Differential Exhumation Research Articles

Cenozoic exhumation of the Danba antiform, eastern Tibet: Evidence from low-temperature thermochronology

The Danba antiform (DA) exposes the highest grade metamorphic rocks in eastern Tibet. The metamorphic grades characterizing the DA evolve from sillimanite-migmatite grade to greenschist grade over a relatively short distance of ∼20 km from core to limb. This metamorphic event indicates an important Mesozoic to Cenozoic doming and exhumation history. However, the Cenozoic history of the antiform is poorly constrained due to a lack of data. Consequently, we used fission track dating on zircon and apatite from 22 samples collected throughout the DA. The zircon fission track (ZFT) data show a transition from Cenozoic non-reset (202 Ma), to partially reset (53–37 Ma), to totally reset (24–8 Ma) ages from the periphery to the core of the DA. The oldest totally reset ZFT ages are ca. 25 Ma and likely indicate the onset of Cenozoic folding of the DA. Compared to the apatite fission track (AFT) ages of ca. 10 Ma in the peripheral region, the youngest AFT ages are younger than 3 Ma in the core of the DA, suggesting that folding could be ongoing. Based on these multithermochronometer data, the cooling rate increases from ∼8 °C/m.y. on the periphery to ∼12–56 °C/m.y. in the core of the DA since ca. 12 Ma. The DA shares a similar cooling history with the Longmen Shan (LMS) fault belt, implying that the detachment fault of the LMS may extend to the DA, resulting in linked uplift histories. The differential exhumation among the samples in the core of the DA and the surrounding area indicates that both upper crustal deformation and crustal channel flow may have developed simultaneously (mainly since ca. 12 Ma) in the DA.

Thermal and exhumation history of Sakhalin Island (Russia) constrained by apatite U-Pb and fission track thermochronology

Sakhalin Island represents a key locality to study the tectonic evolution of the western Pacific. The island is located at the Amur-Okhotsk plate margin and records a complex thermotectonic history. Apatite double dating (U-Pb and fission track) and thermal history modelling were applied to three late Eocene granitoid massifs within central and southern Sakhalin: the Aniva, Okhotsk and Langeri complexes. Apatite U-Pb results yield consistent late Eocene (∼40–37Ma) ages, suggesting rapid post-magmatic cooling. Apatite fission track results reveal bimodal age distributions with late Eocene – early Oligocene (∼38–33Ma) and early Miocene (∼20–17Ma) age populations that can be correlated with variations in Uranium and Chlorine concentrations. Thermal history modelling translates the AFT age bimodality into two-phase cooling histories. The timing of the early cooling phase (∼38–33Ma) corresponds with the apatite U-Pb ages, indicating rapid cooling to at least ∼100°C during the late Oligocene. The second cooling phase at ∼20–17Ma cooled the samples to near-surface temperatures. Both cooling phases correspond with regional unconformities and subsequent accelerations in sedimentation rate, suggesting that cooling was a response to rapid exhumation. In addition, our data suggests that the studied terranes record differential exhumation with respect to the structural architecture. The Miocene exhumation pulse is coeval with the timing of transpressional fault displacement and the subsequent opening of the Kuril Basin.

Thermal history and differential exhumation across the Eastern Musgrave Province, South Australia: Insights from low-temperature thermochronology

Multi–method geo- and thermochronological data obtained for Palaeo- and Mesoproterozoic granitoids traversing the main structural architecture of the eastern Musgrave Province within South Australia reveal multiphase cooling histories. Apatite U-Pb dating on six samples yield consistent ages of ~1075–1025Ma, suggesting a thermal reset coinciding with mantle-derived magmatism of the greater Warakurna Large Igneous Province (~1080–1040Ma). Apatite fission track (AFT) analysis indicate that four discrete thermal events affected the study area, inducing cooling through the AFT partial annealing zone (~60–120°C), supported by apatite and zircon (U-Th-Sm)/He data. Late Neoproterozoic cooling from deep crustal levels to temperatures <200°C was discerned, which is thought to be related to exhumation and denudation during the Petermann Orogeny. Subsequent cooling events at ~450–400Ma (Silurian–Devonian) and ~310–290Ma (Late Carboniferous) are interpreted to represent exhumation associated with the Alice Springs Orogeny. The latter event exhumed the sampled plutons to shallow crustal depths. An additional Triassic – early Jurassic thermal event, likely recording elevated geothermal gradients at that time, was observed throughout the study area, however, more data is needed to further support this interpretation. The high sample density across the structural architecture of the study area furthermore reveals patterns of fault reactivation and resulting differential exhumation, indicating shallower exhumation levels in the centre and deeper exhumation towards the margins of the sampled transect. The observed differential exhumation patterns match with existing seismic data and fit a model of an inverted graben system for the Phanerozoic evolution of the eastern Musgraves.The results highlight a complex Phanerozoic thermal history for the eastern Musgraves and help to elucidate the poorly appreciated tectonic evolution of inland Australia. This study further demonstrates how high-density sample transects across structural architecture can assess the relative crustal level and associated preservation of the thermal history record within fault-reactivated terranes.

The role and extent of dextral transpression and lateral escape on the post-Acadian tectonic evolution of south-central New England

Bedrock mapping, structural analysis, documentation of kinematics and associated petrofabrics, and detailed zircon and monazite geochronology are the basis for a new interpretation of the post-Acadian tectonic evolution of southern New England. Early fabrics in the Central Maine Zone and eastern half of the Bronson Hill Zone are characterized by planar foliations, subhorizontal lineations, and dextral kinematic indicators. As transpression progressed, structures and fabrics reflected an increased component of shortening, marked by tightening of foliations, development of closed to isoclinal folds, localization of reverse high strain zones, steep dip-parallel lineations, and reverse top-to-east kinematic indicators. Syntectonic mineral assemblages show that cooling attended deformation, consistent with a component of vertical extrusion. In contrast, the western half of the Bronson Hill Zone is characterized by sinistral top-to-south deformation from Connecticut to New Hampshire, including two regional sinistral shear zones, and comprise the Western Bronson Hill Shear System. New zircon and monazite geochronology, along with previously published results, show that dextral transpression progressed continuously from ∼350 Ma to ∼295 Ma. Dextral transpression was initiated following ∼360 Ma to ∼355 Ma dioritic, tonalitic, and granitic magmatism. Sillimanite-grade reverse high strain zones developed by ∼330 Ma, while the presence of lower-grade fabrics, metamorphic zircon rims, and published hornblende cooling ages show deformation continued to at least ∼295 Ma in the eastern Bronson Hill and Central Maine. Sinistral top-to-south deformation in the Western Bronson Hill Shear System preceded ∼330 Ma to ∼300 Ma staurolite grade metamorphism, and syntectonic monazite and an abundance of published data show that deformation continued to ∼285 Ma. The conjugate shear zones were spatially and temporally linked, indicating that the dominant fabrics, structures, and mineral assemblages in bedrock exposures in this area of New England were driven by progressive dextral transpression and lateral escape in the Carboniferous, much younger than any previous tectonic reconstruction. This model can explain an assortment of enigmatic features in New England, including contrasting kinematics, differential exhumation of lithologies and lithotectonic zones, and the Pelham dome. These features are attributed to oblique convergence between Laurentian margin and Avalon/Meguma beginning in the Mississippian, rather than the Early to Middle Devonian Acadian orogeny as has long been proposed. The location between the New York promontory and the Quebec embayment could have provided the necessary geometry to focus deformation and lateral escape in New England, and shorten the orogen in this area by almost an order of magnitude.

Pulsed exhumation of interior eastern Tibet: Implications for relief generation mechanisms and the origin of high-elevation planation surfaces

River incision into a widespread, upland low-relief landscape, and related patterns of exhumation recorded by low-temperature thermochronology, together underpin geodynamic interpretations for crustal thickening and uplift of the eastern Tibetan Plateau. We report results from a suite of 11 (U–Th–Sm)/He cooling-age samples. Eight samples comprise a 1.2 km relief section collected from elevations up to 4800 m in the Jiulong Shan, an elevated, rugged region located in the hinterland of the Yalong-Longmen Shan Thrust Belt, and surrounded on three sides by upland low-relief landscape surfaces. Zircon and apatite cooling ages record two episodes of rapid exhumation in the early Oligocene and late Miocene, that were separated by a period of stability from ∼30 to 15 Ma. The first episode is consistent with a similar pulse evident from the Longmen Shan. The second episode is ongoing, and when integrated with adjacent cooling-age data sets, shows that doming of the Jiulong Shan has resulted in 2 to 4 km of differential exhumation of the plateau interior. We show from a compilation of glacial landform-mapping that the elevation of the plateau surface closely tracks global last glacial maximum equilibrium line altitude. We hypothesize that smoothing of highlands by efficient glacial and periglacial erosion, coupled with potential river captures and conveyance of sediments via external drainage, can yield an apparently continuous low-relief plateau landscape formed diachronously at high elevation.

From nappe stacking to exhumation: Cretaceous tectonics in the Apuseni Mountains (Romania)

New Ar–Ar muscovite and Rb–Sr biotite age data in combination with structural analyses from the Apuseni Mountains provide new constraints on the timing and kinematics of deformation during the Cretaceous. Time–temperature paths from the structurally highest basement nappe of the Apuseni Mountains in combination with sedimentary data indicate exhumation and a position close to the surface after the Late Jurassic emplacement of the South Apuseni Ophiolites. Early Cretaceous Ar–Ar muscovite ages from structurally lower parts in the Biharia Nappe System (Dacia Mega-Unit) show cooling from medium-grade conditions. NE–SW-trending stretching lineation and associated kinematic indicators of this deformation phase (D1) are overprinted by top-NW-directed thrusting during D2. An Albian to Turonian age (110–90 Ma) is proposed for the main deformation (D2) that formed the present-day geometry of the nappe stack and led to a pervasive retrograde greenschist-facies overprint. Thermochronological and structural data from the Bihor Unit (Tisza Mega-Unit) allowed to establish E-directed differential exhumation during Early–Late Cretaceous times (D3.1). Brittle detachment faulting (D3.2) and the deposition of syn-extensional sediments indicate general uplift and partial surface exposure during the Late Cretaceous. Brittle conditions persist during the latest Cretaceous compressional overprint (D4).

New (U-Th)/He titanite data from a complex orogen–passive margin system: A case study from northern Mozambique

New titanite (U-Th)/He (He) data on basement rocks from NE Mozambique are presented. The objective was to test the applicability of titanite He thermochronology in an orogen–passive margin setting and to better constrain the exhumation history across the Lurio Belt, a major structural discontinuity in Mozambique. Therefore, samples from existing geochronological and thermochronological studies were dated using titanite He thermochronology. Resulting titanite He data (from abraded crystals) provide average cooling ages from 178 ± 15 to 383 ± 23 Ma. The data fit well into the age pattern obtained from previous thermochronological studies in NE Mozambique, revealing differential exhumation across the Lurio Belt. The basement to the north experienced earlier cooling than that to the south, while overall youngest titanite He ages are from the Lurio Belt, indicating reactivation linked to the post-collisional extension and break-up of Gondwana. Thermal history modelling revealed two possibilities, able to account for the different cooling histories of NE Mozambique since initial Gondwana break-up in Permian times: One involves a transient sedimentary overburden that buried and (re)heated the southern basement, with subsequent basin inversion at ∼250 Ma in response to rift shoulder uplift. The second model implies delayed cooling of the southern basement, possibly due to delamination of the crustal root shortly after Gondwana formation. The formerly upwelling asthenosphere and the subsequently formed sag basin might have caused a prolonged thermal effect. Titanite He ages and thermal histories point to rift shoulder uplift of the southern part and increased thermal activity within the reactivated Lurio Belt, signifying first rifting activities as precursor of Gondwana break-up.

From ocean depths to mountain tops: Uplift of the Troodos ophiolite (Cyprus) constrained by low‐temperature thermochronology and geomorphic analysis

AbstractThe timing and mode of uplift of the Troodos ophiolite are constrained by low‐temperature thermochronology combined with geomorphic analysis. Zircon (U‐Th)/He and apatite fission track cooling ages in the Troodos plutonic sequence are all Cretaceous (83–106 Ma) and within error of published zircon U‐Pb crystallization ages. This indicates early cooling of the oceanic crust and termination of spreading axis magmatism at ~90 Ma. Apatite (U‐Th)/He ages decrease with reconstructed crustal depths from ~40 Ma near the top of the sheeted‐dike complex to ~4 Ma within the mantle sequence. A prominent inflection point in the age versus depth curve defines the bottom of the exhumed helium partial retention zone and records the onset of rapid exhumation of the main Troodos massif at 6 ± 2 Ma. Inverse thermal modeling supports this conclusion, indicating that the timing of uplift is earlier than previously estimated. The boundaries of the mantle sequence exposed in the core of the Troodos structure closely overlap the boundaries of a concentric zone delineated by high local relief and higher channel steepness indices, indicating differential exhumation and uplift of this area relative to its surroundings. This zone also overlaps with a prominent negative Bouguer gravity anomaly. The timing and pattern of the Troodos ophiolite uplift suggest that it is driven by serpentinite diapirism, possibly triggered by Miocene reactivation of subduction along the Cyprean Arc. The worldwide ubiquity of suprasubduction zone ophiolites may thus reflect the importance of extensive serpentinization at the overthrusting mantle wedge in obduction processes.

Post‐breakup burial and exhumation of the southern margin of Africa

AbstractDespite many years of study, the processes involved in the development of the continental margin of southern Africa and the distinctive topography of the hinterland remain poorly understood. Previous thermochronological studies carried out within a monotonic cooling framework have failed to take into account constraints provided by Mesozoic sedimentary basins along the southern margin. We report apatite fission track analysis and vitrinite reflectance data in outcrop samples from the Late Jurassic to Early Cretaceous sedimentary fill of the Oudtshoorn, Gamtoos and Algoa Basins (Uitenhage Group), as well as isolated sedimentary remnants further west, plus underlying Paleozoic rocks (Cape Supergroup) and Permian‐Triassic sandstones from the Karoo Supergroup around the Great Escarpment. Results define a series of major regional cooling episodes. Latest Triassic to Early Jurassic cooling which began between 205 and 180 Ma is seen dominantly in basement flanks to the Algoa and Gamtoos Basins. This episode may have affected a wider region but in most places any effects have been overprinted by later events. The effects of Early Cretaceous (beginning between 145 and 130 Ma) and Early to mid‐Cretaceous (120–100 Ma) cooling are both delimited by major structures, while Late Cretaceous (85–75 Ma) cooling appears to have affected the whole region. These cooling events are all interpreted as dominantly reflecting exhumation. Higher Late Cretaceous paleotemperatures in samples from the core of the Swartberg Range, coupled with evidence for localised Cenozoic cooling, are interpreted as representing Cenozoic differential exhumation of the mountain range. Late Cretaceous paleotemperatures between 60°C and 90°C in outcropping Uitenhage Group sediments from the Oudtshoorn, Gamtoos and Algoa Basins require burial by between 1.2 and 2.2 km prior to Late Cretaceous exhumation. Because these sediments lie in depositional contact with underlying Paleozoic rocks in many places, relatively uniform Late Cretaceous paleotemperatures across most of the region, in samples of both basin fill and underlying basement, suggest the whole region may have been buried prior to Late Cretaceous exhumation. Cenozoic cooling (beginning between 30 and 20 Ma) is focussed mainly in mountainous regions and is interpreted as representing denudation which produced the modern‐day relief. Features such as the Great Escarpment are not related to continental break up, as is often supposed, but are much younger (post‐30 Ma). This history of post‐breakup burial and subsequent episodic exhumation is very different from conventional ideas of passive margin evolution, and requires a radical re‐think of models for development of continental margins.

Veining and post-nappe transtensional faulting in the SW Helvetic Alps (Switzerland)

In the Rawil Depression of the south-western Helvetic Alps, oblique (normal plus dextral strike slip) faults are common but their relative age, regional role and the processes leading to their development are not yet fully determined. This field study establishes the orientation and distribution of these faults and associated veins, the fault geometries and kinematics, and the relationship between veining and faulting. Three post-nappe sets of faults can be distinguished on the basis of their strike: (1) NNW/NW-striking; (2) WNW/W-striking; and (3) WSW-striking ones. Faults sets (1) and (2) generally dip at moderate angle to the SW and typically develop domino-like structures, with a spacing of around 1 km. Fault set (3) is steeper, the strike-slip component is larger, and it is directly associated with the main regional-scale branch of the Simplon–Rhone Fault. Although these faults are broadly coeval, there are clear examples of set (2) cross-cutting set (1), and set (3) cross-cutting (1) and (2), which establishes, at least locally, a relative chronological succession. This transtensional faulting largely post-dates folding related to nappe formation because fold geometry can be matched across the obliquely cross-cutting faults. Regional dextral-transtensional fault development is related to differential exhumation of the External Crystalline Massifs over the last 15–17 Ma, coeval with related movement on the Simplon–Rhone Fault. Locally there is a transition from an initial more ductile mylonitic fabric to cataclasite, accompanied by brittle-ductile veining and intense pressure solution. This progressive embrittlement during faulting is due to exhumation and cooling during faulting, higher strain rates, or increased pore-fluid pressures. Faults of sets (1) and (2) developed across the brittle-ductile transition and may represent fossil seismogenic zones in rocks with high pore-fluid pressure, providing exposed examples of seismic faults in similar rocks currently active at depth north of the Rhone Valley.

3D structural and stratigraphic model of the Perth Basin, Western Australia: Implications for sub-basin evolution

The history of rifting and breakup of eastern Gondwana is recorded by the development and fill of the Perth Basin in Western Australia. Knowledge of the structural architecture and stratigraphic geometry of the Perth Basin is essential to understand the evolution of the Western Australian margin and its applications to hydrocarbon and geothermal prospects, and effective aquifer management. This study integrates existing, publicly available geological, gravity, magnetic and digital elevation data to develop the first refined, regional structural and stratigraphic interpretation of the entire onshore and offshore Perth Basin, Western Australia. This new 3D model offers formation depth and thickness predictions in areas of sparse or no data. The model shows significant heterogeneity in the preserved formation thicknesses and depths at both local and regional scales. These differences may have resulted from differential subsidence and/or differential exhumation, but the formation geometries alone cannot distinguish between these two models if any erosion has occurred. Only the Lower to Middle Jurassic stratigraphy has been minimally eroded and thereby records the net subsidence. This stratigraphic interval shows that subsidence was broadly hinged from south to north, with a greater subsidence rate in the southern and central Perth Basin. Localised differences in thicknesses across adjacent sub-basins were likely controlled by differential displacement along sub-basin bounding faults during subsidence and, subsequently, during exhumation episodes. This new 3D model of the entire Perth Basin provides a framework for numerical simulations of fluid and heat flow and large-scale tectonic analysis, such as stratigraphic forward modelling of the southwestern Australian margin.

Differential exhumation and cooling history of North Qaidam UHP metamorphic rocks, NW China: Constraints from zircon and rutile thermometry and U–Pb geochronology

We analyze zircon and rutile U–Pb ages and trace elements using LA-ICP-MS and SIMS of eclogites from the North Qaidam ultra-high-pressure metamorphic belt in NW China. Zr-in-rutile and Ti-in-zircon thermometries give consistent peak metamorphic temperatures for two fresh eclogite samples. Zircon and rutile U–Pb dating from the Yuka eclogite yielded ages of 430.2 ± 3.7 Ma and 407 ± 16 Ma respectively. Zircons from North Dulan eclogite record two distinct eclogite-facies metamorphic events at 446.6 ± 4.1 and 430.5 ± 4.7 Ma, with T Ti-in-zircon = 684 ± 16 °C and 649 ± 26 °C respectively, while rutile U–Pb dating for the same North Dulan eclogite sample yielded an age of 416 ± 10 Ma. These rutile U–Pb ages are interpreted to record the time when these rocks cooled down to the closure temperature of Pb in rutile (here, taken to be ca . 570 °C). Thus, we obtain a slow cooling rate of ~ 3–4 °C/Ma for Yuka and North Dulan terranes. For one granulite-facies overprinted South Dulan eclogite sample, rutile U–Pb dating gave a younger age of 383 ± 10 Ma. Combined with literature chronological and temperature data for South Dulan, the data imply later exhumation than the other samples with a faster cooling rate of ~ 6.6 °C/Ma, after the granulite-facies event. The T – t paths obtained show that the North Qaidam eclogites experienced considerably slower cooling history than previously assumed, and also that while the thermal histories of the Yuka and North Dulan eclogites resemble one another, the South Dulan eclogite is quite distinct. So, a differential exhumation and emplacement of UHP metamorphic rocks occurred in the North Qaidam. • Rutile U–Pb dating constrains the cooling age of eclogite from North Qaidam. • Decoupling of Zr-in-rutile thermometry and rutile U–Pb dating • Differential exhumation and cooling rate deduced from combination of rutile, zircon U–Pb dating and thermometry

The effect of inherited paleotopography on exhumation of the Central Andes of NW Argentina

Differential exhumation in the Puna Plateau and Eastern Cordillera of NW Argentina is controlled by inherited paleostructures and resulting paleotopography related to the Cretaceous Salta Rift paleomargins. The Ceno zoic deformation front related to the development of the Andean retro-arc orogenic system is generally associated with >4 km of exhumation, which is recorded by Cenozoic apatite fi ssion-track (AFT) and (U-Th-[Sm])/He ages (He ages) in the Eastern Cordillera of NW Argentina. New AFT ages from the top of the Nevado de Cachi document Oligocene (ca. 28 Ma) cooling, which, combined with existing data, indicates exhumation of this range between ca. 28 Ma and ca. 14 Ma. However, some of the highest ranges in the Eastern Cordillera preserve Cretaceous ages indicative of limited Cenozoic exhumation. Samples collected from an similar to 3-km-elevation transect along the northern part of the Sierra de Quilmes paleorift fl ank (Laguna Brava) show AFT ages between ca. 80 and ca. 50 Ma and He ages between ca. 45 and ca. 10 Ma. Another set of samples from an similar to 1-km-elevation transect farther to the southwest (La Quebrada) shows Cretaceous AFT ages between ca. 116 Ma and ca. 76 Ma, and mainly Cretaceous He ages, in agreement with AFT data. Analysis of existing AFT and He ages from the area once occupied by the Salta Rift reveals a pattern characterized by Cretaceous ages along paleorift highs and Cenozoic ages within paleorift hanging-wall basins and later foreland basin depocenters. This pattern is interrupted by the Sierras Pampeanas at similar to 28 degrees S, which record mid-Cenozoic ages. Our data are consistent with a complex inherited pattern of pre-Andean paleostructures, likely associated with paleotopography, which was beveled by the Cenozoic regional foreland basin and reactivated during the late Neogene (ca. <10 Ma), strongly controlling the magnitude of Cenozoic uplift and exhumation and thus cooling age distribution. This, combined with variable lithologic erodibility, resulted in an irregular distribution of themochronological ages.

Thermal history of the Maramureş area (Northern Romania) constrained by zircon fission track analysis: Cretaceous metamorphism and Late Cretaceous to Paleocene exhumation

Abstract This study presents zircon fission track data from the Bucovinian nappe stack (northern part of the Inner Eastern Carpathians, Rodna Mountains) and a neighbouring part of the Biharia nappe system (Preluca massif) in order to unravel the thermal history of the area and its structural evolution by integrating the fission track data with published data on the tectonic and sedimentary evolution of the area. The increase of metamorphic temperatures towards the SW detected by the zircon fission track data suggests SW-wards increasing tectonic overburden (up to at least 15 km) and hence top NE thrusting. Sub-greenschist facies conditions during the Alpine metamorphic overprint only caused partial annealing of fission tracks in zircon in the external main chain of the Central Eastern Carpathians. Full annealing of zircon points to at least 300 °C in the more internal elements (Rodna Mountains and Preluca massif). The zircon fission track central and single grain ages largely reflect Late Cretaceous cooling and exhumation. A combination of fission track data and stratigraphic constraints points to predominantly tectonic differential exhumation by some 7-11 km, connected to massive Late Cretaceous extension not yet detected in the area. Later events such as the latest Cretaceous (“Laramian”) juxtaposition of the nappe pile with the internal Moldavides, causing exhumation by erosion, re-burial by sedimentation and tectonic loading during the Cenozoic had no impact on the zircon fission track data; unfortunately it prevented a study of the low temperature part of the Late Cretaceous exhumation history.

Constraints on Cenozoic tectonics in the southwestern Longmen Shan from low-temperature thermochronology

The Longmen Shan, located at the boundary between the Tibetan Plateau and the Sichuan Basin, has received considerable attention following the 2008 Wenchuan earthquake. However, the tectonic history of the southwestern segment of the range has remained poorly constrained. We present zircon fission-track, zircon (U-Th)/He, and apatite (U-Th)/He data from the Baoxing region in the southwestern Longmen Shan that provide the first constraints on the cooling and exhumation history of the region. All of the measured ages are Cenozoic, and the data suggest that exhumation of the Baoxing region was ongoing by ca. 15 Ma. Zircon (U-Th)/He ages from several samples appear to be affected by radiation damage, suggesting that damage may be a concern even in samples with Cenozoic cooling ages. Samples were collected from two bodies of Precambrian crystalline rocks separated by the Wulong fault, and for all three thermochronometers, ages west of the Wulong fault are systematically younger than ages to the east, indicating that the fault has accommodated differential exhumation since 8–10 Ma. The regions east and west of the Wulong fault have experienced 7–13 km and at least 7–10 km of exhumation, respectively. The magnitude of exhumation in the southwestern Longmen Shan is similar to that reported in the central Longmen Shan, indicating consistency along strike. The thermochronology data also suggest that the Erwangmiao fault in the southwestern Longmen Shan is analogous to the Beichuan fault in the central Longmen Shan, and therefore may represent a source of seismic hazard.

Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology and fault-slip analysis

article i nfo Ar/ 39 Ar muscovite thermochronology 40 Ar/ 39 Ar K-feldspar thermochronology Fault-slip analysis New 40 Ar/ 39 Ar muscovite and K-feldspar thermochronology combined with existing data reveal the timing and patterns of late-stage exhumation across the Western Gneiss Region (U)HP terrane. Muscovite age contours show that exhumation into the mid-upper crust progressed westward over a ~20 Myr period (~400-380 Ma). This exhumation was caused by i) E-W stretching and eastward tilting north of Nordfjord, where muscovite ages decrease from the foreland allochthons westward into the UHP domains, and ii) differential exhumation south of Nordfjord, where muscovite ages depict a NE-SW dome-like pattern and the Western Gneiss Region is bounded by overlying units little affected by the Scandian metamorphism. Exhumation of the UHP domains into the mid-upper crust by late folding continued through ~374 Ma. The smooth gradient of fairly flat muscovite age spectra demonstrates minimal influence of excess Ar, which is relatively unusual for a (U)HP terrane. 40 Ar/ 39 Ar spectra and modeled cooling histories from K-feldspar combined with brittle-ductile

Barrovian metamorphism in the central Kootenay Arc, British Columbia: petrology and isograd geometry

A narrow, partly fault-bounded belt of Barrovian amphibolite facies rocks transects the central Kootenay Arc in the internal zone of the southeastern Canadian Cordillera. The following zones of increasing metamorphic grade are recognised in metapelites: chlorite/biotite, garnet, staurolite, kyanite, sillimanite, and sillimanite + K-feldspar. The garnet and higher-grade zones outline two joined domains: a N- to NNE-trending curved belt, &gt;100 km long and 5–20 km wide, that is approximately parallel to the strike of stratigraphic units; and a SSE-trending belt, &gt;70 km long and 10–15 km wide, that transects strike. Isograds in the N–NNE-trending belt outline an elongate bull’s-eye pattern with highest-grade rocks in the centre, coincident with the position of a structural culmination. Isogradic surfaces have an elongate domal form centred around this culmination. Rocks in the kyanite and sillimanite zones were metamorphosed at ~25 km (∼7 kbar (1 kbar = 100 MPa)) and &gt;650 °C during Early Cretaceous crustal thickening; rocks in the garnet zone experienced peak temperature conditions (∼500 °C) at lower pressure, suggesting the piezothermic array for the belt has a positive slope. The western flank of the N- to NNE-trending belt is cut by the west-dipping Gallagher fault zone (GFZ), whereas the eastern boundary of the SSE-trending fork is marked by the Purcell Trench fault (PTF). These Palaeocene–Eocene normal faults truncate Early Cretaceous isogradic surfaces and juxtapose regions with contrasting structural and metamorphic histories. Low-grade rocks in the hanging wall of the GFZ underwent peak regional metamorphism during the Early–Middle Jurassic, prior to intrusion of the 159–173 Ma Nelson batholith at a depth of ∼12–14 km. With the exception of a zone along the southern “tail” of the Nelson batholith, rocks in the hanging wall of the GFZ were not affected by Cretaceous metamorphism or penetrative deformation. Rocks in the amphibolite-facies belt yield Palaeocene–Eocene K–Ar and 40Ar/39Ar mica cooling ages, whereas K–Ar biotite ages in the hanging wall of the GFZ record Jurassic – Early Cretaceous cooling, and those in the hanging wall of the PTF are mid-Cretaceous. Although the GFZ and PTF accommodated differential exhumation during the Palaeogene, significant relief on isogradic surfaces was established prior to normal faulting, during Early Cretaceous metamorphism and deformation.

Differential exhumation at eastern margin of the Tibetan Plateau, from apatite fission-track thermochronology

New apatite fission-track (AFT) ages from Mesozoic sediments in the Sichuan basin, combined with previous fission-track data, demonstrate differential uplift and exhumation across the basin. Particularly significant change in exhumation (at least ~2000m) was found across the Huaying Mts. Modeled temperature–time histories and the Boomerang plot of AFT dataset across the basin suggest rapid cooling and exhumation events during 120–80Ma and at 20–10Ma. They reflect the start of the basin-scale differential uplift and exhumation which effected the eastern growth of Tibetan Plateau. In particular, nested old-age center separated by Huaying Mts. was found in the center-to-northwest part of the Sichuan basin. A simplified one-dimensional, steady-state solution model was developed to calculate the mean exhumation rate, which is 0.05–0.2mm/yr in most parts of the basin. It suggests a slow exhumation across much of the basin. The regional pattern of AFT age, length and erosion rate supports a progressive change from the nested old-age center towards the southwest. This pattern supports the idea of a prolonged, steady-state uplift and exhumation process across the basin, controlled by cratonic basin structure. The eastern growth of the Tibetan Plateau has exerted a significant effect on the rapid exhumation of the southwestern part of the Sichuan basin, but not on all of the basin during the Late Cenozoic.

Neotectonics and recent uplift at Kamchatka and Aleutian arc junction, Kamchatka Cape area, NE Russia

The tectonic position of the Kamchatka Cape Peninsula at the junction of the active Kuril–Kamchatka and Aleutian arcs exposes the coastline of the peninsula to strong neotectonic activities. Fracture zones have variable influence on uplift of the Kamchatka Cape Peninsula. Relevant morphologic indicators of neotectonic activity are multilevel, highly uplifted marine terraces and terraces displaced along active faults. Recent uplift rates of coastal sediments are determined by remote sensing via ASTER and SRTM DEM combined with optically stimulated luminescence dating (OSL). On the Kamchatka Cape Peninsula, terraces from the same generation are mapped at different elevations by remote sensing methods. After defining different areas of uplifted terraces, four neotectonic blocks are identified. According to apatite fission track data, the mean differential exhumation rates range from 0.2 to 1.2 mm year−1 across the blocks since Late Miocene. The OSL data presented point to significant higher uplift rates of up to 3 ± 0.5 and 4.3 ± 1 mm year−1, which indicates an acceleration of the vertical movement along the coast of Kamchatka Cape Peninsula in Upper Pleistocene and Holocene times.

Spatial variation in Meso-Cenozoic exhumation history of the Longmen Shan thrust belt (eastern Tibetan Plateau) and the adjacent western Sichuan basin: Constraints from fission track thermochronology

The NE-trending Longmen Shan thrust belt, marking the eastern margin of the Tibetan Plateau adjacent to the Sichuan basin, is characterized by the paradox of high topographic gradients but low convergence rates, and thus critical for understanding of geodynamic processes involved in the eastward growth of the plateau. Many low-temperature thermochronological studies focused on the central and southern portions of the Longmen Shan and western Sichuan basin to investigate the Cenozoic exhumation history of the eastern Tibetan Plateau, but little work has been done on their northern portions. Furthermore, the spatial variation in both Cenozoic and Mesozoic thermal histories of this region has been poorly documented. In this paper, zircon (ZFT) and apatite (AFT) fission track analyses for samples collected from three transects and three boreholes covering the northern, central and southern Longmen Shan and the western Sichuan basin provide new data demonstrating several cooling events in this region since the Indosinian deformation. They occurred during the late Triassic (∼200Ma), at the end of early Cretaceous (∼100Ma), during the early Cenozoic (60–40Ma), the early Miocene (20–25Ma) and the late Miocene (9–14Ma), respectively. On regional scale the Longmen Shan experienced a slow cooling between the Mesozoic and early Cenozoic, followed by a rapid cooling during the late Cenozoic, when the rapid regional exhumation began at ∼10Ma, with a sharp increase in exhumation rate from <0.1mm/yr to >0.2mm/yr, up to 0.9mm/yr locally. Comparison of ZFT and AFT ages shows that during the Mesozoic the northern segment of the Longmen Shan belt cooled more rapidly than the central and southern segments, while younger AFT ages indicate more rapid exhumation in central and southern segments of the belt during the late Cenozoic. Similar trend is demonstrated within the western Sichuan basin, where the onset of exhumation changes from ∼45Ma in the northern portion to 20–25Ma in the central and southern portions of the basin. The thermochronological data reveal that the Yingxiu-Beichuan and Guanxian-Anxian faults have accommodated significant differential exhumation by thrusting activity and associated denudation, with a greater amount of differential exhumation from north to south. We propose that the major cause for the rise in topography and rapid exhumation in the Longmen Shan thrust belt during the late Cenozoic is the thrusting activities integrated with the regional uplift of the eastern Tibetan Plateau, a process unrelated to the channel flow within the lower crust.