High Exhumation Rates Research Articles

A new approach to obtaining a 3D shear wave velocity model of the crust and upper mantle: An application to eastern Turkey

We present a new approach to the joint inversion of surface wave dispersion data and receiver functions by utilizing Common Conversion Point (CCP) stacking to reconcile the different sampling domains of the two datasets. Utilizing CCP stacking allows us to suppress noise in the data by waveform stacking, and correct for backazimuthal variations and complex crustal structure by mapping receiver functions back to their theoretical location. When applied to eastern Turkey, this approach leads to a higher resolution image of the subsurface and clearly delineates different tectonic features in eastern Turkey that were not apparent using other approaches. We observe that the slow seismic velocities near the Karliova Triple Junction correlate to moderate strain rates and high heat flow, which leads to a rheologically weak crust that has allowed for the upward propagation of Miocene and younger volcanics near the triple junction. We find seismically fast, presumably rigid blocks located in the southeastern Anatolian Plate and Arabian Plate are separated by a band of low shear wave velocities that correspond to the East Anatolian Fault Zone, which is consistent with the presence of fluids in the fault zone. We observe that the Arabian Plate has underthrust the Eurasian Plate as far as the northern boundary of the Bitlis Massif, which can explain the high exhumation rates in the Bitlis Massif as a result of slab break-off of the Arabian oceanic lithosphere. We also find a shallow (~33km) anomaly beneath eastern Turkey that we interpret as a localized wedge of mantle that was underthrust by a crustal fragment during the collision of Arabia and Eurasia. These observations are possible because of the high-resolution images obtained by combining common conversion point receiver function stacks with ambient noise dispersion data to create a data-driven three-dimensional shear wave velocity model.

Relating orogen width to shortening, erosion, and exhumation during Alpine collision

We investigate along-strike width changes of the thickened, accreted lower plate (TALP) in the Central and in the Eastern Alps. We set the width of the TALP in relation to the inferred amount of collisional shortening and exhumation along six orogen-scale cross sections. Taking the present-day, along-strike gradients in the amount of collisional shortening to represent the temporal evolution of the collisional wedge, it may be concluded that the cross-sectional area of the TALP diminishes during ongoing shortening, indicating that the erosional flux outpaced the accretionary flux. Higher amounts of collisional shortening systematically coincide with smaller widths of the TALP and dramatic increases of the reconstructed eroded rock column. Higher amounts of shortening also coincide with larger amplitudes of orogen-scale, upright folds, with higher exhumation and with higher exhumation rates. Hence, erosion did play a major role in reducing by >30 km the vertical crustal thickness in order to accommodate and allow shortening by folding. Long-term climate differences cannot explain alternating changes of width by a factor of almost 2 along straight segments of the orogen on length scales less than 200 km, as observed from the western Central Alps to the easternmost Eastern Alps. Sedimentary or paleontological evidences supporting such paleo-climatic differences are lacking, suggesting that erosional processes did not directly control the width of the orogen.

Low-temperature thermochronology of the Yakutat plate corner, St. Elias Range (Alaska): bridging short-term and long-term deformation

This study investigates the long-term deformation and rock exhumation of the subducting Yakutat microplate corner at the St. Elias syntaxis region with a special focus on the area east and west of Yakutat Bay, where major coseismic surface uplift occurred during a MW 8.2 earthquake in 1899. We use low-temperature thermochronology to quantify the long-term activity of thrust and reverse faults. We present new apatite and zircon (U–Th–Sm)/He ages from 35 bedrock samples collected along profiles crossing major faults that compose a one-sided flower structure at the northern end of the Fairweather Fault. The cooling ages reveal that rapid rock exhumation is accommodated by the eastern Esker Creek Fault and the Yakutat Fault with rates of 2–3 mm/yr and slower rates of <1.5 mm/yr occur along the Boundary Fault. Exceptionally high exhumation rates of 3–5 mm/yr are recorded by <1 Ma apatite ages at the western part of the Esker Creek Fault and the Chaix Hills Fault. This area is closest to the intersection line of the northwest striking reverse faults associated with the northern Fairweather Fault and the shallow dipping northeast striking thrusts of the Pamplona-Malaspina fault zone. The long-term exhumation patterns derived from this new thermochronology data are compared with the observed coseismic uplift pattern caused by the 1899 earthquake, revealing good agreement and supporting the speculation that the 1899 rupture propagated along the western part of the Esker Creek Fault and the Chaix Hills Fault. Thus, this study illustrates that low-temperature thermochronology can support seismic hazard analyses particularly in area where preservation of paleoseismic field evidence is challenged.

Low-angle normal faulting and focused exhumation associated with late Pliocene change in tectonic style in the southern Apennines (Italy)

In the southern Apennines, low-temperature thermochronometry data indicate that exhumation of previous tectonically buried sedimentary units started at around 10 Ma and took place mostly during the last 6 Ma. Relatively high exhumation rates are obtained from apatite fission track (AFT) and (U-Th)/He (AHe) analysis, pointing to a substantial contribution of tectonic processes to rock exhumation besides erosion. Exhumation rates derived from new apatite (U-Th)/He data (AHe) for the last 3 Ma are generally lower than rates determined by AFT data and almost in line with erosion rates inferred from cosmogenic nuclides and sediment yield, thus suggesting that tectonic exhumation was dominant during the older exhumation stages of this region. However, younger cooling ages in the Monte Alpi area from both AFT and AHe analyses point out focused exhumation during the last 3 Ma. Structural and morphotectonic analyses indicate that fast exhumation occurred specifically in this area—where the Apulian Platform reservoir carbonates, elsewhere buried beneath a several kilometer-thick allochthonous cover, are exposed at the surface—as a result of a complex interplay between steep-rooted reverse faulting and shallow low-angle extension. This deformation involved the development of foreland-dipping low-angle normal faults affecting the allochthonous cover units during the late stages of reverse fault-related anticlinal growth in the underlying buried carbonates. Extension of the region triggered focused exhumation in the footwall of the extensional low-angle faults, which was followed by widespread crustal extension and associated development of high-angle normal faults, leading to surface uplift of Monte Alpi.

Long-lasting tectonic activities of the Lepontine Dome. New evidence from low-temperature thermochronology

To investigate the Neogene exhumation history of the central European Alps, we apply low-temperature thermochronology in combination with thermal history modelling. Fission track and (U–Th–Sm)/He ages on apatites from the central Lepontine Dome (Ticino, Switzerland) indicate higher exhumation rates in the centre of the dome and rather moderate exhumation at the northern and southern boundaries since Neogene times. We present a model for explaining the latest stage exhumation of the central Lepontine Dome and show that (I) both episodic and continuous exhumations are found on small-scale throughout the Neogene, (II) compressional tectonics control the exhumation until the Late Neogene, (III) the exhumation regime changes between 6 and 4 Ma and (IV) increasing hinterland exhumation rates at the Mio-Pliocene boundary cannot be related to tectonic structures of the dome and they are thus explained by climatic changes. • We identified formerly unknown Neogene cooling patterns in the Lepontine Dome. • Exhumation styles changed on small-scale within the study area (less than 40 km). • Large-scale folding in the dome was related to long-lasting north–south compression. • Exhumation patterns across the dome changed towards uniform exhumation at ~ 6–4 Ma.

Implications of Deep-Marine Miocene Deposits on the Evolution of the North Patagonian Andes

The North Patagonian Andes (38°–43°30′S) present topographic and geologic characteristics distinct from those of the Central Andes to the north, including a lower altitude, a predominance of plutonic rocks, and the presence of a major N-S-trending, dextral strike-slip, intra-arc discontinuity known as the Liquiñe-Ofqui Fault Zone (LOFZ). The timing for the uplift of the North Patagonian Andes remains poorly constrained principally because high exhumation rates resulted in the erosion of most of its volcano-sedimentary cover during the late Cenozoic. The strongly deformed Cenozoic marine deposits ascribed to the Ayacara Formation, which crop out on the western flank of this range at ∼42°S, can provide for a better understanding of the recent geologic evolution of this range. To discern this formation’s age and sedimentary environment, we have integrated sedimentology, ichnology, foraminiferal paleontology, and geochronology. Our results indicate that this unit was deposited in a deep-marine environment during the Early-Middle Miocene. Correlation of the Ayacara Formation with coeval deep-marine deposits in the forearc to the west indicate that deposition was caused by a major regional event of tectonic subsidence probably related to subduction erosion. An Early-Middle Miocene age for the Ayacara Formation is a reliable maximum age for the deformation and uplift of the western flank of the North Patagonian Andes as well as for the commencement of transpressional deformation associated with the LOFZ.

Giant non-catastrophic landslides and the long-term exhumation of the European Alps

Landslides influence local slope morphology, affect sediment flux from hillslopes to rivers, and mass wasting in response to tectonics and climate forcing. However, the links between giant, non-catastrophic landslides known as Deep-Seated Gravitational Slope Deformations (DSGSDs) and the long-term evolution of orogenic landscapes are almost unknown. We explore these links in the European Alps using the first orogen-scale inventory of DSGSDs (>900 over an area >105km2) and a dataset of published apatite fission-track ages (>1000) that provides an estimate of the long-term exhumation patterns of the orogen. We show that DSGSDs are more widespread than previously considered, and exhibit an orogen-scale distribution not explained by well-known local lithological and structural controls. We test the hypothesis that this orogen-scale distribution correlates to the long-term evolution of the Alps by subdividing the study area into 37 square sub-areas (50×50km), classified according to combinations of long-term exhumation and mean annual rainfall. On each sub-area we perform a morphometric analysis of topography (hypsometry, relief, slope). Excluding local and regional controls due to rock type and structure, DSGSDs tend to cluster in areas with intermediate exhumation rates (fission-track age between 10 and 40Ma), where large-scale topography is less dissected and incision is localised along major valleys. Here DSGSD abundance correlates positively with the degree of valley incision and related relief. Instead, DSGSDs lack in areas which underwent either low exhumation rates, resulting in insufficient relief production, or high exhumation rates associated to rapid uplift or higher erosional dissection of topography. Negative correlation between DSGSD abundance and mean annual rainfall suggests that effective hydrological surface processes contribute, on the long-term timescale, to the development of large-scale topography unfavourable to DSGSDs, especially in areas of high exhumation rates. Where DSGSDs are abundant, long-lasting slope deformations effectively adjust post-glacial relief by reducing slope inclination values, and are thus expected to significantly contribute to the long-term denudation of active orogens.

Testing models of orogen exhumation using zircon (U–Th)/He thermochronology: Insight from the Ligurian Alps, Northern Italy

Testing models of orogen exhumation requires precise constraint of the time–temperature paths of the exhumed rocks. The zircon (U–Th)/He (ZHe) thermochronometer has a closure temperature of ~140–210°C, and can provide temporal constraints on the exhumation history of rocks through a thermal range which crucially corresponds the onset of brittle behaviour of an exhuming orogen. We performed ZHe analyses to test the existing contradictory models for the exhumation of the Ligurian Alps. The ZHe ages indicate a very rapid (1.3–6.8mm/yr) and southward migrating exhumation of the orogen between ~32 and 25Ma. These high exhumation rates are unique within the Alpine belt and cannot be reconciled with existing geodynamic models of the evolution of the Ligurian Alps. We propose a model of tectonic denudation via detachment accomplished in the shallowest crust (<5km) as a result of the last orogenic phase of extension.

Constraints on brittle field exhumation of the Everest‐Makalu section of the Greater Himalayan Sequence: Implications for models of crustal flow

New apatite and zircon fission track (FT) data from the summit slopes of Everest and along the Barun, Arun, Dudh Kosi, and Kangshung valleys that drain the Everest and Makalu massifs cover a vertical sample transect of almost 8000 m of the Eastern Nepal Greater Himalayan Sequence (GHS). Apatite FT ages range from 0.9 ± 0.3 Ma to 3.1 ± 0.3 Ma in the GHS with ages increasing systematically with elevation. Apatite FT ages in the Everest Series and summit Ordovician limestones are much older, up to 30.5 ± 5.1 Ma. Zircon FT ages from the GHS range from 3.8 ± 0.4 Ma to 16.3 ± 0.8 Ma. The brittle exhumation rates calculated from these data show the GHS was exhumed, since ∼9 Ma, at an average rate of 1.0 ± 0.2 mm/a. Pliocene exhumation rates are higher at 1.7 ± 0.3 mm/a. These values are not significantly different from the estimate of ductile exhumation rates of 1.8 mm/a recorded by metamorphic minerals undergoing decompression between 18.7 and 15.6 Ma but are well below the values (up to 10 mm/a) used by thermomechanical models for ductile channel flow in the GHS. If representative of the GHS these models will therefore require further ‘tuning’. Higher exhumation rates in the Pliocene have also been observed in other parts of the Himalaya and points to a regional cause, likely increased erosion due to the onset of late Pliocene–Pleistocene glaciation of the high Himalaya.

Structure of the actively deforming fold-thrust belt of the St. Elias orogen with implications for glacial exhumation and three-dimensional tectonic processes

Previous studies in the Yakataga fold-thrust belt of the St. Elias orogen in southern Alaska have demonstrated high exhumation rates associated with alpine glaciation; however, these studies were conducted with only a rudimentary treatment of the actual structures responsible for the deformation that produced long-term uplift. We present results of detailed geologic mapping in two corridors across the onshore fold-thrust system: the Duktoth River transect just west of Cape Yakataga and the Icy Bay transect in the Mount St. Elias region. In the Duktoth transect, we recognize older, approximately east-west–trending structures that are overprinted by open, northwest-trending fold systems, which we correlate to a system of northeast-trending, out-of-sequence, probably active thrusts. These younger structures overprint a fold-thrust stack that is characterized by variable structural complexity related to detachment folding along coal-bearing horizons and duplexing within Eocene strata. In the Icy Bay transect, we recognize a similar structural style, but a different kinematic history that is constrained by an angular unconformity at the base of the syntectonic Yakataga Formation. At high structural levels, near the suture, structures show a consistent northwest trend, but fold-thrust systems rotate to east-west to northeast trends in successively younger structures within the Yakataga Formation. We present balanced cross sections for each of these transects where we project the top of basement from offshore seismic data and assume a subsurface structure with duplex systems similar to, but simplified from, structures observed in the onshore transects. These sections can account for 150–200 km of shortening within the fold-thrust system, which is Our section restorations also provide a simple explanation for the observed elongate bullseye pattern of low-temperature cooling ages in the thrust belt as a consequence of exhumation above the growing duplex and/or antiformal stack. Comparison with analog model studies suggests that structural feedbacks between erosion and development of decollement horizons in coal-bearing strata led to this structural style. Although previous studies based on thermochronology suggested an active backthrust at the northern edge of the thrust belt, section restorations indicate that a backthrust is allowable but not required by available data. The Yakataga fold-thrust belt has been treated as a dominantly 2D system, yet our work indicates that 3D processes are prominent. In the Duktoth transect, we interpret a group of northeast-trending thrusts as younger, out-of-sequence structures formed in response to the rapid destruction of the orogenic wedge by glacial erosion and deposition immediately offshore. We infer that these northeast-trending thrusts transfer slip downdip into a duplex system that forms the antiformal stack modeled in cross-section restorations, and we infer that these structures represent thrusting stepping back from the active thrust front attempting to rebuild an orogenic wedge that is being destroyed as rapidly as, or more rapidly than, it is being rebuilt. In the Icy Bay transect, we use the relative chronology provided by an angular unconformity beneath the syntectonic Yakataga Formation to infer that early, northwest-trending fold-thrust systems were formed along the Fairweather transform as transpressional structures. Continued strike slip carried these structures into the tectonic corner between the Fairweather and Yakataga segments of the orogen, producing a counterclockwise rotation of the shortening axis until the rocks reached their present position.

Post-collisional adakite-like magmatism in the Ağvanis Massif and implications for the evolution of the Eocene magmatism in the Eastern Pontides (NE Turkey)

In the Anatolia, the Caucasus and northwest Iran, the Eocene epoch is characterized by widespread basic to acidic magmatism, whose temporal and spatial evolutions and origin are poorly understood. In this paper, we provide geochronological and geochemical data on a suite of Early Eocene intrusions from northeast Turkey and discuss their origin within a regional tectonic framework. Post to late-collisional, moderate to small bodies of quartz diorite and leucogranodiorite as well as later dacite porphyries intrude Permo-Triassic low-grade metamorphic rocks in the southern part of the Eastern Pontides very close to the Neo-Tethyan İzmir–Ankara–Erzincan suture. In places, the intrusives display well-developed subsolidus foliations, indicating on-going penetrative deformation during and shortly after emplacement. Different emplacement depths were inferred based on the contact metamorphic mineral assemblages, Al-in-hornblende barometry and igneous textures (quartz diorite ~14–16km vs dacitic porphyries ≤7km). Biotite and hornblende Ar–Ar and zircon U–Pb data indicate that the compositionally and texturally different intrusives were emplaced at ~51Ma (Ypresian). Three lines of evidence indicate that the Early Eocene represents a tectonically active time with high exhumation rates (≥0.6cma−1): (i) the quartz diorite and dacitic porphyries were emplaced at different crustal depths, (ii) the emplacement ages of these texturally distinct intrusive rocks are geochronologically undistinguishable, and (iii) intrusive rocks locally show a well-developed foliation. The intrusive rocks display high abundances of Al2O3, Sr, Ba, LREEs and low abundances of Y, Sc and HREEs, and are similar to high-silica adakites from supra-subduction zone settings. The geochemical features imply a residual and/or fractionating assemblage rich in hornblende, garnet, pyroxene, and poor in plagioclase. All rock types display similar Sr–Nd isotopic characteristics with initial εNd and 87Sr/86Sr values ranging from 1.0 to −1.1 and from 0.70421 to 0.70494, respectively. These three coeval rock groups (quartz diorite, leucogranodiorite and dacite porphyries) were generated from a similar source by variable degrees of partial melting and fractionation. These data together with those from the literature clearly reveal that the adakitic signature is common in Paleocene to Early Eocene igneous rocks in northern Turkey on both sides of the İzmir–Ankara–Erzincan suture, but disappears towards the Middle Eocene. Regional geological constraints indicate that the continental collision between the Pontides and the Anatolide–Tauride block occurred in the Paleocene and that the Middle Eocene represents a period of major extension, characterized by a marine transgression. We therefore suggest that the Early Eocene adakitic magmatism in northern Turkey resulted from the presence of a thickened continental crust following the continental collision along the Neo-Tethyan İzmir–Ankara–Erzincan suture whereas the voluminous Middle Eocene magmatism was related to extensional collapse of a thickened orogen.

Mesozoic-Cenozoic tectonothermal evolution of the eastern part of the Tibetan Plateau (Songpan-Garzê, Longmen Shan area): insights from thermochronological data and simple thermal modelling

Abstract We present a synthesis of the tectonic and thermochronological evolution of the Eastern Tibet since the Triassic. The long-term cooling histories obtained on magmatic and metamorphic rocks of the South Songpan-Garzê, Kunlun and Yidun blocks are similar showing a very slow and regular cooling during Late Jurassic and Cretaceous, confirming the suspected lack of major tectonic events between c . 150 and 30 Ma. The exhumation linked to the Tertiary growth of the Tibetan Plateau initiated around 30 Ma and concentrates at the vicinity of the major tectonic structures. Exhumation rates increased again from about 7 Ma in the Longmen Shan. To interpret this very slow cooling rate between Late Jurassic and Early Cenozoic from granites of this area, we use a simple 1D thermal model that takes into account the thermal properties of both sediments and crust. The results suggest that: (1) high temperature (500 °C) can be kept over a long period of time; (2) during Cretaceous, cooling is mostly controlled by the thermal properties of sediments of continental origin; and (3) the initial Late Triassic rapid cooling rate was caused by the large thermal contrast between the granite body and the sedimentary rocks rather than by a high exhumation rate.

From nappe stacking to out‐of‐sequence postcollisional deformations: Cretaceous to Quaternary exhumation history of the SE Carpathians assessed by low‐temperature thermochronology

Apatite fission track (AFT) and (U‐Th)/He (AHe) thermochronology have been combined to constrain the exhumation history of the SE Carpathians. Cooling ages generally decrease from Cretaceous for the internal basement nappes (AFT ages), to Miocene–Quaternary (AFT and AHe, respectively) for the external sedimentary wedge. The AFT and AHe data show a Paleogene age cluster, which confirms a suspected but never demonstrated tectonic event. The new data furthermore suggest that the SE Carpathians have been affected by a middle Miocene exhumation phase related to continental collision, which occurred at rates of ∼0.8 mm/yr, similar to the one previously inferred for the East Carpathians. The SE Carpathian tectonic evolution, however, is overprinted by two younger exhumation events in the Pliocene–Pleistocene. The first exhumation phase (latest Miocene–early Pliocene) occurred at high exhumation rates (∼1.7 mm/yr) and is interpreted as a tectonic event and/or associated with a sea level drop in the Paratethys basins during the Messinian low stand. The youngest recorded tectonic phase suggests rapid Pleistocene exhumation (∼1.6 mm/yr) and is interpreted to represent crustal‐scale shortening different in mechanics from collisional processes. The data suggest that the SE Carpathians did not develop as a typical double‐vergent orogenic wedge; instead, exhumation was related to a foreland‐vergent sequence of nappe stacking during collision and was subsequently followed by a large out‐of‐sequence shortening event truncating the already locked collisional boundary.

Erosional variability along the northwest Himalaya

Erosional exhumation and topography in mountain belts are temporally and spatially variable over million year timescales because of changes in both the location of deformation and climate. We investigate spatiotemporal variations in exhumation across a 150 × 250 km compartment of the NW Himalaya, India. Twenty‐four new and 241 previously published apatite and zircon fission track and white mica 40Ar/39Ar ages are integrated with a 1‐D numerical model to quantify rates and timing of exhumation alongstrike of several major structures in the Lesser, High, and Tethyan Himalaya. Analysis of thermochronometer data suggests major temporal variations in exhumation occurred in the early middle Miocene and at the Plio‐Pleistocene transition. (1) Most notably, exhumation rates for the northern High Himalayan compartments were high (2–3 mm a−1) between ∼23–19 and ∼3–0 Ma and low (0.5–0.7 mm a−1) in between ∼19–3 Ma. (2) Along the southern High Himalayan slopes, however, high exhumation rates of 1–2 mm a−1 existed since 11 Ma. (3) Our thermochronology data sets are poorly correlated with present‐day rainfall, local relief, and specific stream power which may likely result from (1) a lack of sensitivity of changes in crustal cooling to spatial variations in erosion at high exhumation rates (&gt;∼1 mm a−1), (2) spatiotemporal variation in erosion not mimicking the present‐day topographic or climatic conditions, or (3) the thermochronometer samples in this region having cooled under topography that only weakly resembled the modern‐day topography.

Generation of Late Cretaceous silicic rocks in SE China: Age, major element and numerical simulation constraints

Rhyolite-dominating bimodal volcanic suites (rhyolite/basalt), mafic dikes and A-type granites distribute from N Zhejiang to S Fujian over 800 km in the Southeast Coast Magmatic Belt (SCMB) – the Late Yanshanian (LY) orogenic belt in SE China. Data of 40Ar/ 39Ar and K–Ar whole-rock ages and LA-ICPMS U–Pb zircon ages indicate that rhyolitic volcanism (101–72 Ma) is contemporaneous with the A-type granitic intrusions (100–90 Ma) and mafic dike injections (94–77 Ma). This time span is used to define the upper volcanic series in Zhejiang–Fujian areas. One striking feature of rhyolites in the SCMB is that many are strongly peraluminous (SP) and others, mostly restrict in Fujian, are peralkaline to mildly peraluminous (P-MP) and chemically resemble A-type granites. The SP character is unique among well-known large rhyolite provinces worldwide. Based on experimental works for a common thermal regime and inherited zircon age information, we suggest that SP and P-MP rhyolites represent low pressure melting of the felsic (quartzofeldspathic) granite (±metapelite) and the accompanied granodioritic, tonalitic and trondhjemitic member of the core complex assemblage, respectively, to account for the decreasing aluminosity. They could have also been contaminated by young igneous rocks, and ancient crust to a lesser degree, during ascent to the surface. Plate subduction and lithosphere extension processes, respectively, are numerically simulated for the magma generation of these rhyolites using the mafic underplating model. Results suggest that the most effective controlling factor to generate SP and associated P-MP (A-type) magmas during 95–80 Ma is thinning of the lithosphere thickness with a high exhumation rate. Under this circumstance, the core complex assemblage can be uplifted to lower level of the crust and match the constraint of experimental works.

(U–Th)/He thermochronometry: Mapping 3D geometry using micro-X-ray tomography and solving the associated production–diffusion equation

(U–Th)/He ages reflect the combined effects of He ( α particles) loss due to diffusion and He ingrowth due to radioactive decay of U and Th series. Furthermore, the α particles are emitted with sufficient kinetic energy that they can travel, in theory, up to 20 μm and as a result, α particles may be ejected beyond the crystal edges. To circumvent this problem a correction factor must be applied, which is essentially based on the geometry of the crystals [Farley, K. A., Wolf, R., and Silver, L. (1996). The effect of long alpha-stopping distances on (U–Th)/He dates. Geochimica et Cosmochimica Acta, 60:4223–4229]. This strongly limits the application of the technique to crystals of appropriate geometries. We present here a method to compute the effects of α-ejection for (U–Th)/He dating by taking into account the 3D morphology of individual crystals determined with micro-X-ray tomography. We also develop a model which solves the diffusion equation for the crystals that have been imaged. We then couple this model with an inversion algorithm to extract the range of thermal histories that the rocks may have experienced. This technique is applied to detrital samples from rocks collected in the Southern Alps of New Zealand, a region of high exhumation rate.

Thermal and baric evolution of garnet granulites from the Kharta region of S Tibet, E Himalaya

Granulite-facies garnet-bearing metapelites, metabasics and calc-silicate rocks from the lower metamorphic complex (Kharta Gneiss) of the Greater Himalayan Crystallines in the Kharta region of S Tibet, E Himalaya, preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–720°C and pressures around 8 kbar during the main event of the Himalayan metamorphism. Post-deformational reaction textures include clinopyroxene (± orthopyroxene) - plagioclase symplectites after garnet in calc-silicate rocks, and cordierite ± spinel coronas on sillimanite and garnet in metapelite granulites. These assemblages indicate a decompressional pressure-temperature path that is confirmed by the geothermobarometry of zoned and symplectite minerals as well as by calculated phase equilibria. Isothermal decompression through ca. 3 kbar occurred at temperatures of about 700°C, and was followed by further decompression to P ∼ 3 kbar, and T ∼ 710°C. At this point, decompression was replaced by quasi-isobaric cooling ending in the andalusite stability field at P ca. 2.5 kbar. The P-T path of the Kharta Gneiss appears to be similar to those inferred for the lower Greater Himalayan Crystallines exposed in the nearby Dudh Kosi and middle Arun valleys of eastern Nepal. This type of clockwise P-T path, with most of the exhumation occurring at relatively constant metamorphic temperatures, requires a high exhumation rate and suggests that extrusion tectonics of crustal-scale wedges may have been operative during post-collisional exhumation of the Greater Himalayan Crystallines.

Pliocene and Pleistocene exhumation and uplift of two key areas of the Northern Apennines

Application of different thermochronological methods to two areas of pronounced relief (Apuane Alps and Mt. Falterona ) of the Northern Apennines chain documents an average exhumation rate of about 0.7 mm/ yr . This result suggests that the general exhumation of the chain is driven mainly by erosion. Nevertheless, the long-term exhumation trend seems to be perturbed by short-term events at higher exhumation rates. The time of increased exhumation rates coincides with onset of intramontane sedimentation, documenting a contribution of tectonics (local normal faulting?) to the surface uplift. The data indicate an eastward shifting of the onset of topographic relief, Early Pliocene in the Apuane Alps and Early Pleistocene in the Mt. Falterona area.

Cooling and exhumation of the Rieserferner Pluton (Eastern Alps, Italy/Austria)

The Defereggen–Antholz–Vals (DAV) Line marks the southern border of alpine greenschist facies metamorphism within the Austroalpine basement to the south of the western Tauern window. During the Oligocene sinistral-strike slip, coeval with vertical displacement, was accompanied by the syntectonic emplacement of the Rieserferner Pluton along the north side of the fault. To the north of the DAV Line Rb/Sr as well as K/Ar-biotite cooling ages continuously increase from 16 Ma at the Rensen Pluton in the west to 30 Ma at the eastern end of the Rieserferner Pluton. Similar trends are apparent from zircon and apatite fission-track data, which increase eastward from 19 to 29 Ma, and 13 to 16 Ma, respectively. Low cooling rates allow a direct transformation into exhumation rates assuming an average geothermal gradient of 28 °C km–1. The resulting exhumation rates of less than 1 mm year–1 do not affect the depth of isotherms in a geologically significant manner. Hence, variable cooling rates within the basement north of the DAV Line are a result of differential exhumation of individual blocks. High exhumation rates bracketed between the intrusion age of the Rieserferner Pluton at 31±3 Ma and the apatite partial annealing zone (PAZ) range between 0.4 and 0.6 mm year–1, whereas final exposure takes place at reduced exhumation rates of 0.2 mm year–1. East-side-down tilting of the block north of the DAV Line, as indicated by variations in exhumation rates, strongly affects the western part of the area studied between the Rensen and the Central Rieserferner Pluton whereas, further east, block rotation becomes less important. A total difference in exhumation of about 2.2 km corresponding to 5° tilting between the Western and Eastern Rieserferner, as indicated by K/Ar-biotite and zircon fission-track data, occurred before going through the PAZ for apatite fission-tracks. Furthermore, the significance of the cooling ages, especially for biotite, is tested by calculating a finite element approximation. Different numerical models indicate fast cooling of the intrusive melts prior to approaching the closure temperature of the K/Ar system in biotite.

Two-dimensional thermal modelling of the early tectonometamorphic evolution in central Himalaya

The Higher Himalayan Crystallines is a major structural feature of the Himalayan belt characterized by a typical collisional and metamorphic evolution from Eocene to Miocene. We use two-dimensional thermal modelling to examine the velocity and the duration of the two main phases of metamorphism in the Higher Himalayan Crystallines. Our calculations suggest that the Eohimalayan metamorphism is not a simple consequence of the burial of the Indian plate below the upper Himalayan wedge. The M1 metamorphic conditions (700 °C, 1 GPa) recorded in the central Himalaya between 37 and 32 Ma corresponds to a period of thermal relaxation of the Higher Himalayan Crystallines. During this period, part of the Himalayan convergence had taken place in the Himalayan wedge along south-vergent thrusting structures, located north and south of the future Main Central Thrust (MCT) zone. Relatively high temperature (∼600 °C) recorded in the MCT between 23 and 18 Ma (Neohimalayan metamorphism) may be explained by high heat production rates, ∼3 μW/m 3, in the upper Himalayan crust and high exhumation rates, greater than 3 mm/year, of the Higher Himalayan Crystalline rocks. By analogy with modern accretionary wedges, we propose that the Eohimalayan phase corresponds to the period of the formation of the Himalayan wedge through several internal and external thrusting. During the subsequent Neohimalayan phase, the Himalayan wedge reached a thermal equilibrium, leading to the localisation of the Himalayan convergence only along the MCT zone.