Paleozoic Tectonic Events Research Articles

Latest Paleoproterozoic (ca. 1.8–1.6 Ga) extensional tectonic setting in the Dunhuang terrane, NW China: Evidence from geochronological and geochemical investigations on A-type granite and metamafic rock

Abstract Latest Paleoproterozoic (ca. 1.8–1.6 Ga) magmatic rocks outcrop in the Dunhuang terrane, represented by A-type granites and mafic (basaltic) rocks that have metamorphosed into amphibolites. The A-type granites, emplaced at ca. 1.79–1.77 Ga, are geochemically characterized by high Na2O + K2O, Fe2O3T, Zr, Nb, and Ce contents, as well as high Fe2O3T/(Fe2O3T + MgO) and Ga/Al ratios. Furthermore, they have Nb/Ta, Y/Nb, Rb/Nb, and Sc/Nb ratios of 12.10–15.56, 1.45–1.79, 3.52–6.51, and 0.11–0.19, respectively, showing affinity to A2-type granite. The A-type granites have negative εNd(t) values (−5.4 to −4.8) with Neoarchean depleted mantle (TDM2) ages (2591–2494 Ma), corresponding to coupling between εHf(t) values (−4.85 to -0.92) and TDM2 ages (2817–2556 Ma) of zircons. Therefore, the A-type granite pluton was mostly generated by partial melting of Neoarchean tonalitic to granodioritic basement rocks of the Dunhuang Complex in a postcollisional tectonic setting following a late Paleoproterozoic continent-continent collisional event. The metamafic rocks have a protolith age of 1605 ± 45 Ma and metamorphic age of 317 ± 20 Ma, indicating a Paleozoic tectonic event. The metamafic rock samples are geochemically characterized by relatively high alkali (Na2O + K2O = 4.39–4.81 wt%) contents and low Nb/Y (0.63–0.66) ratios, and they show steep rare earth element (REE) patterns with light REE enrichment and insignificant Eu anomalies and Nb-Ta, Zr-Hf, and Ti anomalies, resembling subalkaline oceanic-island basalt affinity. They have positive εNd(t) values (+0.8 to +1.8) close to the chondrite evolutionary line and variable εHf(t) values (-1.09 to +9.06) of zircons. Hence, the protolith of the metamafic rocks may have been produced by magma mixing processes between a depleted mantle source and a metasomatized lithospheric mantle source during the initial rifting stage in an extensional setting, completing the formation of the Precambrian Dunhuang Complex. Considering the ca. 1.85–1.80 Ga regional metamorphism in the Dunhuang terrane, the latest Paleoproterozoic (ca. 1.8–1.6 Ga) A2-type granitic magmatism and mafic magmatism documented the postorogenic to initial rifting processes following the global-scale late Paleoproterozoic collisional event, which is comparable with ca. 1.80–1.67 Ga postcollisional and ca. 1.60–1.53 Ga anorogenic magmatism in the North China craton, but different from that of the Tarim craton.

Late Paleozoic – Mesozoic tectonic evolution of the Eastern Taimyr-Severnaya Zemlya Fold and Thrust Belt and adjoining Yenisey-Khatanga Depression

Combined structural, thermochronological and geochronological studies were carried out to unravel the complex late Paleozoic – Mesozoic tectonic evolution of the eastern Taimyr – Severnaya Zemlya Fold and Thrust Belt, along with the adjoining Yenisey-Khatanga Depression and Olenek Fold Zone. New detailed field mapping, a fault and fold geometry and kinematic study, and paleostress reconstruction were undertaken for key areas within the Central and Southern Taimyr domains and the Olenek Fold Zone. Thirty seven samples were used for apatite fission track (AFT) analysis along with two samples for U-Pb zircon granite dating, and three samples for Ar-Ar metamorphic muscovite dating.The late Paleozoic (Early Carboniferous – Permian) tectonic event is recognized only in a relatively narrow zone including the Northern Taimyr Domain and part of the Central Taimyr Domain. It started in the Early Carboniferous (Visean) and is synchronous with the earliest stages of collision in the Uralian Orogen. Our results reveal a widespread distribution of latest Triassic – earliest Jurassic and Cretaceous deformations. The latest Triassic – earliest Jurassic tectonic event significantly overprinted the Northern and Central Taimyr domains and original unaltered late Paleozoic AFT ages are identified only locally. New completely reset AFT cooling ages group at 195-180 Ma, in reasonable agreement with structural data. Older Neoproterozoic and late Paleozoic Ar-Ar ages of metamorphic muscovite are retained, demonstrating that the dated rocks were not heated above 300 °C. An Early Cretaceous tectonic event is inferred from both the structural study and ca. 115–125 Ma AFT cooling ages, and is most widespread in the Southern Taimyr Domain, Olenek Fold Zone and adjoining parts of the Yenisey-Khatanga Depression, modifying older geological structures. A younger Late Cretaceous tectonic event at ca. 100-60 Ma is also inferred from AFT cooling ages, but its distribution is poorly constrained.Our study reveals that the distribution of late Mesozoic tectonic events has been significantly underestimated in previous structural studies of the region. The latest Triassic – earliest Jurassic and Cretaceous tectonic events are easily correlated with those in the Pai-Khoi – Novaya Zemlya and northern Verkhoyansk fold and thrust belts.

Cambrian–Ordovician orogenesis in Himalayan equatorial Gondwana

An early Paleozoic tectonic event, the Kurgiakh orogeny, has long been known from the western Tethyan Himalaya, and it is conspicuously recorded by an angular unconformity between Cambrian marine shelf deposits and coarse Ordovician conglomerate, as well as widespread granitic plutons. Although the lateral extent of this event is poorly known, two regions in the central and eastern Himalaya, the Kumaon of India and klippen of Bhutan, contain conglomerate units that may be correlative with this event. In the Kumaon, the Ralam Formation, which contains a basal polymictic conglomerate unit with detrital zircon grains as young as ca. 512 Ma, is overlain by sandstone containing the arthropod walking trace Diplichnites gouldi , an ichnospecies known from Ordovician and younger strata. The late early Cambrian trilobite Redlichia sp., recovered from the underlying Martoli Group, indicates that the conglomerate is post−early Cambrian and, based on stratigraphic analysis, also likely records the Kurgiakh orogeny. In central Bhutan, the lower Paleozoic succession contains conglomerate units with monomictic quartz sandstone clasts and less mature sandstone matrix. These units have also been assigned to a variety of ages, but rocks that apparently underlie these strata contain the Furongian (late Cambrian) zonal trilobite Kaolishania granulosa . Here, we show that detrital zircon ages of a conglomerate clast (no grains younger than = ca. 781 Ma) are distinctly older than those of the matrix, which contains a large peak at 490−500 Ma. Thus, these conglomerate units are also potential lateral correlatives of the Ordovician conglomerate deposits of the western Himalaya. Our paleontological and geochronologic data suggest that the Kumaon and Bhutan conglomerate units, and possible equivalents in Tibet, correlate with those of the western Himalaya. This increases their known along-strike extent by over 1000 km, bringing the total extent to ∼1200 km. Thus, the Kurgiakh orogeny spanned much of the Himalayan orogen along the northern Gondwanan margin and reflected a widespread tectonic reorganization of the peri-Gondwanan realm to Andean-type margins. Recent work indicates that it also likely extended into the Lhasa block and both the North and South China blocks. This event was associated with widespread ca. 500 Ma arc-related plutonism recorded in the Himalaya, and with abundant detrital zircon grains of that age in younger Himalaya strata. Based on current fossil data, the duration of the hiatus associated with the Cambrian-Ordovician boundary, which separates pre- and postdeformational strata, ranges from 22 to 36 m.y. Available geochronological data do not provide greater constraint. If the event was close in age to the depositional age of the Ordovician molasse, then the main phase of tectonic uplift, erosion, and deposition may have been much later than the presumed Cambrian-Ordovician boundary interval age of ca. 485 Ma.

Far-field effects of Appalachian orogenesis: A view from the craton

The sedimentary cover of the North American craton preserved little evidence of the Paleozoic tectonic events that shaped the Appalachian orogen on its eastern side. A notable exception is the NNW-trending Hudson Bay central high, which corresponds to a normal-fault array extending for a minimum length of 500 km. A working hypothesis is proposed in which stresses applied to the continental margin during the Silurian–earliest Devonian Salinian orogeny were transmitted over a distance of >1400 km in the continental interior, where they induced the normal-fault reactivation of older structural discontinuities. The shutdown of tectonic activity along the Hudson Bay central high during the latest Early Devonian to earliest Middle Devonian is interpreted as resulting from a change in the direction of plate convergence during the Acadian orogeny.

Tectonic evolution of a complex orogenic system: Evidence from the northern Qinling belt, Central China

The Qinling orogenic belt preserves the records of a long evolution history of convergence between the North China Craton and the South China Craton. In this study, we present results from new geological, geochemical, zircon U–Pb geochronological and Lu–Hf isotopic investigations on a suite of orthogneisses from the Taihua Group (THG) and Tietonggou Group (TGG). We also present geochronological data on schist and migmatite from the North Qinling belt (NQB) and metasedimentary rocks from the Wuguan unit (WGU) which is exposed along the southern side of the Shangdan suture zone. Two orthogneisses from the THG define several stages of arc magmatism at ∼2.51Ga, ∼2.34Ga, ∼2.28Ga and ∼2.16Ga, followed by metamorphism at 1930±31Ma. Zircons from the TGG trace a tectonothermal event at 1897±15Ma. Geochemical data on biotite gneiss from the THG and the TTG (tonalite–trondhjemite–granodiorite) gneiss from the TGG classify these rocks as dacite and display volcanic arc affinity. Zircon Lu–Hf isotopic results suggest that the parental magma for the protolith of felsic gneisses were derived from Mesoarchean crustal components (TDMC=2766–3067Ma). The amphibole gneiss from the THG classifies as metabasalt and the zircons from this rock show dominantly negative εHf(t) values varying from −10.3 to −4.6 and TDMC range of 3088–3437Ma, suggesting magma derivation by melting of Paleoarchean–Mesoarchean subducted oceanic crust.Zircons in the schist from the NQB show a wide age population in the range of 937–1131Ma (peak at 1035Ma). Zircons from the melanosomes of the migmatite in the NQB define ages between 405 and 379Ma, correlating with the melting event in the NQB during 450–380Ma. The WGU shows age populations of 854–803Ma (with a peak at 829–824Ma), ∼2460Ma, ∼1802Ma and 1180–1000Ma, which are markedly different from that of the NQB and SQB. The ages obtained in our study correlate with the widely reported Grenvillian-aged magmatism in the NQB, and suggest that the NQB might have been a discrete micro-continent during Paleo- and Mesoproterozoic which has been overprinted by Paleozoic tectonic event.

Early Paleozoic tectonic evolution of the Xing-Meng Orogenic Belt: Constraints from detrital zircon geochronology of western Erguna–Xing’an Block, North China

To better constrain the Early Paleozoic tectonic evolution of the western part of the Erguna–Xing’an Block, detrital zircon U–Pb dating was applied on the Ordovician to Devonian sedimentary strata along the southeast part of the China–Mongolia border. Most of the zircons from five sedimentary samples display fine-scale oscillatory growth zoning and Th/U ratios higher than 0.1, indicating a magmatic origin. All five Ordovician–Devonian samples display the similar age distribution patterns with age groups at ∼440Ma, ∼510Ma, ∼800Ma, ∼950Ma, and few Meso- to Paleo-Proterozoic and Neoarchean grains. This age distribution pattern is similar to those from adjacent blocks in the southeastern Central Asian Orogenic Belt. Considering previous tectonic studies, we propose bidirectional provenances from the Erguna–Xing’an Block and Baolidao Arc.Consequently, a new model was proposed to highlight the Early Paleozoic tectonic evolution of the western Erguna–Xing’an Block, which constrains two main Early Paleozoic tectonic events of the Xing-Meng Orogenic Belt: (a) pre-Late Cambrian collision between Erguna–Kerulen Block and Arigin Sum-Xilinhot-Xing’an Block; (b) the Early Paleozoic subduction of Paleo-Asian Ocean and pre-Late Devonian collision between Erguna–Xing’an Block and Songliao-Hunshandake Block.

Formation Age of the Qinling Complex and the early Paleozoic Tectonic Event

The Qinling orogenic belt (QOB) located between the North China Craton (NCC) and the South China Craton (SCC) is composed of the Northern Qinling Belt (NQB) and the Southern Qinling Belt (SQB). This study presents new geochemical data, zircon U-Pb ages and Hf isotopes from two rocks from the Qinling complex in the NQB. LA-ICP-MS zircon U-Pb dating results suggest that the Qinling complex was formed in early Neoproterozoic and experienced the early Paleozoic metamorphism. High εHf(t) values of 9.0-12.0 for the early Paleozoic zircons indicated that there is mantle-derived magma intruding into the Qinling complex in the early Paleozoic.

Early Jurassic magmatism on the northern margin of CAMP: Derivation from a Proterozoic sub-continental lithospheric mantle

Recent studies have shown that the sub-continental lithospheric mantle (SCLM) can either underlie regions for a long period of time or be replaced during delamination or plume magmatism. The occurrence and timing of mantle replacement may be identified by comparing the Sm-Nd isotopic signature of suites of basaltic rock within the same terrane over a range of time. In mainland Nova Scotia, the early Jurassic North Mountain Basalt (NMB) is widely recognized as the northernmost representative of the Central Atlantic Magmatic Province (CAMP), but it could also have been derived from the underlying SCLM (Avalonian) mantle. The Sm-Nd isotopic data from the NMB yield εNdt values (t = 200 Ma) ranging from + 0.1 to − 2.7. These values are significantly lower than the contemporary depleted mantle and indicate derivation from the underlying Proterozoic SCLM, rather than from the CAMP plume. Comparison with older mafic magmatic events indicates that the SCLM is isotopically indistinguishable from that inferred to yield Ordovician and Devonian mafic magmas in the Meguma terrane and Neoproterozoic to Devonian magmas in the Avalon terrane. These data indicate that the same SCLM underlay mainland Nova Scotia from Neoproterozoic to Jurassic times, and that Paleozoic tectonic events did not significantly detach the mantle from the overlying crust.

Crustal structure of the Gamburtsev Mountains, East Antarctica, from S-wave receiver functions and Rayleigh wave phase velocities

The Gamburtsev Subglacial Mountains (GSM), located in central East Antarctica, are one of the most enigmatic tectonic features on Earth. Buried beneath several kilometers of ice, the mountains are characterized by peak elevations reaching ~ 3000 m above sea level. In this study, new data from the Gamburtsev Antarctic Mountains Seismic Experiment (GAMSEIS) are presented, which substantially improve constraints on the crustal and upper mantle structure in this region. S-wave receiver functions and Rayleigh wave phase velocities are used to analyze data from the GAMSEIS deployment and to improve estimates of crustal thickness beneath the East Antarctic craton and the GSM. Our results indicate that the cratonic crust surrounding the GSM is ~ 40–45 km thick. Beneath the GSM, the crust thickens to ~ 55–58 km and provides isostatic support for the high mountain elevations. It has been suggested that thicker crust beneath the GSM may reflect magmatic underplating associated with a mantle plume. However, considering our results with those from other previous and ongoing studies, we instead favor models in which the GSM are an old continental feature associated with either Proterozoic or Paleozoic tectonic events.

Paleozoic tectonic domains of Nevada: An interpretive discussion to accompany the geologic map of Nevada

The Paleozoic geologic history of Nevada can be viewed in terms of tectonic domains derived from the newly interpreted digital geologic map of Nevada. These domains reveal that Paleozoic tectonic events were shaped by complex interactions between the continental margin in Nevada and accreted terranes outboard of the margin.

Influence of syn-sedimentary faults on orogenic structure: examples from the Neoproterozoic–Mesozoic east Siberian passive margin

The east margin of the Siberian craton is a typical passive margin with a thick succession of sedimentary rocks ranging in age from Mesoproterozoic to Tertiary. Several zones with distinct structural styles are recognized and reflect an eastward-migrating depocenter. Mesozoic orogeny was preceded by several Mesoproterozoic to Paleozoic tectonic events. In the South Verkhoyansk, the most intense pre-Mesozoic event, 1000–950 Ma rifting, affected the margin of the Siberian craton and formed half-graben basins, bounded by listric normal faults. Neoproterozoic compressional structures occurred locally, whereas extensional structures, related to latest Neoproterozoic–early Paleozoic rifting events, have yet to be identified. Devonian rifting is recognized throughout the eastern margin of the Siberian craton and is represented by numerous normal faults and local half-graben basins. Estimated shortening associated with Mesozoic compression shows that the inner parts of ancient rifts are now hidden beneath late Paleozoic–Mesozoic siliciclastics of the Verkhoyansk Complex and that only the outer parts are exposed in frontal ranges of the Verkhoyansk thrust-and-fold belt. Mesoproterozoic to Paleozoic structures had various impacts on the Mesozoic compressional structures. Rifting at 1000–950 Ma formed extensional detachment and normal faults that were reactivated as thrusts characteristic of the Verkhoyansk foreland. Younger Neoproterozoic compressional structures do not display any evidence for Mesozoic reactivation. Several initially east-dipping Late Devonian normal faults were passively rotated during Mesozoic orogenesis and are now recognized as west-dipping thrusts, but without significant reactivation displacement along fault surfaces.

Geophysical and Isotopic Constraints on Crustal StructureRelated to Mineral Trends in North-Central Nevada and Implications forTectonic History

We combined information from Sr and Pb isotope data and magnetotelluric models to develop a new magnetic and gravity interpretation of the crustal structure of north-central Nevada to better understand the origin of mineral trends. The new interpretation suggests a crustal structure that is composed of Precambrian continental crust, transitional crust, and primarily oceanic crust that are separated by northwest- and northeast-striking fault zones. The magnetic expression of the buried Precambrian continental crust is recognized for the first time. Low magnetic values primarily reflect magnetite-poor crystalline crust rather than elevated temperatures at depth. Northwest- and northeast-striking crustal boundaries are defined by isotopic data and abrupt gradients in gravity and magnetic data. The Carlin and Battle Mountain-Eureka mineral trends are associated with two of three northwest-striking boundaries. The Carlin boundary is primarily defined by a change in density and isotopic character of the lower to middle crust. The Battle Mountain-Eureka boundary coincides with a density contrast in the upper crust and a change in isotopic character in the lower to middle crust. Magnetotelluric models suggest that the Battle Mountain-Eureka boundary represents a crustal fault zone for most of its extent, but that deep-rooted faulting is more complex near and northwest of Battle Mountain. Crustal fault zones inferred from the magnetotelluric models near the Carlin trend are oblique to it, suggesting that they may not have been controlled by the deep boundary seen in the gravity and isotopic data. The third northwest-trending boundary is related to the western edge of the buried Precambrian continent in west-central Nevada, but lacks an associated mineral trend. A northeast-striking boundary forms the northern limit of Precambrian continental and transitional crust. The boundaries may have originated as rift or transform faults during Precambrian breakup of Rodinia or as faults accommodating lateral movements or accretion during later Paleozoic tectonic events. Comparing the crustal structure to tectonic elements produced by successively younger events shows that it had a profound influence on subsequent sedimentation, deformation, magmatism, extension, and most important, mineralization.

Geologic and geophysical evidence for the influence of deep crustal structures on Paleozoic tectonics and the alignment of world-class gold deposits, north-central Nevada, USA

Geologic data concur with geophysical and isotopic data that suggest the presence of deep crustal fault zones along the Battle Mountain-Eureka (BME) trend and elsewhere in Nevada. The fault zones may have originated during Proterozoic rifting of the continent and were likely substantially reactivated and modified during Paleozoic tectonism. Five distinct Paleozoic structural and stratigraphic domains are defined that demonstrate the complexity of Paleozoic tectonic events and also lead to hypotheses about ways in which the margin could have been modified. The current locations of these domains adjacent to the geophysically and isotopically defined indicators of the buried continent edge corroborate their interactions with the continental margin. During the Tertiary, preexisting crustal fault zones were intersected and reopened during episodes of extension and served as the conduits for deep-sourced, gold-rich fluids, which were disseminated into Paleozoic slope facies sedimentary rocks, forming sediment-hosted Carlin-type and other deposits. Multiple factors including the locations of these deep-seated structures, the original configuration of the lower Paleozoic continental margin of Nevada, and its subsequent reactivation during the Paleozoic all were fundamental controls on the location of younger mineral deposits. A clearer understanding of the original configuration of the margin and of the effects of subsequent Paleozoic and Mesozoic tectonic events on the margin would provide insight into the locations of these and other prospective mineral belts.

SHRIMP and Conventional U-Pb ages of Ordovician granites and tonalites in the central Appalachian Piedmont: Implications for Paleozoic tectonic events

Discordant Cambrian U-Pb ages of questionable reliability for metamorphosed plutonic rocks in the central Appalachian Piedmont of northern Virginia, Maryland, and the District of Columbia have led to different regional geologic interpretations and controversies. In this study, we use the sensitive high resolution ion microprobe (SHRIMP) for dating 25-μm diameter areas on individual zircons from Piedmont granitoids in an effort to resolve previous controversial ages. Cathodoluminescence images of zoning within the zircons are used to select pristine areas of cores and overgrowths for dating and to help interpret the results. For comparison, hand-picked fractions of zircons from most of the samples were also dated by the conventional isotope dilution thermal ionization mass spectrometry (TIMS) U-Pb method. The combined SHRIMP and TIMS results of this study demonstrate that the Dale City Quartz Monzonite, Norbeck Intrusive Suite, Kensington Tonalite, Goldvein pluton, Lake Jackson pluton, Falls Church Intrusive Suite, Occoquan Granite (main batholith and satellite Bull Run Marina pluton), Georgetown Intrusive Suite, Dalecarlia Intrusive Suite, and related granitoids are parts of an extensive Early to Middle Ordovician magmatic arc. Emplacement of these plutons from about 485 to 450 Ma generally was contemporaneous with major regional deformation and metamorphism, called the Taconic orogeny in rocks of the northern Appalachians. The Ordovician ages supersede most of the discordant Cambrian(?) to Neoproterozoic dates that previously suggested a Potomac or Penobscot orogeny in Piedmont rocks of the Potomac Valley, raising questions about the nature and timing of that event. The Devonian Guilford Granite crosscuts all structures, providing a minimum age for these Paleozoic events.

REMNANT COLLOFORM PYRITE AT THE HAILE GOLD DEPOSIT, SOUTH CAROLINA: A TEXTURAL KEY TO GENESIS

Auriferous iron sulfide-bearing deposits of the Carolina slate belt have distinctive mineralogical and textural features—traits that provide a basis to construct models of ore deposition. Our identification of paragenetically early types of pyrite, especially remnant colloform, crustiform, and layered growth textures of pyrite containing electrum and pyrrhotite, establishes unequivocally that gold mineralization was coeval with deposition of host rocks and not solely related to Paleozoic tectonic events. Ore horizons at the Haile deposit, South Carolina, contain many remnants of early pyrite: (1) fine-grained cubic pyrite disseminated along bedding; (2) fine-grained spongy, rounded masses of pyrite that may envelop or drape over pyrite cubes; (3) fragments of botryoidally and crustiform layered pyrite, and (4) pyritic infilling of vesicles and pumice. Detailed mineral chemistry by petrography, microprobe, SEM, and EDS analysis of replaced pumice and colloform structures containing both arsenic compositional banding and electrum points to coeval deposition of gold and the volcanic host rocks and, thus, confirms a syngenetic origin for the gold deposits. Early pyrite textures are present in other major deposits of the Carolina slate belt, such as Ridgeway and Barite Hill, and these provide strong evidence for models whereby the sulfide ores formed prior to tectonism. The role of Paleozoic metamorphism was to remobilize and concentrate gold and other minerals in structurally prepared sites. Recognizing the significance of paragenetically early pyrite and gold textures can play an important role in distinguishing sulfide ores that form in volcanic and sedimentary environments from those formed solely by metamorphic processes. Exploration strategies applied to the Carolina slate belt and correlative rocks in the eastern United States in the Avalonian basement will benefit from using syngenetic models for gold mineralization.

Canada Basin, Arctic Ocean: Evidence against a rotational origin

The most widely accepted model for the opening of Canada Basin invokes 66° of counterclockwise rotation of Arctic Alaska and Chukotka away from the Canadian Arctic in Early Cretaceous time. Late Paleozoic structural trends and paleogeography have been used in support of the rotation hypothesis. Recent refinements in the ages of Paleozoic tectonic events in Arctic Alaska, Yukon, and the Canadian Arctic Islands provide new controls on correlations of late Paleozoic paleogeography and raise doubts about whether the Paleozoic tectonics of the Arctic Alaska‐Yukon region necessitate a rotational reconstruction of Arctic Alaska against the Canadian Arctic Islands. A rotational restoration of Arctic Alaska requires the Alaskan and Canadian margins to be conjugates of comparable age and evolution. The rift‐drift transition age for the Alaskan margin is most likely Hauterivian (Early Cretaceous), but for the Canadian Arctic margin it is most likely post‐Albian (mid‐Cretaceous). Crustal structure data from the Beaufort Sea continental margin in Canada define a rifted margin segmented by fracture zones which constrain the kinematics of ocean spreading to be northwestward, perpendicular to that required by the rotation hypothesis but subparallel to that suggested by seismic velocity anisotropy in the upper mantle. The Alaska‐Chukotka rotation hypothesis also fails to account for up to 600 km of continental overlap upon restoration of 66° of rotation and the absence of any accommodating contractional structures in northern Yukon and adjacent Northwest Territories. Because the Alaska‐Chukotka rotation hypothesis fails to account for much of the available data, senous doubt is cast on its viability. An existing multistage tectonic model for the evolution of Canada and Makarov basins is summarized as an example of a model which can account for the existing data.

A geologic history of the north-central Appalachians; Part 1, Orogenesis from the Mesoproterozoic through the Taconic Orogeny

The north-central Appalachians lie just southwest of the boundary between the central/southern and northern Appalachians, occupying a critical position within the 3000+ km-long Appalachian orogen. The history of tectonic activity in the north-central Appalachians spans more than one billion years, from the assembly and breakup of a Neoproterozoic supercontinent, through active orogenesis during Laurentia9s Paleozoic northward journey off the western margin of West Gondwana, to the Mesozoic transformation of the active orogen into a passive margin during Pangea9s disassembly. The major tectonic events include five compressional orogenies and two extensional episodes: 1. The late Mesoproterozoic Grenville orogeny assembled several continental masses into the Neoproterozoic supercontinent, Rodinia. It was the most widespread but now least exposed tectonism, and rocks involved in this event underlie as basement most of the exposed north-central Appalachians. Large fragments of Laurentian Grenville rock were subsequently broken off and incorporated in later Appalachian orogenesis. 2. Crustal extension and rifting late in the Neoproterozoic and into the earliest Cambrian separated Laurentia from West Gondwana, thereby forming the intervening Theic ocean and two continental rifts on Laurentia9s eastern margin, the Catoctin rift and, later in the Middle? to Late Cambrian, the so-called Rome trough. The initial siliciclastic sedimentation on the margin migrated westward through time onto the craton, supplanted by a thick and increasingly wide carbonate shelf on Laurentia9s eastern margin. Two microcontinents of Grenville-age, non-Laurentian(?) continental rock became positioned east of Laurentia, thereby creating the Octoraro sea as an arm of Theia. 3. In Theia, east of the two microcontinents, magmatic arcs developed over a subduction zone late in the Cambrian. Convergence within Theia caused the Potomac orogeny, which obduced the arcs (Wilmington Complex, Cecil Amalgamate) over the microcontinents and associated Theic deposits (including accretionary wedge sediments). 4. Continued westward convergence collapsed the Octoraro sea, producing the Middle to Late Ordovician Taconic orogeny in which the Potomac-deformed magmatic arcs and associated Theic elements were obduced onto the Laurentian continental margin. This obduction: drowned the carbonate shelf with siliciclastic sediments (Martinsburg Formation); drove continental rise and basinal deposits over the carbonate shelf on the Martic thrust; slid the Hamburg klippe onto the shelf; and accreted the Potomac package of microcontinent/arc/basinal-sediments onto the Laurentian margin. This orogeny transformed the broad Early Paleozoic carbonate shelf into the Appalachian basin that persisted throughout the Middle and Late Paleozoic. 5. The Middle Devonian Acadian orogeny ended the largely paralic environment that dominated the Appalachian basin during the Late Silurian. Active orogenesis in New England probably extended southward to the north-central Appalachians, because a vast amount of terrigenous sediment was introduced into the Appalachian basin to form the Catskill delta; however, evidence of actual Acadian deformation and metamorphism is lacking at this latitude. These presumed internal Acadides have yet to be found. 6. The convergence of West Gondwana and Laurentia during the Late Carboniferous and earliest Permian produced the Permian Alleghany orogeny in the north-central Appalachians. This widespread decollement tectonism directly affected a larger area of the presently exposed central and southern Appalachians than any earlier Paleozoic tectonic event. An early layer-parallel shortening phase gave way to a fold-thrust development above a basal decollement. This Alleghany fold-and-thrust tectonism created long, arcuate folds in the Appalachian basin. Late in the Alleghany orogeny, rock thrust northward over the Carboniferous rocks in the Anthracite region of northeastern Pennsylvania caused anthracitization of the underlying coals. The internal Alleghanides are not presently exposed. 7. Crustal extension in the Late Triassic and Early Jurassic produced numerous local, closed basins along eastern North America. Igneous intrusions and effusions marked the beginning of the Jurassic. By the end of the Early Jurassic, horizontal crustal rebound in response to opening of the Atlantic Ocean rotated the basins by crustal inversion, which folded some within-basin rocks and produced a prominent topographic ridge along the Piedmont, up-dip of the basins. Subsequent erosion of this Piedmont ridge and other parts of the Appalachian orogen fed large volumes of sediment to offshore basins during the remainder of the Mesozoic and throughout the Cenozoic. Each of these orogenies affected most of the Appalachian orogen. The tectonic expression of each orogeny varied along and across the orogen. The elements and structural bodies involved in each also varied along strike. However, many common elements persist from one part of the orogen to another; only a few features are singular in time and space. The tectonic boundary between the central and northern Appalachians is one of these singular features-it is solely an Alleghanian artifact.

A New Integrated Tectonic Synthesis of the Piceance Basin: Implications for Fractured Reservoir Detection and Characterization: ABSTRACT

Detailed reservoir characterization of Piceance Basin thin-skinned structural traps reveals the importance of fracture-controlled gas production. A complete understanding of basin fracture genesis can be achieved through determination of the regional tectonic evolution. To understand the evolution of thin-skinned and basement-involved structures, high-resolution aeromagnetic data, seismic data, remote sensing imagery analysis, and production history analyses have been integrated with conventional subsurface and surficial dynamic structural analyses. Examination of structural trends in rocks ranging in age from the Precambrian through Holocene show the importance of pre-existing anisotropies in partitioning younger tectonic strain. Because of this strain partitioning, many Laramide structures show complex reactivation histories that obscure older Precambrian and Paleozoic tectonic events. An excellent example of this reactivation and partitioning is provided by NW-trending Precambrian-age structures on the Uncompahgre Uplift that were reactivated during Pennsylvanian-age deformation (Ancestral Rockies) and Laramide events. Because of its importance to reservoir engineering problems such as hydraulic stimulation design and drainage efficiency calculations for fractured reservoirs, the modern stress state throughout the basin has been determined and data suggest that there is significant variability in principal stress orientations throughout the basin. This interpretation demonstrates the complex evolution of multiply-reactivated tectonic structures and the relationship between productionmore » trends, structure, and fractured reservoirs. Most importantly, the integrated exploration approach demonstrates the power of an integrated basin analysis as a deterministic tool for understanding and predicting fractured reservoir conditions in advance of drilling.« less

Lead and sulfur isotope studies of the Wulashan quartz-K feldspar and quartz vein gold deposit, southwestern Inner Mongolia, People's Republic of China

The newly discovered Wulashan gold deposit in southwestern Inner Mongolia, People9s Republic of China, is hosted by Archean metamorphosed volcano-sedimentary sequences of the Wulashan Group and surrounded by a number of late Paleozoic granitoid dikes and a batholith. The gold mineralization occurs in quartz-K feldspar and quartz veins related to these intrusions.Sulfur isotope analyses of 21 sulfide mineral (galena and pyrite) samples from the gold-bearing quartz-K feldspar and quartz veins, hornblende-plagioclase gneiss, sillimanite-biotite gneiss, and granitoid intrusions in the Wulashan district suggest that the sulfur of the ore-bearing fluids was mainly derived from a mixing of sources from late Paleozoic granitoid intrusions and Archean metamorphosed volcano-sedimentary rocks.Lead isotope data for amphibolite, one of the major host rocks of these gold-bearing veins, define a correlation line with a slope of 0.1641 + or - 0.0003 (2Sigma ) which corresponds to an age of 9.498 + or - 76 Ma (2Sigma ). This lead isotope line also overlaps with the late Paleozoic granite line of the Dahuabei granitoid batholith defined by five K feldspar samples. Lead isotope data of 13 galena and pyrite samples from the gold-bearing quartz-K feldspar and quartz veins plot in between data for the amphibolite and Dahuabei granitoid batholith samples, and constitute a mixing line. Plots of all these lead isotope data ( 206 Pb/ 204 Pb vs. 207 Pb/ 204 Pb and 206 Pb/ 204 Pb vs. 208 Pb/ 204 Pb) display a linear trend, with K feldspar of the Dahuabei granitoid batholith at the most radiogenic end and amphibolite of the Wulashan Group at the least radiogenic end. This trend is probably a result of the mixing of mantle-derived lead with lead from a radiogenic granitoid source.The results of lead isotope studies indicate that protoliths of the amphibolite and the Dahuabei granitoid batholith have identical or similar initial lead isotope compositions but that they may possess quite different mu values. The amphibolite was derived from mantle-related mafic igneous rocks erupted or intruded at 2498 Ma, whereas the Dahuabei granitoid batholith was probably derived by selective remelting of previously formed volcano-sedimentary rocks during a late Paleozoic (Variscan) tectonic event. The lead and sulfur of the gold-bearing quartz-K feldspar and quartz veins were derived from multiple sources, including Archean amphibolite and the late Paleozoic granitoid batholith.

Prebreakup geology of the Gulf of Mexico‐Caribbean: Its relation to Triassic and Jurassic rift systems of the region

A review of the prebreakup geology of west central Pangea, comprising northern South America, the Gulf of Mexico, and West Africa, combined with a study of the Mesozoic rift trends of the region confirms a relation between the rift systems and the underlying older grain of deformation. The prebreakup analysis focuses attention on the Precambrian, early Paleozoic, and late Paleozoic tectonic events affecting the region and assumes a Pindell fit. Two late Precambrian orogenic belts are observed in west central Pangea. Along the northern South American margin and Yucatan a paleo northeast trending Pan‐African aged fold belt is documented. A second system is observed along West Africa extending from the High Atlas to the Mauritanides and Rockelides. Similar aged orogenies in the Appalachians are compared. During the late Paleozoic, renewed orogenic activity, associated with the Gondwana‐Laurentia suture, affected large segments of west central Pangea. The general trend of the system is northeast‐southwest and essentially parallels the Guayana craton and West African and eastern North American cratons. Mesozoic rifling closely followed either the Precambrian trends or the late Paleozoic orogenic belt. The Triassic component focused along the western portions of the Gulf of Mexico continuing into eastern Mexico and western South America. The Jurassic rift trend followed along the separation between Yucatan and northern South America. At Lake Maracaibo the Jurassic rift system eventually overlaps the Triassic rifts. The Jurassic rift resulted in the “Hispanic Corridor” that permitted Tethyan and Pacific marine faunas to mix at a time when the Gulf of Mexico underwent continental sedimentation.