Early Jurassic Time Research Articles

Testing the Mojave-Sonora megashear hypothesis: Evidence from Paleoproterozoic igneous rocks and deformed Mesozoic strata in Sonora, Mexico

U-Pb ages and Nd isotope values of Proterozoic rocks in Sonora, Mexico, indicate the presence of Caborca-type basement, predicted to lie only south of the Mojave-Sonora mega-shear, 40 km north of the postulated megashear. Granitoids have U-Pb zircon ages of 1763–1737 Ma and 1076 Ma, with ϵNd(t) values from +1.4 to −4.3, typical of the Caborca block. Lower Jurassic strata near the Proterozoic rocks contain large granitic clasts with U-Pb ages and ϵNd(t) values indistinguishable from those of Caborcan basement. Caborca-type basement was thus present at this location north of the megashear by 190 Ma, the depositional age of the Jurassic strata. The Proterozoic rocks are interpreted as parautochthonous, exhumed and juxtaposed against the Mesozoic section by a reverse fault that formed a footwall shortcut across a Jurassic normal fault. Geochronology, isotope geochemistry, and structural geology are therefore inconsistent with Late Jurassic megashear displacement and require either that no major transcurrent structure is present in Sonora or that strike-slip displacement occurred prior to Early Jurassic time.

Tectonic Controls on Mudrock Geochemisry, Mesozoic Rocks of Eastern Oregon and Western Idaho, U.S.A.: Implications for Cordilleran Tectonics

Abstract Tectonic models for the Mesozoic evolution of accreted island arcs and subduction complexes in the Blue Mountain Province (BMP) of eastern Oregon and western Idaho vary widely. The BMP is situated between coeval accreted terranes of southern British Columbia and the western U.S., and is critical for interpreting Mesozoic paleogeography of the U.S. Cordillera. In this study, we interpret the Triassic–Jurassic evolution of the BMP using geochemistry as a tool to evaluate the provenance of mudrocks in fine-grained turbidites. Temporal and spatial variations in mudrock geochemistry indicate that during Late Triassic to Early Jurassic time the Wallowa terrane was an intra-oceanic island arc and the Olds Ferry terrane was a pericratonic (continental-fringing) island arc. Subsequently, a regionally extensive Middle to Late Jurassic marine basin received input from continental sources and sediment may also have been derived from coeval terranes to the south. Comparison of provenance data from the BMP with similar data from accreted terranes to the north and south suggests that Triassic–Jurassic basins of the western U.S. share a long-term provenance history within a complex system of continent-fringing and intra-oceanic island arcs. In contrast, it appears that the Quesnel terrane of southern British Columbia had a distinct provenance with a more direct link to Laurentia during Triassic time. Early to Middle Jurassic sedimentary rocks from southern British Columbia and the western U.S. show a high degree of geochemical provenance similarity. This may indicate reorganization of formerly discrete tectonic regions into a nascent, integrated western Laurentian margin prior to complete development of a Cretaceous Andean-type margin. The results of this study demonstrate the utility of mudrock geochemistry in tectonic analyses and show that major- and trace-element geochemistry of mudrocks is useful for developing a comprehensive understanding of regional tectonic evolution.

Thermochronological evidence for Mesozoic–Tertiary tectonic evolution in the eastern Sardinia

AbstractApatite fission‐track analyses on samples from eastern Sardinia document a complex tectonic history, whose reconstruction is problematic because of the reactivation of faults and structures at different times from Jurassic to Miocene. The oldest ages (150–154 Ma) have been detected on the southern margin of the Gulf of Orosei and are related to the extensional tectonics that characterize the European passive margin during Early and Middle Jurassic times. Thermal modelling of these data allows reconstruction of the burial history of the Mesozoic basin and estimation of a sedimentary thickness of 2000 m. Part of these sediments was eroded during the following uplift, documented by mid‐Cretaceous fission‐track ages. A further exhumation episode of Eocene age has been revealed by fission‐track data on granite samples, and has been inferred to be related to the Alpine orogenic phase. This tectonic episode caused the exhumation of crustal blocks bound by faults that were finally reactivated during the Late Oligocene–Early Miocene.

Crustal structure of the SW Moroccan margin from wide-angle and reflection seismic data (the Dakhla experiment). Part B — The tectonic heritage

The Dakhla 2002 experiment on the Northwest African margin obtained deep penetration, multichannel reflection (MCS) and wide-angle seismic (OBS) data. These results were combined with total magnetic intensity data, kinematic reconstructions and geological field studies to decipher the early tectonic and sedimentary history of the South Moroccan margin, adjacent to the Reguibat Shield. The structural study of this margin shows that the crust thins, from about 28 km beneath the un-thinned continental domain, to less than 10 km over a lateral distance of about 120 km. The Dakhla margin differs from the structural scheme of the adjacent African margin segments. There, the Moho rise, generally observed just above the deepening of the top of the basement, is achieved with an offset of around 50 km (with respect to the latter). In Early Jurassic time, the break-up of continental crust in what is now the Central Atlantic led to the formation of two basins, one on either side of the Reguibat, which correspond to the older parts of segments of the Late Palaeozoic Hercynian orogen. The Dakhla segment appears as a Precambrian cratonic zone, squeezed between two orogenic segments, which have remained unaffected by break-up processes; and the lower crust of this particular domain has behaved differently from all other neighbouring Appalachian (North America) and Mauritanide domains. This observation points to the important role of tectonic inheritance in the structural development of passive continental margins.

Lithostratigraphy, facies, and deposition environment of the lower Jurassic Ammonitico Rosso type sediments (ARTS) in the Gümüşhane area, NE Turkey: Implications for the opening of the northern branch of the Neo-Tethys Ocean

Ammonitico Rosso type sediments (ARTS) form the bottom level of the Şenköy Formation in the Gümüşhane area. The formation accumulated in the rift-related basins triggered by extensional tectonic regimes in the Early Jurassic times. Six different sections were studied in this area in order to interpret the lithostratigraphy, facies, and deposition environments of the ARTS. Two basic lithofacies were distinguished, nodular-marly and nodular-calcareous. These facies provide useful information regarding depositional conditions in the area and environmental energy. These factors were mainly related to the relative sea-level that in turn was controlled by tectonic and eustasy conditions. In addition, seven different types of microfacies were recognized and described as a result of the detailed analysis of the sections. The results of the analysis show that these sediments in the Gümüşhane area were developed in transgressive phases/events. These sediments share characteristics with other peri-Mediterranean regions and sea-level curves, and show a similarity to the major transgressive pulse during the Pliensbachian stage. Deposition of ARTS was controlled by syn-depositional extensional movements, leading to their accumulation on top of tilted blocks, slopes and horsts in an open marine environment. The ARTS were developed during the rifting of the continental margins pointing up the phases of the Tethys opening. Neptunian dykes verifying this opening developed at different levels of the ARTS and were filled with overlying sediments. These results support the hypothesis that the ARTS were formed in tectonically active environments where extensional tectonic movements were continuously improved during the deposition of the ARTS.

Mesozoic tectonic evolution of the Southeast China Block: New insights from basin analysis

In order to better understand the Mesozoic tectonic evolution of Southeast China Block (SECB in short), this paper describes geological features of Mesozoic basins that are widely distributed in the SECB. The analyzed data are derived from a regional geological investigation on various Mesozoic basins and a recently compiled 1:1,500,000 geological map of Mesozoic–Cenozoic basins. Two types of basin are distinguished according to their tectonic settings, namely, the post-orogenic basin (Type I) and the intracontinental extensional basin (Type II); the latter includes the graben and the half-graben or faulted-depression basins. Our studies suggest that the formation of these basins connects with the evolution of geotectonics of the SECB. The post-orogenic basin (Type I) was formed in areas from the piedmont to the intraland during the interval from Late Triassic to Early Jurassic; and the formation of the intracontinental extensional basin (Type II) connects with an intracontinental crustal thinning setting in the Late Mesozoic. The graben basin was generated during the Middle Jurassic and is associated with a bimodal volcanic eruption; and the half-graben or faulted-depression basin, filled mainly by the rhyolite, tuff and sedimentary rocks during Early Cretaceous, is occupied by the Late Cretaceous–Paleogene red-colored terrestrial clastic rocks. We noticed that the modern outcrops of numerous granites and basins occur in a similar level, and the Mesozoic granitic bodies contact with the adjacent basins by large normal faults, suggesting that the modern landforms between granites and basins were yielded by the late crustal movement. The modern basin and range framework was settled down in the Cretaceous. Abundant sedimentary structures are found in the various basins, from that the deposited environments and paleo-currents are concluded; during the Late Triassic–Early Jurassic time, the source areas were situated to the north and northeast sides of the outcrop region. In this paper, we present the study results on one geological and geographical separating unit and two separating fault zones. The Wuyi orogenic belt is a Late Mesozoic paleo-geographically separating unit, the Ganjiang fault zone behaves as the western boundary of Early Cretaceous volcanic rocks, and the Zhenghe–Dapu fault zone separates the SE-China Coastal Late Mesozoic volcanic-sedimentary basins and the Wuyi orogenic belt. Finally, we discuss the geodynamic mechanisms forming various basins, proposing a three-stage model of the Mesozoic sedimentary evolution.

The extinction of the Metacopina (Ostracoda)

The extinction of the Suborder Metacopina in the early Toarcian (Early Jurassic) was a major event in the macro-evolutionary history of the Ostracoda. The disappearance of this long-ranging, essentially Palaeozoic, group coincides with a change in the composition of marine ostracod faunas from ones with residual Palaeozoic aspect in the pre-Toarcian (due to the presence of the Metacopina) to those dominated by the Cytheroidea, Cypridoidea and Platycopina, a pattern that continues to the present. The Metacopina had been present in many Palaeozoic assemblages although usually at low diversity and abundance but they were particularly successful in Triassic and Early Jurassic times, being diverse, abundant and with an apparently cosmopolitan distribution. Why did such a successful group that had survived four of the ’Big Five’ Phanerozoic extinction events (end Ordovician, Late Devonian, end-Permian, end-Triassic), succumb at this second order and apparently less significant early Toarcian event? The rapid decline and extinction of the Metacopina followed a major regressive phase in the Pliensbachian and broadly coincided with the onset of the early Toarcian Oceanic Anoxic Event (TOAE), which has been linked to global eustatic sea-level rise and is marked by a major, negative carbon isotope (δ13C ) excursion in the global carbon cycle. The nature and likely causes of the Metacopina extinction event (MEE) are reviewed in the context of new environmental data for the Toarcian, with particular reference to evidence from the Mochras Borehole (West Wales) ; the group’s survival of earlier biotic events is also discussed. The one distinct feature that sets the Toarcian extinction event apart from previous evolutionary crises is that the Metacopina never had to compete against relatively advanced ostracod, largely cytheroidean, taxa that were present during the Early Jurassic. We propose that this may have been a key factor in the demise of the Metacopina at this particular time.

SHRIMP Zircon Age and Geochemical Constraints on the Origin of Lower Jurassic Volcanic Rocks from the Yeba Formation, Southern Gangdese, South Tibet

We present SHRIMP zircon dating, bulk-rock geochemical, and Sr-Nd-Pb isotopic results for Yeba volcanic rocks and a mafic dike from Southern Gangdese (SG), southern Tibet, in order to constrain their tectonic setting and origin. Yeba volcanic rocks span a continuous compositional range from basalt to dacite, although andesites are minor, and mafic and felsic rocks are volumetrically predominant. New SHRIMP zircon dating for a dacite coupled with previous SHRIMP zircon dating for a mafic dike and fossil constraints for the sedimentary sequence indicate that Yeba volcanic rocks were emplaced in the Early Jurassic (174-190 Ma). Yeba tholeiitic mafic rocks possess compositional diversity and are divided into three groups based on concentrations of MgO, Al2O3, and La. Mafic samples are all characterized by marked negative Nb, Ta, and Ti anomalies and positive εNd(T) values (+ 2.4 to + 4.5). Yeba calc-alkaline felsic rocks are characterized by coherent, concave-upward MREE patterns and negative anomalies in Nb, Ta, P, and Ti, with positive εNd(T) values (+ 0.3 to + 2.6). Sr-Nd-Pb isotopes overlap among the different groups of Yeba mafic rocks; Pb isotopic compositions in both mafic and felsic rocks are nearly identical. These features are consistent with a subduction-related origin, most likely in an arc built on thin, immature continental crust. Yeba volcanic rocks are interpreted as having been created by northward subduction of Neo-Tethyan oceanic crust in Early Jurassic time. Geochemical signatures and quantitative modeling indicate that fractional crystallization and crustal assimilation played insignificant roles in the generation of Yeba mafic magmas, and that their geochemical diversity was probably produced by variable degrees of partial melting from a common but heterogeneous mantle source, which had been metasomatized by variable contributions of sediments/fluids released from the subducted Neo-Tethyan oceanic crust. Yeba felsic rocks were probably generated by moderate degrees of partial melting of juvenile basaltic lower crust, which consists of dominant underplated magmas (similar to Yeba mafic rocks in composition) and variable contributions from ancient lower crust beneath the Gangdese Back-Arc fault uplift belt (GBAFUB).

Structural controls on orogenic gold mineralisation in the Klondike goldfield, Canada

The Klondike Schist that forms the basement rocks for the famous Klondike placer goldfield was emplaced as km-scale thrust slices in Early Jurassic time, along with some thin (10 to 30 m-scale) slices of greenstone and ultramafic rocks. Permian metamorphic fabrics in the schists were deformed during thrust emplacement by structures formed as the rocks passed through the brittle–ductile transition. Early-formed thrust-related structures were almost-pervasive recumbent folds that affected both the schist and greenstone/ultramafic slices and imposed a spaced cleavage with minor recrystallisation of micas. These structures gave way to shallow-dipping phacoidal cleavage near (within <100 m of) thrust structures. Thrust-related structures have been overprinted locally by well-defined steeply dipping reverse fault-fold zones, and associated upright folding on regional (km) to mesoscopic (m) scales. The fold-fault zones occur as two orthogonal sets of structures oriented NW to N and NE to E. Some of these steeply dipping fault zones have been reactivated by Late Cretaceous normal faulting. Orogenic (mesothermal) gold-bearing veins were emplaced in local sites of extension during or after formation of the compressional fault-fold zones and before normal fault reactivation. Over 400 veins (m to cm-scale) observed in this study imply a general NW strike for mineralised structures (W to N), but with a broad scatter of orientations. Vein emplacement was controlled principally by fold axial surfaces of kink folds of the fault-fold generation. However, some other local extension sites have opened along preexisting structures to host veins locally, including metamorphic foliation and spaced cleavage planes. In addition, irregular extensional fractures with no obvious structural control host some veins. The Klondike mineralised veins formed as swarms with broad regional structural control, but represent relatively diffuse mineralised zones, with numerous scattered small veins, compared to most orogenic vein systems. These diffuse vein swarms appear to be sufficient sources for the rich and geographically localised placer gold deposits that formed in overlying gravels during erosion of the Klondike Schist basement.

Two layer model of lithospheric compression and uplift/exhumation in an intracratonic setting: an example from the Cooper–Eromanga Basins, Australia

Seismic reflection profiles indicate the compressive nature of the structural style associated with the major uplift events in the Cooper–Eromanga Basins. Inversion geometries and reactivated features attest to a period of compression during Late Triassic–Early Jurassic times. In the Eromanga Basin, compressional structural styles associated with Late Cretaceous–Tertiary are apparent. Many of the Late Cretaceous–Tertiary structures coincide with exhumation highs in Late Cretaceous–Tertiary times. The two-layer lithospheric compression model is considered as the most complete explanation of both the uplift of areas subject to compression and crustal thickening, and of the regional uplift of areas not subject to any apparent Late Cretaceous–Tertiary compression. In the model, compression and thickening in the lower lithosphere is decoupled and laterally displaced from that in the upper crust. Thickening of the mantle lithosphere without thickening of the overlying crust can account for the initial subsidence then uplift of not inverted platform areas. The opening of the Tasman Sea and the Coral Seas can lead to stress transmission in the interior of the continent. These stresses are likely to generate uplift but cannot explain the distribution of uplift in areas not subject to compression.

Horizontal movements in the eastern Barents Sea constrained by numerical models and plate reconstructions

SUMMARY The eastern Barents Sea basins, west of Novaya Zemlya, were mildly inverted between Late Permian and Early Jurassic times, as indicated by mild folds in the basin sediments. Previous studies have suggested that the crustal part of Novaya Zemlya was thrust westward, but the magnitude of this compressive movement is not well known. Our aim is to provide an order-of-magnitude constraint on the amount of shortening associated with the displacement of Novaya Zemlya and inversion of the eastern Barents Sea basins by combining numerical models and plate reconstructions in an iterative process. We use a 2-D finite-element method to model inversion of a pre-defined basin. The total amount of shortening imposed on the models is first constrained by plate reconstructions for the Barents Sea region for the Late Palaeozoic to Early Mesozoic. We assume that the shortening is caused by westward movement of the Siberian plate, but the magnitude of this westward displacement in plate reconstructions is highly uncertain due to the allochthonous nature of the rocks of Novaya Zemlya and the scarcity of palaeomagnetic data in the region. Our models show that shortening localizes in the model basin and at Novaya Zemlya and that westward propagation of deformation is more efficient when the strength of the lower crust is reduced. Part of the movement of the Siberian plate could be accommodated by thrusts at Novaya Zemlya and perhaps in the domain of the Kara Sea at the western margin of the Siberian plate. By comparing the inversion obtained in the numerical models to the inferred inversion structures in the eastern Barents Sea basins we further constrain the amount of shortening that caused the inversion and therewith improve the plate reconstructions for the region. Our models indicate that the westward movement of Novaya Zemlya occurred in the Late Triassic–Early Jurassic (220–190 Ma) and was limited in magnitude to 100–200 km, which is considerably less than previous estimates (500–700 km).

Testing palaeo-environmental proxies in Jurassic belemnites: Mg/Ca, Sr/Ca, Na/Ca, δ 18O and δ 13C

We test the hypothesis that the noise in belemnite records through time results from changing climate. We do so by comparing palaeo-proxies (Mg/Ca, Sr/Ca, Na/Ca, δ 18O, δ 13C) in two species of Toarcian belemnites, Acrocoelites ( Odontobelus) vulgaris (14 specimens) and Acrocoelites ( Acrocoelites) subtenuis (10 specimens), deposited in a period of time that was short relative to the timescale of climate change. The specimens are interpreted to have been regurgitated from the stomach of a marine predator and so were deposited together in a geological instant of Early Jurassic time. In A. ( O.) vulgaris, isotopic and elemental palaeo-proxies group tightly, suggesting that this belemnite occupied a restricted environmental niche. In A. ( A.) subtenuis, elemental and isotopic compositions spread more widely, suggesting that this species occupied a greater range of habitats. Possible sexual dimorphism in A. ( O.) vulgaris is reflected in bimodal Mg/Ca and differing intra-rostral distribution of Mg in putative dimorphs; concentrations decline by 30% from apical line to rim in specimens with high Mg/Ca, but are largely invariant radially in specimens with lower Mg/Ca. Sexual dimorphism in A. ( A.) subtenuis yields a bimodality in size, Mg/Ca, and intra-rostral distribution of Mg; larger specimens have a higher Mg/Ca, and Mg concentrations that increase along radial traverses from apical line to rim, whilst the smaller specimens show a mid-rostral minimum in Mg concentration. Radial profiles of concentrations for Sr and Na vary little in all forms. Toarcian belemnites of these species thus show strong biofractionation effects on Mg/Ca, and little or no observable biofractionation of Sr/Ca or Na/Ca. It follows that much of the noise in belemnite records results from biofractionation, and from a cosmopolitan life-style of some belemnite species, rather than climate-change. For Toarcian belemnites, we suggest that Sr/Ca is the best indicator of palaeo-temperature.

Detrital record of Mesozoic shortening in the Yanshan belt, NE China: testing structural interpretations with basin analysis

ABSTRACTThe Yanshan fold‐thrust belt is an exposed portion of a major Mesozoic orogenic system that lies north of Beijing in northeast China. Structures and strata within the Yanshan record a complex history of thrust faulting characterized by multiple deformational events. Initially, Triassic thrusting led to the erosion of a thick sequence of Proterozoic and Palaeozoic sedimentary strata from northern reaches of the thrust belt; Triassic–Lower Jurassic strata that record this episode are deposited in a thin belt south of this zone of erosion. This was followed by postulated Late Jurassic emplacement of a major allochthon (the Chengde thrust plate), which is thought to have overridden structures and strata associated with the Triassic event and is cut by two younger thrusts (the Gubeikou and Chengde County thrusts). The Chengde allochthon is now expressed as a major east–west trending, thrust‐bounded synform (the Chengde synform), which has been interpreted as a folded klippe 20 km wide underlain by a single, north‐vergent thrust fault. Two sedimentary basins, defined on the basis of provenance, geochronology and palaeodispersal trends, developed within the Yanshan belt during Late Jurassic–Early Cretaceous time and are closely associated with the Chengde thrust and allied structures. Shouwangfen basin developed in the footwall of the Gubeikou thrust and records syntectonic unroofing of the hanging wall of that fault. Chengde basin developed in part atop Proterozoic strata interpreted as the upper plate of the Chengde allochthon and records unroofing of the adjacent Chengde County thrust. Both the Chengde County thrust and the Gubeikou thrust are younger than emplacement of the postulated Chengde allochthon, and structurally underlie it, yet neither Shouwangfen basin nor Chengde basin contain a detrital record of the erosion of this overlying structure. In addition, facies, palaeodispersal patterns and geochronology of Upper Jurassic strata that are cut by the Chengde thrust suggest only limited (ca. 5 km) displacement along this fault. We suggest that the units forming the Chengde synform are autochthonous, and that the synform is bounded by two limited‐displacement faults of opposing north and south vergence, rather than a single large north‐directed thrust. This conclusion implies that the Yanshan belt experienced far less Late Jurassic shortening than was previously thought, and has major implications for the Mesozoic evolution of the region. Specifically, we argue that the bulk of shortening and uplift in the Yanshan belt was accomplished during Triassic–Early Jurassic time, and that Late Jurassic structures modified and locally ponded sediments from a well‐developed southward drainage system developed atop this older orogen. Although Upper Jurassic strata are widespread throughout the Yanshan belt, it is clear that these strata developed within several discrete intermontane basins that are not correlable across the belt as a single entity. Thus, the Yanshan has no obvious associated foreland basin, and determining where the Mesozoic erosional products of this orogen ultimately lie is one of the more intriguing unresolved questions surrounding the palaeogeography of North China.

Petrogenesis of the Middle Devonian Gushan diorite pluton on the northern margin of the North China block and its tectonic implications

The Gushan diorite pluton, located at the northern margin of the North China block, was emplaced during Middle Devonian times (SHRIMP U–Pb zircon age of 390 ± 5 Ma). Rocks from the pluton are characterized by low SiO2 and high alkali contents, and they show monzodiorite compositions in a total alkali v. silica (TAS) plot. They exhibit light REE-enrichment, no to slightly positive Eu anomalies, strong depletion in Rb, Th, U, Nb, Ta, P, Zr, Hf and Ti, enrichment in Ba, K and Sr, low contents of Y and Yb, and high Sr/Y ratios. They have a relatively narrow range of isotopic compositions with initial 87Sr/86Sr ratios of ∼ 0.7050, εNd(T) values of −9.5 to −7.5 and zircon εHf(T) values from −11.8 to −5.8. These features are remarkably similar to another Middle Devonian intrusion, the Shuiquangou syenitic complex at the northern margin of the North China block. These similarities suggest that the two intrusions probably have a common origin. They were considered to be derived from a type I enriched mantle, ultimately with some involvement of ancient lower crustal components, and were likely emplaced in a back-arc extension environment related to southward subduction of the Palaeo-Asian oceanic plate or during the cessation of the subduction. Aluminium-in-hornblende barometry studies of the Middle Devonian Gushan pluton yielded emplacement depths of about 18 km. Combined with previous geobarometry results on the Carboniferous plutons within the Inner Mongolia Palaeo-uplift on the northern margin of the North China block, it is inferred that the uplift and exhumation of the plutons within the Inner Mongolia Palaeo-uplift during Middle Devonian to Late Carboniferous times were not as distinct as those during Late Carboniferous to Early Jurassic times, and the strong uplift and exhumation of the Inner Mongolia Palaeo-uplift were achieved during Late Carboniferous to Early Jurassic times.

Late Triassic and Early Jurassic palaeogeography of the world

A new version of global and regional palaeogeographic maps is presented for two time intervals. These maps depict the plate tectonic configuration, palaeoenvironment and lithofacies during the Late Triassic (Carnian–Norian) and Early Jurassic (Hettangian–Toarcian) time. The individual maps illustrate the general conditions present during the maximum marine transgressions of higher frequency cyclicity within the Absaroka sequence of Sloss. During Triassic time Pangaea began to stretch, initiating the rifting and future break-up of the supercontinent. The continued northward drift of the Cimmerian continent corresponded with the progressive closure and consumption of Palaeotethys oceanic crust, and the opening of the Neotethys Ocean. The most significant Late Triassic convergent event was the Indosinian orogeny, occurring as a result of the consolidation of South China and North China blocks. Also, Indochina and Indonesia were sutured to South China. At the same time the Qiangtang block approached the Eurasian margin. The consolidation of the North Chinese and Amurian blocks left open a large embayment of Panthalassa, between Amuria and Laurasia, the so-called Mongol-Okhotsk Ocean. Active subduction existed along the margin of this ocean, dipping cratonwards towards East Siberia. The last collisional events of the Uralian orogeny took place during the Triassic and Early Jurassic time. The conclusion of the Uralian orogeny was accompanied by uplift of the adjacent areas of Eastern Europe and Western Siberia. During the Early Jurassic the Palaeotethys Ocean was finally closed and the Cimmerian continent collided with Asia causing the Cimmerian orogeny. The time around the Triassic–Jurassic boundary marked an important biotic extinction event. Plate tectonic activity caused palaeogeographic and palaeoclimatic change, which may have contributed to the mass extinction. From the plate tectonic and palaeogeographic point of view the following events could have influenced the extinction: 1) the closure of Palaeotethys and assembly of the Asian part of Pangaea; 2) the break-up of Pangaea in the future Central Atlantic area and transition from rifting to drifting; and 3) the very extensive basaltic volcanism of the Central Atlantic Magmatic Province.

Burial/exhumation histories for the Cooper–Eromanga Basins and implications for hydrocarbon exploration, Eastern Australia

ABSTRACTThe Cooper–Eromanga Basins of South Australia and Queensland are not at their maximum burial depth due to Late Cretaceous–Tertiary, and Late Triassic–Early Jurassic exhumation. Apparent exhumation (maximum burial depth–present burial depth) for the Cooper Basin has been quantified using the compaction methodology. The results show that exhumation of the Cooper Basin for the majority of the wells is greater than the exhumation of the Eromanga Basin. Using the compaction methodology, apparent exhumation of Early to Middle Triassic age Arrabury and Tinchoo Formatios has been quantified. Both units yield similar results and do not support that the Arrabury/Tinchoo boundary represents the Cooper–Eromanga boundary. Hence, the Cooper Basin is believed to have reached its maximum burial depth in Late Triassic times. Sonic log data are not available for the units overlying the Late Cretaceous Winton Formation; thus, it is not possible to date exhumation beyond the Late Cretaceous–Tertiary using the compaction methodology. Tertiary sequences as are preserved are relatively thin and separated by marked unconformities and weathered surfaces; hence, exhumation rather than sedimentation dominated the Tertiary, and in exhumed areas, maximum burial depth was attained in Late Cretaceous times. The burial/exhumation history of representative wells was synthesized using sediment decompaction and establishing porosity/depth relations for the Cooper–Eromanga units. Considering the relative significance of the major periods of exhumation in the Cooper/Eromanga Basins, three broad types of burial/exhumation histories can be distinguished. Maximum burial depth of the Cooper Basin sequence was attained before the deposition of the Eromanga Basin sequence, i.e. Late Triassic–Early Jurassic times; maximum burial depth of the Cooper and Eromanga Basin sequences attained in Late Cretaceous times; and Cooper and Eromanga Basin sequences are currently at maximum burial‐depth. Incorporation of exhumation into burial history has major implications for hydrocarbon exploration.

Petroleum migration and mixing in the northwestern Junggar Basin (NW China): constraints from oil-bearing fluid inclusion analyses

Abundant oil-bearing fluid inclusions are present in four reservoir sandstone samples from the slope and fault zone areas in the northwestern margin of the Junggar Basin (NW China). Free oil in intergranular pores, adsorbed oil on grain surfaces, and petroleum in inclusions hosted by mineral grains of these samples were analyzed by gas chromatography and gas chromatography–mass spectrometry. Analytical results indicate similar biomarker distributions in the three bitumen fractions of the samples collected from the slope area, which correlate well with characteristic of the source rocks in the Lower Permian Fengcheng Formation. This is in contrast to the fault zone, where the three bitumen fractions vary significantly in their geochemical signatures, suggesting that all of the three Permian source sequences rocks (including the Lower Permian Jiamuhe and Fengcheng formations, and the Middle Permian Lower Wuerhe Formation) have been mature and contributed to the oils in the fault zone. The presence of an unresolved hydrocarbon hump and a full range of unaltered n-alkanes in the gas chromatograms of samples collected from the fault zone indicate an earlier hydrocarbon biodegradation and subsequent injection of fresh oils, distinctly different from that in the slope area. Petrographic and microthermometric investigations of oil-bearing fluid inclusions suggest a single oil charge in the slope area (to the Lower Triassic reservoir, occurring during the Late Triassic to Early Jurassic time). In the fault zone, oil migration and mixing took place mainly during the Late Triassic to Early Jurassic and Cretaceous, followed by gas migration in the Tertiary period. The different petroleum migration and mixing histories in the slope and fault zones are attributed to the effect of fault-controlled oil and gas migration.

Quantification of Exhumation from Sonic Velocity Data, Cooper Basin, Australia, and Implications for Hydrocarbon Exploration

Exhumation (defined as rock uplift minus surface uplift) in the Cooper Basin of South Australia and Queensland has been quantified using the compaction methodology. The sonic log, which is strongly controlled by the amount of porosity, is an appropriate indicator of compaction, and hence is used for quantifying exhumation from compaction. The traditional way of estimating exhumation based on the degree of overcompaction of a single shale unit has been modified and five units ranging in age from Permian to Triassic have been analysed. The results reveal that exhumation increases eastwards from the South Australia into the Queensland sector of the basin. The results show that exhumation in Late Triassic – Early Jurassic times, after the Cooper Basin deposition, seems to be 200–400 m higher than exhumation in Late Cretaceous – Tertiary times, after the Eromanga Basin deposition. This study has major implications for hydrocarbon exploration. Maturation of source rocks will be greater for any given geothermal history if exhumation is incorporated in maturation modelling. Exhumation values can also be used to improve porosity predictions of reservoir units in undrilled targets.

Zircon U-Pb and Hf isotope constraints on the Mesozoic tectonics and crustal evolution of southern Tibet

The first in situ Hf and U-Pb isotope analyses of zircon separates from Mesozoic granites in southern Tibet identify a significant, previously unknown stage of magmatism. Igneous zircons (n = 34) from a granite within the Gangdese batholith show a weighted mean 206 Pb/ 238 U age of 188.1 ± 1.4 Ma and e Hf (T) (the parts in 10 4 deviation of initial Hf isotope ratios between the zircon sample and the chondritic reservoir) values between +10.4 and +16.8, suggesting predominantly Early Jurassic intrusive activity with a juvenile mantle contribution. Of 40 inherited zircons from two Cretaceous S-type granites in the northern magmatic belt, 23 delineate a slightly older 206 Pb/ 238 U age cluster between 188 and 210 Ma. These zircons have e Hf (T) values from −3.9 to −13.7, yielding crustal Hf model ages from ca. 1.4 to 2.1 Ga, suggesting a major episode of crustal growth in Proterozoic time and remelting of this crust in Early Jurassic time. Combining these with literature data, we interpret the Jurassic Gangdese magmatism as an early product of the Neo-Tethyan subduction that played a long-lasting role in the tectonic evolution of southern Tibet prior to the India-Asia collision.

Sedimentary evidence from the south Mediterranean region (Sicily, Crete, Peloponnese, Evia) used to test alternative models for the regional tectonic setting of Tethys during Late Palaeozoic-Early Mesozoic time

Abstract The south Mediterranean region, including western Sicily, Crete and mainland Greece (southern Peloponnese and Evia), is critical to an interpretation of the Late Palaeozoic-Early Mesozoic tectonic evolution of Tethys. Several contrasting tectonic models compete to explain the regional evolution. In a divergence-related hypothesis (Model 1) the south Aegean region experienced pulsed rifting along the northern margin of Gondwana that culminated in break-up to form the Pindos ocean in the region of Greece. In an alternative convergence-related hypothesis (Model 2) the south Aegean experienced Late Palaeozoic Early Mesozoic northward subduction, accretion and arc magmatism, culminating in ‘Cimmerian’ suturing of a Palaeotethyan ocean in latest Triassic time. In a third model, southward subduction of a Palaeotethyan ocean took place beneath the North Gondwana margin during Late Palaeozoic-Triassic time, giving rise to back-arc magmatism in an extensional setting. In addition, a more complex setting involving two opposing subduction zones (Andean-type and intra-oceanic) has also been suggested (Model 4), mainly based on lava geochemistry. To test these tectonic alternatives, mainly sedimentary studies were carried out in western Sicily, western and eastern Crete, the Peloponnese and Evia (eastern central Greece). Western Sicily was studied as a proxy for the unexposed deep Mediterranean south of Crete. Most of the available evidence supports the divergence-related (pulsed rift) hypothesis (Model 1). There is no clear evidence of sea-floor spreading (e.g. ophiolites) to the south of what became the Pindos ocean, or of plate convergence (e.g. magmatic arcs, subduction complexes), or collisional deformation in the south Aegean region that could be related to subduction or collision during the Mid-Carboniferous to Triassic, as in Model 2. Model 3 is not supported by evidence from the wider region (northern Greece, Turkey). Model 4 is not supported by evidence independent of igneous geochemistry. In the proposed interpretation, the northern margin of Gondwana initially rifted during Mid-Carboniferous to Early Permian time to form a wide deep-water basin. This was followed by further rifting, associated with volcanism during the Early Triassic; final continental break-up and spreading to form the Pindos ocean to the north during Late Triassic to Early Jurassic time then followed. Mid-Triassic uplift of part of the rift basin is explained as a flexural response to rifting as a precursor to opening of the Pindos ocean. Passive margin subsidence during the Early Mesozoic relates to opening of the Pindos ocean to the north. A subduction geochemical signature within some Triassic volcanic rocks, in this interpretation, is explained by melting of heterogeneous sub-crustal mantle, following an earlier, possibly Hercynian, subduction event.