Orogenic Loading Research Articles

Surface Processes Driving Intracontinental Basin Subsidence in the Context of India–Eurasia Collision: Evidence from Flexural Subsidence Modeling of the Cenozoic Southern Tarim Basin along the West Kunlun Foreland, NW Tibetan Plateau

AbstractThe India–Eurasia collision has produced a number of Cenozoic deep intracontinental basins, which bear important information for revealing the far‐afield responses to the remote collision. Despite their significance, their subsiding mechanism remains the subject of debate, with end‐member models attributing it to either orogenic or sedimentary load. In this study, we conduct flexural subsidence modeling with a two‐dimensional finite elastic plate model on the Hotan–Mazatagh section along the southern Tarim Basin, which defines a key region in the foreland of the West Kunlun Orogen, along the NW margin of the Tibetan Plateau. The modeling results indicate that the orogenic load of West Kunlun triggers the southern Tarim Basin to subside by up to less than ∼6 km, with its impact weakening towards the basin interiors until ∼230 km north from the Karakax fault. The sedimentary load, consisting of Cenozoic strata, forces the basin to subside by ∼2 to ∼7 km. In combination with the retreat of the proto‐Paratethys Sea and the paleogeographic reorganization of the Tarim Basin, we propose that surface processes, in particular a shift from an exorheic to an endorheic drainage system associated with the consequent thick sedimentary load, played a decisive role in forming deep intracontinental basins in the context of the India–Eurasia collision.

Subsidence and Uplift History of the UAE and the Western Flank of the UAE‐Oman Mountain Range

AbstractThe western flank of the UAE‐Oman mountain range offers a unique geodynamic setting to study the development of a cratonic rift into a mature passive margin and its subsequent flexure under orogenic load. However, the geodynamic processes driving this evolution are not fully understood. In this study, seismic and biostratigraphic data from 283 exploration wells were utilized to assess the regional subsidence and uplift history of the United Arab Emirates (UAE) and neighboring areas of the western flank of the UAE‐Oman mountain range. Three major sequences have been identified: pre‐Permian, Permian‐Turonian rifted margin, and Coniacian‐Pleistocene active margin. Backstripping of biostratigraphic data reveals Early Permian (ca. 272 Ma) and Late Jurassic (ca. 160) rift phases, that have been linked to Gondwana's early and final fragmentation. A NE‐SW‐oriented basin in central UAE suggests a Jurassic intracratonic rift that has been influenced by pre‐existing structures. Compressional phases were identified during the Late Cretaceous and in the Oligocene‐Miocene, coinciding with the emplacement of the Semail ophiolite and the Arabian‐Eurasian plates collision, respectively. These events caused additional subsidence and flank uplift, forming the Aruma foreland basin in the east and the Pabdeh foreland basin in the eastern and northern UAE. Crustal thickness following the rift episodes ranges from 30 to 36 km, resulting in a sedimentary cover thickness of 11–14 km. These estimates are in accord with Moho depths derived from teleseismic receiver functions and gravity inversion. These findings can be used to better understand Gondwana fragmentation and the opening and closing of the Neo‐Tethys Ocean.

Formation and evolution of an Early Cambrian foreland basin in the NW Yangtze Block, South China

The Ediacaran to Cambrian succession in the NW Yangtze Block has long been considered to have formed in a passive margin. The Ediacaran is dominated by thick carbonate rocks, whereas the Lower Cambrian comprises a thick clastic succession. The transition from carbonate to clastic rocks and the provenance and tectonic setting of the clastic succession are poorly understood. Stratigraphic correlation shows a distinct stratigraphic absence from the Lower Cambrian to Devonian in the Bikou terrane. A regional seismic profile shows a wedge geometry of the Lower Cambrian from NW to SE. Outcrops reveal an overall coarsening-upward Lower Cambrian succession. The petrographic analysis of clastic rocks indicates immaturity, implying a proximal source. Palaeocurrent measurements of clastic rocks point to dominant SE-vergent orientations. The age spectra of detrital zircons imply that they were derived from Early Cambrian continental arcs and older continental crust. Geological, geophysical and geochemical evidence indicates that an Early Cambrian foreland basin was formed in the NW Yangtze Block. This foreland basin appears to have been strongly influenced by orogenic loading northwestward. We propose that this orogenic event should be named the Motianling orogeny, the origin of which may be related to subduction of the Proto-Tethys ocean beneath the NW Yangtze Block. Supplementary material: Supplementary tables A–D and a petrological description of detrital zircons are available at https://doi.org/10.6084/m9.figshare.c.6332980

Geomorphic signature of segmented relief rejuvenation in the Sierra Morena, Betic forebulge, Spain

Abstract. The foreland relief of alpine orogenic belts is often rejuvenated due to the intraplate propagation of orogenic deformation. Thus, in these long-lived areas, the localisation of relief rejuvenation may be largely controlled by the reactivation of previous mechanical discontinuities. In this regard, we explored the relationship between the relief rejuvenation pattern and the distribution, geometry, and kinematics of faults in a wide segment of the Betic foreland (Sierra Morena, southern Spain). Specifically, we focused on the forebulge, a WSW–ENE flexural relief that formed, paired to the Betic foreland basin, in response to orogenic load. For this purpose, we applied both qualitative and quantitative geomorphological tools, including geomorphic indices and knickpoint pattern modelling in χ space. We found that the pattern of relief rejuvenation responds to large-scale flexural uplift coupled with the tectonic activity of two groups of faults that often show evidence of reactivation, namely overall WSW–ENE faults contributing to both regional NNW–SSE relief segmentation and vertical extrusion of the forebulge, and NW–SE reverse faults associated with an outstanding WSW–ENE topographic segmentation in the west of the study area. In addition, our knickpoint modelling suggests that the faults related to the southernmost Sierra Morena mountain front have been particularly active in recent times, although their activity span and the relative uplift that they accommodate differ along the Sierra Morena/foreland basin limit. The knickpoint pattern also suggests a significant reorganisation of the analysed drainage basins. The strain partitioning accommodated by the structures involved in relief rejuvenation suggests the intraplate propagation of the transpressional deformation reported from the Betic external fold and thrust belt.

The Miocene deep‐seated spontaneous coal‐seam fire in the Upper Silesian Coal Basin (S Poland) and its geotectonic trigger mechanism

AbstractThe deep‐seated spontaneous coal‐seam fire occurred in the southwestern part of the Upper Silesian Coal Basin (S Poland), confirmed by oxygenated compounds emission to over‐ and underlying rocks. The fire was dated at 19.2–19.8 Ma using the 39Ar‐40Ar method and palaeomagnetism. It was initiated in the Carpathian foreland thanks to a normal faulting in response to thrust‐loaded deflections of the Carpathian foreland lithosphere during collision of tectonic plates (Alcapa and stable Europe). The orogenic loading caused this normal faulting in the foreland and triggering the flexural mechanism of foredeep subsidence and forebulge uplift. Thanks to the dating of rocks affected by the spontaneous coal fire initiated during the rapid uplift of coal‐bearing rocks, when they became exposed to the aeration zone, the age of these mountain‐building processes in the Carpathians has been precisely defined.

Investigating the formation of the Cretaceous Western Interior Seaway using landscape evolution simulations

Abstract Transient intraplate sedimentation like the widespread Late Cretaceous Western Interior Seaway, traditionally considered a flexural foreland basin of the Sevier orogeny, is now generally accepted to be a result of dynamic topography due to the viscous force from mantle downwelling. However, the relative contributions of flexural versus dynamic subsidence are poorly understood. Furthermore, both the detailed subsidence history and the underlying physical mechanisms remain largely unconstrained. Here, we considered both Sevier orogenic loading and three different dynamic topography models that correspond to different geodynamic configurations. We used forward landscape evolution simulations to investigate the surface manifestations of these tectonic scenarios on the regional sedimentation history. We found that surface processes alone are unable to explain Western Interior Seaway sedimentation in a purely orogenic loading system, and that sedimentation increases readily inland with the additional presence of dynamic subsidence. The findings suggest that dynamic subsidence was crucial to Western Interior Seaway formation and that the dominant control on sediment distribution in the Western Interior Seaway transitioned from flexural to dynamic subsidence during 90–84 Ma, coinciding with the proposed emplacement of the conjugate Shatsky oceanic plateau. Importantly, the sedimentation records require the underlying dynamic subsidence to have been landward migratory, which implies that the underlying mechanism was the regional-scale mantle downwelling induced by the sinking Farallon flat slab underneath the westward-moving North American plate. The simulated landscape evolution also implies that prominent regional-scale Laramide uplift in the western United States should have occurred no earlier than the latest Cretaceous.

Detrital zircons from the Upper Três Marias Formation, São Francisco basin, SE Brazil: Record of foreland deposition during the Cambrian?

The Bambuí Group, a succession of mixed carbonate and siliciclastic rocks, accumulated in the intracratonic and poly-historic São Francisco successor basin by the end of the Neoproterozoic. The São Francisco basin, which occupies a substantial portion of the eponymous craton in southeastern Brazil, was converted into a major foreland depocenter in response to orogenic loads generated along the craton margins during the assembly of West Gondwana in the Ediacaran Period. Various studies carried out in recent years have led to a better understanding of many aspects of the evolution of the São Francisco basin during the Neoproterozoic. However, the age of the Bambuí Group, critical for a better understanding of the basin history and establishment of regional and global correlations, remains poorly constrained and matter of a long-standing debate in the literature. In order to contribute to this debate, we performed U-Pb LA-ICPMS age determinations on detrital zircons extracted from the Três Marias Formation, the youngest unit of the group. Our study, based on a stratigraphic survey, was conducted in an occurrence of the formation located near to the eastern border of the basin, i.e., in a proximal position in relation to its potential source, the Brasiliano Araçuaí orogen. The detrital zircon age spectra (involving 432 concordant ages) of the lower alluvial and upper marine units of the Três Marias Formation are characterized by a unimodal distribution with c.83% of the grains dated between 735 and 522 Ma. These results indicated that lithostratigraphic assemblages of the Araçuaí orogen, namely the 630-580 Ma granitoids of Rio Doce magmatic arc and 585-540 Ma syn-collisional granites were the most important sources of the Três Marias sediments. Moreover, post-collisional granite suites younger than 530 Ma also contribute with sediments to the formation. Using four different metrics, we obtained 527 ± 4 Ma as the most accurate and reliable estimation for maximum depositional age of the Upper Três Marias Formation, an age that has major implications for the developmental history of both the São Francisco basin and Araçuaí orogen.

Anticosti foreland basin offshore of western Newfoundland: Concealed record of northern Appalachian orogen development

AbstractThe Anticosti Basin, largely hidden beneath the Gulf of St. Lawrence, includes foreland basin successions that record multiple tectonic events associated with the Ordovician to Devonian evolution of the northern Appalachian orogen. Due to the lack of well ties and minimal onshore exposure, geophysical data must be used in mapping the offshore stratigraphy. Outcropping geologic boundaries are tied to magnetic lineaments that parallel stratigraphy. These lineaments are correlated with reflections on seismic profiles in order to interpret the subsurface. Seismic isochron maps for successive basin development episodes display differences in geometry, implying that orogenic loading varied through time. The geometry and subsidence rates recorded by the Middle Ordovician Goose Tickle Group imply that it formed in a pro‐arc setting associated with loading during arc‐continent collision that was most intense in the northern Newfoundland Appalachians. The geometry and subsidence recorded by the overlying Long Point Group imply pro‐arc loading by Taconian allochthons in the Québec segment of the orogen. Diachronous subduction polarity reversal along the margin placed the Long Point Group in a combined retro‐arc and pro‐arc setting, comparable to that experienced by parts of the north Australian margin at the present day. The uppermost Silurian to Lower Devonian Clam Bank Formation and Lower Devonian Red Island Road Formation represent foreland basin successions associated with the later Salinian and Acadian orogenies. Their consistent thickness implies a broad, shallow basin, suggesting that the lithosphere was cooler and stronger than during earlier subsidence, and are consistent with a retro‐arc setting.

Distinct Responses of Intraplate Sedimentation to Different Subsidence Mechanisms: Insights From Forward Landscape Evolution Simulations

AbstractBoth the surface processes associated with large intracontinental basins and their underlying geodynamic mechanisms remain unclear. Proposed models include flexural deformation due to orogenic loading, isostatic subsidence in response to lithosphere extension, and dynamic subsidence arising from sublithospheric mantle downwelling. Here we use forward landscape evolution modeling to quantitatively investigate the surface process response to various types of tectonic subsidence during intraplate sedimentation. We find that isostatic subsidence causes continuous accumulation of sediment through time with a largely stable pattern of fluvial drainage. In contrast, dynamic subsidence results in fast intraplate sedimentation followed by regional unconformities and continental‐scale reorganization of fluvial networks. Our quantitative simulations reveal that the traditional 1‐D isostatic backstripping approach may lead to erroneous estimates of the past tectonic subsidence from the preserved sedimentary records: for lithospheric elastic thickness (Te) < 50 km, the flexural effect leads to larger (smaller) apparent subsidence at the center (rim) of the basin, compared to the true subsidence; for Te > 50 km, broadscale flexural support leads to underestimated subsidence throughout the basin. Subparallel tilted strata are a unique stratigraphic feature associated with spatially migratory dynamic subsidence followed by dynamic rebound. Their characteristic slope provides a quantitative constraint on the spatiotemporal pattern of dynamic subsidence. The Cretaceous Western Interior Seaway provides an example of this type of basin fill. We demonstrate that quantitative, forward landscape evolution modeling can help decipher the relationship between intraplate sedimentation and the underlying tectonic drivers.

First-order foreland cycles: Interplay of flexural tectonics, dynamic loading, and sedimentation

Foreland systems on overriding plates (i.e., retroarc settings) form through the deflection of the lithosphere in response to a combination of supra- and sublithospheric loads. Supracrustal loading by orogens leads to the partitioning of foreland systems into foredeep, forebulge, and back-bulge flexural provinces. Thrusting in the orogenic belt results in foredeep subsidence and forebulge uplift, and the reverse occurs as orogenic load is removed by erosion or extension. This pattern generates out-of-phase ('reciprocal') stratigraphies across the flexural hingeline that separates the foredeep from the forebulge. Coupled with flexural tectonics, accommodation is also modified by dynamic (sublithospheric) loading. The latter mechanism operates at larger, continental scales, with rates controlled by the velocity and the angle of subduction.The interplay of accommodation and sediment supply during the lifespan of a foreland system controls the long-term shift from underfilled to overfilled accommodation conditions, which defines the first-order foreland cycle. Predictable shifts in the balance between flexural tectonics and dynamic loading during the evolution of a foreland system allow the subdivision of the first-order foreland cycle into early and late stages dominated by flexural tectonics, separated by an intermediate stage dominated by system-wide dynamic subsidence. The early stage is defined by an underfilled foredeep that hosts a deep-water environment, and an exposed forebulge whose progradation and erosion generates the ‘forebulge unconformity' at the base of the first-order foreland sequence. Submergence of the forebulge during the intermediate stage establishes an interior seaway across the entire foreland system. The late stage marks a shift to a system-wide continental setting, which defines the overfilled phase of the foreland cycle.

Deciphering the Late Cretaceous‐Cenozoic Structural Evolution of the North Peruvian Forearc System

AbstractThe link between plate tectonics and the evolution of active margins is still an ongoing task to challenge since the acceptance of plate tectonic paradigm. This paper aims at deciphering the structural architecture and uplift history of the North Peruvian forearc system to better understand the evolution and the mechanics that govern the Late Cretaceous‐Cenozoic building of this active margin. In this study, we report surface structural geology data, interpretation of seismic reflection profiles, apatite fission track data, and the construction of two offshore‐onshore crustal‐scale balanced cross sections. The structure of the North Peruvian forearc system is dominated by an accretionary style with northwestward propagation of thrust‐related structural highs involving continental/oceanic basement rocks and off‐scrapped sediments. The thrust systems bound thick thrust‐top forearc depocenters mainly deformed by crustal normal to strike‐slip faults and thin‐skinned gravitational instabilities. The sequential restoration of the margin calibrated with apatite fission track data suggests a correlation between uplift, shortening, and plate convergence velocity during Late Cretaceous and Miocene. Pliocene‐Quaternary shortening and uplift of the coastal zone is rather related to the subduction of asperities during convergence rate decrease. The development of crustal normal to strike‐slip faulting and subsidence zones might be the consequence of slab flexure, local basal erosion along subduction fault, and/or oblique subduction associated with sediment loading control. We conclude that the evolution of the North Peruvian forearc system was controlled by subduction dynamics, strong sediment accumulation, and recent ridge subduction, and it recorded the orogenic loading evolution of the Andes over the Cenozoic.

Subsidence History and Seismic Stratigraphy of the Western Musandam Peninsula, Oman–United Arab Emirates Mountains

AbstractSeismic reflection profiles, exploratory well, and outcrop data are used to determine the stratigraphy and tectonic subsidence and uplift history of the western Musandam peninsula. Five major regional megasequences have been recognized: (1) Permian to Late Cretaceous rifted margin sequence, (2) Late Cretaceous Aruma foreland basin sequence that evolved in response to the obduction of the Semail Ophiolite, (3) early–mid‐Cenozoic Pabdeh foreland basin sequence that formed due to orogenic loading associated with the early continent–continent collision of the Arabian and Eurasian plates in the Zagros mountains of central Iran, (4) a sequence above the mid‐Miocene unconformity that marks the final stage of continent–continent collision, and (5) a sequence above the late Pliocene unconformity interpreted as tilting due to the latest stage of continent–continent collision. In addition, seismic and outcrop data captured multiple west‐verging and east‐dipping thrust faults associated with the deformation of the Hagab thrust, which causes repetition of the Permian–Mesozoic shelf sequence and the early–mid Cenozoic foreland basin sequence. The tectonic subsidence and uplift derived from backstripping can be explained by a model in which the margin developed by uniform depth extension with an initial age of rifting of 260 Ma and a final age of rifting of 185 Ma. Moreover, the tectonic subsidence indicates two compressional events that commenced at ~94 Ma and ~25 Ma, respectively. These events are attributed to the obduction of the Semail Ophiolite and the culmination of the Musandam peninsula, respectively.

Neuquén Group (Upper Cretaceous): A case of underfilled-overfilled cycles in an Andean foreland basin, Neuquén basin, Argentina

The Nenquén Group was deposited during a period dominated by the Cretaceous Greenhouse and can be divided in three cycles correlated with large-scale changes in the evolution of the Andean foreland basin. The filling of the Neuquén Group is constituted by a complete cycle and two incomplete cycles of underfilled-overfilled, separated by first-order discontinuities assigned to the uplift of the Agrio fold-and-thrust belt during the Chasca/Catequil, Mid Ocean Ridge (CCMOR) collision, coinciding with first-order climatic changes within the Cretaceous greenhouse cycle.The Candeleros Formation in the base of this group was deposited in late underfilled conditions, showing prominent forebulge zones. It is demonstrated that during the Albian, with the cratonward migration of the uplifting forebulge zones, the axis of backbulge zones also migrated cratonwards and a wide uplifted forebulge zone was formed. On top, the Huincul Formation was deposited in an overfilled period without orogenic load, while the Cerro Lisandro Formation was deposited in early underfilled conditions with orogenic load. The Río Neuquén Subgroup started with a late underfilled period (Portezuelo Formation -second-order discontinuity), and after wards the Plottier Formation was deposited in an overfilled period without orogenic load. Finally, the Río Colorado Subgroup was deposited under late and early underfilled conditions (Bajo de la Carpa and Anacleto Formations respectively).

New Late Permian tectonic model for South Africa\u2019s Karoo Basin: foreland tectonics and climate change before the end-Permian crisis

Late Permian Karoo Basin tectonics in South Africa are reflected as two fining-upward megacycles in the Balfour and upper Teekloof formations. Foreland tectonics are used to explain the cyclic nature and distribution of sedimentation, caused by phases of loading and unloading in the southern source areas adjacent to the basin. New data supports this model, and identifies potential climatic effects on the tectonic regime. Diachronous second-order subaerial unconformities (SU) are identified at the base and top of the Balfour Formation. One third-order SU identified coincides with a faunal turnover which could be related to the Permo-Triassic mass extinction (PTME). The SU are traced, for the first time, to the western portion of the basin (upper Teekloof Formation). Their age determinations support the foreland basin model as they coincide with dated paroxysms. A condensed distal (northern) stratigraphic record is additional support for this tectonic regime because orogenic loading and unloading throughout the basin was not equally distributed, nor was it in-phase. This resulted in more frequent non-deposition with increased distance from the tectonically active source. Refining basin dynamics allows us to distinguish between tectonic and climatic effects and how they have influenced ancient ecosystems and sedimentation through time.

Basin inversion and supercontinent assembly as drivers of sediment-hosted Pb–Zn mineralization in the Mount Isa region, northern Australia

Deep seismic reflection imaging combined with geochronological, palaeomagnetic and stratigraphic data indicate that the world class sediment-hosted Pb–Zn deposits of northern Australia are preferentially concentrated in the post-extensional syn-inversion fraction of the Calvert and Isa superbasins. These fractions were deposited at 1650–1640 Ma and 1615–1575 Ma respectively, and overlap the age of known orogenic events in both Australia and western Laurentia, pointing to a common origin linked to crustal shortening and supercontinent assembly. Crustal shortening resulted in thrust faulting and reactivation of earlier-formed extensional faults leading to upward expulsion of mineralizing fluids at or close to the seafloor while basin inversion and sedimentation were still in progress. This is contrary to most existing models for Pb–Zn ore genesis in northern Australia where fluid flow and mineralization have been attributed to syn-extensional processes accompanying continental breakup or thermal sag. Instead, mineralization post-dates passive margin formation and more probably occurred in a contractional or foreland basin setting involving orogenic loading of the continental crust from the east during assembly or reassembly of the Nuna supercontinent. A strikingly similar history of basin inversion and Pb–Zn mineralisation is shared by the late Neoproterozoic-early Paleozoic Selwyn and older epicratonic basins of western North America.

Lithospheric velocity model across the Southern Central Iberian Zone (Variscan Iberian Massif): The ALCUDIA wide-angle seismic reflection transect

A P wave seismic velocity model has been obtained for the Central Iberian Zone, the largest continental fragment of the Iberian Variscan Belt. The spatially dense, high-resolution, wide-angle seismic reflection experiment, ALCUDIA-WA, was acquired in 2012 across central Iberia, aiming to constrain the lithospheric structure and resolve the physical properties of the crust and upper mantle. The seismic transect, ~310 km long, crossed the Central Iberian Zone from its suture with the Ossa-Morena Zone to the southern limit of the Central System mountain range. The energy generated by five shots was recorded by ~900 seismic stations. High-amplitude phases were identified in every shot gather for the upper crust (Pg and PiP) and Moho (PmP and Pn). In the upper crust, the P wave velocities increase beneath the Cenozoic Tajo Basin. The base of the upper crust varies from ~13 km to ~20 km between the southernmost Central Iberian Zone and the Tajo Basin. Lower crustal velocities are more homogeneous. From SW-NE, the traveltime of PmP arrivals varies from ~10.5 s to ~11.8 s, indicating lateral variations in the P wave velocity and the crustal thickness, reflecting an increase toward the north related with alpine tectonics and the isostatic response of the crust to the orogenic load. The results suggest that the high velocities of the upper crust near the Central System might correspond to igneous rocks and/or high-grade metamorphic rocks. The contrasting lithologies and the increase in the Moho depth to the north evidence differences in the Variscan evolution.

Late‐stage orogenic loading revealed by contact metamorphism in the northern Appalachians, New York

AbstractPelitic schists from contact aureoles surrounding mafic–ultramafic plutons in Westchester County, NY record a high‐P (~0.8 GPa) high‐T (~790 °C) contact overprint on a Taconic regional metamorphic assemblage (~0.5 GPa). The contact metamorphic assemblage of a pelitic sample in the innermost aureole of the Croton Falls pluton, a small (<10 km2) gabbroic body, consists of quartz–plagioclase–biotite–garnet–sillimanite–ilmenite–graphite–Zn‐rich Al‐spinel. Both K‐feldspar and muscovite are absent, and abundant biotite, plagioclase, sillimanite, quartz and ilmenite inclusions are found within subhedral garnet crystals. Unusually low bulk‐rock Na and K contents imply depletion of alkalic components and silica through anatexis and melt extraction during contact heating relative to typical metapelites outside the aureole. Thermobarometry on nearby samples lacking a contact overprint yields 620–640 °C and 0.5–0.6 GPa. In the aureole sample, WDS X‐ray chemical maps show distinct Ca‐enriched rims on both garnet and matrix plagioclase. Furthermore, biotite inclusions within garnet have significantly higher Mg concentration than matrix biotite. Thermobarometry using GASP and garnet–biotite Mg–Fe exchange equilibria on inclusions and adjacent garnet host interior to the high‐Ca rim zone yield ~0.5 ± 0.1 GPa and ~620 ± 50 °C. Pairs in the modified garnet rim zone yield ~0.9 ± 0.1 GPa and ~790 ± 50 °C. Thermocalc average P–T calculations yield similar results for core (~0.5 ± ~0.1 GPa, ~640 ± ~80 °C) and rim (~0.9 ± ~0.1 GPa, ~800 ± ~90 °C) equilibria. The core assemblages are interpreted to record the P–T conditions of peak metamorphism during the Taconic regional event whereas the rim compositions and matrix assemblages are interpreted to record the P–T conditions during the contact event. The high pressures deduced for this later event are interpreted to reflect loading due to the emplacement of Taconic allochthons in the northern Appalachians during the waning stages of regional metamorphism (after c. 465 Ma) and before contact metamorphism (c. 435 Ma). In the absence of contact metamorphism‐induced recrystallization, it is likely that this regional‐scale loading would remain cryptic or unrecorded.

Microprobe analysis and dating of monazite from the Potsdam Formation, New York: A progressive record of chemical reaction and fluid interaction

It has been recognized for several decades that REE-phosphates (monazite and xenotime) can grow during diagenesis and low-grade metamorphism. Growth of REE-bearing accessory phases at low-grade conditions commonly involves pervasive fluid-rock interaction, dissolution of detrital grains, transportation, and precipitation of REEs, typically facilitated by an increase in temperature. The occurrence of low-grade REE-phosphate offers a rare opportunity to date crystallization/mineralization and possibly fluid percolation. We report here the results of in situ dating by electron microprobe of Paleozoic authigenic and low-grade monazite and xenotime overgrowths on detrital monazite and zircon, respectively. Samples are from the Potsdam Formation, a basal sandstone deposited uncomformably on Proterozoic basement of the Adirondack Mountains of New York State. This study also focuses on the textural and chemical relationships of these REE-bearing accessory phases. Textures include rounded and fractured detrital monazite and zircon, which contrast with new sub-euhedral REE-phosphate overgrowths. Monazite overgrowths are enriched in LREE and depleted in HREE compared to detrital cores. The U and Th concentrations are low, typical of low-grade metamorphic conditions. Monazite core ages yield Proterozoic ages between 1.17 and 0.90 Ga (Shawinigan and Ottawan orogeny). Monazite overgrowth and xenotime ages indicate four to five major overgrowth events between ca. 500 Ma (shortly after the time of deposition) and ca. 200 Ma. As these ages are relatively young and the actinide content is low (∑ < 2 wt%), the radiogenic Pb content of monazite overgrowths and xenotime is low (<400 ppm). Therefore, EPMA dates have relatively large uncertainties. Nevertheless, the ages determined broadly correlate with major Paleozoic orogenic events recorded in the Appalachian Orogen to the East (Taconic, Salinic, Acadian, Neo-Acadian, and Alleghanian). Fluid percolation, driven by orogenic loading, may induce dissolution of detrital monazite and zircon. Subsequent precipitation of new monazite and xenotime probably results from changes in fluids or metamorphic conditions. This study demonstrates the power of the EMPA technique to resolve the fluid-related growth history of REE-phosphates in low-grade metasediments.

A crust-scale 3D structural model of the Beaufort-Mackenzie Basin (Arctic Canada)

The Beaufort-Mackenzie Basin was initiated in the Early Jurassic as part of an Arctic rifted passive continental margin which soon after became overprinted by Cordilleran foreland tectonics. Decades of industrial exploration and scientific research in this petroliferous region have produced a wide spectrum of geological and geophysical data as well as geoscientific knowledge. We have integrated available grids of sedimentary horizons, well data, seismic reflection and refraction data, and the observed regional gravity field into the first crust-scale 3D structural model of the Beaufort-Mackenzie Basin. Many characteristics of this model reflect the complex geodynamic and tectonostratigraphic history of the basin.The Mesozoic–Cenozoic sedimentary part of the model comprises seven clastic units (predominantly sandy shales) of which the modelled thickness distributions allow to retrace the well-established history of the basin comprising a gradual north(east)ward shift of the main depocentres as well as diverse phases of localised erosion. As a result of this development, the present-day configuration of the basin reveals that the sedimentary units tend to be younger, more porous, and thus less dense towards the north at a constant depth level.By integrating three refraction seismic profiles and performing combined isostatic and 3D gravity modelling, we have modelled the sub-sedimentary basement of the Beaufort-Mackenzie Basin. The continental basement spans from unstretched domains (as thick as about 42km) in the south to extremely thinned domains (of less than 5km thickness) in the north where it probably represents transitional crust attached to the oceanic crust of the Canada Basin. The uppermost parts of the continental crust are less dense (ρ=2710kg/m3) and most probably made up by pre-Mesozoic meta-sediments overlying a heavier igneous and metamorphic crust (ρ=2850kg/m3).The presented crust-scale 3D structural model shows that the greatest thicknesses of Mesozoic–Cenozoic sediments (almost 17km) are not found in the north where the sub-sedimentary crust is thinnest, but farther south where the crust is thicker and the Moho quite deep. We causally relate the huge amounts of foredeep deposits overlying a Moho depression to a flexural response of the lithosphere to orogenic loading induced by the Brooks Range orogen in the south. Our 3D model provides an ideal base and reference for future numerical studies including reconstructions of the development from a passive margin to a foreland basin and simulations of the present-day thermal field of the basin.

Sedimentology and foreland basin paleogeography during Taiwan arc continent collision

The Western foreland basin in Taiwan originated through the oblique collision between the Luzon volcanic arc and the Asian passive margin. Crustal flexure adjacent to the growing orogenic load created a subsiding foreland basin. The sedimentary record reveals progressively changing sedimentary environments influenced by the orogen approaching from the East. Based on sedimentary facies distribution at five key stratigraphic horizons, paleogeographic maps were constructed. The maps highlight the complicated basin-wide dynamics of sediment dispersal within an evolving foreland basin.The basin physiography changed very little from the middle Miocene (∼12.5Ma) to the late Pliocene (∼3Ma). The transition from a passive margin to foreland basin setting in the late Pliocene (∼3Ma), during deposition of the mud-dominated Chinshui Shale, is dominantly marked by a deepening and widening of the main depositional basin. These finer grained Taiwan derived sediments clearly indicate increased subsidence, though water depths remain relatively shallow, and sedimentation associated with the approach of the growing orogen to the East.In the late Pleistocene as the shallow marine wedge ahead of the growing orogen propagated southward, the proximal parts of the basin evolved into a wedge-top setting introducing deformation and sedimentation in the distal basin. Despite high Pleistocene to modern erosion/sedimentation rates, shallow marine facies persist, as the basin remains open to the South and longitudinal transport is sufficient to prevent it from becoming overfilled or even fully terrestrial.Our paleoenvironmental and paleogeographical reconstructions constrain southward propagation rates in the range of 5–20km/Myr from 2Ma to 0.5Ma, and 106–120km/Myr between late Pleistocene and present (0.5–0Ma). The initial rates are not synchronous with the migration of the sediment depocenters highlighting the complexity of sediment distribution and accumulation in evolving foreland basins.