Exhumation Of Rocks Research Articles

Fluid generation in the Boucheville Basin as a consequence of the North Pyrenean metamorphism

Metamorphic fluids transport heat and matter. In the Boucheville Basin (North Pyrenean Zone), the circulation of metamorphic fluids is attested by abundant synmetamorphic quartz – calcite veins. The Boucheville Basin formed during the Albian extensional regime and was filled by the so-called “Albian flysch”. The basin underwent a thermal overprint (the North Pyrenean Metamorphism) related to the exhumation of nearby mantle rocks that advected large amounts of heat to the upper crustal levels. The oxygen and carbon isotope compositions of quartz – calcite veins and their host rocks show strong buffering of the composition of the fluids by the Albian metamorphic host rocks. Some host rocks are depleted in calcite near vein contacts showing that some of the elements implicated in crystal growth in veins were derived from the local host rock. The Albian rocks display a range of oxygen and carbon isotope compositions potentially related to closed-system processes of decarbonation–dehydration. We argue then that the fluids that circulated throughout the Boucheville Basin were generated within the basin itself. Their upward migration led to heat advection in the metamorphic pile, a consequence of which was some homogenization of the recrystallization temperatures in the basin.

Permian age of orogenic thickening and crustal melting in the Garm Block, South Tien Shan, Tajikistan

In this paper we present new data for the metamorphic Garm Block (150×40km) that was traditionally shown on regional maps as fragment of a Precambrian basement. The Garm Block comprises migmatized gneisses and schists for some of which peak metamorphic conditions (600–660°C at 7–9kbar) characteristic for medium P/T series metamorphic belts were determined. U–Pb ages of overgrown rims of zircon grains and newly grown magmatic zircon grains from four migmatitic samples indicate that melting occurred during peak metamorphic conditions between 299 and 290Ma. Thus the Garm Block is the first Early Permian regional metamorphic terrane established in the southern part of the Central Asian Orogenic Belt. A crosscutting granite intrusion yields a concordia age of 290Ma identical to the age of metamorphism and similar to ages of post-collisional intrusions elsewhere in the Tien Shan. Detrital zircon grains from the Garm samples yield an age spectrum with major peaks at 700–550Ma and 1150–700Ma, and smaller ones at ca. 2.7–2.4Ga and 3.5–3.3Ga. The youngest grains define an Ediacarian maximum depositional age of the sedimentary protoliths. The presence of Grenvillian ages of 1150–800Ma, which are characteristic for the Tarim Craton, and the lack of ca. 1.6–1.2Ga ages, characteristic for the Northern Tien Shan, suggest an origin of the Precambrian zircon grains from southern continental blocks similar to the Tarim Craton. Six granitoid samples yield negative εNd values of −10.6 to −3.7 consistent with the reworking of crustal material with 1.8–1.1Ga average crustal residence times and the formation of the South Tien Shan belt on a continental basement with Paleoproterozoic or older crust. The Garm Block formed as a consequence of lithospheric thickening during the Late Carboniferous collision of two continental blocks that created the South Tien Shan fold belt. Lateral crustal melt flow coupled with extension may have resulted in exhumation of the Permian metamorphic rocks.

Low-temperature thermochronology of the northern Thomson Orogen: Implications for exhumation of basement rocks in NE Australia

The Tasmanides of eastern Australia record much of the Phanerozoic tectonic development of the retreating Pacific-Australia plate boundary and are an oft-cited example of an orogen that has undergone “tectonic mode switching.” To begin to constrain the timing of exhumation of basement rocks that are now exposed in portions of the NE Tasmanides, we measured apatite and zircon (U–Th)/He ages from the Thomson Orogen and overlying Paleozoic strata in the back-arc of the New England Orogen in NE Australia. Zircon (U–Th)/He ages from basement samples (including those recovered from boreholes at depths of up to 1.1km) are characterized by large inter- and intra-sample variability and range from approximately 180Ma (Early Jurassic) to 375Ma (Late Devonian). (U–Th)/He zircon ages from several individual samples are negatively correlated with effective uranium (eU), a pattern that is also true of the dataset as a whole, suggesting that variations in U and Th zoning and radiation damage are partially responsible for the age variability. The oldest zircon (U–Th)/He cooling ages coincide with the formation of regionally extensive Late Devonian–early Carboniferous back-arc basins, suggesting that Late Devonian extension played a significant role in exhumation of parts of the northern Thomson Orogen. Apatite (U–Th)/He ages from a basement sample and a late Permian sandstone in the overlying Bowen Basin, which are also marked by intra-sample variability and age–eU correlations, span from the Early Cretaceous through Oligocene, in general agreement with previous apatite fission track data. In conjunction with observations of key geologic relationships and prior K–Ar and 40Ar/39Ar data, our results suggest four overall phases in the thermal history of the northern Thomson Orogen: (1) Cambrian–early Silurian metamorphism during the Delamerian and Benambran Orogenies; (2) protracted cooling during the Late Devonian through mid-Permian that likely resulted from extensional exhumation; (3) Permian–Triassic reheating during burial beneath thick sedimentary basins; and (4) Cretaceous and Paleogene cooling during uplift and erosion.

Timing of Partial Melting and Cooling across the Greater Himalayan Crystalline Complex (Nyalam, Central Himalaya): In-sequence Thrusting and its Implications

The timing of crustal melting and cooling has been investigated across the migmatites of the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, central Himalaya. Monazite U–Pb ages vary from 32 to 14 Ma and are linked to metamorphic conditions on the basis of monazite internal zoning, mineral inclusions, and changes in heavy rare earth element and Y composition. Metamorphic temperatures were estimated by Zr-in-rutile thermometry and cooling rates were further constrained by rutile U–Pb ages. The results reveal two distinct blocks within the GHC of the Nyalam region. The upper GHC experienced higher peak metamorphic temperatures (730–750°C) and a higher degree of melting (15–25%). Partial melting was dominated by muscovite dehydration melting, which lasted from ∼32 to 25 Ma, possibly until ∼20 Ma. The lower GHC experienced lower peak metamorphic temperatures (640–675°C) and a lower degree of melting (0–10%) mainly via H2O-saturated melting from 19 to 16 Ma. At different times, both upper and lower blocks experienced initial slow cooling (rates 35 ± 8 and 10 ± 5°C Ma–1, respectively) followed by rapid cooling (100 ± 20°C Ma–1). The documented diachronous metamorphism implies the presence of the ‘High Himalayan Thrust’ that was active at ∼25–16 Ma within the GHC of the central Himalaya. Different degrees and durations of partial melting in the investigated section suggest that a channel flow process dominated the exhumation of the upper GHC migmatites at 25–16 Ma, whereas a critical taper process dominated the exhumation of the relatively lower-grade lower GHC rocks and cooled upper GHC migmatites at 16–10 Ma. We suggest that propagating thrusts along large tectonic boundaries together with low-viscosity lateral crustal flow could contribute to exhumation of high-grade metamorphic rocks in the Himalaya and other similar collisional orogens.

Amalgamation of the Qiangtang terranes, central Tibet, and new insights into the mechanism of exhumation of high-pressure rocks

Amalgamation of the Qiangtang terranes, central Tibet, and new insights into the mechanism of exhumation of high-pressure rocks

Exumação das Rochas Mantélicas no Arquipélago de São Pedro e São Paulo, Oceano Atlântico Equatorial, e sua Implicação na Possível Geração de Hidrocarbonetos Abiogenéticos por Serpentinização

This paper presents the mantle rock exhumation at Saint Peter and Saint Paul Islets, Equatorial Atlantic Ocean, and its implication to possible generation of abiogenic hydrocarbons by serpentinization. In Saint Paul transform fault zone, there are two contrasting tectonic processes for mantle exhumation: Extension at amagmatic spreading centre and compression along a pressure ridge. On the ocean bottom, the ultramafic rocks of exhumed mantle react chemically with sea-water generating heat and abiogenic hydrocarbons, the phenomenon called serpentinisation. The amagmatic spreading with megamullion formation occurs along inter-transform ridge segments. The ultramafic rocks are originated from uppermost mantle and have advanced serpentinisation and little expressive plastic deformation. The olivine porphyroblasts show brittle fracturing. On the other hand, the pressure-ridge occurs along the transform fault span where the fault directions and relative plate motion are oblique. Due to the direction discordance, the strike-slip movement of the transform fault generates compression stress perpendicular to the fault. This stress squeezes out the underlying deep mantle from the ductile deformation depth up to the Earth's surface. The pressure ridge at Saint Peter and Saint Paulo Islet, namely Brachiosaurus Ridge, is the only confirmed example of mantle exhumation above sea level in Atlantic Ocean. The mantle rocks have little expressive serpentinisation and extremely developed plastic deformation, showing mylonitic texture. The porphyroclasts and matrix show either brittle fracturing or plastic deformation.

Geomorphological–thermo-mechanical modeling: Application to orogenic wedge dynamics

Coupled geomorphological–thermo-mechanical modeling is presented in a new implementation that combines two established thermo-mechanical and landscape evolution models. A finite-difference marker-in-cell technique is used to solve for the thermo-mechanical problem including complex visco-plastic rheologies in high resolution. Each timestep is synchronously solved with a fluvial landscape evolution model that includes numerical solution of fluvial incision and analytical hillslope processes for both diffusive and slope-limited processes on an adaptive grid. The implementation is successful in modeling large deformation at different scales. We demonstrate high degrees of coupling through processes such as exhumation of rocks with different erodibilities. Sensitivity of the coupled system evolution to surface parameters, and mechanical parameters, is explored for the established case of development of compressive wedges. The evolution of wedge models proves to be primarily sensitive to erodibility and the degree of river network integration. Relief follows deformation in propagating forward with wedge growth. We apply the method to a large-scale model of continental collision, in which a close relationship between deep tectonics, fluvial network evolution, and uplift and erosion can be demonstrated.

Eduction, extension, and exhumation of ultrahigh‐pressure rocks in metamorphic core complexes due to subduction initiation

AbstractThe controversy over the exhumation of ultrahigh‐pressure (UHP) rocks centers on whether it involves rising of pieces of crust detached from subducted continental lithosphere or an entire subducted plate that undergoes “eduction,” i.e., reverse subduction. We present a new thermomechanical model of continental subduction showing that these apparently contrasting mechanisms can occur together: crust subducted deep enough is heated and weakened, causing limited diapiric rise, while crust subducted to shallower depths retains strength and is exhumed only by eduction. The model also shows for the first time how eduction followed by seafloor spreading can occur in a zone of regional convergence. This occurs spontaneously when subduction of buoyant crust causes a subduction zone to “lock up” in one place causing a new subduction zone to form in another. The model is consistent with many features of the youngest region of UHP rock exhumation on earth: the D'Entrecasteaux Islands. UHP exhumation and the amount of regional extension, as well as the seismic structure around the islands, can be explained by eduction. Ductile flow fabrics, seen on the islands, would result from exhumation of the most deeply subducted crust heated enough to undergo partial melting. Reversal of motion on the north‐dipping continental subduction zone, required by this model, was likely triggered by initiation of the New Britain Trench, as suggested previously. Our model implies that the crust of Goodenough Basin, south of the islands, was exhumed by eduction in the last 5 Ma and this hypothesis can be tested by drilling.

H2O-fluid-saturated melting of subducted continental crust facilitates exhumation of ultrahigh-pressure rocks in continental subduction zones

We present two-dimensional numerical models of plate subduction and collision inspired by the Scandinavian Caledonian orogeny to investigate the possible impact of continental crust partial melting on the exhumation of ultra-high pressure metamorphic rocks. Three possible reactions were tested: low temperature solidus representing H2O-fluid-saturated partial melting, and two end-member reaction curves for dehydration melting. Thermo-mechanical effects of partial melting were implemented as (1) a viscosity decrease as a determined rheologically critical melt percentage was reached (here 0.1), (2) a change in effective heat capacity and adiabatic heating/cooling accounting for a latent heat term in the heat equation. Among the 3 tested reactions, only H2O-fluid-saturated partial melting drastically modifies the collision dynamics from the non-melting reference model holding all other parameters constant. A substantially low general viscosity truncation (here 1017 Pa s) is needed to properly resolve the effect of partial melting on deep collision processes. Low temperature melting indeed induces the development of a low viscosity buoyant plume prior to slab detachment, where migmatites exhume from UHP conditions at rates and with pressure–temperature paths similar to the natural values acknowledged for the Norwegian Caledonides. High temperature melting has no drastic influence on early collision dynamics. While positive buoyancy remains the first order driver for the exhumation of buried continental rocks, exhumation initiates in these cases with eduction subsequent to slab detachment. Melting and formation of a migmatite plume can later occur along decompression path while continental crust undergoes thermal reequilibration at temperatures above 900 °C. Some of the partially molten material can also relaminate in the overriding plate rather than exhume within the collision zone. Even if minor in terms of amount of magma produced, H2O-fluid-saturated partial melting at UHP conditions could therefore have a dramatic rheological effect and actually limits continental rocks subduction and facilitates their exhumation.

Cooperation among tectonic and surface processes in the St. Elias Range, Earth's highest coastal mountains

AbstractInvestigations of tectonic and surface processes have shown a clear relationship between climate‐influenced erosion and long‐term exhumation of rocks. Numerical models suggest that most orogens are in a transient state, but observational evidence of a spatial shift in mountain building processes due to tectonic‐climate interaction is missing. New thermochronology data synthesized with geophysical and surface process data elucidate the evolving interplay of erosion and tectonics of the colliding Yakutat microplate with North America. Focused deformation and rock exhumation occurred in the apex of the colliding plate corner from > 4 to 2 Ma and shifted southward after the 2.6 Ma climate change. The present exhumation maximum coincides with the largest modern shortening rates, highest concentration of seismicity, and the greatest erosive potential. We infer that the high sedimentation caused rheological modification and the emergence of the southern St. Elias, intercepting orographic precipitation and shifting focused erosion and exhumation to the south.

Current challenges for explaining (ultra)high‐pressure tectonism in the Pennine domain of the Central and Western Alps

AbstractThe Pennine domain of the Central and Western (PCW) Alps, including the Dora‐Maira, Monte Rosa, Gran Paradiso, Adula/Cima Lunga nappes and the Zermatt‐Saas zone underwent ultrahigh‐ or high‐pressure [(U)HP >1.5 GPa] metamorphism during the Alpine orogeny. We review structural, petrological and geochronological data for the (U)HP units in the PCW Alps (i) to clarify the relationship between (U)HP metamorphism and deformation, (ii) to confront published exhumation models for the (U)HP units with the reviewed data and (iii) to evaluate consequences of different pre‐Alpine paleogeographic settings (Penrose‐type ocean v. hyperextended margins) on the Alpine orogeny. The review indicates that (i) peak pressures are recorded only in minor volumes of the corresponding tectonic nappes; (ii) (U)HP rocks occur within coherent and imbricate thrust sheets which show substantial pressure jumps; (iii) peak pressures are mostly associated with a top‐to‐the‐foreland kinematics; (iv) decompression from (U)HP (4 to >1.5 GPa) to greenschist or amphibolite facies (~1 GPa) metamorphic conditions was fast (< c. 2 Ma); and (v) accreted fragments of the Alpine–Tethyan basins reflect Mesozoic hyperextended margins rather than mature Penrose‐type oceans. The comparison of these characteristics with numerical exhumation models suggests that exhumation of (U)HP rocks by buoyancy‐driven return flow within a subduction channel under near‐lithostatic pressure is unlikely because (i) models predict volumes of (U)HP rocks that are more than an order of magnitude larger as observed in the PCW Alps and (ii) the required top‐to‐the‐hinterland (U)HP kinematics in the hangingwall of (U)HP units has not been observed until now. Also, the buoyancy‐driven exhumation of the individual crustal (U)HP units within weak (≤1019 Pa s) and denser (> ~3180 kg m−3) rocks is questionable, because there is no evidence of such rocks around the (U)HP units. An alternative model, which could explain the main characteristics of the (U)HP units in the PCW Alps, is an orogenic wedge model that (i) involves dynamic stresses deviating from lithostatic pressure and (ii) is formed during the convergence of hyperextended margins. Deviations of dynamic stresses from the lithostatic pressure and local pressure variations cannot be excluded during the Alpine orogeny, but these deviations and variations have not been clearly identified until now.

Middle–Late Mesozoic sedimentary provenances of the Luxi and Jiaolai areas: Implications for tectonic evolution of the North China Block

Provenances of sedimentary rocks may provide important constraints on the tectonic evolution of the North China Block (NCB). Previous studies have demonstrated that the northern NCB (NNCB) and the Xing-Meng orogenic belt (XMOB) supplied massive detritus southward into the hinterland of the NCB during the Jurassic. In order to study the evolution of sedimentary provenance during the Middle–Late Mesozoic, U–Pb geochronology and Hf isotopic geochemistry of detrital zircon grains and chemical compositions of detrital garnets from sandstones in the Luxi and Jiaolai areas, eastern NCB, were analyzed in combination with published data on the Jurassic sandstones. The Late Paleozoic–Mesozoic (367–139Ma) zircons in the lowermost Cretaceous Mengyin Formation samples from the Luxi area show εHf(t) values of −15.3 to −3.2 and +1.3 to +10.0, which are very similar to the results of analyses of the Jurassic formations. Further, the increased amount of Mesozoic zircons and granulite-derived garnets in the Mengyin Formation samples, compared to those in the Jurassic samples, indicates there was more detritus supply from the NNCB than from the XMOB. In the overlying Qingshan Formation samples, zircon grains do not exhibit Paleozoic ages, but most of them have Early Cretaceous ages and negative εHf(t) values, which are similar to the zircon grains extracted from the widespread Early Cretaceous igneous rocks in the NCB. This suggests that the provenance might have changed to a locally derived source. In contrast, the zircon population of the Early Cretaceous sandstones from the Jiaolai basin is dominated by grains of mid-Neoproterozoic age (700–900Ma) which signifies contribution from the Sulu orogen. Moreover, the detrital garnet assemblages of sandstones in the Luxi area are not consistent with those from representative metamorphic rocks in the Sulu orogen. The above results seem to confirm that the Mesozoic sedimentary provenance of the Luxi area had no evident connection with the Sulu orogen. Instead, the differential uplifting and exhumation of deep crustal rocks in the NNCB and the XMOB, associated with closure of the Mongol–Okhotsk Ocean, might exert a dominant control on sedimentary provenances in the wide hinterland of the NCB from the Jurassic period to ∼136Ma (the upper boundary age of the Mengyin Formation).

Imaging high-pressure rock exhumation in eastern Taiwan

Increasingly detailed studies of out crops of high-pressure rock terranes in combination with rapidly evolving numerical modeling studies have given rise to a number of possible explanations for the processes by which these rocks are exhumed. Imaging actively exhuming high-pressure terranes remains one of the fundamental, but elusive, tasks that could advance the understanding of how these important rocks reach Earth’s surface. Seismic tomography along the active arc-continent collision in eastern Taiwan images a high P- and S-wave velocity zone that extends from the shallow subsurface beneath a high-pressure metamorphic terrane to ∼50 km depth. We present a petrophysical analysis of this high-velocity zone that indicates the presence of rock types common to high-pressure terranes. The high-velocity zone is seismically active throughout. We determine focal mechanisms for 57 earthquakes, and carry out full waveform modeling on 10; these have double-couple focal mechanisms with a compensated linear vector dipole component up to 20.6%. We suggest that the high-velocity zone comprises an exhuming high-pressure terrane. Focal mechanisms for earthquakes within it indicate that shear faulting dominates in the deformation, but high fluid pressure may also play a role.

Unroofing of fore-arc ranges along the Hikurangi Margin, New Zealand: Constraints from low-temperature thermochronology

The Axial Ranges of North Island, New Zealand, parallel the Hikurangi subduction margin. They consist of uplifted and exhumed Mesozoic meta-sedimentary basement rocks of the overriding Australian Plate, beneath which the Pacific Plate has been subducting since at least the Late Oligocene. We investigate the unroofing histories of these fore-arc mountains during the evolution of the Hikurangi Margin, based on new and previous zircon and apatite fission-track, and apatite (U–Th–Sm)/He data which we interpret based on inverse modelling. The results suggest that the exhumation of rocks in the Axial Ranges initiated in the west and migrated trench-wards towards the east. Onsets of accelerated exhumation in different parts of the ranges indicate significant eastwards thrusting on the margin-parallel Ngamatea Fault before ~20–17Ma and on the Wellington–Mohaka Fault before ~10–7Ma. The exhumation rate has varied significantly along-strike since the Late Miocene, lower in the central part of the Axial Ranges and significantly higher to the south. Since the Late Miocene, the increasing exhumation rate from the central to southern Axial Ranges is consistent with the clockwise vertical-axis rotation of eastern North Island relative to the Australian Plate. In the Hikurangi Margin, although underplating of subducted material at the basal upper plate may have contributed to localised rock uplift (e.g., in the Raukumara Range), we suggest that the shortening of the fore-arc upper plate was the chief driver of unroofing of the (proto-) Axial Ranges.

Contrasting styles of (U)HP rock exhumation along the Cenozoic Adria‐Europe plate boundary (Western Alps, Calabria, Corsica)

AbstractSince the first discovery of ultrahigh pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel‐flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria‐Europe plate boundary, and along three different transects (Western Alps, Calabria‐Sardinia, and Corsica‐Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging toward the foreland. Both mechanisms were active to create boundary divergence along the Corsica‐Northern Apennines transect, where European southeastward subduction was progressively replaced along strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present‐day eastern Papua New Guinea, which is presented as a modern analog of the Paleogene Adria‐Europe plate boundary.

Zircon 4He/3He thermochronometry

Multiple thermochronometric methods are often required to constrain time-continuous rock exhumation for studying tectonic processes or development of km-scale topography at Earth’s surface. Here, we explore 4He/3He thermochronometry of zircon as a method for constraining continuous time–temperature (t–T) paths of individual samples through a temperature range that is complementary to methods such as 40Ar/39Ar thermochronometry of K-feldspar and 4He/3He thermochronometry of apatite. For different cooling rates and diffusion domain size, the temperature sensitivity of zircon 4He/3He thermochronometry ranges from slightly less than 100°C to slightly greater than 250°C; a typical sample provides continuous thermal constraints over ∼100°C within that range. Outside these temperatures, 4He in zircon will either be quantitatively retained or completely lost by volume diffusion. As proof-of-concept, we present stepwise release 4He/3He spectra and associated U and Th concentration maps measured by laser ablation ICP-MS analysis of individual crystal aliquots of Fish Canyon Tuff (FCT) zircon and of a more complex setting in the Sierra Nevada batholith that experienced reheating from a proximal basaltic intrusion, the Little Devil’s Postpile (LDP). The FCT zircon 4He/3He release spectra are consistent with a 4He spatial distribution dominated by alpha-ejection from crystal surfaces. The spatial distributions of U and Th measured in the same crystals do not substantially influence 4He/3He release spectra that are predicted for the known thermal history, even when incorporating spatially variable diffusivity due to accumulation of radiation damage. Conversely, the LDP 4He/3He release spectra are strongly influenced by the observed parent nuclide zonation. A three-dimensional (3D) numerical model of 4He production and diffusion, which incorporates crystal geometry, U and Th zonation, and spatially variable He diffusion kinetics, substantially improves the fit between measured and modeled 4He/3He release spectra for the independently known thermal history of the sample. We conclude that zircon 4He/3He thermochronometry provides accurate thermal constraints through geologic time, but certain applications of this technique may require 3D numerical modeling approaches.

Tectonic evolution and high-pressure rock exhumation in the Qiangtang terrane, central Tibet

Abstract. Conflicting interpretations of the > 500 km long, east–west-trending Qiangtang metamorphic belt have led to very different and contradicting models for the Permo–Triassic tectonic evolution of central Tibet. We define two metamorphic events, one that only affected pre-Ordovician basement rocks and one subduction-related Triassic high-pressure metamorphism event. Detailed mapping and structural analysis allowed us to define three main units that were juxtaposed due to collision of the north and south Qiangtang terranes after closure of the Ordovician–Triassic ocean that separated them. The base is formed by the Precambrian–Carboniferous basement, followed by non-metamorphic ophiolitic mélange containing mafic rocks that range in age from the Ordovician to Middle Triassic. The top of the sequence is formed by strongly deformed sedimentary mélange that contains up to > 10 km size rafts of both unmetamorphosed Permian sediments and high-pressure blueschists. We propose that the high-pressure rocks were exhumed from underneath the south Qiangtang terrane in an extensional setting caused by the pull of the northward subducting slab of the Shuanghu–Tethys. High-pressure rocks, sedimentary mélange and margin sediments were thrust on top of the ophiolitic mélange that was scraped off the subducting plate. Both units were subsequently thrust on top of the south Qiantang terrane continental basement. Onset of Late Triassic sedimentation marked the end of the amalgamation of both Qiangtang terranes and the beginning of spreading between Qiantang and north Lhasa to the south, leading to the deposition of thick flysch deposits in the Jurassic.

Oil migration driven by exhumation of the Canol Formation oil shale: A new conceptual model for the Norman Wells oil field, northwestern Canada

Thermal history, petroleum system, structural, and tectonic constraints are reviewed and integrated in order to derive a new conceptual model for the Norman Wells oil field, and a new play type for tectonically active foreland regions. The thermal history recorded by Devonian rocks suggests that source rocks experienced peak thermal conditions in the Triassic–Jurassic, during which time oil was likely generated. After initial oil generation and expulsion, the Canol Formation oil shale retained a certain fraction of hydrocarbons. The shallow reservoir (650–350 m) is a Devonian carbonate bank overlain by the Canol Formation and resides within a hanging wall block of the Norman Range thrust fault. Both reservoir and source rocks are naturally fractured and have produced high API non-biodegraded oil. Thrust faults in the region formed after the Paleocene, and a structural cross-section of the field shows that the source and reservoir rocks at Norman Wells have been exhumed by over 1 km since then.The key proposition of the exhumation model is that as Canol Formation rocks underwent thrust-driven exhumation, they crossed a ductile–brittle transition zone and dip-oriented fractures formed sympathetic to the thrust fault. The combination of pore overpressure and new dip-directed subvertical fractures liberated oil from the Canol Formation and allowed for up-dip oil migration. Reservoir rocks were similarly fractured and improved permeability enhanced charging and pooling of oil. GPS and seismicity data indicate that strain transfer across the northern Cordillera is a response to accretion of the Yakutat terrane along the northern Pacific margin of North America, which is also the probable driving force for foreland shortening and rock exhumation at Norman Wells.

Multistage exhumation and partial melting of high-T ultrahigh-pressure metamorphic rocks in continental subduction-collision zones

High-temperature (HT; >850°C) metamorphism in continental collision orogens, particularly for those ultrahigh-pressure (UHP) metamorphic rocks, has become one of the remarkable topics in Earth science. It has bearing on the element and isotope behaviors of UHP rocks, their partial melting and related geodynamic effects during exhumation. In this paper, five representative continental collision orogens with typical HT/UHP rocks, including the Dabie orogen in China, the Kokchetav in Kazakhstan, the Caledonides in Greenland, the Rhodope in Greece, and the Erzgebirge in Germany are introduced, and their HT/UHP metamorphism and evolution processes are summarized. In addition, metamorphic P-T-t paths, multistage exhumation processes, and partial melting and preservation and retrogression of UHP index minerals during exhumation and their possible mechanisms are discussed. On this basis, the forthcoming key fields and scientific subjects of HT/UHP rocks within continental subduction channel are proposed.

Monazite, zircon, and garnet growth in migmatitic pelites as a record of metamorphism and partial melting in the East Humboldt Range, Nevada

Monazite and zircon thermometry and geochronology were applied to anatectic pelites of the northern East Humboldt Range, Nevada. The study area is an exhumed portion of the Sevier orogenic root and is characterized by two crustal blocks with different prograde metamorphic histories: the Winchell Lake nappe and the underlying Lizzies Basin block. U/Th-Pb secondary ion mass spectrometry (SHRIMP) results from zircon and monazite indicate that in the Lizzies Basin block prograde metamorphism began by 96.5 ± 8.0 Ma. Cooling and melt crystallization was initiated by 80.1 ± 1.4 Ma. Possible reheating and a monazite growth event occurred again at 76.2 ± 2.8 to 68.2 ± 2.0 Ma. In the upper limb of the overlying Winchell Lake nappe, prograde metamorphism began by 82.8 ± 1.3 Ma. Cooling and melt crystallization recorded by zircon and monazite growth/recrystallization at 77.4 ± 12.4 to 58.9 ± 3.6 Ma. Melt may have been present in the Winchell Lake nappe upper limb for a protracted period, or may have formed during several unresolved melting events. Thin Eocene-Oligocene (~40-32 Ma) high-Y monazite rims are found in both crustal blocks. These probably represent a phase of heating during Eocene-Oligocene magmatism and extensional deformation. U/Th-Pb geochronology results are consistent with differential burial, heating, and exhumation of different crustal blocks within the East Humboldt Range. Significantly fast exhumation of upper limb rocks of the Winchell Lake nappe and somewhat slower exhumation of Lizzies Basin block rock occurred prior to the “core complex” phase of the exhumation of the Ruby Mountains-East Humboldt Range metamorphic core complex. Differences in the timing and tectonic significance of exhumation episodes within the East Humboldt Range may reflect how the localized presence of partial melt can affect the evolution of an exhumed orogenic terrane.