T14C-01
Palaeozoic Orogeneses Around the Siberian Craton : Structure and Evolution of the Patom Belt and Foredeep
The Patom fold and thrust belt draws an arc along the south-eastern edge of the Siberian craton, and is a segment of the Central Asian Orogenic Belt (CAOB) also known as the Altaides. It is supposed to be Caledonian and to result from the accretion of volcanic arcs and microcontinents against the craton. A field study in the Patom foreland and highland allows us to precise the geodynamic history of the area. The observed sedimentary succession confirms the existence of a passive margin setting in the Late Riphean (around 900 Ma), followed during the Vendian (650-600 Ma) by the obduction of the Baikal-Muya ophiolites belt and a foredeep inversion. Then, a period of relative tectonic quiescence is recorded until the main collision stage (post Devonian). The sedimentary record is thus marked by a slow evolution with very long stable periods of more than 250 Ma. We interpret it in the light of the geodynamic evolution of Siberia, which is dominated by successive continental collages against the Siberian craton. The analysis of field data reveals homogeneous directions of compression from the inner metamorphic areas to the outermost foreland domain. But whereas the inner range displays highly deformed rocks and metamorphosed units, deformation is much weaker in the foreland which developed above the cratonic crust. We propose to relate this high deformation gradient to the presence of the stiff craton which impeded strain propagation in the foreland. In addition this gradient, but also the topography, seem to evolve laterally from a strong contrast in the south to more progressive variations in the northern arc. This evolution is consistent with a rather thick-skin tectonic in the south and a thin-skin style in the north. The curved shape of the craton's boundary and the pre-existent basement topography inherited from Palaeoproterozoic rifting stages can explain these different tectonic styles observed along the belt, as well as its particular tightened shape.
T14C-02 INVITED
Interconnections Between the Mantle and the Near-Surface System Above the Aspen Anomaly, Central Colorado, and Implications for Cenozoic Uplift of the Rocky Mountains
Teleseismic studies indicate that the upper mantle beneath the Colorado Rocky Mountains has a small percentage of partial melt and dramatic velocity variations that occur across sharp domain boundaries. The CREST (Colorado Rockies Experiment and Seismic Transects) project is investigating an enigmatic low velocity mantle domain that we refer to as the Aspen anomaly. This region has geologic as well as geophysical similarities to the Yellowstone and Jemez Mountain anomalies, including complex magmatic histories, an association with highest topography of the Rockies, radial drainage patterns, high active mantle devolatilization documented by 3He/4He ratios, and high heat flow. Collectively and individually, these low velocity mantle domains constitute enigmatic, world-class, upper mantle features that can be used to test models for the origin of upper mantle velocity variations in the western U.S. and the formation and stabilization of continental lithosphere. Near 3-D teleseismic images will be produced of the geometry of the Aspen Anomaly. The planned CREST experiment involves 83 sites, recording for 14 months. There will be 60 passive IRIS PASSCAL-supported stations coordinated with 23 Transportable Array stations. Siting is complete, installation begins in 2008, first data results will be in 2009, final data analysis is anticipated for 2010. Station lattice spacing of 15-40 km will allow good images of Moho topography as well as mantle structure. Integration with the Transportable Array sites will provide excellent regional context for CREST images. Geologic studies are ongoing and emphasize the time-space correlations among Cenozoic rock and surface uplift, denudation patterns, magmatism, and the modern day mantle anomaly. We seek to understand when and why changes in lithospheric buoyancy occurred in the Rockies and how these changes have been expressed in the lithosphere and at the surface. Recent modeling of AFT results from the MWX well on the edge of the anomaly in Colorado indicate onset of rapid exhumation about 6 Ma, perhaps related to Neogene rock and surface uplift in the Rockies. We postulate that this and other Cenozoic changes in lithospheric buoyancy have resulted in epeirogenic uplift such that the highest elevation region of the Rocky Mountains may be directly responding to active mantle tectonism. Lithospheric structure of continents is the time-integrated composite of structures that formed during lithosphere assembly and later modification, including active tectonism. The CD-ROM experiment interpreted some velocity variations to be associated with old lithospheric structure as shown by dipping velocity boundaries found beneath paleosutures, dipping anisotropy, and a depth-extent of mantle anomalies of >200 km. The Aspen anomaly and related (?) Four Corners anomaly may extend to depths of > 400 km and hence are at least partly in the asthenosphere and may reflect Cenozoic small scale asthenospheric convection. For Yellowstone, Aspen, and Jemez mantle anomalies, lowest mantle velocities coincide with the intersection of postulated NE-trending Proterozoic paleosutures and active Cenozoic mantle tectonism. This suggests a geodynamic process in which Cenozoic and still-active asthenospheric upwelling and uplift are influenced by Proterozoic lithospheric heterogeneity. As a new geodynamic process, we infer that reactivation of old structures can induce material and energy exchange across the base of the lithosphere and thus influence upper asthenospheric circulation.
T14C-03
U-Pb Basement and Detrital Zircon Geochronology of the Lhasa and Qiangtang Terranes in Tibet
U-Pb ages of >3500 zircons from Paleozoic sandstones and basement exposures in the Qiangtang and Lhasa terranes of Tibet provide new constraints on the pre-Mesozoic geological framework and paleogeography of Tibetan terranes. 1. Carboniferous-Permian strata exposed to the north and south of the E-W trending high- pressure central Qiangtang metamorphic belt (CQMB) show no obvious differences in their detrital zircon age spectra. These results, together with stratigraphic and sedimentologic similarities, indicate that the CQMB does not separate strata of Cathaysian affinity in the north from that of Gondwana affinity in the south as previously suggested. 2. Upper Paleozoic strata in the southern Qiangtang terrane are locally exposed structurally above kyanite/silliminate-bearing orthogneisses and metasedimentary assemblages. The orthogneisses yielded crystallization ages in the 470-480 Ma range and metasandstones show minimum ages of ~540 Ma. We suggest that these metamorphic rocks represent early Ordovician and older basement of the Qiangtang terrane which was deformed and metamorphosed during an early- to mid-Paleozoic orogeny and then unconformably overlain by upper Paleozoic strata. 3. In the central Lhasa terrane, an exposure of ~509 Ma granitic basement is unconformably overlain by a straight section of Cambrian (?) through Permian strata. Detrital zircon age spectra determined for the Paleozoic strata are distinct from those of Qiangtang strata, but strikingly similar to Tethyan strata of the Tibetan Himalaya. These results are unexpected because it is generally thought the Lhasa and Qiangtang terranes were separated by a relatively narrow and short-lived Meso-Tethys Ocean whereas the Lhasa terrane and Tethyan Himalaya were separated by the large and long-lived Neo-Tethys Ocean. 4. The youngest ages of detrital zircons in all Paleozoic sandstones are significantly older (>100 Ma) than their depositional ages, consistent with their deposition in passive margin settings. Our and previous results suggest that Tibetan basement rocks are largely Neo-Proterozoic to Ordovican in age. In addition, the Paleozoic cover sequence sandstones are immature and dominated by <1.0 Ga detritus, indicating little input from older continental sources. Consequently, we suggest that the present Tibetan Plateau is made up mainly, if not entirely, of juvenile terranes. In contrast, it is bounded by blocks that include significantly older (and by inference rheologically stronger) basement. Hence, the spatial extent of the Tibetan Plateau was likely predetermined by initial conditions.
T14C-04
The role of basement relief in the structural evolution of the Naryn intramontane basin, Kyrgyz Tian Shan
We examine the emergence of basement topographic relief and the distribution of shortening through time in the Naryn intramontane basin, located within the Tian Shan orogen. At present the entire basin is undergoing erosion by the Nayrn River system, resulting in remarkable exposures of its structural architecture. The basin is bounded by basement-cored uplifts with 1500-2500 m of topographic relief above the Naryn River. Growth strata shed into a Neogene lacustrine basin record the activity of these basement-cored uplifts. This setting differs markedly from Late Quaternary deformation, which largely occurs on folds and thrust faults that deform the Neogene lacustrine strata and Quaternary stream gravels. Rates of uplift, erosion, and resulting landscape evolution differ significantly between structures hosted in basement versus Neogene strata. Despite rates of relative rock uplift approaching ~2 mm/yr, easily eroded Neogene strata sustain topographic relief of at most 300-500 m above river level. In contrast, relief production on basement is dominated by erosional stripping of pre-existing structures; evidence of Late Quaternary deformation of the basement is sparse. We calibrate a ~1 mm/yr vertical rate of exposure of basement from cosmogenic 36Cl dating of a progressively exposed, pristine unconformable contact with Neogene cover strata. Patterns of river incision into this surface are also consistent with sustained rapid erosional stripping of the unconformity surface. We conclude that (1) exhumation of pre- existing, largely inactive basement massifs dominates relief production in the Naryn basin and (2) the presently rapid rate of convergence (~5 mm/yr) across the basin is almost completely focused on structures hosted in Neogene strata with modest topographic relief where shortening is largely balanced by erosion. This pattern is consistent with mechanical models of thrust-belt evolution where topographic relief above reverse faults exerts a first-order control on the loci of shortening.
T14C-05
Links between erosion and tectonics in basement-cored uplift provinces: Theory and applications to the central Andes
Recently, modeling and field studies have explored the potential interactions between erosion and tectonics of simple, mechanically homogeneous orogens. In this contribution, we explore the potential influence of pre- existing geologic structures on the development of ranges in basement-cored uplift provinces. These provinces are often characterized by high-angle reverse faulting along preexisting crustal heterogeneities, which may greatly affect the mechanics of deformation and the coupling between erosion and orogenic structure. To understand how deformation and erosion may be coupled in such situations, we model the mechanics and erosion of mountain belts in which the spatial distribution of deformation is largely influenced by the presence of preexisting high-angle faults. In this case, deformation is accommodated along, and topography is built above the weakest of these structures. However, topographic loading increases lithostatic stresses beneath these regions, and as a result, our models indicate that active deformation may migrate to frictionally stronger structures in adjacent regions where lithostatic loading is lower. In cases where erosional processes are more vigorous and may subdue surface slopes, the lower lithostatic loads inhibit the migration of deformation to frictionally stronger structures in adjacent regions. We apply this theory to the development of two basement-cored uplifts in the central Andes where tectonic circumstances are similar, but climate (and presumably erosion rates) are different. In the wetter and more erosive mountain range, deformation appears localized on specific structures over Myr time-scales, but in the dryer, less erosive range, deformation is broadly distributed across several high-angle reverse faults as our model predicts. Our model results and field observations suggest that while pre-existing geologic structures may largely determine the potential loci of deformation in such situations, erosional processes may play a role in determining the longevity and sequence of uplift within such ranges. Thus, in these types of tectonic environments, it is possible that both the long-term geologic history and current erosional processes may play important roles in moderating deformation.
T14C-06
Controls on the architecture of the Sierra Madre Oriental fold-thrust belt
Geologic mapping and structural analysis of east-central Mexico were conducted to assess the architecture and structural evolution of the SMO fold-thrust belt. We present a regional map that synthesizes our results from the hinterland to foreland of the SMO fold-thrust belt over a distance of 180 km. Our results show that several regional east-directed thrust faults step-up structurally via flats and ramps. The deepest rocks exhumed in their hanging walls are Paleozoic metasedimentary rocks and early Mesozoic rift deposits in the cores of hanging wall antiforms. At structurally shallow levels thrust faults sole into a regionally extensive evaporite horizon which forms the basal detachment to fold trains within Late Jurassic through Cretaceous marine strata. Field mapping shows that structurally shallow short-wavelength folds rooted into the evaporite horizon are locally folded by structurally deeper long-wavelength folds that expose Paleozoic metasedimentary rocks and early Mesozoic rift deposits. Stratal geometries of the rift deposits suggest that west-dipping normal faults are reactivated by east-directed thrust faults. Structural restorations indicate that east-west shortening is concentrated where crystalline basement has been exhumed while only modest amounts of shortening has occurred between them where deformation is localized along evaporite floored detachments. Comparisons between our field mapping and a regional DEM show that short wavelength folds display a curved mapview pattern and developed above evaporite. These folds are cross cut by linear trending longer wavelength folds that correlate to reactivated rift-bounding faults. This result implies that the architecture and structural history of the SMO fold-thrust belt is contingent upon events that occurred during the opening of the Gulf of Mexico basin. Results of this study permit an assessment of the spatial distribution of Gulf of Mexico basin-related rifts and evaporite deposits. The distribution we interpret provides a picture of the rift geometry that is best explained by early opening of the Gulf of Mexico as a rift-rift-rift triple junction. One running along the Texas-Louisiana Gulf Coast, another running along the Eastern SMO and a third corresponding to the Chihuahua trough. Cessation of the third rift arm is interpreted to shift the kinematics along the eastern SMO arm from east-west stretching to right-lateral strike-slip faulting which lead to the development of the Tehuantepec transform plate boundary.
T14C-07
Tectonic vs. climatic controls on accelerated Mio-Pliocene denudation of the Western European Alps
A sharp increase in deposited sediment volume since Pliocene times has been observed worldwide (Zhang et al., Nature 2001) and in particular around the European Alps (Kuhlemann et al., Int. J. Earth Sci. 2002). The phenomenon has been linked to a rise in denudation rates controlled by an increase of either climatic or tectonic forcing. We exploit the unique density of apatite fission-track ages in the Western European Alps to reconstruct cooling isoage surfaces and to estimate exhumation rates between 14 and 2 Ma. Our methodical approach is based on the association of isoage contours with age-elevation relationships. We use map-view interpolation, enabling a spatio-temporal analysis of exhumation rates over the entire Western Alps. Exhumation histories were reconstructed for eight areas of the Western Alps and display strong similarities in timing and rates with orogen- wide average denudation rates inferred from sediment volumes. This consistency validates the use of both techniques for the study of an orogen characterised by strong relief and high recent exhumation rates. We conclude that exhumation rates in the Western Alps have increased more than twofold during the Pliocene. Our spatialisation of inferred denudation rates through time allows us to study possible controls on these by spatial correlation with morphological (e.g., relief, mean elevation, unit stream power), tectonic (current strain rates, seismic energy released) and climatic (precipitation, LGM ice thickness) forcing variables. Whereas clear correlations exist between long-term exhumation rates and both morphologic variables and geodetically determined present-day rock uplift, the controls exerted by either tectonic or climatic forcing on the spatial distribution of exhumation appear more subtle.
T14C-08
Palaeogene Exhumation of the Pyrenean Orogen: Magnetostratigraphic and Thermochronological Constraints From the South-Central Pyrenees
The Pyrenees is an asymmetric, doubly vergent orogen formed by the collision between the European and Iberian plates from late Cretaceous to Miocene times. The Pyrenean orogen comprises a central duplex structure of Hercynian basement, the Axial Zone, which is flanked north and south by oppositely vergent fold and thrust belts and associated foreland basins. The syntectonic continental conglomerates of the South-Central Pyrenees record both the late stages of thin-skinned transport of the South-Pyrenean Central Units and the onset of exhumation of the Pyrenean Axial Zone. The magnetostratigraphic dating of the syntectonic conglomerates of Sis and La Pobla de Segur/Senterada basins has yield a continuous record of the evolving orogenic system from 41 to 27 My (C18r to C9n). Apatite fission track thermochronology on Axial Zone derived granite pebbles within these conglomerates has revealed 3 groups of detrital ages centred around 45, 60 and 27 My, with track lengths that reflect a first stage of slow cooling followed by a period of rapid cooling. In the light of our results 4 stages in the tectonic evolution of the Pyrenean orogen during the Palaeogene have been recognized, which involve variations in the amount and rate of exhumation, as well as changes in the sedimentation rate, clast provenance and position of the water divide.