sr and Nd isotopes in sediments of the Nile river and its major Sudanese branches (Blue Nile, White Nile, Atbara), focusing on the cohesive mud fraction (< 0.04 mm). In one sample for each branch, the 0.125-0.180 mm sand fraction was also analysed. Marked variations in Sr and Nd isotopic records were expected between White Nile sediments, mostly derived from cratonic sources, and Blue Nile and Atbara sediments, chiefly derived from Tertiary Ethiopian volcanic highlands. Epsilon Nd values, eps(Nd), range between –8.8 ± 0.1 and +3.2 ± 0.1. Nd model ages (tDM) range from 0.8 to 1.6 Ga, duplicating the eps(Nd) trend. Eps(Nd) of main Nile sediments decreases sharply upstream Khartoum, indicating that the Nile sediments are predominantly contributed by the Blue Nile. The Sr isotopic ratios range between 0.70402 ± 2 and 0.72128 ± 2, in anticorrelation with eps(Nd), and reproduce the dichotomy between White and Blue Nile sources. Results from sand samples are more heterogeneous with respect to mud samples. This may be explained by the variable mechanical strength of the main Nd carriers and the petrographic observation of their differential preservation as a function of grain size.
T23C-1531
Eocene-Oligocene aridification and uplift of the Tibetan Plateau, insight from chronostratigraphic and pollen analysis
Continental aridification and the intensification of the monsoons in Asia are generally attributed to uplift of the Tibetan plateau and to the land–sea redistributions associated with the continental collision of India and Asia, whereas some studies suggest that past changes in Asian environments are mainly governed by global climate. The most dramatic climate event since the onset of the collision of India and Asia is the Eocene-Oligocene transition, an abrupt cooling step associated with the onset of glaciation in Antarctica 34 million years ago. However, the influence of this global event on Asian environments is poorly understood. Using magnetostratigraphy and cyclostratigraphy, we showed that aridification, which is indicated by the disappearance of playa lake deposits in the northeastern Tibetan plateau, occurred precisely at the time of the Eocene–Oligocene transition (Dupont-Nivet et al., 2007, Nature vol. 445, p. 637-638). This result suggest that this global transition is linked to significant aridification and cooling in continental Asia recorded by palaeontological and palaeoenvironmental changes, and thus support the idea that global cooling is associated with the Eocene–Oligocene transition. New insight is provided by preliminary pollen data recovered from gypsiferous beds of the playa deposits. The sudden and regional appearances of representatives of the Pinaceae family -and in particular that of Picea- which dominate the palynological record is interpreted to indicate a change to cooler and/or higher altitude conditions in surrounding paleoenvironments. This change occurring at ca. 38 Ma, predates by 4 My the major Eocene-Oligocene aridification but is in close correspondence to increasing sediment accumulation rates and tectonic rotations (see Dai et al., this meeting, Session GP12) and may thus be related to regional Tibetan uplift. http://www.geo.uu.nl/~forth/people/Guillaume
T23C-1532
Detrital zircon evidence from Burma for reorganization of the eastern Himalayan river system
For studying the sedimentary source to sink relation and reorganizing of the mountain rivers around eastern Himalayas, we conducted a detrital zircon study from the Irrawaddy and Inner-Burma Basin by combined SHRIMP U-Pb dating and in-situ Hf isotope measurements. These results, together with U-Pb and Hf isotope data of igneous zircons from the eastern Transhimalayan batholith from southern Tibet to Burma, allow a more quantitative examination of the source provenance of sediments deposited in the drainage since Miocene time. For example, among 47 out of 62 dated zircons from Upper Miocene sandstone that show Cretaceous to Paleogene 206Pb/238U ages, 24 grains have positive εHf(T) values up to +16. Whilst zircons of such ages are common in the Transhimalayan plutons, those showing high εHf(T) values have been observed so far only in the Gangdese batholith. Our results, therefore, support the notion that by Late Miocene time the Yarlu Tsangpo, which flows past the Gangdese batholith in southeastern Tibet, drained into the Irrawaddy River. We attribute this river routing to not only regional topographic control but also the dextral movement of the Jiali-Gaoligong-Sagaing fault system that appears most active during the Middle Miocene. Subsequent reorganization of these mountain rivers was affiliated with headward erosion of the Brahmaputra River that eventually cut across the Namche Barwa Syntaxis and captured the Yarlu Tsangpo drainage to form the modern eastern Himalayan river system.
T23C-1533
Structural and drainage pattern evolution of the Alborz Mountains (N Iran) inferred from provenance data and magnetostratigraphy
The Alborz Mts, N Iran, constitute an active, E-W-oriented, double verging orogen, within the continental collision zone between Arabia and Eurasia. GPS data show that currently convergence is partitioned in the Alborz by 6 mmyr-1 of NNE directed shortening and 4 mmyr-1 of left-lateral motion. The growth of the Alborz was associated with the development of adjacent foreland basins. Here, we discuss the evolution of the southern Alborz and its foreland basin. We present a sandstone and conglomerate provenance study, combined with sedimentary facies analysis, and magnetostratigraphy of a 7200-m-thick sedimentary section in the foreland basin, previously thought to be Mio- Pleistocene in age. This section includes three first-order coarsening and thickening upward units and documents a gradual transition from restricted marine to lacustrine, braided river (U1 and U2), and alluvial fan depositional environments (U3). These units were deposited between 17.5 and 7.8 Ma. Assuming a constant accumulation rate for the alluvial-fan conglomerates of U3, the top of the section is approximately 6 m.y. old. The derived accumulation rates correlate with the cyclicity of the recognized units: at the bottom of each cycle in association with the fine-grained facies, the accumulation rates are faster, while with the appearance of coarse- grained facies the accumulation rates decrease. Fifty sandstone samples were analyzed using the Gazzi-Dickinson sandstone provenance method. The detrital modes of lithic grains indicate at least two first-order variations in the source area. The first major change occurred at the bottom of U1 and is characterised by an increase of low-grade metamorphic clasts and a concomitant decrease in volcanoclastic grains. The second major change is manifested at the base of U3 with a progressive decrease in low-grade metamorphic clasts and the increase of carbonates and volcanic lithics. The analysis of conglomerate clasts shows a relatively uniform composition with dominant volcanoclastic pebbles up to U3, where an important change occurs with the appearance of low-grade metamorphic clasts, carbonates, and intrabasinal sandstones. Faster accumulation rates combined with fine-grained facies and variations in sandstone petrography suggest tectonically controlled flexural loading associated with erosional unroofing. The composition of conglomerate clasts indicates the southward propagation of the Alborz deformation front coupled with the erosion of an adjacent source and the onset of basin inversion and clast recycling. In addition, our data suggest that the locus of tectonic activity and unroofing shifted forward and backward and/or that the shortening rate varied through time. Interestingly, the accumulation rates available for northern foreland basin in the realm of the South Caspian Basin, do not record any important changes earlier than Pliocene time. This suggests that either the flexural load was focused on the southern foreland rather than the Caspian side during the Miocene or that most of the sediments eroded from the Alborz were delivered to the southern sectors before the Pliocene. In any case, important drainage pattern reorganization must have taken place before the Pliocene. This reorganization appears to be coeval with of the onset of pronounced exhumation at 7-6 Ma.
T23C-1534
Detrital zircon fission track analysis reveals the thermotectonic history of ice-covered rocks of the Chugach-St. Elias orogen, SE-Alaska
Investigating the exhumation history of the Chugach-St. Elias orogen (SE Alaska) using low-temperature thermochronometers is challenged by significant ice cover. Assuming exhumation drove cooling, cooling ages increase with elevation in an orogenic belt, and as such the youngest ages occur in valley bottoms. Cooling and exhumation rates are expected to be very high in the Chugach-St. Elias orogen due to efficient glacial erosion and the most intense erosion occurs under the major ice fields. To study the cooling history of rapidly exhuming rocks underneath this ice cover, we analyzed detrital zircon fission track (DZFT) ages of Recent sand samples from modern rivers that drain the central Bagley Ice field and smaller glaciers draining north (Chitina valley) and south (Pacific) of the mountain range. A distinct advantage of DZFT is that it allows one to sample a landscape regardless of accessibility. The youngest ZFT component populations of samples north and south of the Bagley Ice field record a Late Miocene (5-13 Ma) cooling of the orogen. The pattern of cooling ages shows symmetry across the orogen predates the earliest record of the collision of the Yakutat terrane with Alaska. This result contrasts with the asymmetric cooling pattern displayed by low- temperature thermochronological ages (AFT and AHe) of the exposed bedrock within the range. Apatite FT and U- Th/He ages of bedrock samples south of the Bagley Ice field record the syn-collisional (<5 Ma) fast exhumation whereas apatite ages to the north reveal more heterogeneous exhumation and vary widely from Miocene to Eocene. The bedrock samples from throughout the orogenic belt thus display predominantly the effects of the recent climatic situation of the mountain range with very high precipitation on the south, seaward side versus a more arid north side. Our ZFT results from the northern drainages highlight the relative sense and timing of two important fault zones, both accommodate south-side-up exhumation. The Steward Creek fault zone, located north of the Bagley Ice field, limits the Late Miocene exhumation, whereby samples north of it yielded age populations that are Late Eocene to Cretaceous (30-120 Ma) or older. The Border Ranges fault zone, located farther north, limits the Late Eocene cooling and exhumation of the low-P and high-T Chugach Metamorphic Complex that is inferred to have formed during Eocene ridge subduction. This study provides the first insights on the exhumation history of the Chugach- St. Elias orogen between the time of Eocene ridge subduction and full collision of the Yakutat terrane with North America in the latest Miocene.
T23C-1535
Using the Foreland Basin Record to Constrain Orogenic Evolution: Examples from the Alps, Andes and North America Cordillera
Foreland basin deposits are a unique archive of information regarding the nature of the rocks once present in the hinterland and the processes responsible for erosion, deformation and deposition within the source-sink coupled system. Different thermochronometers, because of their different closure temperatures, can answer different but complementary questions when applied to foreland basin strata. Detrital 40Ar/39Ar on white micas from the peripheral Alpine foreland deposits highlight important differences in the rates of erosion between the retro-wedge (Po Plain foreland basin: NW Italy) and the pro-wedge (Bârreme wedge-top basin: SE France). Whereas 40Ar/39Ar ages from the Po Plain (pro-wedge) document fast Eocene-Oligocene erosion coherent with active tectonic deformation and high-grade metamorphism, 40Ar/39Ar ages from the Bârreme Basin (retro-wedge) show that Eocene-Oligocene foreland deposits were mainly derived from sedimentary cover that did not experience enough tectonic burial and erosion during the Alpine orogeny. This documents how orogenic growth can be strongly asymmetric putting important constraints on numerical modeling input parameters. Detrital 40Ar/39Ar ages on white micas from the Tertiary retroarc foreland basin within the central Andes constrain sediment provenance and pre-Tertiary (360-390 Ma) cooling. Apatite fission track (AFT) ages can instead constrain Tertiary rapid erosion (0.4 to >1mm/yr) and show that during the Eocene the foreland basin was receiving material from a growing orogenic wedge. The fact that AFT ages are not reset after deposition combined with the progressive shallowing upward of the foreland stratal dip suggests syn-depositional deformation and structural growth within a wedge-top depozone. Detrital AFT ages from the North America Cordilleran retroarc foreland basin deposits are younger than the depositional age of the hosting strata documenting total annealing (i.e., T > ca. 110° C) owing to significant burial after deposition. The AFT ages in this case record cooling and erosion during forelandward propagation of the Absaroka and Hogsback thrusts. AFT ages from the Frontier (78.9 ± 4.6 Ma), Gannett (63.6 ± 4.1 Ma), and Morrison (58.6 ± 6.5 Ma) Formations thus constrain the timing of thrusting. Detrital thermochronology applied to foreland basin deposits has proven to be successful in tackling key geological issues such as the relationships between tectonics and erosion by constraining provenance, rates and patterns of erosion and deformation both within the hinterland and foreland regions.
T23C-1536
The timing and implications of late Miocene uplift in the Coast and Rocky Mountains of Canada
The topography of the western North American continent crucially affects the characteristics of Northern Hemisphere climate. General circulation models (GCM) with a much reduced orography highlight that without the current mountain barrier to westerly atmospheric circulation winters in North America would be significantly warmer and the continental interior would be much wetter. Given the well documented climatic deterioration that has characterised the last 50 million years these observations raise the question - when did the current mountain barrier achieve its present height? And what role, if any, did mountain uplift play in the evolution of Cenozoic climate? It is relatively well established that the Colorado Plateau has been an area of high elevation since its formation in the Late Cretaceous. In contrast, a growing body of evidence suggests that the coastal mountain ranges of the American west (i.e. the Coast Mountains of Canada and adjacent North Cascades of the US) developed in the Late Miocene (7-12 Ma). In this study we examine ancient and modern detritus shed from the Canadian Coast Range and Rocky Mountains in order to better determine the spatial extent and nature of this Miocene uplift. By examining the Nd isotopic composition of sediments of the Queen Charlotte and Tofino Basins, coupled with a regional study of apatites and zircons from modern rivers we are able to establish that Miocene uplift was not restricted to the coastal ranges alone but also affected the Rocky Mountains of Canada. Since uplift is associated with enhanced erosion and the establishment of relief, we conclude that the modern orographic situation was created in the late Miocene by ~7 Ma. Prior to this time a reduced mountain barrier presented itself to westerly atmospheric circulation. A fully coupled ocean-atmosphere GCM of this situation demonstrates that these relatively modest orographic changes had a significant impact on Northern Hemisphere climate and may have contributed to the Miocene expansion of the North American Prairie and the cooling of the Northern Hemisphere that ultimately led to bi-polar continental glaciation.
T23C-1537 [WITHDRAWN]
Detrital-zircon fission-track geochronology of the Lower Cenozoic sediments, NW Himalayan foreland basin: Clues for exhumation and denudation of the Himalaya during the India-Asia collision
Detrital-zircon fission-track geochronology of the synorogenically-deposited Subathu–Dagshai–Kasauli–Lower Siwalik Formations of the Sub-Himalayan Lower Cenozoic foreland basin reflects progressive effects of the Himalayan tectonometamorphic events on the Proterozoic–Paleozoic source rock as a consequence of the India–Asia collision. The oldest transgressive marine Subathu Formation (57.0–41.5 Ma) contains a very dominant 302.4 ± 21.9 Ma old detrital zircon FT suite with a few determinable 520.0 Ma grains. This old suite was derived by mild erosion of the Zircon Partially Annealed Zone (ZPAZ) of 240–180 oC, which affected the Himalayan Proterozoic basement and its Tethyan sedimentary cover as a consequence of first imprint of the collision. In addition, 50.0 Ma old detrital zircons in this formation were derived possibly from the Indus Tsangpo Suture Zone and the Trans-Himalayan Ladakh Batholith. Sudden source rock changes and unroofing are manifested in the overlying fluvial Dagshai (~30–20 Ma) and Kasauli (20–13 Ma) molassic sediments, which are characterised by dominant 30.0 and 25.0 Ma old youngest zircon FT peaks, respectively. A distinct unconformity spanning for about 10 Myr gets established between the Subathu–Dagshai formations on the basis of detrital- zircon FT ages. Molassic sedimentation since ~30 Ma coincides with the depletion of detritus from the suture zone, and the bulk derivation from the main Higher Himalayan source rock, which has undergone sequentially the UHP–HP–amphibolite facies metamorphism (53–40 Ma) in the extreme north and widespread Eo– and Neo–Himalayan tectonothermal events in the middle. Strength of the Pre–Himalayan Peaks (PHP) >50 Ma in these younger sediments gradually decreases with the intensification of the Himalayan thermal events till the end of the Kasauli sedimentation. Widespread Eo– and Neo–Himalayan metamorphic events (40.0–30.0 and 25.0–15.0 Ma) have almost remobilised the provenance and obliterated most of the evidences of the Pre–Himalayan Peaks in zircon FT ages and appear to be responsible for incoming of the Himalayan (HP) ~30.0 Ma and Young Himalayan Peaks (YHP) of ~15.0 Ma, respectively; the latter appears only between 13.0 and 11.0 Ma sedimentation of the Lower Siwalik Formation. Three distinct metamorphic events get recognised in source area of the Himalayan Metamorphic Belt. Though the Dagshai-Kasauli-Lower Siwalik sequence records uninterrupted fluvial sedimentation since 30 Ma, distinct breaks in zircon FT ages ~5.0-7.0 Myr at the beginning of each formation records pulsative exhumation of the source area in response to the collision between India and Asia.
T23C-1538
Transient incision of the Yellow River: a response to drainage basin integration across the northeastern margin of the Tibetan Plateau
Although the character, provenance and accumulation rate of detritus shed from active mountain ranges contains a rich archive of the interplay between tectonics, climate and erosion, the subsequent excavation of such basins offers additional insight into the processes that govern the topographic evolution of orogens. In northeastern Tibet, growth of the Tibetan Plateau was associated with a protracted period of basin development and sediment accumulation extending from Oligocene to Pliocene time. The present-day course of the Yellow River transects a number of these basins as it descends off the high plateau; from upstream to downstream these are the Tongde, Gong He, Guide, Xunhua, and Linxa basins. In the vicinity of the plateau margin, in the Linxia basin, the youngest portions of the stratigraphic record are characterized by an abrupt transition from lacustrine sediments to fluvial gravels that herald the onset of relatively rapid fluvial incision at ~1.7 Ma [Fang et al., 2003; Li et al., 1997]. Little is known, however, about the timing, rates and patterns of fluvial incision along the Yellow River as it flows through basins upstream of the plateau margin. We address this question through a combination of new constraints on the uppermost basin fill from cosmogenic isotope burial ages, analysis of tributary channel profiles, and volumetric reconstruction of the basin fill. The highest basins along the Yellow River (Gong He and Tongde) are characterized by a thick (>400 m) sequence of conglomerates and fluvial sands that appear to reflect filling by an ancestral Yellow River. Well preserved remnants of aggradational surfaces atop the basin fill allow us to reconstruct the volume of eroded material from these basins. Over a ~200 km long reach of the river, both the depth of fluvial incision and the total volume of eroded material decrease in the upstream direction, consistent with headward migration of a wave of incision. Moreover, cosmogenic radionuclide burial ages of several samples near the top of the basin fill indicate that aggradation ceased at approximately 0.5-0.7 Ma, and suggest subsequent incision rates of ~0.7 – 1.0 mm/yr in this region. Finally, upstream of these basins, the Yellow River has incised a deep canyon through bedrock in the Anyemaqen Shan; analysis of tributary profiles and strath terrace profiles reveal a transient wave of incision propagating into the higher elevations of the plateau. Ages of the highest terrace surfaces near the rim of the inner canyon suggest that the onset of incision occurred only in the past 0.15 Ma. Together these results indicate that incision along the Yellow River propagated upstream, integrating fluvial and lacustrine systems along the way. The wave of incision appears to have migrated a streamwise distance of >500km in the past 1.7 Ma, at average rates of nearly 300 km/Myr. We hypothesize that the breach of drainage boundaries was facilitated by 1) a change to a more erosive climatic condition and/or 2) the steady accumulation of potential energy during basin filling and growth of the plateau. Whereas the timing of incision along large rivers in eastern and southeastern Tibet reflects the development of high topography associated with the Tibetan Plateau, incision along the Yellow River appears to significantly postdate growth of the plateau in northeastern Tibet.
T23C-1539
Magnetochronology of the Plio-Pleistocene Sediments and Multi-pulsed Folding and Thrusting in the Northern Qilian Shan, Tibetan Plateau
Understanding the tectonics of the large-scale crustal shortening and thickening that produced the Tibetan Plateau, and control its continued growth, has been the focus of many studies in recent years. The NE margin of the plateau is actively growing and is related to the northeastward propagation of the lithospheric-scale Altyn Tagh Fault. The easternmost mountains, the Northern Qilian Shan, rise ca. 4000 meters above the Gobi desert exposing Cambrian-Cretaceous sedimentary strata in thrust-bounded slices. The Jiuxi Basin is one of the small foreland basins and borders the Qilian Shan at its NW margin. Towards the south deformation in the Jiuxi basin is accommodated by a NW-SE fold train (the Laojunmiao fold-and-thrust belt (LFTB)) developed during late Cenozoic. Nearly continuous exposure of deformed Plio-Pleistocene sediments reveal the LFTB particularly favorable for determining the geometry, style, timing, and rate of active faulting and folding within the basin. Paleomagnetic investigations of two Plio-Pleistocene terrestrial successions provide detailed magnetostratigraphy for the upper Cenozoic strata in the basin. Results show that the conglomeratic Yumen Formation is time-transgressive with the basal age ranging from ca. 4.0 Ma in the Niugetao section to ca. 3.5 Ma in the Qingcaowan West section ca.14 km to the northwest at the forelimb of the anticline. Northwestward lateral propagation and growth of the LFTB initiated at ca. 3.0 Ma in the Niugetao, and ca.1.2 Ma in the Qingcaowan West with a rate of ca.7.8 km/Ma. The cross section in the Niugetao provides a conservative estimate of shortening rate of 1.2 mm/yr and uplift rate of 1.1 mm/yr since ca. 3 Ma. Detailed mapping of the rotational offlap-onlap geometrical growth strata at the forelimb indicates multi-pulsed folding and thrusting, which suggests that the LFTB developed as a result of northeastward migration and propagation of thrusting and folding of the NE edge of the Tibetan plateau.
T23C-1540
Spatial and Temporal Evolution of Erosion in the Bhutanese Himalaya. What can we Learn by Combining Several Proxies?
Surface erosion in the Himalaya is mostly controlled by rainfall during the Indian Summer Monsoon (ISM) season. While the ISM is a complex atmospheric phenomenon, rainfall in the Himalaya is controlled by orographic barriers with a 10-fold south to north gradient. The Kingdom of Bhutan in the eastern Himalaya is characterized by different distributions of topographic barriers that result in varying rainfall patterns. Furthermore, the Shillong Plateau ~250 km to the south of the Himalayan front imposes a significant moisture barrier for rainfall reaching eastern Bhutan. In order to investigate the different spatial distribution of erosion, we have undertaken an extensive study of 16 modern river sands collected during autumn 2004 in sandbars of the main streams. We adopted a multi chronometer approach involving cosmogenic nuclides dating, detrital apatite and zircon fission track analyzes combined with petrographic and mineralogical data in order to: 1) assess the fidelity of the detrital signal by directly comparing detrital apatite and zircon fission-track grain age distributions from a river sample collected at the outlet of a spatially restricted test catchment (Singhe Dzong valley, NE Bhutan) to modern bedrock cooling ages obtained across this catchment, 2) examine downstream development of detrital cooling ages with the changing contributing areas along the Kuri- and Koma Chhu, Mo- Po- and Puna Tsang Chhu and Thimphu- Paro- and Wang Chhu river systems (eastern, central and western Bhutan, respectively), 3) quantify the spatial distribution of erosion in the different catchments drained by these major rivers using multiple geochronometers and methods characteristic for surface processes acting at different timescales. Ultimately, our large dataset will be appropriate to test different existing models predicting the distribution, in modern landscapes, of bedrock cooling and cosmogenic nuclides ages from modern sands.
T23C-1541
Quantifying transient erosion of orogens with detrital thermochronology from syntectonic basin deposits: insights from the central Pyrenees, Spain
Detrital thermochronology represents a powerful tool with which to constrain the exhumational history of mountain belts. In steadily eroding mountain belts, the long-term erosion rate may be calculated from low-temperature thermochronometer ages in syn-orogenic sediments. The difference between a thermochronomter age and the depositional age – the "lag-time" – is a function of the erosion rate, with faster erosion rates associated with shorter lag-times. Although widely applied, this approach systematically leads to errors if the erosion rate of the source region is not steady. Geologically reasonable variations in erosion rate create perturbations in the thermal field of a mountain belt that distort the expected relationship between lag-time and erosion rate. In such cases, field data are best interpreted by numerical models that account for the evolution of the thermal field with time. In this study we integrate new detrital thermochronometer samples with a 1D transient thermal finite element model to characterize the exhumation history of the orogen over ~15 m.y. The thermal model predicts cooling rate dependent thermochronometer ages in a stratigraphic section as a function of variable source region erosion histories and thermophysical material properties. New thermochronometer data are presented from a syn-orogenic foreland basin deposit exposed in the Spanish Pyrenees (Sierra de Sis conglomerate). A several km thick section of conglomeratic deposits preserves approximately 15 million years of erosion in the core of the orogen, from ~42 to 27 Ma. Apatite fission-track and apatite and zircon (U-Th)/He data from this sequence therefore provide an opportunity to constrain the long-term evolution of the source region. Apatite fission-track data, including track-length distributions, indicate that conglomeratic clasts from throughout the section record rapid cooling between 48 and 42 Ma. Apatite and zircon (U-Th)/He data also show generally invariant ages throughout the section, consistent with rapid exhumation of the source terrane during the mid to late Eocene. (U-Th)/He ages show a similar trend, although significant larger variability in grain-ages is present. Thermochronometer ages are constant throughout the stratigraphic section, requiring that the lag-times increased throughout time. For example, apatite fission-track lag-times increase from ~0-2 m.y. at the base of the section to about 15 m.y. near the top. This increase in lag-time implies a gradual but significant reduction in the erosion rate of the source terrane. Results from numerical models of a wide range of transient erosion histories suggest a significant decrease in the erosion rate of the source terrane from around 1.0 mm/year to ~0.5 to 0.2 mm/yr. This conclusion is consistent with the results from bedrock thermochronometric studies from the source region that similarly indicate a decrease in erosion rate in the late Eocene and early Oligocene, demonstrating the potential of the analysis of syn-orogenic deposits.
T23C-1542
Coupled Basement-Detrital Thermochronology from the southern Gulf of California
We have developed a thermo-kinetic numerical model to simultaneously interpret detrital and basement thermochronology based upon variational methods designed to better constrain the initiation time, slip history, and erosion history of rifted arc crust. The San Jose del Cabo (SJDC) normal fault is a major east-dipping normal fault associated the San Jose del Cabo rift basin and mid- to late Cretaceous arc basement (Los Cabos block). Preliminary results indicate that approximately half of the 10 km total basement denudation is due to Late Miocene-Recent rift-related denudation. Peak exhumation occurred at 7 m.y. at 1 km/m.y. K-feldspar multi- diffusion domain results limit total denudation to approximately 4 km and indicate that virtually all slip along the SJDC fault took place after 10 Ma of similar to the age of the oldest known sediments within the SJDC basin.
T23C-1543
Isotopic and Thermochronological Evidence for Origin and Erosion History of the Gamburtsev Mountains, East Antarctica
The Gamburtsev Subglacial Mountains (GSM) in East Antarctica are located near the South Pole of Inaccessibility and are covered by 0.6-4 km of ice. Their ice-free elevation is approximately 2 km, and they are considered enigmatic due to their great elevation in the center of a continent that is surrounded by passive rift margins. Understanding the origin and erosion history of these mountains is important for constraining the tectonic history of East Antarctica. Additionally, these mountains are considered one of the key postulated source points for development of post-Eocene glaciation. However, they have not yet been sampled directly because there are no outcrops. Topography and ice flow suggest that terrigenous sediment to the Lambert Graben-Prydz Bay Basin were derived from flow paths consistent with drainage from the GSM. We have taken sediment samples from young glacial diamict (from NBP01-01 JPC34) and from Eocene fluvial-deltaic sediments (ODP-1166A) in order to evaluate their provenance, with the goal of constraining the origin and uplift history of the GSM. In particular the Eocene fluvial-deltaic sediments should have received a significant fraction of their detritus from the GSM. For both sample types, zircon (U-Th)/Pb ages are dominated by ca. 550 Ma ages, with a subordinate ca. 900 Ma peak and scattered older grains, while the 40Ar/39Ar ages of hornblende and biotite are ca. 500 and 490 Ma, respectively. The Nd isotope composition of the Eocene fluvial sediments are approximately 2 epsilon units higher than those of the Quaternary glacial sediments, but are not consistent with a substantially distinct source formation age or with a young volcanic source. In order to constrain the erosion history of these sediments, we measured (U-Th)/He ages on detrital apatite and zircon grains. The ages range from 110 to 316 Ma on detrital apatite and 197 to 397 Ma on detrital zircon (of pan- African (U-Th)/Pb age). Thermal models that fit the mineral ages suggest rapid uplift and erosion in pan-African times followed by gradual erosion since about 490 Ma of ~10-18 km of crust (~.02-.04 km/Ma). Collectively, if the assumption holds that the sediments were derived from the GSM, then the crust that forms them has an average mantle extraction age of ca. 2 Ga, and experienced major tectonothermal reworking during pan-African orogenesis. Although more data are clearly needed to make a firm conclusion, the rapid, recent exhumation that would be expected if these mountains were young and quickly eroding was not observed.
T23C-1544
Provenance and basin evolution, Zhada basin, southwestern Tibet
The Zhada basin is a late Miocene – Pliocene intermontane basin situated at high elevations in the Himalayan hinterland. The fluvial and lacustrine sediments of the Zhada formation are undeformed and sit in angular unconformity above the deformed Tethyan Sedimentary Sequence (TSS). The basin sits just south of the Indus suture in a structural position occupied elsewhere in the Himalayan orogen by some of the highest mountains on earth, including Everest. The occurrence of a basin at this location demands explanation. Currently, the Sutlej River flows parallel to the structural grain of the Himalaya, westward through the basin, towards the Leo Pargil (Qusum) range. Near the range front it takes a sharp southward turn, cuts across the structural grain of the Himalaya and out into the Gangetic foreland. Palaeocurrent indicators in the lower part of the Zhada formation show that the basin originated as a northwest flowing axial river. Palaeocurrent indicators are consistently northwest oriented, even to within to within 10 km of the Leo Pargil range front in the north-western end of the basin. This implies that at the onset of sedimentation in Zhada basin the Leo Pargil range was not a barrier as it is today. In the upper part of the Zhada formation, palaeocurrent indicators are generally directed towards the centre of the basin. In the central and southern portions of the basin this indicates a transition from an axial, northwest flowing river to prograding fluvial and alluvial fans. However, in the north-western part of the basin the change between lower and upper Zhada formation involves a complete drainage reversal. This change in palaeocurrent orientation is also reflected in the detrital zircon signal from basin sediments. Low in the Zhada formation the detrital zircon signal is dominated by zircons from the Kailash (Gangdese) batholith (or associated extrusives, see below). However, higher in the sections, a local source, either from the TSS or the core of the Leo Pargil range dominates the detrital zircon signal. Finally, there is a shift in the sandstone composition from unmetamorphosed sedimentary lithic fragments and extrusive felsic volcanic fragments in the lower part of the Zhada formation to metasedimentary and metaigneous fragments in the upper part of the Zhada formation. This is likely linked either to unroofing of the source terrain or a change to another source terrain. Based on the palaeocurrent and detrital zircon data, a change to another source terrain is favoured. This combination of evidence suggests that the Zhada basin evolved from a through-going fluvial plain to a dammed lake primarily due to uplift of the Leo Pargil range. This uplift would have dammed and ponded the river, and exposed higher grade metamorphic rocks at the surface for incorporation into Zhada formation sandstones. It also would have introduced a new source for detrital zircons. Uplift of the Leo Pargil range along a low angle normal fault would also have evacuated portions of the mid-crust, providing a mechanism for subsidence in the Zhada region. Lacustrine sedimentation would have coincided with progradation of marginal alluvial fans and would have continued until the basin was filled in to the level of a new spill point. At this time incision and re- establishment of the Sutlej River would have occurred.
T23C-1545
Climatic and lithologic influences on erosional efficiency in Fiordland, New Zealand
Landscape evolution in collisional orogens is predominantly driven by spatial and temporal patterns of climatically induced differential erosion in concert with tectonic forcing. Here we investigate the influence of climate, lithology, topography and the primary erosive processes on the landscape morphology of Fiordland, South Island, New Zealand. Spatial patterns of differential erosion, derived from detrital cosmogenic radionuclides (CRNs), across the width of Fiordland reveal an inverse relationship with rainfall gradients. Erosion rates appear highest in the east and decrease towards the rain-soaked west coast. This decoupling of basin averaged erosion rates and modern rainfall gradients suggest that Fiordland morphology is not controlled by stream-power styled river incision. We suggest instead, that glacial and peri-glacial processes are the primary erosive agents governing landscape morphology of the region. Correlation of topographic swath profiles and paleo-ELA gradients, determined from cirque outlets, indicate that the elevation of the mean and maximum topography may be controlled by long-term average gradients in the paleo-ELA. Hypsometric analysis of the range indicates that the amount of landmass diminishes significantly above the paleo-ELA. Slope-altitude distributions reveal that hillslopes steepen with elevation to reach and maintain threshold slopes above the paleo-ELA. The correlation of topographic profiles, hypsometry, and slope distributions with paleo-ELA gradients suggests that topographic limits and the spatial distribution of threshold hillslopes may be controlled by a glacial- or peri-glacial buzzsaw process. The lithologic influence on morphology and erosional efficiency is dominated by the degree of fracturing within underlying bedrock. Initial measurements of bedrock fracture densities in the shallow subsurface appear to follow broad patterns of catchment averaged erosion rates across the range. Additionally, bedrock fracture densities show an altitudinal dependence, with increased fracturing above modern snowline. The eastward verging topographic asymmetry of the range and the similarity in spatial patterns of erosion derived from detrital CRNs and long-term denudation patterns from thermochronologic data suggest that spatial patterns of erosion and the resulting orogenic geometry are controlled by differential patterns of surface uplift raising high peaks into an altitude dependant zone of enhanced erosional efficiency.
T23C-1546
Pieces of Laurentia in East Antarctica
East Antarctica figures prominently as a central cratonic piece in reconstructions of the Neoproterozoic supercontinent Rodinia, yet these fits variously position East Antarctica next to (so-called SWEAT fit), or distant from, the western margin of Laurentia. Paleomagnetic poles are lacking for East Antarctica during the time period between assembly at about 1.3-1.1 Ga and subsequent breakup by 700-650 Ma, and there is uncertainty about the reliability and/or applicability of data from Laurentia and Australia. Despite controversial paleomagnetic results, several lines of geologic, age and isotopic evidence from the central Transantarctic Mountains provide a reasonable Rodinia fit between East Antarctica and western Laurentia: (1) similar Nd-isotope crustal age provinces; (2) similarity of ~1.7 Ga crustal events; (3) provenance link between ~1.4 Ga detrital zircons in Antarctic rift-margin strata and Mesoproterozoic A-type granites in Laurentia; (4) associated 1.8-1.6 Ga detrital zircons in these same strata; and (5) similarity in ages of rift-margin formation, by 668 Ma in East Antarctica and ~717-685 Ma in western Laurentia. New isotopic and age data further support such a fit: (6) ~1.4 Ga Antarctic-margin detrital zircons have εHf(i) values (-2 to +7) that match those of coeval A-type Laurentian granites; and (7) a newly discovered A-type rapakivi granite boulder in glacial till at Nimrod Glacier has a U-Pb zircon age of ~1440 Ma and εHf(i) = +7, indicating the presence of Mesoproterozoic granites beneath the East Antarctic ice sheet. We suggest that these detrital-mineral and rock clasts represent distinctive pieces of Laurentia in East Antarctica, providing further support for a Rodinia connection. These two areas thus share similar crustal, rift- margin, and sedimentary histories, and there are several lines of lithologic correlation strengthened by distinctive age and isotopic signatures. From these integrated geological, isotopic and geochronological datasets we also infer that some significant part of the East Antarctic shield is comprised by Paleoproterozoic orogenic belts punctuated with geochemically and temporally distinctive ~1.4 Ga A-type rapikivi granites. Although various other Rodinia geometries have been suggested, none appear as geologically compelling as the modified SWEAT model.
T23C-1547
Paleoproterozoic Detrital Zircons in Mesozoic Sediments From the Yangtze Craton: Where did They Come From?
Detrital zircons from the Late Triassic to Early Jurassic continental sandstones in the Yangtze craton have been dated by SHRIMP and LA-ICP-MS methods. The samples show similar age patterns clustering in the 1.7-2.0 Ga range, with three minor age probability peaks at ca. 2.4-2.6 Ga, 700-850 Ma and 210-250 Ma. The occurrence of 1.7-2.0 Ga detrital zircons in South China were commonly interpreted as associated with the North China craton. However, the present Paleoproterozoic zircons may have no relation with the North China craton because (1) the unimodal age distribution of the Late Paleoproterozoic (1.8-1.9 Ga) zircons and the paucity of Neoarchean-Early Paleoproterozoic zircons from samples are not consistent with the formation of the major blocks of the North China craton prior to 2.5 Ga; (2) the Triassic Qinling-Dabie Orogen is possibly a high-relief uplift compared to the southern margin of North China craton and likely served as a barrier to southward transportation of detritus; and (3) detrital zircon age patterns of the Late Proterozoic to Early Paleozoic clastic rocks from the North China craton are different form those of the present Triassic rocks. Likewise, the Yangtze craton is not a major source as it comprises numerous Neoproterozoic granitoids, incompatible with the Predominance of Paleoproterozoic zircons in most of the Mesozoic sediments. Alternatively, the Cathaysia block is a more favorable provenance of the 1.7-2.0-Ga zircons as various Paleoproterozoic crystalline rocks have been recognized there. This result is consistent with paleogeographic studies and Paleocurrent data. Abundant Paleoproterozoic detrital zircons were also found in the Middle-Late Triassic Songpan-Ganzi Complex, west of the Yangtze craton, which might also have been sourced by the Cathaysia block. The extensive occurrence of 1.8-1.9 Ga zircons is indicative of the existence of a vast Cathaysia Oldland that has formed largely in the Late Paleoproterozoic and a westward draining pan-Yangtze river system in the Early Mesozoic which have fed the contemporaneous basins in and outside South China.