Thermochronological Ages Research Articles

The Exhumation History of North Qaidam Thrust Belt Constrained by Apatite Fission Track Thermochronology: Implication for the Evolution of the Tibetan Plateau

AbstractDetermining the spatio‐temporal distribution of the deformation tied to the India‐Eurasian convergence and the impact of pre‐existing weaknesses on the Cenozoic crustal deformation is significant for understanding how the convergence between India and Eurasia contributed to the development of the Tibetan Plateau. The exhumation history of the northeastern Tibetan Plateau was addressed in this research using a new apatite fission track (AFT) study in the North Qaidam thrust belt (NQTB). Three granite samples collected from the Qaidam Shan pluton in the north tied to the Qaidam Shan thrust, with AFT ages clustering in the Eocene to Miocene. The other thirteen samples obtained from the Luliang Shan and Yuka plutons in the south related to the Luliang Shan thrust and they have showed predominantly the Cretaceous AFT ages. Related thermal history modeling based on grain ages and track lengths indicates rapid cooling events during the Eocene‐early Oligocene and since late Miocene within the Qaidam Shan, in contrast to those in the Cretaceous and since the Oligocene‐Miocene in the Luliang Shan and Yuka region. The results, combined with published the Cretaceous thermochronological ages in the Qaidam Shan region, suggest that the NQTB had undergo rapid exhumation during the accretions along the southern Asian Andean‐type margin prior to the India‐Eurasian collision. The Cenozoic deformation initially took place in the North Qaidam thrust belt by the Eocene, which is consistent with the recent claim that the deformation of the northeastern Tibetan Plateau initiated in the Eocene as a response to continental collision between India and Eurasia. The immediate deformation responding to the collision is tentatively attributed to the pre‐existing weaknesses of the lithosphere, and therefore the deformation of the northeastern Tibetan Plateau should be regarded as a boundary‐condition‐dependent process.

Latitudinal variation in glacial erosion rates from Patagonia and the Antarctic Peninsula (46°S–65°S)

We use extensive sedimentary and marine geophysical data to derive sediment volume−based millennial time-scale glacial erosion rates ( Ē ) from glacially influenced fjords and bays across a broad latitudinal transect, from central Patagonia (46°S) to the Antarctic Peninsula (65°S), and to determine how glacial erosion rates change with increasing latitude and decreasing atmospheric temperatures. We also calculate million-year time-scale erosion rates for the western Antarctic Peninsula cordillera and inner continental shelf from seismic stratigraphic analysis of the continental margin. These results are complemented by erosion rates derived from existing thermochronology data sets (apatite fission-track and apatite [U-Th]/He) for both Patagonia and the Antarctic Peninsula regions. Despite considerable regional variability, our results show a clear trend of decreasing Ē with increasing latitude. Millennial Ē values span two orders of magnitude, from 0.02 mm/yr for Illiad glacier on Anvers Island, Antarctica (∼64.5°S), to 0.83 mm/yr for San Rafael glacier in northern Patagonia (∼46.5°S). Regional averages are three times higher for the Patagonian areas than the Antarctic Peninsula areas. This trend is interpreted to result from a general decrease in temperature and water availability at the ice-bedrock interface. For the Antarctic Peninsula study sites, erosion rates are highly clustered around 0.1 mm/yr, with the exception of Maxwell Bay, for which the Ē value is 0.36 mm/yr. In Patagonia, erosion rates are more variable than in the Antarctic Peninsula, with Ē ranging between 0.14 mm/yr (Europa glacier area) and 0.83 mm/yr (San Rafael glacier area). This regional variability in Ē is interpreted as due to differences in hypsometry and bedrock resistance to erosion. Million-year time-scale Ē values derived from thermochronology ages also decrease with latitude, with maximum values decreasing from ∼0.9−1.1 mm/yr north of 46°S to ∼0.1−0.2 mm/yr south of 48°S in Patagonia, and reaching ∼0.2−0.3 mm/yr in the Antarctic Peninsula. The sediment-based million-year time-scale Ē estimates for the western Antarctic Peninsula cordillera indicate that glacial erosion rates increased by 25%−30% after 5.3 Ma, from ∼0.09 mm/yr (5.3−9.5 Ma) to ∼0.11−0.12 mm/yr (<5.3 Ma). For Patagonia, the decrease in long-term erosion rates south of ∼46°S is interpreted to result from relatively long periods of slow glacial erosion associated with the ice masses having been colder (subpolar) on the southern Patagonian cordillera, and having eroded at rates comparable to those we obtained for the Antarctic Peninsula. These long-term erosion rates are 1−2 orders of magnitude lower than estimates based on recent sediment yields, highlighting the transient nature of high-sediment-flux events. However, our sediment volume−derived millennial time-scale Ē closely approximates the maximum values of tectonic time-scale Ē values derived from thermochronology ages. Our combined millennial and million-year time-scale glacial erosion data quantify the significant decrease in rates of glacially driven denudation at geological (tectonic) and millennial time scales with increasing latitude from Patagonia to the Antarctic Peninsula, highlighting the influence of climate on mountain denudation.

Constraining provenance, thickness and erosion of nappes using low‐temperature thermochronology: the Northland Allochthon, New Zealand

AbstractThe Northland Allochthon, an assemblage of Cretaceous–Oligocene sedimentary rocks, was emplaced during the Late Oligocene–earliest Miocene, onto the in situ Mesozoic and early Cenozoic rocks (predominantly Late Eocene–earliest Miocene) in northwestern New Zealand. Using low‐temperature thermochronology, we investigate the sedimentary provenance, burial and erosion histories of the rocks from both the hanging and footwalls of the allochthon. In central Northland (Parua Bay), both the overlying allochthon and underlying Early Miocene autochthon yield detrital zircon and partially reset apatite fission‐track ages that were sourced from the local Jurassic terrane and perhaps Late Cretaceous volcanics; the autochthon contains, additionally, material sourced from Oligocene volcanics. Thermal history modelling indicates that the lower part of the allochthon together with the autochthon was heated to ca. 55–100°C during the Late Oligocene and Early Miocene, most likely due to the burial beneath the overlying nappe sequences. From the Mesozoic basement exposed in eastern Northland, we obtained zircon fission‐track ages tightly bracketed between 153 and 149 Ma; the apatite fission‐track ages on the other hand, generally young towards the northwest, from 129 to 20.9 Ma. Basement thermochronological ages are inverted to simulate the emplacement and later erosion of the Northland Allochthon, using a thermo‐kinematic model coupled with an inversion algorithm. The results suggest that during the Late Oligocene, the nappes in eastern Northland ranged from ca. 4–6‐km thick in the north to zero in the Auckland region (over a distance &gt;200 km). Following the allochthon emplacement, eastern Northland was uplifted and unroofed during the Early Miocene for a period of ca. 1–6 Myr at the rate of 0.1–0.8 km/Myr, leading to rapid erosion of the nappes. Since Middle Miocene, the basement uplift ceased and the erosion of the nappes and the region as a whole slowed down (ca. 0–0.2 km/Myr), implying a decay in the tectonic activity in this region.

FetKin: Coupling kinematic restorations and temperature to predict thrusting, exhumation histories, and thermochronometric ages

FetKin is a C++ program for forward modeling thermochronological ages on a two-dimensional geological cross section. Modeled ages for various thermochronometers are computed from time–temperature histories that result from coupling the modeled kinematics of deformation obtained from commercial software for balanced reconstructions (2DMove) and a finite element computation of temperatures. Additional capabilities include the ability to accommodate (1) a smooth change of topological relief; (2) the influence of variation in rock physical properties; and (3) multikinetic modeling of fission-track ages and length distributions, as well as apatite and zircon (U-Th)/He and muscovite systems. A joint first-order analysis of the impact of erosion parameters and material properties improves age predictions and allows for a more complete analysis of observed cooling ages based on their modeled thermal histories. Thus, this paper presents a new software tool that has been developed as a basic support for the methodological approach used to build the kinematic restorations shown in this volume, which are the basic input for petroleum systems modeling and prediction in the Colombian Eastern Cordillera and Llanos foothills basin.

A Fourier approach for estimating and correcting the topographic perturbation of low-temperature thermochronological data

Thermochronology is a unique tool to reconstruct the long-term exhumation history of outcropping rocks. Pronounced (palaeo-) topography can markedly perturb near-surface isotherms, which can result in erroneous exhumation histories derived from age–elevation relationships but also offers the possibility to reconstruct palaeo-topography. Here we use a synthetic dataset to illustrate the complex non-linear relationships between the degree of topographic perturbation of thermochronological ages on one hand, and exhumation rate, geothermal gradient, and topographic wavelength and relief on the other. The dataset reveals that, in theory, relief changes can be retrieved for wavelengths as low as 5km, and wavelength changes are possible to detect for relief as low as 0.5km. In addition, the data attest that even in regions characterised by very slow exhumation rates (e.g. 0.03km/Ma), changes in palaeo-topography can be successfully retrieved. Coupling of this dataset with a Fast Fourier Transform (FFT) algorithm to decompose complex 2D topography into sinusoidal functions allows a rapid and accurate estimation of the topographic perturbation and resulting thermochronological ages assuming steady-state exhumation. This coupled method was successfully implemented to (i) predict most promising sample sites for the estimation of palaeo-topography and (ii) correct exhumation rates derived from non-vertical age–elevation profiles.

Topographic relief driven by variations in surface rock density

Earth’s surface topography is generated by tectonically induced variations in crustal thickness combined with erosion and, to a lesser degree, by vertical stresses caused by convection in the underlying mantle. Rock hardness and resistance to erosion are also commonly thought to influence topography because hard rocks, such as granites and basalts, usually form topographic highs in the landscape. Here we use analytical and numerical models to simulate the erosion-induced isostatic rebound of rocks. We find that the isostatic rebound that accompanies erosion causes denser rocks to occupy higher elevations in the landscape, thereby creating topographic relief that is proportional to surface rock density differences rather than rock hardness. We quantify this effect, taking into account the flexural strength of the continental lithosphere. We show that in a steady-state erosional setting, density-dependent isostatic rebound can cause the densest rocks to be exhumed at double the rate of surrounding, less-dense rocks and has a stronger effect than typical rock hardness variations. The results imply that denser rock formations should erode faster and therefore be characterized by younger thermochronological ages. Thermochronological data sets from the Kinabalu granite in Borneo and the Shakhdara–Alichur gneiss domes in Pamir confirm this counter-intuitive result. Our findings imply that lateral variations in surface rock density have significant control on the shaping of the large-scale features of Earth’s surface. The high elevation in Earth’s topography of hard rocks, such as granites and basalts, was thought to be caused by their inherent resistance to erosion. Numerical modelling now demonstrates, counterintuitively, that erosion-induced isostatic rebound of rocks, which is density dependent, causes granites and basalts to occupy high elevations because they are more dense than surrounding rocks.

Cenozoic erosion of the Transantarctic Mountains: A source-to-sink thermochronological study

The formation of the Transantarctic Mountains (TAM) is strictly related to the evolution of the West Antarctic Rift system, but the timing of their exhumation is still not fully assessed. In this work, we provide new apatite fission-track data collected on the region between the Royal Society Range and the Britannia Range. Cooling ages are late Eocene–Oligocene in the center of the region but they get older both northwards and southwards. We infer that exhumation was strictly controlled by TAM-parallel fault strands that were active after the Oligocene. The Royal Society Range and the Britannia Range represent transition zones corresponding to transverse structures, probably inherited from early basement crustal discontinuities and reactivated as transfer regions during rift propagation. The exhumation of the investigated region has been then modeled and predicted thermochronological ages have been compared with detrital data from the Miocene sedimentary succession drilled in the Victoria Land Basin. Results indicate that this sector of the TAM is the most probable candidate for the source of sediments and that during the Neogene 3km (but up to 5km) of rocks was exhumed.

Outward-growth of the Tibetan Plateau during the Cenozoic: A review

The surface uplift history of the Tibetan Plateau (TP) offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the Asian monsoon. Various models have been proposed to reveal the surface uplift history of the TP, but controversies remain. We evaluate these models using data from sedimentology and stratigraphy, structural geology, magmatism, exhumation, and paleoaltimetry studies. Structural analyses indicate that thrust belts, which spread from the central TP outward toward its surrounding margins, accommodated most of the India–Asia convergence, and facilitated crustal shortening and thickening in the central TP. Eocene adakitic rocks located in the Qiangtang and the Lhasa blocks likely were generated by partial melting of an eclogitic source. Paleogene (50–30 Ma) potassic rocks only occur in the Qiangtang block, whereas Late Oligocene–Late Miocene (26–8 Ma) potassic rocks occur both in the Qiangtang and Lhasa blocks. Low-temperature thermochronologic ages in the central TP are older than 40–35 Ma, whereas those in the margins are younger than 20 Ma (mostly Late Miocene, and Pliocene/Pleistocene in age). Independent paleoaltimetry estimates suggest that the Lhasa and Qiangtang terranes attained their current elevations during the Eocene, most likely due to the initial collision between India and Lhasa, whereas the Hoh Xil basin area to the north and Himalayas to the south were still low, even below sea level in the latter case. We argue for an inside-out growth pattern for the Tibetan Plateau. The TP grew southward and northward from a nucleus of high topography and is likely to continue expanding along the Mazar Tagh fault to the northwest, the Kuantai Shan-Hei Shan-Longshou Shan to the northeast, the Longquan Shan to the east and the Shillong plateau to the south if the northward force of India would not diminished.

Geometry and kinematics of the Main Himalayan Thrust and Neogene crustal exhumation in the Bhutanese Himalaya derived from inversion of multithermochronologic data

Both climatic and tectonic processes affect bedrock erosion and exhumation in convergent orogens, but determining their respective influence is difficult. A requisite first step is to quantify long-term (~106 year) erosion rates within an orogen. In the Himalaya, past studies suggest long-term erosion rates varied in space and time along the range front, resulting in numerous tectonic models to explain the observed erosion rate distribution. Here, we invert a large data set of new and existing thermochronological ages to determine both long-term exhumation rates and the kinematics of Neogene tectonic activity in the eastern Himalaya in Bhutan. New data include 31 apatite and five zircon (U-Th)/He ages, and 49 apatite and 16 zircon fission-track ages along two north-south oriented transects across the orogen in western and eastern Bhutan. Data inversion was performed using a modified version of the 3-D thermokinematic model Pecube, with parameter ranges defined by available geochronologic, metamorphic, structural, and geophysical data. Among several important observations, our three main conclusions are as follows: (1) Thermochronologic ages do not spatially correlate with surface traces of major fault zones but appear to reflect the geometry of the underlying Main Himalayan Thrust; (2) our data are compatible with a strong tectonic influence, involving a variably dipping Main Himalayan Thrust geometry and steady state topography; and (3) erosion rates have remained constant in western Bhutan over the last ~10 Ma, while a significant decrease occurred at ~6 Ma in eastern Bhutan, which we partially attribute to convergence partitioning into uplift of the Shillong Plateau.

Neogene rock uplift and erosion in northern Borneo: evidence from the Kinabalu granite, Mount Kinabalu

Thermochronological data from the Kinabalu granite, emplaced betweenc. 7.2 and 7.8 Ma, provide a unique record of northern Borneo’s exhumation during the Neogene. Biotite40Ar/39Ar ages (c. 7.32–7.63 Ma) record rapid cooling of the granite in the Late Miocene as it equilibrated with ambient crustal temperatures. Zircon fission-track ages (c. 6.6–5.8 Ma) and apatite (U–Th–Sm)/He ages (central agec. 5.5 Ma) indicate rapid cooling during the Late Miocene–Early Pliocene. This cooling reflects exhumation of the granite, uplift and erosion bringing it closer to the Earth’s surface. Thermochronological age versus elevation relationships suggest exhumation rates of more than 7 mm a−1during the latest Miocene and Early Pliocene. Neither the emplacement of the Kinabalu granite nor its exhumation is related to the Sabah orogeny, which terminated in the Early Miocene. Instead, granite magmatism was caused by extension related to subduction rollback of the Sulu Arc, and Mio-Pliocene exhumation of the Kinabalu granite was driven either by lithospheric delamination or break-off of a subducted slab beneath Sabah. Plio-Pleistocene tectonism offshore and onshore northern Borneo reflects continuing large-scale gravity-driven tectonics in the region.Supplementary material:Full40Ar/39Ar, fission-track and (U–Th–Sm)/He analytical data and40Ar/39Ar age spectra plots can be found atwww.geolsoc.org.uk/SUP18613.

Relations between denudation, glaciation, and sediment deposition: implications from the Plio‐Pleistocene Central Alps

AbstractDespite abundant data on the early evolution of the Central Alps, the latest stage exhumation history, potentially related to relief formation, is still poorly constrained. We aim for a better understanding of the relation between glaciation, erosion and sediment deposition. Addressing both topics, we analysed late Pliocene to recent deposits from the Upper Rhine Graben and two modern river sands by apatite fission‐track and (U‐Th‐Sm)/He thermochronology. From the observed age patterns we extracted the sediment provenance and paleo‐erosion history of the Alpine‐derived detritus. Due to their pollen and fossil record, the Rhine Graben deposits also provide information on climatic evolution, so that the erosion history can be related to glacial evolution during the Plio‐Pleistocene. Our data show that Rhine Graben deposits were derived from Variscan basement, Hegau volcanics, Swiss Molasse Basin, and the Central Alps. The relations between glaciation, Alpine erosion, and thermochronological age signals in sedimentary rocks are more complex than assumed. The first Alpine glaciation during the early Pleistocene did not disturb the long‐term exhumational equilibrium of the Alps. Recent findings indicate that main Alpine glaciation occurred at ca. 1 Ma. If true, then main Alpine glaciation was coeval with an apparent decrease of hinterland erosion rates, contrary to the expected trend. We suggest that glaciers effectively sealed the landscape, thus reducing the surface exposed to erosion and shifting the area of main erosion north toward the Molasse basin, causing sediment recycling. At around 0.4 Ma, erosion rates increased again, which seems to be a delayed response to main glaciation. The present‐day erosion regime seems to be dominated by mass‐wasting processes. Generally, glacial erosion rates did not exceed the pre‐glacial long‐term erosion rates of the Central Alps.

Multi-stage tectono-magmatic events of the Eastern Kunlun Range, northern Tibet: Insights from U–Pb geochronology and (U–Th)/He thermochronology

To understand the magmatic and tectonic events in the Eastern Kunlun Range, we conducted systematic zircon U–Pb geochronology, and paired zircon (ZHe) and apatite (AHe) (U–Th)/He themchronology investigations in the central and eastern segments of this range. Zircon U–Pb ages show two-stage magmatic events of Late Silurian and latest Permian–Early Jurassic. ZHe and AHe ages and multi-system thermochronometers reveal multi-stage rapid cooling events. From a combination of these data with previously published geochronological and thermochronological ages, and regional geological setting, we confirm several important Phanerozoic tectono-magmatic events in this region. Whereas the Silurian granites might be related to a small paleo-ocean subduction and subsequent collision, the latest Permian–Early Triassic granitoids were produced by northward subduction of the Songpan–Ganzi Paleo-Tethyan ocean. The Late Triassic–earliest Jurassic granitoids and Late Triassic–Early Jurassic exhumation are interpreted as response to the collision between Kunlun–Qaidam and Qiangtang. Samples close to the Kunlun Fault show rapid Late Oligocene–Early Miocene exhumation (1.3–1.6 km/Ma), whereas those from the northern part of central segment display extremely low exhumation rates (0.02–0.05 km/Ma). This heterogeneous denudation during 30–20 Ma between northern and southern part of the central segment requires that crustal thickening was completed by lower-crustal underthrusting with little exhumation or crustal thickening in pre-Cenozoic times. The activity of the Kunlun Fault and associated normal faulting component were probably responsible for this stage of rapid exhumation in the southern part. The complex tectono-magmatic evolution of the Eastern Kunlun Range is controlled by pre-Cenozoic paleo-ocean subductions and subsequent continent collisions and Paleogene India–Asia collision. • Zircon U-Pb, apatite and zircon (U-Th)/He ages from the Eastern Kunlun Range. • Zircon U-Pb ages confirm two-stage magmatic events. • Multi-system thermochronometers reveal multi-stage rapid cooling events. • The complex exhumation related to multi-stage tectono-magmatic events.

Exhumation History of the Gangdese Batholith, Southern Tibetan Plateau: Evidence from Apatite and Zircon (U-Th)/He Thermochronology

To test previously suggested exhumation histories of the Gangdese Batholith in the central part of the Transhimalayan plutonic belt, we conducted paired apatite and zircon (U-Th)/He thermochronological investigations of the Yarlung Zangbo gorge in the central part of the batholith. Age-elevation relationships and multisystem thermochronometers showed three periods of accelerated exhumation (∼46–48, ∼22–18, and ∼11–8 Ma). Combining these data with previously published thermochronological ages and synthesizing these ages with regional geological events provides an entire exhumation history. The Cretaceous–Early Paleogene exhumation of the Gangdese Batholith was probably caused by both the northward subduction of the Neo-Tethys and the collision between the Lhasa and Qiangtang blocks. The Early Miocene rapid exhumation might be a response to shortening caused by the Gangdese Thrust or erosion driven by dynamic uplift following lithospheric delamination. In contrast, the Late Miocene exhumation is coincident with both the proposed capture of the Yarlung Zangbo gorge by a foreland draining catchment and the intensification of the Asian monsoon, as well as normal faulting. Hence, the latest stage of exhumation might be attributed to the incision of the Yarlung Zangbo gorge, the activity of a north-south fault, or both.

Exhumation and relief development in the Pelvoux and Dora‐Maira massifs (western Alps) assessed by spectral analysis and inversion of thermochronological age transects

We have used dedicated sampling and analysis of apatite fission track (AFT) and apatite (U‐Th)/He (AHe) thermochronological data sets in an attempt to quantify relief evolution and exhumation rates in the Pelvoux and Dora‐Maira massifs (western European Alps). A dual approach comparing spectral analysis and thermal‐kinematic model inversion was applied. We sampled age‐elevation relationships at a range of topographic wavelengths along two north‐south transects crossing these massifs. For the 40‐km‐long Pelvoux transect we report 35 new AFT ages that range between 3.0 ± 0.4 Ma and 12.6 ± 1.0 Ma, and 8 new AHe ages between 3.5 ± 1.5 and 4.7 ± 0.7 Ma. The Dora‐Maira transect spans a distance of 60 km and includes 28 (23 new and 5 previously published) significantly older AFT ages, which vary between 13.1 ± 0.2 and 27.3 ± 0.3 Ma. Inferred exhumation rates of 0.7 km m.y.−1 since ∼8 Ma in the Pelvoux and only 0.1 km m.y.−1since ∼20 Ma in Dora‐Maira are consistent between methods as well as with previous estimates, although they are associated with large uncertainties due to imperfect sampling and analytical errors. Neither massif displays clear evidence for relief change during the intervals constrained by the data. Data from the Dora‐Maria massif suggest moderate recent relief increase, whereas the results from the Pelvoux are inconclusive but could imply relief decrease. Our study highlights the difficulties of applying thermochronology techniques to constrain relief changes. We show that an unreasonable number of samples is needed to perform reliable spectral analysis of age‐elevation transects and discuss the overall limitations of both techniques to address relief evolution. We suggest that a more efficient approach would be to apply thermal‐kinematic model inversion to relatively small study areas using spatially distributed sampling and multiple thermochronometer analyses per sample.

Plio-Plistocene in-sequence thrust propagation along the Main Central Thrust zone (Kumaon–Garhwal Himalaya, India): New thermochronological data

The Kumaon and Garhwal Himalayas, NW-India have similar geologic, tectonic, topographic and precipitation pattern. However, low-temperature thermochronological age pattern varies significantly along the strike as well as across the strike of major faults. Here, we interpret the new set of thermochronological age data across the strike of major faults namely Vaikrita Thrust (VT), Munsiari Thrust (MT), Berinag Thrust (BT) and the Baijnath nappe along Pindari valley in the Kumaon–Garhwal Himalaya. In the present work, 18 new apatite fission track (AFT) ages of samples collected along a north–south transect have been reported. Ages in the hanging wall of VT which range from 2.1±0.2 to 4.2±0.7Ma, have been found to be becoming younger linearly with distance from north to the VT. This trend crosses the VT with significant jump in ages. In the south of the VT, ages are lying between 0.3±0.1 and 3.9±0.8Ma, and show linear variation with distance from the VT to BT. No significant jump in ages across the MT is observed and the linear trend of younging southward is continuing till the BT. The break and displacement in age pattern with youngest ages close to thrusts possibly indicate progressive late thrust movement. It reflects sequential uplift and cooling towards the south from the VT to BT which is consistent with an in-sequence style of thrust propagation. We compare our new data to the published thermochronological data from its adjacent traverses along the Goriganga and Dhauliganga valleys (~25km away from the present traverse towards east and west respectively) in the Kumaon–Garhwal Himalaya in order to understand potential along-strike variations in the kinematics. Our observations indicate a dynamic coupling between tectonic and surface processes in the Garhwal–Kumaon Himalaya and support that the geometry of crustal scale faults and their associated kinematics control exhumation pathways of rocks.

Late Cretaceous Tectonic Change of the Eastern Margin of the Tibetan Plateau – Results from Multisystem Thermochronology

Abstract Partially due to lack of structural and sedimentary records to constrain the Jurassic-to-Cretaceous evolution, there was a missing process here in the eastern margin of Tibetan Plateau as it changed from the Paleo-Tethyan to Neo-Tethyan regime. Based on the analysis of 125 thermochronology ages (U/Pb, 40Ar/39A, 87Rb/86Sr, FT, U-Th/He) of igneous rocks from the eastern margin of Tibet, we propose a multisystem thermochronology approach to restore the cooling and emplacement of granites and decipher the missing process. Our integrated study suggests that a key Late Cretaceous (about 100Ma) tectonic change from the Paleo-Tethyan to Neo-Tethyan regime took place there. In the Late Triassic period, the initial emplacement of granite in the Songpan-Ganzi Fold Belt (SGFB) was characterized by a decrease in emplacement age and depth from west to east, and from north to south. Subsequently, all were followed by a very long period of slow cooling, which was followed by a rapid emplacement of about 100Ma. The intensive granite emplacement took place all over except northeastern SGFB, with a decrease in emplacement depth from west to east, which was linked with the far-field effect of Lhasa-Qiangtang collision. After this episode, the cooling history of granite in SGFB had a rapid emplacement on the subsurface under the control of the Neo-Tethyan regime. This process has control of the post Late Cretaceous regional magmatic activity and tectonics, as well as the sedimentary response in Sichuan and Xichang basin.

Surprisingly young Rb/Sr ages from the Simav extensional detachment fault zone, northern Menderes Massif, Turkey

The present paper demonstrates that exposed semi-brittle–brittle detachment fault zones, in addition to footwall mylonites and syn-extensional granitoids, can be used to date the timing of faulting and constrain the history and evolution of metamorphic core complexes. We employed Rb–Sr geochronology on micas, sampled directly from a part of the Simav detachment fault (SDF) zone in the northern Menderes Massif (western Turkey). The exposed part of the fault zone is marked by ∼3-m thick zone of low-grade mylonites/foliated cataclasites, in which mylonitic fabrics in orthogneisses are overprinted by fabrics of semi-brittle deformation. The low-grade mylonites/foliated cataclasites are characterized by coexistence of brown and green biotites. Rb–Sr ages on muscovite and brown and green biotite from the low-grade mylonites/foliated cataclasites are ca. 30 Ma, 17–13 Ma and 12–10 Ma, respectively; green biotite ages are interpreted as dating fluid-assisted deformation-induced dynamic recrystallization and suggest that a part of the SDF was active during a 12–10 Ma interval. The ca. 30 Ma muscovite ages date dynamic crystallization and probably beginning of extensional exhumation of the northern Menderes Massif. The coexistence of brown and green biotites in the same sample indicates retrogressive processes associated within a detachment faulting during which green biotites have recrystallized from primary brown biotites with an age of 19 ± 2 Ma in this area. This further means that the isotopic system became opened during faulting and that the green biotite ages therefore record the activity of the SDF. We have also dated an orthogneiss sample exposed well away from the detachment fault zone (devoid of any retrogressive processes); muscovites and biotites from this sample yield Rb–Sr ages of 45.7 ± 0.6 and 18.17 ± 0.18 Ma, respectively. The biotite age is in accord with regional biotite ages (19 ± 2 Ma) and record cooling of the footwall rocks of the detachment fault. We argue that the ca. 46 Ma muscovite age record synkinematic recrystallization during, and lend credibility to interpretation of, the prograde Barrovian-type regional Main Menderes Metamorphism as an Eocene event and that, prior to the onset of extensional detachment formation, the northern Menderes Massif (or at least the rocks we measured) experienced Eocene temperatures in excess of 500 °C to get this age. New brown biotite and secondary green biotite ages together with previously published age data argues that the footwall rocks reached to biotite cooling temperature conditions by ca. 18 Ma and stayed at similar conditions for a longer period until ca. 12 Ma. The ca. 18–12 Ma time interval corresponds to a period of tectonic quiescence during the Neogene and is attributed to the locking of SDF activity by the intrusion and rapid cooling of I-type syn-extensional granites. Secondary green biotite Rb–Sr ages indicate that the fault then become active during 12–10 Ma. Published low-temperature apatite–zircon fission track and apatite (U–Th)/He ages further suggest that displacement along the entire fault zone has continued until 8.0 ± 0.5 Ma. Combined with previous radiometric and low-temperature thermochronologic ages, the new ages argue for episodic core-complex formation and the Simav detachment fault activity between ca. 30 and 8 Ma. The data therefore argues for a complex history of core-complex formation in the northern Menderes Massif; it occurs in pulses and involves periods of alternating rapid and slow cooling/denudation rates.

Control of detachment geometry on lateral variations in exhumation rates in the Himalaya: Insights from low-temperature thermochronology and numerical modeling

[1] The Himalayan range is commonly presented as largely laterally uniform from west to east. However, geological structures, topography, precipitation rate, convergence rates, and low-temperature thermochronological ages all vary significantly along strike. Here, we focus on the interpretation of thermochronological data sets in terms of along-strike variations in geometry and kinematics of the main crustal detachment underlying the Himalaya: the Main Himalayan Thrust (MHT). We report new apatite fission track (AFT) ages collected along north-south transects in western and eastern central Nepal (at the latitudes of the Annapurna and Langtang massifs, respectively). AFT ages are consistently young (<3 Ma) along both N-S transects in the high-relief zone of the Higher Himalaya and increase (4 to 6 Ma) toward the south in the Lesser Himalaya. We compare our new data to published low-temperature thermochronological data sets for Nepal and the Bhutan Himalaya. We use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the Pecube code in order to constrain potential along-strike variations in the kinematics of the Himalayan range. Our results show that lateral variations in the geometry of the MHT (in particular the presence or absence of a major crustal-scale ramp) strongly control the kinematics and exhumation history of the orogen.

Síntesis geocronológica del magmatismo, metamorfismo y metalogenia de Costa Rica, América Central

Una recopilación completa de 651 determinaciones de edades radiométricas desde 1968 (415 dataciones 40Ar/39Ar, 211 K/Ar, 5 U/Th, 4 Rb/Sr, 2 U/Pb y de fisión termocronología con zircón), tanto aquellas publicadas desde 1968 como muchas nuevas, proporcionan un marco completo de la estratigrafía ígnea de Costa Rica y las inferencias acerca de la edad del metamorfismo y los eventos metalogenéticos. Las rocas ígneas del Jurásico Superior temprano al Eoceno Medio (~ 160 a ~ 41 Ma), corresponden principalmente con acreciones ofiolíticas. Durante el Campaniano (~ 71 Ma) comienza a establecerse la actual zona de subducción, conformada por rocas volcano-sedimentarias, con una composición desde básica a félsica. Sin embargo, las rocas volcánicas primarias, subalcalinas, in situ y abundantes, aparecen hasta después de los ~ 29 Ma. Los intrusivos e hipoabisales de granitoides hasta gabroides (plutones, stocks, diques y sills), están presentes de SE a NW en: la cordillera de Talamanca (~ 12,4 a 7,8 Ma), cerros de Escazú (~ 6,0 a 5,9 Ma), fila Costeña (~ 18,3 a 16,8 y ~ 14,8 a 11,1 Ma), Tapantí-montes del Aguacate-Carpintera (~ 4,2 a 2,2 Ma) y Guacimal (~ 6,4 a 5,2 Ma). Las rocas asociadas al arco con edades entre los 29 y 11 Ma (denominado Frente Proto- Volcánico), están presentes en la llanura de San Carlos y en el sur de Costa Rica. La ubicación y la edad de las rocas ígneas indican que el arco dio un giro de 20° al NW entre 15 y 8 Ma, con un polo de rotación que se centró en el sur de Costa Rica. Esta rotación se atribuye a la deformación en la placa superior (acortamiento en el sur y extensión en el noroeste), acompañado por el retroceso de la trinchera hacia el sur. A los ~ 3,45 Ma, el arco volcánico en el sur cesó su actividad, mientras que el vulcanismo adakítico localizado persistió en la cordillera de Talamanca (4,2 - 0,95 Ma) debido a la subducción de las Placa Cocos. El Frente Paleo-Volcánico está representada por rocas del arco (8 - 3,5 Ma) que se extienden a lo largo de Costa Rica, de forma paralela al arco volcánico moderno y fue seguido por el evento efusivo Monteverde (2,1 - 1,1 Ma), que progresivamente se retrocedió hacia el NE. El Frente Neovolcánico se estableció entre el 2,1 y el presente, con el crecimiento de los actuales volcanes, mayoritariamente hace unos 0,6 Ma y, en general, están representados por tres episodios, no isocrónicos entre volcanes: volcanes ancestrales entre 1,61 y 0,85 Ma (Proto-Cordillera), en parte coetánea con Monteverde; un evento importante cerca de ~ 0,74 - ~ 0,2 Ma (Paleo-Cordillera), y una actividad volcánica relativamente pequeña, pero activa aún en 0,25 - 0 Ma (Neo-Cordillera). El vulcanismo submarino en aguas del Pacífico costarricense, está representado por la serranía Fisher (30,0 Ma y 19,2 Ma), la cordillera del Coco (14,5 - 0,6 Ma) y el vulcanismo subaéreo de la isla del Coco (2,2 - 1,5 Ma). Los principales eventos magmáticos, metamórficos y metalogéneticos están relacionados con los principales eventos geotectónicos, incluyendo discordancias regionales en Cretácico Superior (~ 82 Ma), Eoceno Medio (~ 45 Ma) y el Mioceno Superior (~ 8 Ma).

Episodic exhumation and relief growth in the Mont Blanc massif, Western Alps from numerical modelling of thermochronology data

The Pliocene–Quaternary exhumational and topographic evolution of the European Alps and its potential climatic and tectonic controls remain a subject of controversy. Here, we apply inverse numerical thermal–kinematic modelling to a spatially dense thermochronological dataset (apatite fission-track and (U–Th)/He) of both tunnel and surface samples across the Mont Blanc massif in the Western Alps, complemented by new zircon fission-track data, in order to better quantify its Neogene exhumation and relief history. Age–elevation relationships and modelling results show that an episodic exhumation scenario best fits the data. Initiation of exhumation in the Mont Blanc massif at 22 ± 2 Ma with a rate of 0.8 ± 0.15 km/Myr is probably related to NW-directed thrusting during nappe emplacement. Exhumation rates decrease at 6 ± 2 Ma to values of 0.15 ± 0.65 km/Myr, which we interpret to be the result of a general decrease in convergence rates and/or extensive exposure of less erodible crystalline basement rocks from below more easily erodible Mesozoic sediments. Finally, local exhumation rates increase up to 2.0 ± 0.6 km/Myr at 1.7 ± 0.8 Ma. Modelling shows that this recent increase in local exhumation can be explained by valley incision and the associated increase in relief at 0.9 ± 0.8 Ma, leading to erosional unloading, isostatic rebound and additional rock uplift and exhumation. Given the lack of tectonic activity as evidenced by constant thermochronological ages along the tunnel transect, we suggest that the final increase in exhumation and relief in the Mont Blanc massif is the result of climate change, with the initiation of mid-Pleistocene glaciations leading to rapid valley incision and related local exhumation.