Extensional Exhumation Research Articles

Paleogene mid-crustal intrusions in the Ruby Mountains–East Humboldt Range metamorphic core complex, northeastern Nevada, USA

Abstract Middle Eocene to early Oligocene intrusions, widespread in the Ruby Mountains–East Humboldt Range metamorphic core complex, Nevada, USA, provide insights into a major Paleogene magmatic episode and its relation to tectonism in the northeastern Great Basin. These intrusions, well-exposed in upper Lamoille Canyon, range in composition from gabbro to leucomonzogranite. They form small plutons, sheets, and dikes that intrude the metamorphic and granitic infrastructure of the core complex. Two types of Paleogene monzogranite were recognized. The first is exemplified by two of the larger intrusive bodies, the Snow Lake Peak and Castle Lake intrusions, which occur as sheet-like bodies near and at the structural base of metamorphosed Neoproterozoic and Cambrian Prospect Mountain Quartzite where it is inverted above Cambrian and Ordovician marble of Verdi Peak in the Lamoille Canyon nappe. Swarms of dikes are associated with these intrusions. U-Pb (zircon) ages range ca. 40–33 Ma and typically display relatively simple and minor inheritance. The rocks have the lowest εHf (zircon) and εNd (whole rock) of any of the middle Cenozoic granites. The second type of monzogranite, Overlook type, typically occurs as thin, isolated dikes and leucosome-like bodies in Late Cretaceous granites of the infrastructure, with no obvious relationship to the large monzogranite bodies. Overlook-type monzogranite displays complex zircon inheritance, yields igneous ages ca. 37–32 Ma, and has εHf (zircon) and εNd (whole rock) identical to those of Late Cretaceous granites in the core complex. These isotopic and field data indicate that Overlook-type monzogranite formed in situ through anatexis of host Cretaceous granites. A pervasive thermal event was required to stimulate this crustal melting. Gabbros from Lamoille Canyon and quartz diorite dated from 32 km away signal mantle-derived magmatism ca. 39–37 Ma (U-Pb, zircon) was a driver of crustal melting and hybridization. Eocene 40Ar/39Ar apparent ages on hornblende and biotite are consistent with syn- to post-magmatic extensional exhumation and decompression. Thus, the core complex provides a window into trans-crustal magmatism and insight into how such magmatism affected the Nevadaplano orogenic plateau. This Paleogene thermal pulse, which may relate to removal of the Farallon slab by delamination of mantle lithosphere, involved partial melting of the upper mantle and transfer of magma and heat to the Nevadaplano crust. Lower-crustal melting of Archean(?) to Paleoproterozoic rocks resulted in Snow Lake Peak–type magmas, and middle-crustal melting of granite in the infrastructure yielded Overlook-type magmas. This crustal magmatism, as exemplified by the intrusions in the core complex, likely played a role in destabilizing the Nevadaplano and its later collapse during middle Miocene extension. The Paleogene intrusions and associated structural features also provide insight into the evolution of the core complex through either the buoyant upwelling of a melt-rich diapir (gneiss-dome model) or buoyant upwelling of the melt-rich middle crust synchronous with a west-rooted mylonitic shear zone (extensional shear-zone model). We favor a hybrid that incorporates both models.

Extensional exhumation of cratons: insights from the Early Cretaceous Rio Negro–Juruena belt (Amazonian Craton, Colombia)

Abstract. This study presents results from apatite fission track (AFT) thermochronology to investigate the thermal history and exhumation dynamics of the Rio Negro–Juruena basement, situated within the western Guiana Shield of the Amazonian Craton. AFT dating and associated thermal history modeling in South America has largely been restricted to the plate's margins (e.g., Andean active margin, Brazilian passive margin, and others). Our paper reports on low-temperature thermochronological data from the internal part of the western Guiana Shield for the first time. This area is part of a vast cratonic lithosphere that is generally thought to be stable and little influenced by Mesozoic and Cenozoic tectonics. Our data, however, show AFT central ages ranging from 79.1 ± 3.2 to 177.1 ± 14.8 Ma, with mean confined track lengths of ca. 12 µm. Contrary to what might be expected of stable cratonic shields, inverse thermal history modeling indicates a rapid basement cooling event in the early Cretaceous. This cooling is interpreted as a significant exhumation event of the basement that was likely driven by the coeval extensional tectonics associated with back-arc rifts in the Llanos and Putumayo–Oriente–Maranon basins. The extensional tectonics facilitated both basement uplift and subsidence of the adjoining basins, increasing erosional dynamics and consequent exhumation of the basement rocks. The tectonic setting shifted in the late Cretaceous from extensional to contractional, resulting in reduced subsidence of the basins and consequential diminishing cooling rates of the Guiana Shield basement. Throughout the Cenozoic, only gradual, slow subsidence occurred in the study area due to regional flexure linked to the Andean orogeny. Comparative analysis with low-temperature thermochronology data from other west Gondwana cratonic segments highlights that exhumation episodes are highly controlled by tectonic inheritance, lithospheric strength, and proximity to rift zones. This study underscores the complex interplay between tectonic events and the response of cratonic lithosphere over geological timescales and highlights extensional settings as an important geological context for craton exhumation.

The Central Indian Tectonic Zone: A Rodinia supercontinent-forming collisional zone and analogy with the Grenville and Sveconorwegian orogens

Abstract In the paleogeographic reconstructions of the Rodinia supercontinent, the circum-global 1.1–0.9 Ga collisional belt is speculated to skirt the SE coast of India, incorporating the Rodinian-age Eastern Ghats Province. But the Eastern Ghats Province may not have welded with the Indian landmass until 550–500 Ma. Instead, the ~1500-km-long, E-striking Central Indian Tectonic Zone provides an alternate option for linking the 1.1–0.9 Ga circum-global collisional belt through India. The highly tectonized Central Indian Tectonic Zone formed due to the early Neoproterozoic collision of the North India and the South India blocks. Based on a summary of the recent findings in the different crustal domains within the Central Indian Tectonic Zone, we demonstrate that the 1.03–0.93 Ga collision involved thrusting that resulted in the emplacement of low-grade metamorphosed allochthonous units above the high-grade basement rocks; the development of crustal-scale, steeply dipping, orogen-parallel transpressional shear zones; syn-collisional felsic magmatism; and the degeneration of orogenesis by extensional exhumation. The features are analogous to those reported in the broadly coeval Grenville and Sveconorwegian orogens. We suggest that the 1.1–0.9 Ga circum-global collisional belt in Rodinia swings westward from the Australo-Antarctic landmass and passes centrally through the Greater India landmass, which for the most part welded at 1.0–0.9 Ga. It follows that the paleogeographic positions of India obtained from paleomagnetic data older than 1.1–0.9 Ga are likely to correspond to the positions of the North and South India blocks, respectively, and not to the Greater India landmass in its entirety.

Coupling between Cenozoic extensional exhumation in North China and the subduction of the Pacific Plate

Both the timing and mechanism for the formation of the topographic stepping in East Asia along the North-South Gravity Lineament (NSGL) are critical to understand the coupled influence of tectonics, climate, and surface processes on landscape evolution. The Yanshan Mountains lie in the central portion of the NSGL and offer a chance to reveal the link between surficial tectonic deformation and the underlying subduction to understand the continental dynamics and paleoclimate of NSGL. In this study, we report 22 new apatite (UTh)/He (AHe) data from the Yanshan Mountains. Age-elevation profiles indicate a period of rapid exhumation initiated at ∼27–26 Ma, while the AHe ages generally increase with the distance from the fault coincides with northwestward tilting of the fault footwall. Combining a series of shallow extensional events in East Asia with deep process of the Pacific plate subduction, we suggest that the widely-distributed late Oligocene rapid exhumation represents a phase of Cenozoic re-destruction of the North China Craton (NCC). The Cenozoic re-destruction of the NCC and topographic stepping along the NSGL may be related to the retreat, dehydration, and decarbonization of the underlying stagnant Pacific slab.

The Lower Cretaceous sequence of western Alaska—demise of the Koyukuk terrane?

Lower Cretaceous marine sedimentary rocks, deposited in shallow shelf and basin settings and unconformity-bound, are well exposed in southwest Alaska. Collections of Early Cretaceous fossils from across western Alaska show that similar and coeval Lower Cretaceous clastic rocks are widely distributed though only locally exposed. Volcanic rocks become an important part of the Lower Cretaceous sequence in the Yukon-Koyukuk basin, where they have been interpreted to represent a mobile intra-oceanic island arc, the Koyukuk terrane, that collided with Arctic Alaska to form the Brooks Range orogen. The volcanic rocks are chemically unlike Aleutian arc rocks but share compositional characteristics with spatially related, mid-Cretaceous alkaline intrusive rocks. The volcanic-bearing sequence was also deposited on an angular unconformity, includes both shallow shelf and basin depositional settings, and is unconformably overlain by mid-Cretaceous clastic rocks. The volcanic rocks are therefore considered part of the Lower Cretaceous sequence now identified across western Alaska. In this interpretation, the Lower Cretaceous volcanic rocks are an initial expression of the mid-Cretaceous tectonic regime that included extensional exhumation and subsidence, crustal and upper mantle melting, and high-temperature metamorphism in the hinterland of the Brooks Range orogen. The Cretaceous heating that led to hinterland crust and upper mantle change may have been caused by deep mantle disturbances in a postsubduction setting. This interpretation has implications for the timing of contractional orogenesis, the location and nature of the related continental borderland, and the tectonic setting for development of the Anguyucham and related oceanic terranes.

Structure, chronology, kinematics, and geodynamics of tectonic extension in the greater Catalina metamorphic core complex, southeastern Arizona, USA

Abstract Oligocene and early Miocene displacement on the Catalina–San Pedro detachment fault and its northern correlatives uncovered mylonitic fabrics that form the greater Catalina metamorphic core complex in southeastern Arizona, USA. Gently to moderately dipping mylonitic foliations in the complex are strongly lineated, with a lineation-azimuth average of 064–244° and dominantly top-southwest shear sense over the entire 115-km-long mylonite belt. Reconstruction of detachment fault displacement based on a variety of features indicates 40–60 km of displacement, with greater displacement in more southern areas. Widespread 26–28 Ma volcanism during early extensional basin genesis was followed by 24–26 Ma granitoid magmatism. Cooling of footwall mylonites continued until 22–24 Ma, as indicated by 40Ar/39Ar mica dates. Lower temperature thermochronometers suggest that footwall exhumation was still underway at ca. 20 Ma. Tectonic reconstruction places a variety of unmetamorphosed supracrustal units in the Tucson and Silver Bell Mountains above equivalent units that were metamorphosed and penetratively deformed in the Tortolita and Santa Catalina Mountains. This restored juxtaposition is interpreted as a consequence of older Laramide thrust burial of the deformed units, with northeast-directed thrusting occurring along the Wildhorse Mountain thrust in the Rincon Mountains and related but largely concealed thrusts to the northwest. Effective extensional exhumation of lower plate rocks resulted from a general lack of internal extension of the upper plate wedge. This is attributed to a stable sliding regime during the entire period of extension, with metamorphic core complex inflation by deep crustal flow leading to maintenance of wedge surface slope and detachment fault dip that favored stable sliding rather than internal wedge extension.

Anorthosite formation and emplacement coupled with differential tectonic exhumation of ultrahigh-temperature rocks in a Sveconorwegian continental back-arc setting

The tectonic setting and mechanisms and duration of emplacement of Proterozoic massif-type anorthosites and the significance of typically associated ultrahigh-temperature (UHT) host rocks have been debated for decades. This is particularly true of the Rogaland Anorthosite Province (RAP) in the SW Sveconorwegian Orogen. Earlier studies suggest that the RAP was emplaced over 1–3 Myr around 930 Ma towards the end of orogenesis, resulting in an up to 15–20 km-wide contact metamorphic aureole. However, our structural observations show that the RAP is located in the footwall of a 15 km-wide extensional detachment (Rogaland Extensional Detachment, RED), separating the intrusions and their UHT host rocks from weakly metamorphosed rocks in the hanging wall. U–Pb zircon dating of leucosome in extensional pull-aparts associated with the RED yields ages of 950–935 Ma, consistent with Re–Os molybdenite ages from brittle extensional structures in the hanging-wall block that range between 980 and 930 Ma. A metapelite in the immediate vicinity of the RAP yields a 950 Ma U–Pb age of matrix-hosted monazite, and part of the RAP was intruded by the Storgangen norite dike at ca. 950 Ma, providing a minimum age of emplacement. These ages are consistent with Ar–Ar hornblende and biotite ages that show rapid cooling of the footwall before 930 Ma, but slow cooling of the hanging wall. Field and geochronologic data suggest that the RAP formed and was emplaced over a long period of time, up to 100 Myr, with different emplacement mechanisms reflecting an evolving regional stress regime. The distribution of UHT rocks around the RAP reflects differential extensional exhumation between 980 and 930 Ma, not contact metamorphism. The duration and style of orogenic activity and externally (as opposed to gravitationally) driven extension suggest that the RAP formed in a continental back-arc setting.

Contrasting zircon and garnet behaviors during metamorphic transformation from eclogite to granulite facies: Constraints from orogenic metabasites from North Qaidam in northern Tibet

A combined study of garnet and zircon from the same growth stages can provide effective constraints on the pressure–temperature-time path of metamorphic processes. This is illustrated by distinguishing different episodes of garnet and zircon in metabasites from North Qaidam. Two different garnet domains grown under eclogite- and granulite-facies conditions were recognized to have different trace elements and inclusions. The variable trace elements are ascribed to the coexisting MREE-rich minerals and whole-rock compositions under eclogite-facies conditions, and to variable growth of feldspar and consumption of precursor garnet at the continuous granulite facies metamorphism. The majority of zircon cores with ages of 430–441 Ma and zircon rims with ages of 419–424 Ma record metamorphism at eclogite-facies and granulite-facies, respectively. They exhibit different trace elements and Hf-O isotopes, indicating zircon growth via different mechanisms with differential involvement of precursor zircon and other minerals as well as anatectic melts under eclogite- and granulite-facies conditions. The metabasites experienced an increase of metamorphic thermal gradients with time and thus the tectonic transition from deeply continental subduction for Alpine-type HP to UHP eclogite-facies metamorphism at 2.6–3.1 GPa and 827–864 °C through decompressional exhumation to the lower crust for Barrovian-type HP granulite-facies overprinting at 1.05–1.70 GPa and 725–872 °C, eventually to extensional exhumation to the mid-crustal level for Buchan-type low-pressure amphibolite-facies overprinting at 0.34–0.56 GPa and 528–667 °C in the collisional orogen. While HREE disequilibrium between the two minerals is common, the equilibrium HREE partitioning may be achieved between zircon and garnet rims during crustal anatexis at the late HP granulite-facies metamorphism.

Formation of a Composite Albian–Eocene Orogenic Wedge in the Inner Western Carpathians: P–T Estimates and 40Ar/39Ar Geochronology from Structural Units

The composite Albian–Eocene orogenic wedge of the northern part of the Inner Western Carpathians (IWC) comprises the European Variscan basement with the Upper Carboniferous–Triassic cover and the Jurassic to Upper Cretaceous sedimentary successions of a large oceanic–continental Atlantic (Alpine) Tethys basin system. This paper presents an updated evolutionary model for principal structural units of the orogenic wedge (i.e., Fatricum, Tatricum and Infratatricum) based on new and published white mica 40Ar/39Ar geochronology and P–T estimates by Perple_X modeling and geothermobarometry. The north-directed Cretaceous collision led to closure of the Jurassic–Early Cretaceous basins, and incorporation of their sedimentary infill and a thinned basement into the Albian–Cenomanian/Turonian accretionary wedge. During this compressional D1 stage, the subautochthonous Fatric structural units, including the present-day higher Infratatric nappes, achieved the metamorphic conditions of ca. 250–400 °C and 400–700 MPa. The collapse of the Albian–Cenomanian/Turonian wedge and contemporary southward Penninic oceanic subduction enhanced the extensional exhumation of the low-grade metamorphosed structural complexes (D2 stage) and the opening of a fore-arc basin. This basin hemipelagic Coniacian–Campanian Couches-Rouges type marls (C.R.) spread from the northern Tatric edge, throughout the Infratatric Belice Basin, up to the peri-Pieniny Klippen Belt Kysuca Basin, thus tracing the south-Penninic subduction. The ceasing subduction switched to the compressional regime recorded in the trench-like Belice “flysch” trough formation and the lower anchi-metamorphism of the C.R. at ca. 75–65 Ma (D3 stage). The Belice trough closure was followed by the thrusting of the exhumed low-grade metamorphosed higher Infratatric complexes and the anchi-metamorphosed C.R. over the frontal unmetamorphosed to lowest anchi-metamorphosed Upper Campanian–Maastrichtian “flysch” sediments at ca. 65–50 Ma (D4 stage). Phengite from the Infratatric marble sample SRB-1 and meta-marl sample HC-12 produced apparent 40Ar/39Ar step ages clustered around 90 Ma. A mixture interpretation of this age is consistent with the presence of an older metamorphic Ph1 related to the burial (D1) within the Albian–Cenomanian/Turonian accretionary wedge. On the contrary, a younger Ph2 is closely related to the late- to post-Campanian (D3) thrust fault formation over the C.R. Celadonite-enriched muscovite from the subautochthonous Fatric Zobor Nappe meta-quartzite sample ZI-3 yielded a mini-plateau age of 62.21 ± 0.31 Ma which coincides with the closing of the Infratatric foreland Belice “flysch” trough, the accretion of the Infratatricum to the Tatricum, and the formation of the rear subautochthonous Fatricum bivergent structure in the Eocene orogenic wedge.

Late Cretaceous Crustal Shortening in the Northern Snake Range Metamorphic Core Complex: Constraints on the Structural Geometry and Magnitude of Pre‐Extensional Footwall Burial

AbstractDetailed geologic mapping and structural investigation of the footwall of the northern Snake Range (NSR) metamorphic core complex reveal km‐scale Late Cretaceous fold and thrust relationships of the northern Snake Range Fold and Thrust System (NSRFTS) that are broadly coeval with the deep burial and amphibolite facies metamorphism of Eocambrian strata. The O'Neill Peak Recumbent Syncline (OPRS) ‐ an NNW‐trending, non‐cylindrical, eastward‐opening, inclined to recumbent syncline and its overlying anticlinal closure, the O'Neill Peak Anticline, affect the entire Middle to Upper Cambrian stratigraphy, effectively doubling its structural thickness from ∼3.5 to ∼8 km. West of the OPRS, pervasive isoclinal folding in lower Cambrian schistose units and local thrust duplication of lower and middle Cambrian units of the Eightmile Thrust System record layer‐parallel shear and shortening. Detailed cross sections are used to create an interpretive pre‐extensional reconstruction of the NSR that integrates the NSRFTS with associated supracrustal shortening structures now exposed in the Confusion Range to the east. We conclude that the NSRFTS and overlying structures accommodated ≥10 km of horizontal shortening, resulting in ≥7 km of structural burial of the deepest structural levels in the NSR footwall, thus explaining the localized kyanite‐grade metamorphism and elevated thermobarometric P‐T estimates. This burial was apparently quite localized as evidenced by decreasing metamorphism to the north and south. The highest magnitude of subsequent extensional exhumation of the NSR footwall appears to coincides with the area of deepest burial, suggesting that Mesozoic crustal thickening influenced both the magnitudes and geometry of subsequent Cenozoic extensional exhumation.

Thermometry and Microstructural Analysis Imply Protracted Extensional Exhumation of the Tso Morari UHP Nappe, Northwestern Himalaya: Implications for Models of UHP Exhumation

AbstractDocumenting the processes that facilitate exhumation of ultrahigh‐pressure (UHP) rocks at convergent margins is critical for understanding orogen dynamics. Here, we present structural and temperature data from the Himalayan UHP Tso Morari nappe (TMN) and overlying nappes, which we integrate with published pressure‐temperature‐time constraints to refine interpretations for their structural evolution and exhumation history. Our data indicate that the 5.5‐km‐thick TMN is the upper portion of a penetratively deformed ductile slab, which was extruded via distributed, pure shear‐dominated, top‐down‐to‐east shearing. Strain in the TMN is recorded by high‐strength quartz fabrics (density norms between 1.74 and 2.86) and finite strain data that define 63% transport‐parallel lengthening and 46% transport‐normal shortening. The TMN attained peak temperatures of ~500–600°C, which decrease in the overlying Tetraogal and Mata nappes to ~150–300°C, defining a field gradient as steep as 67°C/km. Within the overlying nappes, quartz fabric strength decreases (density norms between 1.14 and 1.21) and transport‐parallel lengthening and transport‐normal shortening decrease to 14% and 18%, respectively. When combined with published 40Ar/39Ar thermochronometry, quartz fabric deformation temperatures as low as ~330°C indicate that the top‐to‐east shearing that exhumed the TMN continued until ~30 Ma. Peak temperatures constrain the maximum depth of the overlying Mata nappe to 12.5–17.5 km; when combined with published fission‐track thermochronometry, this provides further support that the TMN was not underplated at upper crustal levels until ~30 Ma. The long‐duration, convergence‐subnormal shearing that exhumed the TMN outlasted rapid India‐Asia convergence by ~15 Myr and may be the consequence of strain partitioning during oblique convergence.

Early Permian tectonic evolution of the Last Chance thrust system: An example of induced subduction initiation along a plate boundary transform

AbstractThe late Paleozoic is an important precursor stage in the development of the Mesozoic Cordilleran subduction system along the western margin of North America, but the tectonic history remains ambiguous due to complex overprinting deformation and magmatism. Determining the driving mechanism of large magnitude Permian shortening in southwest Laurentia is critical to understanding the late Paleozoic transition from transform margin to subduction zone. We investigated the driving mechanism of the Permian Last Chance thrust system in east-central California to understand this transition prior to the development of the Mesozoic Cordilleran arc. Here, we present the results of new geological mapping, detrital zircon U-Pb geochronology, and a synthesis of regional tectonics to inform a kinematic model of the Last Chance thrust system and outline the Permian–Triassic tectonic evolution of the plate boundary during induced subduction initiation. The record of subduction initiation along an inferred late Paleozoic transform fault (the California-Coahuila transform) is preserved by (1) Permian arc magmatism, (2) the onset of volcaniclastic sedimentation, and (3) the development of a regional transpressional system in present-day east-central California. The evolution of this transpressional system and subduction zone is recorded by development of the Last Chance thrust system of the Death Valley region. Geological mapping in the Last Chance Range, northern Death Valley National Park, and the Inyo Mountains reveals the east-directed Last Chance thrust system was constructed by repetitive out-of-sequence deformation consistent with transpressional strain. The Last Chance thrust system accommodated a minimum of >75 km (60%) shortening, based on cross-section restorations guided by regional stratigraphic relationships and restoration of subsequent Mesozoic deformation. Our revised model of Jurassic extensional exhumation of the Snow Lake terrane argues the Last chance thrust was not reactivated during the Mesozoic. Large-magnitude shortening along the California-Coahuila transform accommodated a significant component of the convergent plate motion as the Panthalassan crust was thrust below the continental margin before initial slab sinking. Numerical models show the forces resisting subduction are greatest before initial slab sinking takes place, and compression is transmitted in board from the plate boundary. We argue the Last Chance thrust system developed in response to this compression. Early-middle Permian plutons and late Permian detrital zircons in coeval basins suggest subduction was well established by the early Permian. Collectively, the preservation of a thrust system, early arc magmatism, and syntectonic sedimentary basins, which are features typically destroyed by subduction magmatism and deformation, allow for the evaluation of subduction initiation mechanisms based on field observations.

Thermobarometric constraints on burial and exhumation of 2-billion-year-old eclogites and their metapelitic hosts

One of the first appearances of eclogite-facies mineral assemblages in the geological record occurs in the c. 2000 Ma Palaeoproterozoic Usagaran Belt in central Tanzania, where the extended margin of the Tanzanian Craton is interpreted to have been subducted. Mafic rocks are interpreted to have contained the mineral assemblage garnet + omphacite + rutile + quartz ± amphibole. This high-pressure assemblage has been overprinted by a secondary mineral assemblage containing clinopyroxene + plagioclase + hornblende + ilmenite ± orthopyroxene. Mineral equilibria forward modelling indicates that the eclogite-facies assemblages reached minimum peak pressure–temperature (P–T) conditions of ~17 kbar and ~700 °C. Inclusions in garnet document a prograde P–T history consistent with burial through upper amphibolite-facies conditions and possible partial melting. Petrological and compositional evidence from garnet suggests that following peak metamorphism, the eclogite-facies rocks were heated while stalled at approximate peak pressures. Temperature estimates derived from Zr concentrations in interpreted texturally retrograde rutile support a near-isothermal post-peak P–T evolution for the eclogite-facies rocks – an evolution that terminates at retrograde P–T conditions of approximately 7.6–8.2 kbar and 680–790 °C. The relict eclogite domains form part of a larger assemblage with enclosing migmatitic metapelitic lithologies (the Isimani Suite). The metapelitic gneisses contain garnet + kyanite + biotite + staurolite + hornblende + plagioclase + muscovite + rutile + quartz and preserve minimal evidence of a high-pressure history, conceivably due to post-peak mineralogical recrystallisation. P–T modelling, inclusion assemblages and compositional zonation patterns in porphyroblastic garnet suggests the metapelitic gneisses — similarly to the relict eclogites — experienced burial to minimum peak pressures of approximately 16.5–17 kbar. Compositional zoning patterns in eclogitic garnet suggest the Isimani system was buried, reached peak metamorphic conditions, and was subsequently exhumed within a timeframe of up to 20 Myr. A tectonic regime involving crustal thickening and subduction, followed by extensional exhumation of the entire Isimani Suite is our preferred model for the development of the c. 2000 Ma Usagaran Belt.

The petrology, geochronology and tectono-magmatic setting of igneous rocks in the Suckling-Dayman metamorphic core complex, Papua New Guinea

Southeastern Papua New Guinea has hosted magmatism throughout the Cenozoic, with the latest phase being concurrent with active extension in the Woodlark Rift. There, the Suckling-Dayman metamorphic core complex (SDMCC) has exhumed middle-crustal rocks in the past few million years along a still-active low-angle normal fault, the Mai'iu Fault. Uplift of the SDMCC has exposed metasedimentary and metaigneous rocks of Late Cretaceous-Pleistocene age that record an evolution from oceanic spreading to subduction, to collision, and finally to subduction-inversion and extensional exhumation. We present new petrographic, whole-rock geochemical and geochronologic (zircon U–Pb LA-ICP-MS) data from the SDMCC to reconstruct its long-term evolution. The dominant footwall-lithology of the SDMCC, the Goropu Metabasalt, has a MORB composition. Detrital zircons from metasedimentary beds intercalated with these basalts yield U–Pb-based maximum estimates for deposition of ~103 and ~72 Ma, suggesting a Late Cretaceous spreading age of the oceanic protolith. Bulk compositions of low-grade tholeiitic meta-gabbroic and -tonalitic rocks (Yau Igneous Complex) that intrude the Goropu Metabasalt reveal both enriched and depleted light rare earth element patterns. Zircon U–Pb ages from the Yau Igneous Complex range between ~60 and ~57 Ma, providing a minimum age for the formation of the Goropu Metabasalt. Much younger syn-extensional granitoids in the mafic footwall of the SDMCC have calc-alkaline to high-K compositions and U–Pb ages on zircon between ~3.7 and ~2.0 Ma. Our data indicate that the Mai'iu Fault had re-activated a Paleogene thrust as an extensional detachment fault by 3.7 ± 0.2 Ma. U–Pb ages of detrital zircons in modern streams draining the footwall of the SDMCC essentially reflect the ages revealed by the Paleocene and Plio-Pleistocene intrusions. Xenocrystic zircons in the Plio-Pleistocene granitoids imply that the crust underlying the metabasaltic carapace of the SDMCC consists chiefly of Australian-continent derived sedimentary rocks.

The Alichur Dome, South Pamir, Western India–Asia Collisional Zone: Detailing the Neogene Shakhdara–Alichur Syn‐collisional Gneiss‐Dome Complex and Connection to Lithospheric Processes

AbstractNeogene, syn‐collisional extensional exhumation of Asian lower–middle crust produced the Shakhdara–Alichur gneiss‐dome complex in the South Pamir. The <1 km‐thick, mylonitic–brittle, top‐NNE, normal‐sense Alichur shear zone (ASZ) bounds the 125 × 25 km Alichur dome to the north. The Shakhdara dome is bounded by the <4 km‐thick, mylonitic–brittle, top‐SSE South Pamir normal‐sense shear zone (SPSZ) to the south, and the dextral Gunt wrench zone to its north. The Alichur dome comprises Cretaceous granitoids/gneisses cut by early Miocene leucogranites; its hanging wall contains non/weakly metamorphosed rocks. The 22–17 Ma Alichur‐dome‐injection‐complex leucogranites transition from foliation‐parallel, centimeter‐ to meter‐thick sheets within the ASZ into discordant intrusions that may comprise half the volume of the dome core. Secondary fluid inclusions in mylonites and mylonitization‐temperature constraints suggest Alichur‐dome exhumation from 10–15 km depth. Thermochronologic dates bracket footwall cooling between ~410–130 °C from ~16–4 Ma; tectonic cooling/exhumation rates (~42 °C/Myr, ~1.1 km/Myr) contrast with erosion‐dominated rates in the hanging wall (~2 °C/Myr, <0.1 km/Myr). Dome‐scale boudinage, oblique divergence of the ASZ and SPSZ hanging walls, and dextral wrenching reflect minor approximately E–W material flow out of the orogen. We attribute broadly southward younging extensional exhumation across the central South Pamir between ~20–4 Ma to: (i) Mostly northward, foreland‐directed flow of hot crust into a cold foreland during the growth of the Pamir orocline; and (ii) Contrasting effects of basal shear related to underthrusting Indian lithosphere, enhancing extension in the underthrust South Pamir and inhibiting extension in the non‐underthrust Central Pamir.

Cretaceous‐Paleogene Tectonics of the Pelagonian Zone: Inferences From Skopelos Island (Greece)

AbstractWe have investigated subduction‐exhumation processes in the Pelagonian zone, exposed on the Northern Sporades islands (Aegean Sea) related to successive episodes of ocean continent and continent‐continent convergence through integrating multiscale structural analysis, metamorphic petrology, and white mica 40Ar/39Ar dating. Two major progressive phases of ductile deformation are documented, which are related to distinct episodes of tectonic burial and exhumation of the Pelagonian continental margin, which was facing the Neotethys/Vardar ocean. Review of existing data sets from neighboring regions shows that both deformation phases can be correlated along strike up to the Dinarides. The first phase of tectonic burial and exhumation (D1) is characterized by NW‐SE tectonic transport, greenschist facies metamorphism, and Early Cretaceous (~105‐135Ma) 40Ar/39 Ar white mica single fusion ages. D1 is correlated with the initial closure of the Vardar ocean by top‐to‐the‐W to NW ophiolite obduction and the underthrusting of the Pelagonian margin below the oceanic upper plate. Underthrusting was followed by exhumation and the deposition of Late Cretaceous‐Paleogene sediments. The second phase of burial and exhumation (D2) is characterized by NE‐SW tectonic transport, greenschist to blueschist facies metamorphism, and latest Cretaceous‐Early Eocene 40Ar/39 Ar white mica ages of S2 fabrics. Top‐to‐the‐SW shearing is correlated with the tectonic burial of the Pelagonian zone below the Eurasian continent (Rhodopia), while top‐to‐the‐NE shearing is attributed to subsequent extensional exhumation. D2 fabrics record low‐grade P‐T conditions suggesting that the decoupled cover formations exposed on Skopelos were incorporated in an accretionary wedge that formed above the subducting Pelagonian basement during Paleogene times.

Early flank uplift along the Suez Rift: Implications for the role of mantle plumes and the onset of the Dead Sea Transform

The Suez Rift is the northern extension of the Red Sea Rift and is an ideal location to study young continental breakup tectonics. Yet, despite much effort to constrain the timing of rift flank uplift and extensional exhumation in this region, the emerging picture remains uncertain. Here we present new thermochronologic constraints including apatite (U–Th)/He and fission track data from a transect across the eastern flank of the Suez Rift in SW Sinai. The use of two different thermochronometers in conjunction with thermal history modeling and well constrained kinetic parameters delineates a pronounced episode of relatively rapid cooling at 25-18 Ma followed by subdued post ∼18 Ma cooling. In contrast to previous estimates, this cooling pattern suggests that the most rapid uplift and exhumation phase along the Gulf of Suez took place during the early stage of rift formation, prior to the middle Miocene, with over 2.4 km of exhumation occurring during this stage. Uplift and exhumation along the Suez Rift flank slowed substantially post ∼18 Ma, as deformation migrated to the newly formed plate boundary along the Dead Sea Transform. This finding complements and strengthens previous indications for the initiation timing of the Dead Sea Transform plate boundary. The inferred late Oligocene–early Miocene timing of rapid exhumation, associated with tectonic extension and rift flank uplift coincides with the onset of regional alkali-basalt magmatism, suggesting lithospheric heating prior to the rift formation. This observation, along with the elevated topography around the Sinai triple junction and indications for some pre-rifting uplift, supports a more active mantle plume role in the rifting process.

Deformation and extensional exhumation of 1.9 Ga high-pressure granulites along the Wholdaia Lake shear zone, south Rae craton, Northwest Territories, Canada

Deformation and extensional exhumation of 1.9 Ga high-pressure granulites along the Wholdaia Lake shear zone, south Rae craton, Northwest Territories, Canada

Miocene UHT granulites from Seram, eastern Indonesia: a geochronological–REE study of zircon, monazite and garnet

Abstract The island of Seram, eastern Indonesia, incorporates Miocene ultrahigh-temperature (UHT; >900°C) garnet–sillimanite granulites that formed by extensional exhumation of hot mantle rocks behind the rolling-back Banda Arc. UHT metamorphic conditions are supported by new Zr-in-rutile thermometry results and the Miocene age of the UHT event is confirmed by closely-matched heavy rare earth element (HREE) abundances between garnet and c. 16 Ma zircon. Monazites also record identical U–Pb ages, within uncertainty. However, these geochronometers do not date peak UHT metamorphism; instead, they date retrograde, garnet-consuming (Zr- and rare earth element (REE)-liberating) reactions that produced the granulites’ post-peak cordierite + spinel reaction microstructures. Zircons shielded within garnet did not crystallize c. 16 Ma rims and so were unaffected by the entire UHT event. Miocene UHT metamorphism overprinted a Late Triassic–Early Jurassic upper-amphibolite facies event that grew garnet cores and 216–173 Ma zircon. In the Miocene, these garnet cores were overgrown by peritectic garnet rims during UHT metamorphism, with some rutiles recording c. 900°C Zr-in-rutile temperatures. Garnet Lu–Hf ages of 138 Ma – produced by core–rim mixing – demonstrate that a component of Hf 4+ produced since c. 200 Ma was retained through the c. 16 Ma UHT event. Accordingly, UHT conditions must have been very short-lived and exhumation of the granulite complex very rapid.

Alps to Apennines zircon roller coaster along the Adria microplate margin

We have traced the particle path of high-pressure metasedimentary rocks on Elba Island, Northern Apennines, with the help of a U-Pb-Hf detrital zircon study. One quarter of the analysed zircons are surprisingly young, 41-30 Ma, with a main age peak at ca. 32 Ma, indicating an unexpected early Oligocene maximum deposition age. These Oligocene ages with negative εHf indicate a volcanic source region in the central-southern Alps. Though young by geological means, these zircons record an extraordinary geodynamic history. They originated in a volcanic arc, during the convergence/collision of the the Adria microplate with Europe from ca. 65 to 30 Ma. Thereafter, the Oligocene zircons travelled ca. 400 km southward along the Adria margin and the accretionary prism to present-day Tuscany, where they were subducted to depths of at least 40 km. Shortly thereafter, they were brought to the surface again in the wake of hinge roll back of the Apennine subduction zone and the resulting rapid extensional exhumation. Such a zircon roller coaster requires a microplate that has back-to-back subduction zones with opposing polarities on two sides.