Subsequent Thermal Subsidence Research Articles

A Deformed Wedge‐Top Basin Inverted During the Collapse of the Variscan Belt: The Permo‐Carboniferous Lorraine Basin (NE France)

AbstractA new structural model is presented for the Permo‐Carboniferous Lorraine Basin (NE France), a major intramountain basin that developed during the latest stages of the Variscan orogeny (ca. 315–270 Ma). Digitalized well logs and reprocessed seismic data were used to decipher the kinematic evolution of this basin located along the Rhenohercynian orogenic suture zone. The basin was initiated during the late collision stage (Early to Middle Pennsylvanian) in a wedge‐top position upon the Saxothuringian retro‐wedge. The syn‐orogenic sequence is delimited to the north by the major SE‐verging Metz Thrust, which is part of the backthrust system that propagated during Middle Pennsylvanian (Late Westphalian). Seismic data provide evidence of negative tectonic inversion, allowing the formation of syn‐rift depocenters (Late Pennsylvanian‐Early Permian) above the former anticlines. Erosion of these anticlines results in a major unconformity marking the onset of post‐orogenic collapse. The late Early Permian shortening (Saalian phase) is suggested to reactivate former thrusts and normal faults, thus generating late uplift of the basin. The post‐orogenic phase is complex and diachronous at basin scale, and both compression and extension can be recorded in the same area over a short period (<10 Myr). The Late Carboniferous negative tectonic inversion along the Rhenohercynian suture zone is proposed to result from the lithospheric delamination of the Variscan orogenic roots. The associated upwelling of asthenospheric material is recorded by intense magmatic activity, and can be, in turn, considered as the main trigger for the subsequent thermal subsidence of the Mesozoic Paris Basin.

Palaeogeographic and geodynamic control on the Iranian karst-type bauxite deposits

The Iranian karst bauxite deposits formed in northwestern Iran, the Alborz Mountain Chain, Central Iranian Plateau, and Zagros Simply Folded Belt during warm periods of the middle–late Permian, Permo–Triassic, middle–late Triassic, late Triassic–early Jurassic, and late Cretaceous. According to the literature, mafic igneous and argillaceous carbonate rocks have been considered to be a source rock for these deposits. We show here that, according to palaeogeographic reconstructions, bauxite deposits are directly linked to the tectonic settings of the Iranian area through time. From the middle–late Permian to the late Triassic, the formation of the Iranian karst bauxite deposits coincides with the uplift of the rift and passive margin shoulders, and their disappearance through the subsequent thermal subsidence and sea-level rise. The occurrence of bauxite deposits in the late Cretaceous is coeval with relief formation associated with the arc–continent collision and obduction of the Semail plate with the Arabian margin. The present study supports the idea that palaeogeographic reconstructions can potentially be used as predictive tools for the possible occurrence of bauxites elsewhere in time slices when climatic prerequisites of bauxite formation prevailed.

Reconstruction of the Cenozoic tectono-thermal history of the Dongpu Depression, Bohai Bay Basin, China: Constraints from apatite fission track and vitrinite reflectance data

The Dongpu Depression is an important oil-producing area in the Bohai Bay Basin, China. The exploration focus in this area has recently shifted to intervals with greater depths (>3500 m) because of decreasing hydrocarbon production at depths shallower than 3500 m. Deep petroleum resource evaluation is becoming important and the tectono-thermal history is a major part of this evaluation. In this study, Cenozoic tectono-thermal histories of the Dongpu Depression are reconstructed by coupling an inversion method of vitrinite reflectance and apatite fission track data from 13 wells. Moreover, the maturation history of the deep source rocks was modeled based on these histories. The results clearly show that the Dongpu Depression experienced two heat flow maxima during the deposition of the middle parts of the Eocene Shahejie 3 member and the Oligocene Dongying Formation, which exhibit maximum heat flow values of 70.5–77.5 mW/m2 and 64.8–70.0 mW/m2, respectively. These two periods of high heat flow are linked to the rifting events of the Dongpu Depression, corresponding to the intense rifting substage (45-38 Ma) and the later rifting substage (38-27 Ma). Tectonic subsidence analysis revealed that the Dongpu Depression experienced initial syn-rift subsidence during the Paleocene, followed by subsequent thermal subsidence since the Neogene. The maturation history indicates that these source rocks experienced only one hydrocarbon generation phase in the mid-northern sags, and experienced a hydrocarbon generation peak during the Oligocene Dongying Formation deposition.

Post-rifting magmatism and the drowned reefs in the Xisha Archipelago domain

Fourteen isolated drowned reefs have been identified around the Xisha Uplift by multibeam and seismic data. The drowning processes of these reefs can be divided into three different stages, which correspond to three different accelerated tectonic subsidence periods. The drowning of the Xisha reefs is the result of the combined action of tectonic subsidence and sea level fluctuations, and the tectonic subsidence rate had to remain above 0.2 mm yr−1 for a long time. Three abrupt accelerated tectonic subsidence events that occurred in the late Miocene, Pliocene and early Quaternary in the Xisha Uplift were closely related to the thermal subsidence processes after three stages of post-rifting magmatism. The magmatism of the middle Miocene and the following thermal subsidence resulted in the drowning of reefs in the northwestern Xisha uplift (Zone A). During the early Pliocene, massive magmatic intrusions and volcanic eruptions occurred in the Xisha Uplift. Then, the subsequent thermal subsidence started the drowning process of reefs in the northeastern and western regions of the Xisha Uplift (Zone B and C). During the early Quaternary, large-scale magmatism also occurred in the Xisha Uplift. The subsequent thermal subsidence resulted in a new rapid tectonic subsidence, which caused the reefs in the southern and southeastern regions of the Xisha Uplift to drown (Zone D and E).

Geothermal regime and source rock thermal evolution in the Chagan sag, Inner Mongolia, northern China

The Chagan sag has the greatest oil and gas exploration potential among other sags in the Yingen Ejinaqi Basin, Inner Mongolia. The average geothermal gradient in the Chagan sag is 33.6 °C/km, whereas the heat flow ranges from 65.9 mW/m2 to approximately 85.5 mW/m2, with an average value of 74.5 mW/m2. Thermal history reveals that the Chagan sag experienced the following 4 stages of thermal evolutions: (1) a rapidly increasing geothermal gradient stage from the Early Cretaceous Bayingebi Formation depositional period to the Early Cretaceous Suhongtu Formation depositional period; (2) a geothermal gradient peak stage during the Early Cretaceous Yingen Formation depositional period; (3) a high geothermal gradient continuation stage during the Late Cretaceous Wulansuhai Formation depositional period; and (4) a thermal subsidence stage during the Cenozoic. Tectonic subsidence analysis reveals that the area experienced an initial synrift subsidence during the Early Cretaceous followed by a subsequent long-term thermal subsidence since Late Cretaceous. Thermal and tectonic subsidence histories of this area are of great significance to petroleum exploration and hydrocarbon resource assessment because they bear directly on issues of source rock maturation. The maturation histories of the 3 sets of source rocks in the sag were modeled on the basis of the present geothermal field, thermal history and tectonic subsidence history. The results reveal that the hydrocarbon generation of the Chagan sag was controlled by the Early Cretaceous geothermal fields, and the source rock maturity reached the maximum at the end of the Yingen Formation depositional period. Moreover, the maturation evolution degree shows a difference for the 3 sets of source rocks. The source rocks of the Bayingebi 1 and 2 Formations reached the middle mature and dry-gas stage. In contrast, the source rocks of the Suhongtu 1 Formation only reached the early mature and middle mature stage. This work may provide new insights for the understanding of the oil and gas exploration potential of the Chagan sag.

Geothermal regime and hydrocarbon kitchen evolution in the Jianghan Basin

The present geothermal gradient and terrestrial heat flow was calculated of 18 wells in the Jianghan Basin. Thermal gradient distribution of the Jianghan Basin was obtained based on data of systematical steady-state temperature and oil-test temperature. The basin-wide average thermal gradient in depth interval of 0–4000 m is 33.59°C/km. We report nine measured terrestrial heat flow values based on the data of detailed thermal conductivity and systematical steady-state temperature. These values vary from 41.9 to 60.9 mW/m2 with a mean of 52.3 ± 6.3 mW/m2. However, thermal history analyses based on vitrinite reflectance (VR) and apatite fission track (AFT) data indicate that thermal gradient in the northern and southern Qianbei Fault reached its peak of ∼36 and ∼39°C/km respectively in the Middle Jurassic and the Oligocene, and it descended during the early Miocene to the present-time value. Furthermore, tectonic subsidence analysis reveals that the tectonic subsidence of the Jianghan Basin in the Cretaceous to early Miocene was characterized by synrift initial subsidence followed by the subsequent thermal subsidence. The thermal history and tectonic subsidence history of Jianghan Basin are of great significance to petroleum exploration and hydrocarbon source assessment, because they bear directly on issues of petroleum source rock maturation. Based on the thermal history and tectonic subsidence history, with the combination of geochemical and thermal parameters, the maturation and the hydrocarbon generation intensity evolution history of the P2 d source rocks are modeled. The results show that the P2 d source rocks are in a higher degree of maturation at present, and the Yuan’an and Herong sags are the two most important kitchens in the Late Jurassic, Xiaoban Sag is another most important kitchen during the Late Cretaceous to late Paleogene, and the Zhijiang and Mianyang sags are other two important hydrocarbon kitchens in the Late Cretaceous. The Mianyang Sag and Yichang Ramp are the favorable exploration targets in the future. This study may provide new insight for the understanding of the oil and gas exploration potential for the Jianghan Basin.

Is there a remnant Variscan subducted slab in the mantle beneath the Paris basin? Implications for the late Variscan lithospheric delamination process and the Paris basin formation

The Paris basin (northern France) is a Late Paleozoic-Mesozoic intracratonic basin that settled upon the collapsed Variscan collisional belt. The lithospheric roots of the Variscan orogenic system, below the Paris basin, have been investigated using a European-scale P-wave velocity tomographic model. Tomography points out the existence of a significant high velocity anomaly in the upper mantle below the western part of the basin. At ~150–200km depth, the anomaly extends with a NW–SE trend along the buried Northern France trace of the Northern Variscan Suture Zone i.e. the Bray segment of the Upper Carboniferous Lizard–Rhenohercynian (LRH) suture. Moreover, the high-velocity anomaly is spatially correlated with the prominent Paris Basin Magnetic Anomaly. Its downdip extent reaches depths greater than 200km below the southern margin of the Paris basin. As suggested in previous tomographic studies below ancient suture zones, these data argue for such anomaly being the remnant of a Variscan subducted slab that escaped the extensive late orogenic delamination process affecting the lithospheric roots by Late Carboniferous-Early Permian times and that was preserved stable over 300Ma at the base of the lithosphere. On a general geodynamical perspective, these results provide a new insight into the long-term evolution of subducted lithosphere into the mantle. In the case of the Western European Variscan orogenic belt, they suggest that the subduction of the LRH slab below the previously thickened Variscan crust, and its final detachment from the orogenic root, have played an important role in the collapse of the belt, inducing thermal erosion and extension of the overriding lithosphere. The spatial evolution of late orogenic extension across the belt and of subsequent thermal subsidence in the Paris basin is suggested to result from the heterogeneous delamination of the lithospheric roots along strike and from the resultant pattern of asthenospheric rise.

Nature of and controls on volcanism in the ca. 2.32–2.06 Ga Pretoria Group, Transvaal Supergroup, Kaapvaal Craton, South Africa

The time of deposition of the Pretoria Group between 2.32 and 2.06Ga on South Africa's Kaapvaal Craton was characterized by the first major increase in atmospheric oxygen. It was accompanied by the extrusion of significant thicknesses of volcanic deposits, namely the Bushy Bend lavas of the Timeball Hill Formation, the Hekpoort Formation and the Machadodorp Volcanic Member of the Silverton Formation, marking the three major volcanic events within the Transvaal Supergroup which are thought to be precursors to the succeeding Bushveld Complex magmatism. The Bushy Bend Lava Member of the Timeball Hill Formation is characterized by subaqueous basaltic–andesitic fissure eruptions with fumarolic activity and probably minor subaerial explosive eruptions in the hinterland of the Transvaal Supergroup basin. The subaerial volcanism of the Hekpoort Formation appears to have been dominated by fissure eruptions with a preponderance of lava flows and locally important pyroclastic material. Intermittent hiatuses in volcanism were marked by local lacustrine shale deposition. Finally, the Machadodorp Lava Member can be interpreted as the deposits of several seamounts aligned along a fissure, probably within an extensional environment.The geodynamic control inferred for the evolution of the Pretoria Group basin encompasses two cycles of prerift uplift, subsequent mechanical rifting and long lived thermal subsidence. The limited extent of the Bushy Bend lavas in the south of the Pretoria Group depository attests to the likelihood that volcanism accompanying the first rifting event was short-lived. In contrast, the second rifting cycle postulated for the Pretoria Group basin has a strong association with widespread and large scale volcanism of the Hekpoort–Ongeluk flood basalts, which may have been plume-related. The scale of the second cycle volcanism and its importance in influencing the upper part of the Pretoria Group basin-fill is underlined by the Machadodorp volcanism, probably related to hot spot volcanism as the Ongeluk–Hekpoort plume waned.

Geothermal regime and hydrocarbon kitchen evolution of the offshore Bohai Bay Basin, North China

The offshore Bohai Bay Basin, located in the central Bohai Bay Basin, north China, one of the most petroliferous basins in China. In this article, based on formation-testing temperature, drill-stem tests, and bottom-hole temperature data, 80 thermal gradient values at the depth interval of 0 to approximately 3000 m (~9843 ft) in the offshore Bohai Bay Basin were obtained. The basinwide average thermal gradient is 31.8 4.6C/km. Based on the above thermal gradient data and the corresponding average weighted thermal conductivity data, 80 measured terrestrial heat flow values were obtained. These values range from 33.5 to 84.0 mW/m2, with an average value of 60.8 8.7 mW/m2. The heat flow and thermal gradient distribution in this region generally show higher values in the uplifts and lower ones in the sags. A thermal history, derived from vitrinite reflectance and apatite fission-track) data, indicates that Paleogene cooling occurred after a period of much higher paleogeothermal gradient (3854C/km). Tectonic subsidence analysis reveals that the area experienced initial synrift subsidence during the Paleogene followed by subsequent thermal subsidence since the Neogene. Thermal and tectonic subsidence histories of this area are of great significance to petroleum exploration and hydrocarbon resource assessment because they bear directly on issues of petroleum source rock maturation. The maturation and hydrocarbon expulsion histories of the Paleogene Shahejie 3 Formation (E2s3), which is the most important source rock in the offshore Bohai Bay Basin, are modeled. Results show that the Shahejie 3 Formation is in a high mature stage at the present day, and the Bozhong and Qikou sags are the most important kitchens. The Huanghekou sag became the third most important hydrocarbon kitchen in the early Neogene. Based on this hydrocarbon kitchen evolution, oil and gas mainly accumulated after 12 Ma. The evolution of kitchen areas may provide new insights for the understanding of the oil and gas exploration potential of the offshore Bohai Bay Basin.

The Kaapvaal Craton, South Africa: no evidence for a supercontinental affinity prior to 2.0 Ga?

We briefly examine the possible antiquity of the supercontinental cycle while noting the likely unreliability of palaeomagnetic data >ca.1.8 Ga, assuming a gradual change from a magmatically dominated Hadean Earth to a plate tectonically dominated Neoarchaean system. A brief review of one of Earth's oldest cratons, Kaapvaal, where accent is placed on the lithostratigraphic and geodynamic-chronological history of its cover rocks from ca. 3.1 to 2.05 Ga, forms the factual basis for this article. The ca. 3.1–2.8 Ga Witwatersrand–Pongola (Supergroups) complex retroarc flexural foreland basin developed while growth and stabilization of the craton were still underway. Accretion of relatively small composite granite-gneiss-greenstone terranes (island arc complexes) from both north and west does not support the formation of a Neoarchaean supercontinent, but may well have been related to a mantle plume which enhanced primary gold sources in the accreted terranes and possibly controlled the timing and rate of craton growth through plate convergent processes. Subsequent deformation of the Witwatersrand Basin fill with concomitant loss of ≤1.5 km of stratigraphy must have been due to far-field tectonic effects, but no known mobile belt or even greenstone belts can be related to this contractional event. At ca. 2714–2709 Ma, a large mantle plume impinged beneath the thinned crust underlying the Witwatersrand Basin forming thick, locally komatiitic flood basalts at the base of the Ventersdorp Supergroup, with subsequent thermal doming leading to graben basins within which medial bimodal volcanics and immature sediments accumulated. Finally (possibly at ca. 2.66–2.68 Ga), thermal subsidence enabled the deposition of uppermost Ventersdorp sheet-like lavas and sediments, with minor komatiites still present. Ongoing plume-related influences are thus inferred, and an analogous cause is ascribed to a ca. 2.66–2.68 Ga dike swarm to the north of the Ventersdorp, where associated rifting allowed formation of discrete ‘protobasinal’ depositories of the Transvaal (ca. 2.6–2.05 Ga Supergroup, preserved in three basins). Thin fluvial sheet sandstones (Black Reef Formation, undated) above these lowermost rift fills show an association with localized compressive deformation along the palaeo-Rand anticline, north of Johannesburg, but again with no evidence of any major terrane amalgamations with the Kaapvaal. From ca. 2642 to 2432 Ma, the craton was drowned with a long-lived epeiric marine carbonate-banded iron formation platform covering much of it and preserved in all three Transvaal Basins (TB). During this general period, at ca. 2691–2610 Ma, the Kaapvaal Craton collided with a small exotic terrane [the Central Zone (CZ), Limpopo Belt] in the north. Although farfield tectonic effects are likely implicit in TB geodynamics, again there is no case to be made for supercontinent formation. Following an 80–200 million years (?) hiatus, with localized deformation and removal of large thicknesses of chemically precipitated sediments along the palaeo-Rand anticline, the uppermost Pretoria Group of the Transvaal Supergroup was deposited. This reflects two episodes of rifting associated with volcanism, and subsequent thermal subsidence within a sag basin setting; an association of the second such event with flood basalts supports a plume affinity. At ca. 2050 Ma the Bushveld Complex intruded the northern Kaapvaal Craton and reflects a major plume, following which Kaapvaal–CZ collided with the Zimbabwe Craton, when for the first time, strong evidence exists for a small supercontinent assembly, at ca. 2.0 Ga. We postulate that the long-lived evidence in favour of active mantle (cf. plume) influences with subordinate and localized tectonic shortening, implicit within the review of ca. 3.1–2.05 Ga geological history of the Kaapvaal Craton, might reflect the influence of earlier Precambrian mantle-dominated thermal systems, at least for this craton.

Strange bedfellows: glacial diamictite and cap carbonate from the Marinoan (635 Ma) glaciation in Namibia

AbstractThe Otavi Group is a Neoproterozoic carbonate‐dominated succession up to 4 km thick, which blankets the southern promontory of the Congo craton in northern Namibia. This succession was deposited between 770 and 580 Ma in response to north–south crustal stretching and subsequent thermal subsidence. The main shallow‐water platform has a well‐defined southern limit, beyond which is a distally tapered foreslope wedge of deep‐water carbonate facies. The Ghaub Formation represents the younger of two Cryogenian glaciations of the platform and was deposited during the period of thermal subsidence. A deep negative δ13C excursion, accompanied by increased size, abundance and variety of stromatolites, occurs in the last 10 to 80 m of shallow‐water carbonate on the platform beneath the Ghaub glacial erosion surface. The same phenomena are observed before the older (Sturtian) glaciation in other areas, suggesting temporal proximity of the δ13C excursions to glaciation. Growth of ice sheets is manifested by emergence of the platform and development of a falling‐stand wedge on the foreslope, composed of upward‐coarsening carbonate turbidites and debrites. Rafts of very coarse‐grained, well‐sorted oolite have no source on the platform. The oolite probably originated at the strandline on the foreslope and was redeposited gravitationally downslope as sea‐level fell. The Ghaub Formation is a laterally continuous wedge of carbonate diamictite, limited to the distal foreslope andca80 m in average thickness. Tongues of massive to weakly stratified diamictite, representing proglacial rain‐out and subglacial tillite, are bounded by thinner, well‐bedded units consisting of hypopycnal plume fallout, ice‐rafted debris, turbidites and debrites, sorted sands and gravels, and westward‐directed contourites. Debris is derived from the falling‐stand wedge and the top 80 m of the inner platform. The wedge rests on a laterally continuous erosion surface, presumably cut by ice, and its sedimentary makeup and stratal organization are diagnostic of an ice grounding‐line wedge. The subglacial erosion surface cuts a steep‐walled trough on the distal foreslope, presumably once occupied by a transverse ice‐stream. In the middle of the trough stands a doubly crested moraine composed of amalgamated unstratified diamictites. Terminal deglaciation is recorded by a fining‐upward, 10 m thick drape of Fe‐rich carbonate debrite and turbidite, loaded with far‐travelled ice‐rafted debris of all sizes. If deglaciation began with the collapse of ‘sea‐glacier’ ice on the tropical ocean, the loss of this buttress could have triggered catastrophic ice‐sheet drainage, concomitant with surface ocean ventilation. Complete deglaciation presumably was driven by ice‐albedo, ice‐elevation and greenhouse‐gas feedbacks. The grounding‐line wedge and bare upper foreslope and platform are overlain by a transgressive ‘cap dolostone’, which features shallow‐water sedimentary structures (sorted peloids, low‐angle cross‐bedding, tubestone stromatolite and giant wave ripples) from the distal foreslope to the inner platform. The structures and isotopic profiles show that it was deposited diachronously on the time scale of ice‐sheet melting globally (kyr), implying very high sedimentation rates. Temperature‐dependent isotope fractionation could account for the observed secular and lateral δ13C changes. This interpretation requires sea water pH lower than 7·3 (highpCO2) and warming of at least 45°C in the tropics, consistent with the change in planetary albedo accompanying global deglaciation. The thickness of the highstand part of the cap‐carbonate sequence on the platform implies >3 to 5 Myr of tectonic subsidence during the glacial period to create permanent accommodation. The highstand sequence prograded inward from the raised rim of the platform, inherited from karstic and glacial erosion. Sea floor cements, formerly aragonitic, are localized over palaeobathymetric highs (ice‐stream moraine, platform rim and inner platform highs). Subsequent aggradation of the platform and coeval foreslope shedding were accompanied by a 0·5 km thick stratigraphic interval in which platform strata are depleted in13C by up to 2·5‰, compared with coeval foreslope strata; this again suggests temperature‐dependent fractionation at low pH. Above this interval, the δ13C gradient reverts to normal (platform strata more enriched in13C than foreslope equivalents), consistent with CO2drawdown due to silicate weathering.

Late Neoproterozoic rise and fall of the northern Arabian–Nubian shield: The role of lithospheric mantle delamination and subsequent thermal subsidence

Review of the Late Neoproterozoic history of the northern Arabian–Nubian Shield (ANS) reveals that rapid and extensive erosional denudation, widespread late-orogenic calc–alkaline and alkaline igneous activity and intramontane basin formation affected this terrane at 630–590 Ma. Core-complex extension is also recorded at this time interval. To account for this coupling it is suggested that the mantle lithosphere was removed/delaminated from below the northern ANS subsequent to significant crust–mantle thickening in the course of the Late Neoproterozoic orogeny. Because the physical properties of the delamination process are not clear, we use it as a conceptual term accounting for the replacement of mantle lithosphere by an asthenosphere. Removal and replacement of the thickened lithospheric mantle roots potentially caused uplift of the northern ANS to elevations of more than 3 km, thus triggering exceptionally-rapid erosional unroofing of a ca. 10 km thick rock carapace, and some lateral extension. Delamination and erosional decompression potentially caused partial melting of ANS residual upper mantle and lower crust to produce the widespread post-orogenic magmas. We emphasize that removal of the lithospheric mantle also played a key role in lowering ANS topography to sea level. Our data show that lowering ANS down to sea level was significantly accelerated by post-delamination cooling and thermal subsidence. Unlike erosional denudation which is followed by isostatic uplift, thermal contraction causes net surface lowering. We show that thermal subsidence plays a critical role in lowering mountain belts, particularly in the latest stages, when the topography reaches a threshold of ∼ 1.0 km and erosion rates greatly decrease. Post-delamination thermal subsidence can cause 1.3 km of net surface lowering within 100 m.y. regardless of the state of relief and elevation. Therefore, orogenic belts that experienced removal or delamination of their mantle lithosphere (hot orogens) would rapidly rise, but would relatively rapidly fall too.

Sequence stratigraphy and fill history of the Bowen Basin, Queensland

A regional seismic synthesis of the 10 km-thick continental and shallow-marine succession of the Bowen Basin has revealed the basin-filling episodes and nine depositional supersequences (A–I). An Early Permian extensional phase, characterised by thick volcanics and half-graben development in a number of separated troughs, allowed fluviolacustrine sediments, including coal, to accumulate (Supersequence A). In the subsequent thermal subsidence phase, four marine supersequences (B–E) were generated. The onsets of supersequences B and C appear to be primarily tectonically controlled, but there is no evidence of tectonism initiating D, E and F. Supersequences B, D and E start with marine-flooding events, and in the Denison Trough end with either non-marine progradation (B), or high-energy nearshore sands (D and E). The first evidence of the compressional deformation associated with the development of a foreland basin occurs at the base of deltaic to marine Supersequence C. The thrust-induced loading of the foreland caused rapid subsidence in the adjacent Taroom Trough. It took about 10 Ma for the effects of thrust loading to spread across the basin from east to west. The foreland loading first affected the entire basin at the time of Late Permian Supersequence F, and lasted until the end of Triassic deposition. Four supersequences (F–I) have been identified in this phase of basin evolution. The later part of Supersequence F is almost wholly non-marine coal measures. The tectonism and base-level fall that generated the sequence boundary between Supersequences F and G, and ended coal deposition, represents a significant hiatus, and in places is characterised by low-angle unconformity and erosion. The pattern of the remaining units of the foreland basin phase indicates episodic basin filling as accommodation space became available, related to pulses of thrust loading. The regional geometry of the units, and the overstepping of Supersequence G by H, and then by I, indicates continuing westward progradation of foreland loading and corresponding changes in accommodation. It is striking that although the late foreland-loading phase displays the greatest rate of subsidence affecting the basin since the initial rift phase, there is little evidence of widespread flooding by the sea. The reasons were probably that the Late Permian–Early Triassic was a time of global first-order lowstand, and that foreland loading was able to provide simultaneously high rates of subsidence in the basin and greatly increased sediment supply from erosion in the mountains thrust up to the east.

Provenance of the Meguma terrane, Nova Scotia: rifted margin of early Paleozoic Gondwana

Detrital zircon ages from the lower part of the Late Proterozoic(?) to Middle Cambrian Goldenville Group in the Meguma terrane of Nova Scotia suggest derivation from local sources in the Avalonian and Pan-African orogens on the margins of Early Cambrian Gondwana. Samples from near the top of the group show a broader distribution, including ages back to Archean. The εNd data show a corresponding trend, from slightly positive in the lower Goldenville Group to highly negative in the upper Goldenville Group and overlying Upper Cambrian to Lower Ordovician Halifax Group. The trends are consistent with deposition of the lower part of the Meguma succession in a rift, in which uplifted rift-flanks were the main source of the early basin fill, whereas subsequent thermal subsidence of rift margins allowed for more widespread sediment sourcing in younger units. The rift was possibly located between Gondwana and Avalonia, and may have been the locus for separation of Avalonia from Gondwana to form part of the Rheic Ocean.

Kaapvaal craton: Changing first- and second-order controls on sea level from c. 3.0 Ga to 2.0 Ga

Well preserved supracrustal basin-fills including the oldest large depository (Witwatersrand) known makes the Kaapvaal craton an ideal example for studying controls on first- and second-order stratigraphic cyclicity. The duration of the three basins studied (Witwatersrand, Ventersdorp and Transvaal), from c. 3.0 to c. 2.0 Ga is almost twice that of the entire Phanerozoic period, from which the paradigms of classical sequence stratigraphy have been derived. Amongst these is an assumption that cyclicity at specific hierarchical levels is predictable and repetitive through Earth history. This does not appear to be borne out by the Precambrian record studied here; instead, a hierarchical order of cycles based on relative importance of sequences allied to a more flexible approach to controls on stratigraphic cyclicity at first- and second-orders is preferred. First-order cycles within the c. 3.0–2.0 Ga period on Kaapvaal are synonymous with the entire evolution of a supracrustal basin-fill defined by a specific basin-type; second-order cycles define basin-fill packages of unique character, which reflect large scale shifts in the accommodation–sedimentation balance. Early to Middle Archaean evolution of the Kaapvaal craton encompassed amalgamation of a mosaic of subdomains, to produce an early south–southeastern nucleus by c. 3.1 Ga. Accretion of composite terranes from both north and west followed from c. 3.0–2.7 Ga; this was contemporaneous with evolution of the c. 3.0–2.8 Ga Witwatersrand basin. The high-grade Limpopo terrane was amalgamated with the Kaapvaal craton between 2.95 and 2.69 Ga, although a younger age for this orogensis is also possible. The Witwatersrand first-order depositional cycle was controlled largely by subduction-related tectonic loading and had a duration of c. 190 My. The c. 5 My first-order cycle of the c. 2.7 Ga Ventersdorp basin was essentially controlled by thermal uplift-induced extensional subsidence, and that of the c. 650 My Transvaal basin by extensional and subsequent thermal subsidence. It is noteworthy in the three basins examined that first-order cycles controlled by plate tectonics (Witwatersrand and Transvaal basins) were about two orders of magnitude longer relative to the Ventersdorp “plume tectonic” cycle; second-order cycles also varied greatly, from c. 100 My to c. 1 My. Within the Witwatersrand and Transvaal basins, subsidence allowed extensive seaways to develop, within which classical sequence stratigraphic systems tracts can be recognized, reflecting different stages of shoreline shifts. Within the short-lived Ventersdorp continental basin system, sedimentological criteria can be used to discriminate high and low accommodation systems tracts. The Transvaal first-order cycle was terminated at c. 2.0 Ga by the Bushveld Complex plume event.

Tertiary Subsurface Facies, Source Rocks and Hydrocarbon Reservoirs in the SW Part of the Pannonian Basin (Northern Croatia and South-Western Hungary)

The Neogene sedimentary successions of the Drava, Sava and Slavonija–Srijem depressions in the SW part of the Pannonian Basin Systemare built up of three 2nd order megacycles separated by four majorerosional unconformities. The first megacycle contains terrestrial tomarine syn-rift and early post-rift sediments of Early to Mid-Mioceneage. The second is built up of Late Miocene Lake Pannon deposits,while the third contains those sediments which were deposited inthe remnants of Lake Pannon and in the subsequent fluvial systems,in areas of continuous subsidence associated with basin inversionfrom the Pliocene onwards. Most of the petroleum source rocks andreservoir rocks are of Miocene age and were formed during the firstand second depositional megacycle. Conditions for the accumulationand preservation of large quantities of marine and terrigenousorganic matter were most favourable during the Badenian, Sarmatianand Early Pannonian, in deep basin settings, partly associated withrifting. The generation of hydrocarbons was promoted by relativelyhigh geothermal gradients during the initial and subsequent thermalsubsidence. Various sedimentary environments produced depositswith good reservoir characteristics: e.g. fault-related talus breccia(mainly Lower Miocene), reefs (mainly Badenian), coastal, shallowmarine (Karpatian, Badenian) and deltaic (Pannonian–Pontian) sandbodies or turbiditic sand lobes (mainly Pannonian). The hydrocarbon(HC) migration paths were often provided by the major unconformitiesbounding the three megacycles, as well as by faults, particularlyaround the basement highs.

Major influences on the evolution of the 2.67–2.1Ga Transvaal basin, Kaapvaal craton

The Transvaal is one of three structural basins of the Transvaal Supergroup preserved on the Kaapvaal craton. The evolution of the Neoarchaean–Palaeoproterozoic Transvaal basin is ascribed predominantly to magmatism, palaeoclimate and eustasy, with plate tectonics playing a sporadic role. The supergroup comprises basal ‘protobasinal’ rocks, followed by the Black Reef Formation, Chuniespoort Group and uppermost Pretoria Group. Immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c. 2.7 Ga Ventersdorp (Supergroup) mantle plume. Individual protobasinal successions were laid down in separate fault-bounded basins, controlled at least partially by greenstone belt orientations in the Kaapvaal basement. Post-magmatic, post-rifting thermal subsidence accommodated Black Reef fluvial sheet sandstones and the subsequent thick carbonate-BIF epeiric platform succession of the Chuniespoort Group. Subordinate mechanical subsidence accompanied this long-lived thermal relaxation. Intense weathering due to Neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation. Globally enhanced sea levels, due to enhanced mid-ocean ridge growth consequent upon either global magmatic events or supercontinent break-up, also played a pivotal role in Chuniespoort epeiric basin evolution. Pretoria Group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence. The first cycle appears to reflect plate tectonically induced rifting, with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation. The second Pretoria cycle is most likely related to a major continental flood basalt event, with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern Kaapvaal craton. Evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic Transvaal basin depends to a large degree on the age assigned to the Limpopo orogeny during which the Zimbabwe and Kaapvaal cratons collided; recent evidence for a c. 2.0 Ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during Transvaal basin evolution.

Structure of the Lithosphere and Mesozoic Sedimentary Basins in Western Liaoning, Northern Liaoning, and Songliao, Northeast China

The western Liaoning, northern Liaoning, and Songliao areas represent three different tectonic units with different lithospheric thermal-mechanical properties in terms of composition, thickness of the thermal lithosphere, effective elastic thickness, depth of the detachment surface, and strength of the lithosphere. Western Liaoning is characterized by a thick lithosphere with high rigidity and strength, whereas Songliao and northern Liaoning are characterized by relatively thin lithosphere with low rigidity and strength. These differences controlled the development of distinct types of basins under an extensional regime during the Early Cretaceous. The basins in western Liaoning are controlled by faulting rather than by thermal subsidence. The Songliao basin is controlled both by faulting and by thermal subsidence, and thus both syn- and post-rift sequences developed with the same thickness in the center of the basin. In contrast, in northern Liaoning, the basins are mainly controlled by faulting, and to a lesser extent by subsequent thermal subsidence. As a part of east China, where more than 93% of the proven oil reserves of China are distributed, the western Liaoning-northern Liaoning-Songliao area also has attracted considerable attention for its hydrocarbon potential. The Songliao basin is proven to be the largest non-marine petroliferous basin and one of the most important oil- and gas-producing basins in China. In addition, the Tieling-Changtu basin and the Zhezhong depression in northern Liaoning show some petroleum potential. In contrast, basins in western Liaoning represent a limited prospect for oil and gas.

Tertiary foreland sedimentation in the Southern Subalpine Chains, SE France: a geodynamic appraisal

Tertiary foreland sedimentation in SE France occurred along the western sidewall of the Alpine orogen during collision of the Apulian indentor with the European passive margin. A detailed reappraisal of the stratigraphy and structure of the Southern Subalpine Chains (SSC) in SE France shows that Tertiary depocentres of differing character developed progressively toward the foreland during ongoing SW‐directed shortening. The geodynamic controls on each of four stages of basin development are evaluated using a flexural isostatic modelling package of thrust sheet emplacement and foreland basin formation. (1) The initial stage (mid to late Eocene) can be explained as a flexural basin that migrated toward the NW, closing off to the SW against the uplifting Maures–Esterel block. This broad, shallow basin can be reproduced in forward modelling by loading a lower lithospheric plate with an effective elastic thickness of 20 km. (2) The end of detectable flexural subsidence in the early Oligocene coincides with the emplacement of the internally derived Embrunais–Ubaye (E‐U) nappes, which caused 11 km of SW‐directed shortening in the underlying SSC. The lack of Oligocene flexural subsidence dictates that the E‐U units were emplaced as gravitational nappes. Within the SSC, Oligocene sedimentation was restricted to small thrust‐sheet‐top basins recording mainly continental conditions and ongoing folding. Further west, Oligocene to Aquitanian NNW–SSE extension generated the Manosque half‐graben as part of the European graben system that affected an area from the Gulf of Lion to the Rhine graben. (3) Following the Burdigalian breakup of the Gulf of Lion rift, a marine transgression migrated northward along the European graben system. Subsequent thermal subsidence allowed 1 km of marine sediments to be deposited across the Valensole and Manosque blocks, west of the active SSC thrust belt. (4) Mio‐Pliocene conglomeratic deposits (2 km thick) were trapped within the Valensole basin by the uplifting Vaucluse block to the west and the advancing Alpine thrust sheets to the east. Late Pliocene thrusting of the SSC across the Valensole basin (approx. 10.5 km) can be linked along a Triassic detachment to the hinterland uplift of the Argentera basement massif.

Vendian-Cambrian subsidence of the passive margin of western Baltica - application of new stratigraphic data from the Scandinavian Caledonian margin

Stratigraphic information from the Neoproterozoic to Cambrian cover sequences at the Caledonian margin in central Scandinavia has been compiled from the literature and our own data. Four flooding events can be recognized in the autochthonous cover rocks and parts of the eastern Caledonian Lower Allochthon: one, at the base of the Vendian at 590 Ma, two Early Cambrian events (540 and 530 Ma), and the fourth at the base of the Mid Cambrian and the alum shales (518 Ma). Stratigraphic successions of the western Baltica margin from northern Sweden to southern Norway are correlated using these flooding events. Based on these correlations, depth and time sections are constructed and subsidence curves calculated. Although Early Cambrian flooding events lead to temporarily higher sedimentation rates, the subsidence appears to have decreased through time. Such a decrease is consistent with models of lithospheric stretching and subsequent thermal subsidence. A review of available age data on tectonic events suggests a transition from continental rifting to ocean-floor formation off western Baltica at ca. 600 Ma ago. Accordingly, the Vendian to Cambrian evolution of the western Baltica continental margin is interpreted as a stage of post-rift subsidence showing the 'steer's head' geometry characteristic of sequences onlapping from an older zone of active rifting and of ocean-floor formation farther west. The gradual decrease in thermal subsidence through Cambrian time also shows that the Baltica lithosphere was essentially thermally re-equilibrated prior to earliest Caledonian tectonic activity in Early Ordovician time.