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Abstract: The Connemara region is a world-class example of a regional-scale, high-temperature metamorphic terrain. Its rock record documents formation of a bi-vergent orogenic wedge and associated calkalkaline magmatism in a [...]

[1] Structural and thermochronologic studies of the western margin of the central Andean Plateau show changing styles of deformation through time that give insights into tectonic evolution. In southwest Peru, uplift of the plateau proceeded in several distinct phases. First, NW striking, NE dipping reverse faults accommodated uplift prior to ~14-16 Ma. Subsequent uplift of the plateau relative to the piedmont (between the plateau and the Pacific Ocean) occurred between ~14 and 2.2 Ma and was accommodated by NW striking, SW dipping normal faults and subparallel monoclinal folds. The youngest phase of uplift affected the piedmont region and the plateau margin as a coherent block. Although the uplift magnitude associated with phase 1 is unknown, phases 2 and 3 resulted in at least 2.4-3.0 km of uplift. Up to 1 km of this may have occurred during phase 3. Geodynamic processes occurring in both the continental interior and the subduction zone likely contributed to uplift. Citation: Schildgen, T. F., K. V. Hodges, K. X. Whipple, M. S. Pringle, M. van Soest, and K. Cornell (2009), Late Cenozoic structural and tectonic development of the western margin of the central Andean Plateau in southwest Peru, Tectonics, 28, TC4007, doi: 10.1029/2008TC002403.

The relationship between thermochronometer age and structural elevation is commonly used to infer long-term exhumation histories. Previous studies suggest that inferred exhumation rates from the conventional (one-dimensional, 1-D) age-elevation approach are sensitive to topography and variations in exhumation rate and pathway. Here we evaluate the magnitude of these effects by (1) using a 3-D thermal-kinematic model of the central Nepalese Himalaya to predict age-elevation profiles for multiple thermochronometers as a function of exhumation rate and pathway (vertical, oblique, or thrust fault), and (2) calculating the probability that the true exhumation rate will be recovered from an age-elevation profile for sample uncertainties of different magnitudes. Results suggest that profiles oriented orthogonal to long-wavelength topography and the direction of lateral transport are relatively insensitive to their influence. For profiles oriented parallel to the transport direction, horizontal transport during exhumation partly counteracts topographic effects. The difference between model imposed and 1-D exhumation rates from the slope of a best fit line through an age-elevation plot is greatest when rocks are exhumed vertically and low-temperature thermochronometers are used. The magnitude of error in 1-D exhumation rate estimates varies dramatically as a function of sample uncertainty, particularly when exhumation is rapid. The nature of this variation can be used to design sampling strategies for which 1-D interpretations of age-elevation gradients are likely to be within error of the true exhumation rate. Alternatively, if sample uncertainties can be reduced, studies that combine thermal modeling with age-elevation data can potentially provide important constraints on thermal and kinematic fields at depth. doi:10.1029/2007TC002108.

We explore canyon incision history of the western margin of the Andean (Altiplano-Puna) plateau in the central Andes as a proxy for surface uplift. (U-Th)/He apatite data show rapid cooling beginning at ca. 9 Ma and continuing to ca. 5.1 Ma in response to incision. A minimum of 1.0 km of incision took place during that interval. The youngest apatite date and a volcanic flow perched 125 m above the present valley floor dated at 2.261 [+ or -] 0.046 Ma ([sup.40]Ar/[sup.39]Ar) show that an additional ~1.4 km of incision occurred between ca. 5.1 and 2.3 Ma. Thus, we infer that a total of at least 2.4 km, or 75 % of the present canyon depth was incised after ca. 9 Ma. (U-Th)/He zircon data collected along the same transect imply that the western margin of the plateau was warped upward into its present monoclinal form, rather than uplift being accommodated on major surface-breaking faults. Keywords: Altiplano, geochronology, tectonics, Peru, geomorphology, helium.

[1] The southern flanks of the central Nepalese Himalaya correspond to a sharp transition in landscape morphology and bedrock mineral cooling ages that suggests a change in rock uplift rate. This transition can be explained by either (1) accretion of footwall material to the hanging wall across a ramp in the decollement separating India from Eurasia, thereby enhancing rock uplift rates above the zone of accretion or (2) out-of-sequence surface thrust faulting at the physiographic transition. Here we use geomorphic data, 649 new detrital [sup.40]Ar/[sup.39]Ar cooling ages, and a simple thermokinematic model to evaluate which of these tectonic configurations is most appropriate for the central Nepalese Himalaya. We first define and delineate the physiographic transition in central Nepal using maps of knickpoints, fiver steepness indices, local relief, and the distribution of thick alluvial fill deposits. We then report new detrital [sup.40]Ar/[sup.39]Ar data from two trans-Himalayan transects, each of which suggests a rapid northward increase in the total amount of exhumation across the physiographic transition. Thermokinematic modeling suggests that either of the two developmental scenarios for the transition is plausible but that an accretion model is viable only under an extremely narrow range of conditions. We contend that the physiographic and thermo-chronologic data in our study area are most simply explained by recent out-of-sequence surface thrusting within the Lesser Himalayan metasedimentary sequence, approximately 15-30 km south of the mapped surface trace of the Main Central Thrust system. An important finding of this work is that there are substantial along-strike variations in physiography and thermal history that reflect along-strike changes in the degree and location of out-of-sequence surface thrusting. Citation: Wobus, C. W., K. X. Whipple, and K. V. Hodges (2006), Neotectonics of the central Nepalese Himalaya: Constraints from geomorphology, detrital [sup.40]Ar/[sup.39]Ar thermochronology, and thermal modeling, Tectonics, 25, TC4011, doi: 10.1029/2005TC001935.

[1] The Indus River is the only major drainage in the western Himalaya and delivers a long geological record of continental erosion to the Arabian Sea, which may be deciphered and used to reconstruct orogenic growth if the modern bedload can be related to the mountains. In this study we collected thermochronologic data from river sediment collected near the modern delta. U-Pb ages of zircons spanning 3 Gyr show that only ~5% of the eroding .crust has been generated since India-Asia collision.' The Greater Himalaya are the major source of zircons, with additional contributions from the Karakoram and Lesser Himalaya. The [sup.39]Ar/[sup.40]Ar dating of muscovites gives ages that cluster between 10 and 25 Ma, differing from those recorded in the Bengal Fan. Biotite ages are generally younger, ranging 0-15 Ma. Modern average exhumation rates are estimated at ~0.6 km/m.y. or less, and have slowed progressively since the early Miocene (~20 Ma), although fission track (FT) dating of apatites may indicate a recent moderate acceleration in rates since the Pliocene (~1.0 km/m.y.) driven by climate change. The [sup.39]Ar/[sup.40]Ar and FT techniques emphasize the dominance of high topography in controlling the erosional flux to the ocean. Localized regions of tectonically driven, very rapid exhumation (e.g., Nanga Parbat, S. Karakoram metamorphic domes) do not dominate the erosional record. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 1625 Global Change: Geomorphology and weathering (1824, 1886); 1094 Geochemistry: Instruments and techniques; 1035 Geochemistry: Geochronology; 9320 Information Related to Geographic Region: Asia; KEYWORDS: Himalaya, Indus River, fission track, exhumation, erosion, thermochronology. Citation: Clift, P. D., I. H. Campbell, M. S. Pringle, A. Carter, X. Zhang, K. V. Hodges, A. A. Khan, and C. M. Allen (2004), Thermochronology of the modern Indus River bedload: New insight into the controls on the marine stratigraphic record.

The geomorphic character of major river drainages in the Himalayan foothills of central Nepal suggests the existence of a discrete, west-northwest-trending break in rock uplift rates that does not correspond to previously mapped faults. The [sup.40]Ar/[sup.39]Ar thermochronologic data from detrital muscovites with provenance from both sides of the discontinuity indicate that this geomorphic break also corresponds to a major discontinuity in cooling ages: samples to the south are Proterozoic to Paleozoic, whereas those to the north are Miocene and younger. Combined, these observations virtually require recent (Pliocene-Holocene) motion on a thrust-sense shear zone in the central Nepal Himalaya, ~20-30 km south of the Main Central thrust. Field observations are consistent with motion on a broad shear zone subparallel to the fabric of the Lesser Himalayan lithotectonic sequence. The results suggest that motion on thrusts in the toe of the Himalayan wedge may be synchronous with deeper exhumation on more hinterland structures in central Nepal. We speculate that this continued exhumation in the hinterland may be related to intense, sustained erosion driven by focused orographic precipitation at the foot of the High Himalaya. Keywords: tectonics, Himalayas, Nepal, geomorphology, argon-argon, Main Central thrust.

[1] Recent field investigations in the central East Greenland Caledonides (72[degrees]-74[degrees]N) resulted in the identification of an orogen-scale extensional fault system called the Fjord Region Detachment (FRD). Previous geochronologic constraints on this deformation indicated that the FRD was active circa 430-425 Ma, a time when the Baltica-Laurentia collision was thought to be occurring, and continued to be active for up to 80 million years. We present new [sup.40]Ar/[sup.39]Ar thermochronologic data from an E-W transect that cuts across two splays of the FRD. Our data demonstrate that at least two distinct episodes of faulting were responsible for extension in the East Greenland Caledonides: an earlier phase (circa 425-423 Ma) that was synorogenic and penetrated to middle-crustal levels, followed by a post-Caledonian phase of reactivation (~414 to 380 Ma) that affected even deeper structural levels. Furthermore, we present in situ UV laser [sup.40]Ar/[sup.39]Ar data for pseudotachylite collected along the deepest splay of the FRD that indicate this fault was active again as recently as ~357 Ma (coeval with Devonian basin formation). Altogether, our data suggest that rather than being active continuously for 80 million years, the FRD consisted of multiple splays that were active for shorter intervals over discrete time periods separated by as much as 60 million years. Finally, our data provide evidence that young extensional deformation associated with postorogenic collapse in East Greenland was not restricted to the formation of sedimentary basins in the far eastern part of the orogen, but also resulted in deformation of the Archean-Paleozoic crystalline basement. INDEX TERMS: 8109 Tectonophysics: Continental tectonics--extensional (0905); 1035 Geochemistry: Geochronology; 8010 Structural Geology: Fractures and faults; 9315 Information Related to Geographic Region: Arctic region; KEYWORDS: extensional tectonics, East Greenland Caledonides, [sup.40]Ar/[sup.39]Ar, UV laser, thermochronology, synorogenic extension. Citation: White, A. P., and K. V. Hodges, Multistage extensional evolution of the central East Greenland Caledonides, Tectonics, 21(5), 1048, doi: 10.1029/2001TC001308 2002. Received 19 June 2001; revised 11 December 2001; accepted 25 March 2002; published 24 October 2002.

U-Pb geochronological data from the Connemara region of Ireland indicate that continental arc magmatism along the southern margin of Laurentia was short-lived, lasting only from ca. 475 to 463 Ma. Previous work has demonstrated that intrusive activity in Connemara was broadly synchronous with Grampian mid-crustal deformation and upper amphibolite facies metamorphism of Neoproterozoic - lower Paleozoic Dalradian Supergroup rocks. The two oldest intrusions, the Currywongaun and Cashel-Lough Wheelaun gabbros, have U-Pb zircon ages of 474.5 [+ or -] 1.0 and 470.1 [+ or -] 1.4 Ma, respectively, whereas the U-Pb xenotime age of the postdeformational Oughterard granite is 462.5 [+ or -] 1.2 Ma. Thus, the implied age of the Grampian orogeny in Connemara is substantially younger than generally acknowledged, but consistent with other age constraints from Ireland and Scotland. A revised age for the Grampian orogeny helps to solve a long-standing controversy about the age of contractional deformation and arc magmatism in Dalradian Supergroup rocks of the British Isles. This event now demonstrably postdates Neoproterozoic-lower Paleozoic rift deposits and Laurentian passive margin sediments, a finding consistent with deposition of Dalradian Supergroup rocks in an extensional environment at or near the Laurentian margin. The Grampian orogeny is also now demonstrably synchronous with the Taconian orogeny of the northern Appalachians. In Middle Ordovician time, the Laurentia-Iapetus plate boundary was characterized by ophiolite obduction, arc-continent collision, and associated short-lived subduction beneath Laurentia from at least the New England Appalachians to the Irish Caledonides.

Tectonic and surface processes (e.g., erosion) act in concert to denude and flatten elevated topography during and following collisional uplift. In orogens that collapsed rapidly (<10 Myr), the effectiveness and relative contribution of erosion remains poorly understood. Here, we use 1‐D and 2‐D finite‐difference models to test plausible magnitudes of erosion during denudation of the Scandian orogenic wedge, in northern Scotland, which previous work suggests may have been very rapid. Thermochronologic, stratigraphic, and thermobarometric data constrain initial post‐collision uplift rates as well as the cumulative post‐collisional exhumation magnitude. Using these constraints, we consider an idealized orogen subjected to fluvial incision, hillslope diffusion, and subsequent isostatic compensation for a duration of 10 Myr. Using a range of geomorphic parameters, we evaluate whether erosion alone is sufficient to achieve the observed cumulative exhumation. Additionally, we apply a thermal model during the same period to determine the viability of the previous erosion estimates in the presence of a transient geothermal gradient. For most cases with initial uplift rates of 0.5–2.5 mm yr−1, total erosion is insufficient to account for the observed Scandian denudation history, regardless of whether a fixed or transient geothermal gradient is assumed. Average elevations near the crest of the modeled orogen after 10 Myr are >1.4 km, which is inconsistent with the post‐Scandian landscape relief interpreted from the reconstructed depositional surface of the Emsian (407–403 Ma) Old Red Sandstone, Collectively these findings imply a combination of rapid erosion and tectonic denudation likely accomplished the rapid collapse of the Scandian wedge. Plain Language Summary: The destruction of mountain ranges is accomplished by numerous physical processes, which include erosion of the landscape and subsurface deformation of the Earth's crust. In mountains where this destruction occurred very rapidly, such as in northern Scotland, there is poor understanding of the relative effectiveness of each process in wearing mountains down. In this paper, we use computer models to calculate how much erosion might have occurred in the geologically short time frame after the mountains in Scotland stopped growing. These models track the effects of river erosion, hillslope movement, and crustal buoyancy on elevation change after mountain‐building has essentially ceased. We compare the model results to pressure‐temperature‐time data recorded in rocks exposed at the surface today that give us information about how fast the rocks were brought up to the Earth's surface from depth. Our inference based on these models is that erosion alone cannot remove material quickly enough to explain available geologic data, implying the importance of crustal thinning by deformation in the Scottish Highlands. Key Points: Published thermochronologic data for metamorphic rocks of the Scottish Highlands indicate rapid denudation following orogenesisThe models in this work suggest that denudation was not accomplished by surface processes alone, but required additional tectonic processesThis may indicate a role for channelized crustal flow similar to that proposed for the Himalaya and the East Greenland Caledonides [ABSTRACT FROM AUTHOR]

Estimates of the time-temperature history of geologic samples are often based on 40Ar/39Ar ages obtained through incremental-heating experiments on minerals with different Ar retentivities. Laser incremental heating of single crystals of muscovite and biotite from an approximately 1700 Ma monzogranite from a low-pressure-high-temperature metamorphic terrain in central Arizona suggested an essentially uniform distribution of radiogenic 40Ar in each crystal and yielded ages of 1412 [+ or -] 5 Ma for the muscovite and 1410 [+ or -] 10 Ma for the biotite. The similarity in these ages implies rapid (approximately 25 K/m.y.) cooling of the monzogranite after a reheating episode slightly before 1412 Ma. In contrast, detailed age mapping of single crystals from the same samples with the laser microprobe revealed large radiogenic 40Ar gradients indicative of very slow ([less than]1 K/m.y.) cooling and a closure interval for Ar diffusion lasting [less than]400 m.y. Evidence for such low cooling rates has important implications for the longevity of high thermal gradients in the Early-Middle Proterozoic lithosphere. In addition, inconsistency between the laser mapping and incremental-heating results for these micas suggests caution when interpreting incremental-heating data for slowly cooled samples.

In the northern Ruby Mountains-East Humboldt Range metamorphic core complex, a kinematically coupled mylonitic shear zone and brittle-plastic detachment fault compose a low-angle extensional fault system that is mid-Tertiary in age. Geothermobarometry of pelitic schists from the mylonitic shear zone yields a temperature and pressure estimate of 820-900 K (580-620 [degrees]C) and 310-370 MPa (3.1-3.7 kbar) for mylonite petrogenesis. The shear-zone-detachment-fault system excised 11-12 km of crustal section and accommodated at least 15-17 km of normal-sense dip slip.

[1] In the Himalaya and other active convergent orogens, linear relationships between thermochronometer sample age and elevation are often used to estimate long-term exhumation rates. In these regions, high-relief topography and nonvertical exhumation pathways may invalidate such one-dimensional (1-D) interpretations and lead to significant errors. To quantify these errors, we integrate apatite fission track (AFT) ages from the central Himalaya with a 3-D coupled thermokinematic model, from which sample cooling ages are predicted using a cooling-rate-dependent algorithm. By changing the slip partitioning between faults near the Main Central thrust and the Main Frontal thrust system at the Himalayan range front, we are able to explore the significance of different tectonic scenarios on predicted thermochronometer ages. We find that the predicted AFT cooling ages are not sensitive to the different slip partitioning scenarios but depend strongly on erosion rate: Predicted ages are most consistent with kinematic models that produce erosion rates of 1.8-5.0 mm/yr. This range is considerably smaller than that derived from regression lines through the data (-2.6-12.2 mm/yr). The pattern of observed AFT ages shows more complexity than our models predict. None of the kinematic scenarios are able to fit >80% of all of the AFT data, most likely because erosion is spatially variable. Such complexities notwithstanding, we conclude that the use of thermokinematic modeling and thermochronologic data sets to explore detailed fault kinematics in rapidly eroding active orogens has great promise but requires integration of higher-temperature (>~350[degrees]C) data sets to maximize effectiveness. doi: 10.1029/2006TC001991.