6,039 research outputs found

    Early Cenozoic denudation of central west Britain in response to transient and permanent uplift above a mantle plume

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    Upwelling mantle plumes beneath continental crust are predicted to produce difficult to quantify, modest uplift and denudation. The contribution of permanent and transient components to the uplift is also difficult to distinguish. A pulse of denudation in Britain in the Early Paleogene has been linked, although with some controversy, with the arrival of the proto-Iceland mantle plume. In this contribution we show that combining apatite and zircon (U-Th-Sm)/He and apatite fission track analyses from central west Britain with numerical modeling clearly identifies a pulse of early Cenozoic denudation. The data indicate that rock uplift and denudation were centered on the northern East Irish Sea Basin and 1.0–2.4 km of rocks were removed during the latest Cretaceous-early Paleogene. Uplift and erosion appears to have started a few million years before the earliest magmatism in the region. The regional denudation pattern mirrors the distribution of low-density magmatic rocks that has been imaged in the deep crust. However, the injection of the underplating melt is not enough to account for the total denudation. An additional regional uplift of at least 300 m is required, which is consistent with a transient thermal effect from the hot mantle plume. The rapid exhumation event ceased by ~40 Ma and the data do not require significant Neogene exhumation

    How local crustal properties influence the amount of denudation derived from low-temperature: Comment

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    Late Cenozoic metamorphic evolution and exhumation of Taiwan

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    The Taiwan mountain belt is composed of a Cenozoic slate belt (Hsuehshan Range units, HR, and Backbone Slates, BS) and of accreted polymetamorphic basement rocks (Tananao Complex, TC). Ongoing crustal shortening has resulted from the collision between the Chinese continental margin and the Luzon volcanic arc, which initiated ~6.5 Ma ago. The grade and age of metamorphism and exhumation are a key record of the development of the orogenic wedge. Because the Taiwan mountain belt is mostly composed by accreted sediments lacking metamorphic index minerals, quantitative constraints on metamorphism are sparse. By contrast, these rocks are rich in carbonaceaous material (CM) and are therefore particularly appropriate for RSCM (Raman Spectroscopy of CM) thermometry. We apply this technique in addition to (U-Th)/He thermochronology on detrital zircons to assess peak metamorphic temperatures (T) and the late exhumational history respectively, along different transects in central and southern Taiwan. In the case of the HR units, we find evidence for high metamorphic T of at least 340°–350°C and locally up to 475°C, and for relative rapid exhumation with zircon (U-Th)/He ages in the range of 1.5–2 Ma. Farther east, the BS were only slightly metamorphosed (T < 330 °C), and zircons are not reset for (U-Th)/He. From the eastern BS to the inner TC schists, T gradually increases from ~350°C up to ~500°C following an inverted metamorphic gradient. Available geochronological constraints and the continuous thermal gradient from the BS to the basement rocks of the TC suggest that the high RSCM T of the TC were most probably acquired during the last orogeny, and were not inherited from a previous thermal event. Zircons yield (U-Th)/He ages of ~0.5–1.2 Ma. Peak metamorphic T and the timing of exhumation do not show along-strike variations over the TC in the studied area. In contrast, exhumation is laterally diachronous and decreases southward in the case of the HR units. In particular, our data imply that the HR units have been exhumed by a minimum of 15 km over the last few Ma. In the case of the BS, they show far less cumulated exhumation and much slower cooling rates. We propose that most of the deformation and exhumation of the Taiwan mountain belt is sustained through two underplating windows located beneath the Hsuehshan Range and the TC. Our data show significant departures from the predictions of the prevailing model in Taiwan, which assumes a homogeneous critical wedge with dominant frontal accretion. Our study sheds new light on how the mountain belt has grown as a possible result of underplating mostly

    Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography

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    Two end‐member kinematic models of crustal shortening across the Himalaya are currently debated: one assumes localized thrusting along a single major thrust fault, the Main Himalayan Thrust (MHT) with nonuniform underplating due to duplexing, and the other advocates for out‐of‐sequence (OOS) thrusting in addition to thrusting along the MHT and underplating. We assess these two models based on the modeling of thermochronological, thermometric, and thermobarometric data from the central Nepal Himalaya. We complement a data set compiled from the literature with 114 ^(40)Ar/^(39)Ar, 10 apatite fission track, and 5 zircon (U‐Th)/He thermochronological data. The data are predicted using a thermokinematic model (PECUBE), and the model parameters are constrained using an inverse approach based on the Neighborhood Algorithm. The model parameters include geometric characteristics as well as overthrusting rates, radiogenic heat production in the High Himalayan Crystalline (HHC) sequence, the age of initiation of the duplex or of out-of-sequence thrusting. Both models can provide a satisfactory fit to the inverted data. However, the model with out-of-sequence thrusting implies an unrealistic convergence rate ≥30 mm yr^(−1). The out-of-sequence thrust model can be adjusted to fit the convergence rate and the thermochronological data if the Main Central Thrust zone is assigned a constant geometry and a dip angle of about 30° and a slip rate of <1 mm yr^(−1). In the duplex model, the 20 mm yr^(−1) convergence rate is partitioned between an overthrusting rate of 5.8 ± 1.4 mm yr^(−1) and an underthrusting rate of 14.2 ± 1.8 mm yr^(−1). Modern rock uplift rates are estimated to increase from about 0.9 ± 0.31 mm yr^(−1) in the Lesser Himalaya to 3.0 ± 0.9 mm yr^(−1) at the front of the high range, 86 ± 13 km from the Main Frontal Thrust. The effective friction coefficient is estimated to be 0.07 or smaller, and the radiogenic heat production of HHC units is estimated to be 2.2 ± 0.1 µWm^(−3). The midcrustal duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr^(−1). We also run 3-D models by coupling PECUBE with a landscape evolution model (CASCADE). This modeling shows that the effect of the evolving topography can explain a fraction of the scatter observed in the data but not all of it, suggesting that lateral variations of the kinematics of crustal deformation and exhumation are likely. It has been argued that the steep physiographic transition at the foot of the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate that the best fit duplex model derived from the thermochronological and thermobarometric data reproduces the present morphology of the Nepal Himalaya equally well

    Thermal structure and exhumation history of the Lesser Himalaya in central Nepal

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    The Lesser Himalaya (LH) consists of metasedimentary rocks that have been scrapped off from the underthrusting Indian crust and accreted to the mountain range over the last ~20 Myr. It now forms a significant fraction of the Himalayan collisional orogen. We document the kinematics and thermal metamorphism associated with the deformation and exhumation of the LH, combining thermometric and thermochronological methods with structural geology. Peak metamorphic temperatures estimated from Raman spectroscopy of carbonaceous material decrease gradually from 520°–550°C below the Main Central Thrust zone down to less than 330°C. These temperatures describe structurally a 20°–50°C/km inverted apparent gradient. The Ar muscovite ages from LH samples and from the overlying crystalline thrust sheets all indicate the same regular trend; i.e., an increase from about 3–4 Ma near the front of the high range to about 20 Ma near the leading edge of the thrust sheets, about 80 km to the south. This suggests that the LH has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm/yr. For a convergence rate of about 20 mm/yr, this implies underthrusting of the Indian basement below the Himalaya by about 15 mm/yr. The structure, metamorphic grade and exhumation history of the LH supports the view that, since the mid-Miocene, the Himalayan orogen has essentially grown by underplating, rather than by frontal accretion. This process has resulted from duplexing at a depth close to the brittle-ductile transition zone, by southward migration of a midcrustal ramp along the Main Himalayan Thrust fault, and is estimated to have resulted in a net flux of up to 150 m^2/yr of LH rocks into the Himalayan orogenic wedge. The steep inverse thermal gradient across the LH is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units

    Crustal structure beneath the Trondelag Platform and adjacent areas of the Mid-Norwegian margin, as derived from wide-angle seismic and potential field data

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    The outer mid-Norwegian margin is characterized by strong breakup magmatism and has been extensively surveyed. The crustal structure of the inner continental shelf, however, is less studied, and its relation to the onshore geology, Caledonian structuring, and breakup magmatism remains unclear. Two Ocean Bottom Seismometer profiles were acquired across the Trøndelag Platform in 2003, as part of the Euromargins program. Additional-land stations recorded the marine shots. The P-wave data were modeled by ray-tracing, supported by gravity modeling. Older multi-channel seismic data allowed for interpretation of stratigraphy down to the top of the Triassic. Crystalline basement velocity is ~6 km s-1 onshore. Top basement is difficult to identify offshore, as velocities (5.3-5.7 km s-1) intermediate between typical crystalline crust and Mesozoic sedimentary strata appear 50-80 km from the coast. This layer thickens towards the Klakk-Ytreholmen Fault Complex and predates Permian and later structur-ing. The velocities indicate sedimentary rocks, most likely Devonian. Onshore late- to post-Caledonian detachments have been proposed to extend offshore, based on the magnetic anomaly pattern. We do not find the expected correlation between upper basement velocity structure and detachments. However, there is a distinct, dome-shaped lower-crustal body with a velocity of 6.6-7.0 km s-1. This is thickest under the Froan Basin, and the broad magnetic anomaly used to delineate the detachments correlates with this. The proposed offshore continuation of the detachments thus appears- unreliable. While we find indications of high density and velocity (~7.2 km s-1) lower crust under the Rås Basin, similar to the proposed igneous underplating of the outer margin, this is poorly constrained near the end of our profiles. The gravity field indicates that this body may be continuous from the pre-breakup basement structures of the Utgard High to the Frøya High, suggesting that it could be an island arc or oceanic terrane-accreted during the Caledonian orogeny. Thus, we find no clear evidence of early Cenozoic igneous underplating of the inner part of the shelf

    Mountain building in Taiwan: A thermokinematic model

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    The Taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening. However, a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge. We propose here a new thermokinematic model of the Taiwan mountain belt reconciling existing kinematic, thermometric and thermochronological constraints. In this model, shortening across the orogen is absorbed by slip on the most frontal faults of the foothills. Crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge (Tananao Complex, TC), where the uplift rate is estimated to ~6.3 mm a^(−1), and beneath the westernmost internal region of the orogen (Hsueshan Range units, HR), where the uplift rate is estimated to ~4.2 mm a^(−1). Our model suggests that the TC units experienced a synchronous evolution along strike despite the southward propagation of the collision. It also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures. Exhumation of the HR units increases northward but has not yet reached an exhumational steady state. Presently, frontal accretion accounts for less than ~10% of the incoming flux of material into the orogen, although there is indication that it was contributing substantially more (~80%) before 4 Ma. The incoming flux of material accreted beneath the TC significantly increased 1.5 Ma ago. Our results also suggest that the flux of material accreted to the orogen corresponds to the top ~7 km of the upper crust of the underthrust Chinese margin. This indicates that a significant amount (~76%) of the underthrust material has been subducted into the mantle, probably because of the increase in density associated with metamorphism. We also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field. By combining available geological data on the thermal and kinematic evolution of the wedge, our study sheds new light onto mountain building processes in Taiwan and allows for reappraising the initial structural architecture of the passive margin

    Styles of underplating in the Marin Headlands Terrane, Franciscan Complex, California

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    This is a pre-copy-editing, author-produced PDF of an article accepted for publication in The Geological Society of America Special Papers following peer review. The definitive publisher-authenticated version: "Regalla, C., Rowe, C., Harrichhausen, N., Tarling, M. and Singh, J., 2018. Styles of underplating in the Marin Headlands Terrane, Franciscan Complex, California. GSA Special Publications no. 534" is available online at: http://rock.geosociety.org/Store/detail.aspx?id=spe534.Geophysical images and structural cross-sections of accretionary wedges are usually aligned orthogonal to the subduction trench axis. These sections often reveal underplated duplexes of subducted oceanic sediment and igneous crust that record trench-normal shortening and wedge thickening facilitated by down-stepping of the décollement. However, this approach may under-recognize trench-parallel strain and the effects of faulting associated with flexure of the downgoing plate. New mapping of a recently exposed transect across a portion of the Marin Headlands terrane, California, USA documents evidence for structural complexity over short spatio-temporal scales within an underplated system. We document the geometry, kinematics, vergence and internal architecture of faults and folds along ~2.5 km of section, and identify six previously unmapped intra-formational imbricate thrusts and thirteen high-angle faults that accommodate shortening and flattening of the underthrust section. Thrust faults occur within nearly every lithology without clear preference for any stratigraphic horizon, and fold vergence varies between imbricate sheets by ~10-40°. In our map area, imbricate bounding thrusts have relatively narrow damage zones (≤5-10 m), sharp, discrete fault cores, and lack veining, in contrast to the wide, highly-veined fault zones previously documented in the Marin Headlands terrane. The spacing of imbricate thrusts combined with paleo-convergence rates indicates relatively rapid generation of new fault surfaces on ~10-100 ka timescales, a process which may contribute to strain hardening and locking within the seismogenic zone. The structural and kinematic complexity documented in the Marin Headlands are an example of the short spatial and temporal scales of heterogeneity that may characterize regions of active underplating. Such features are smaller than the typical spatial resolution of geophysical data from active subduction thrusts, and may not be readily resolved, thus highlighting the need for cross-comparison of geophysical data with field analogues when evaluating the kinematic and mechanical processes of underplating

    The lithospheric mantle and lower crust-mantle relationships under Scotland: a xenolithic perspective

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    In the British Isles the majority of volcanic rocks containing upper mantle and lower crustal xenoliths occur in Scotland. Most of the occurrences are of Carboniferous–Permian age. This paper presents new data on the mineral chemistry of spinel lherzolite xenoliths from the five principal Scottish tectonic terranes. Compositional variations among the minerals emphasize the broad lateral heterogeneity of the subcontinental lithospheric mantle across the region. The remarkable range of Al2O3 v. CaO exhibited by the clinopyroxenes compared with data from other ‘xenolith provinces' emphasizes the extremely complex tectonomagmatic history of the Scottish lithosphere. The generalized age increase from southern and central Scotland to the Northern Highland and Hebridean terranes of the north and NW, with concomitant complexity of geological history, is reflected also by trace element and isotopic studies. Reaction relationships in lherzolites from the Hebridean Terrane, owing to pervasive metasomatism, involve secondary growth of sodic feldspar. This, and light REE enrichment of clinopyroxenes, points to involvement of a natro-carbonatitic melt. Most pyroxenitic xenoliths are inferred to form a basal crustal layer with a generally sharp discontinuity above the underlying (dominantly lherzolitic) mantle. A second discontinuity is inferred to separate these ultramafic cumulates from overlying, broadly cognate metagabbroic cumulates
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