Tectonic Subsidence History Research Articles

Rare earth elements and origin of Fe[sbnd]Mn ores from the Lower Carboniferous Um Bogma Formation, Southwest Sinai, Egypt: A mixed source of metals and multiple formation processes

The economic FeMn ores of the Lower Carboniferous Um Bogma Formation, Southwest Sinai are an important mineral resource, and are also of interest because of their significant enrichment in Cu and U compared to other Phanerozoic Mn deposits. However, their origin remains controversial. Genetic theories range from sedimentary, to hydrothermal, to karst and to laterization origins. This study integrates geological, sedimentological, mineralogical, and geochemical data, particularly rare earth elements, to elucidate the origin of these deposits. Field relations showed the occurrence of FeMn deposits as lenses of Mn-rich material surrounded by Fe-rich clastic deposits, which then are overlain by dolostones, which, in the subsurface, are known to also contain MnFe lenses. The occurrence of the largest of these deposits at a major transgressive surface is consistent with syndepositional Mn mineralization, but not of ironstones. Geochemical proxies, based on modern hydrothermal, diagenetic and hydrogeneous Mn deposits, are mixed: some suggest a hydrogenous and some a hydrothermal source of Fe and Mn. The sweater (hydrogenous) source of Mn and Fe is indicated by the plotting of these ores in the hydrogenous field of the SiAl discrimination plot, by high ΣREE (42–327 ppm), low La/Ce ratios (0.261.5), relatively high Y/Ho ratios for Mn but not for Fe (averages of 35 and 28), and positive YSN anomalies (1–1.8). The ores plot in the sweater fields of the (La/Sm)SN-(La/Yb)SN and (Sm/Yb)SN-Y/Ho diagrams, which also supports this interpretation. However, the hydrothermal input to the source of these ores cannot be ignored and is indicated by high Ba, Cu, Zn, and by high LREE/HREE ratio as well as the plotting of the FeMn samples in the hydrothermal field of the Fe-Mn-(Ni + Co + Cu)*10, (Co + Ni)-(As+Cu + Mo + Pb + V + Zn), (Co + Ni + Cu)-Co/Zn, CeSN/CeSN*-YSN/HoSN and CeSN/CeSN*-NdSN diagrams. In addition, lower Er/Nd ratios than that of seawater and similarity of REE parameters such as ΣREY, Nd and CeSN/CeSN* between the diagenetic FeMn nodules and the FeMn ores of Um Bogma Formation suggest a post-depositional overprint on the REEs geochemistry of these ores. We propose, as most consistent with the evidence from tectonic subsidence history, sequence stratigraphy, and our geochemical-mineralogical data combined with that of others, an initial synsedimentary deposition of Mn, derived from a distally-located back-arc spreading center where modified seawater at relatively low temperature was the hydrothermal fluid released into the surrounding seawater. This plume of Mn-rich water, which can be far-travelled, deposited Mn on the unconformity surface during a rapid transgression. This accumulation of Mn was subsequently modified and significantly upgraded during karstification of the host dolostones soon after deposition of the dolostones. It is likely that the Fe was introduced at this stage via low-oxygen ground water flow along with As and possibly Ba. Finally, much later, there was a flow of warm, saline oxidizing water through the dolomitic aquifer that precipitated Cu, U and Zn in the weathered material accumulated in the karst cavities.

The fault activity, tectonic subsidence history, and geodynamics of syn‐depositional faults during the Palaeogene in Liaodong Bay, Bohai Bay Basin

The fault activity, tectonic subsidence history, and geodynamics are analysed based on borehole information and seismic datasets from Liaodong Bay. The results reveal that the faults are intensely active (resulting in the faults that are characterized by more lengths and more throws) both in the northern and southern portions of the study area, while the faults are not active in the central portion of the study area during Kongdian Formation and Shahejie Formation deposition. However, the faults are generally more active in the northern portion of the study area than that in the southern portion of the study area. In contrast, the faults are intensely active in central and southern portion of the study area during Dongying Formation deposition. The numerous syn‐depositional faults, which are intensely active, result in southward movement of depositional centre. The tectonic subsidence history of wells (including well JZ‐A, well JZ‐B, well JZ‐C, well JZ‐D, well JX‐A, and well SZ‐A) in Liaodong Bay indicates that it shows obvious episodic tectonic evolution during the Palaeogene in Liaodong Bay. The episodic tectonic movement (initial rifting, rifting climax, thermal subsiding, and strike‐slip rifting) corresponds to Kongdian Formation and Shahejie Formation deposition, and Dongying Formation deposition, respectively. The spatio‐temporal variation of tectonic subsidence history is closely related to fault activity. The analysis results of balanced cross‐sections show that faults are intensely active during Dongying Formation deposition. However, a few faults are much active than that during Shahejie Formation deposition. There is a slight decrease in tectonic activity intensity during Kongdian Formation and Shahejie Formation deposition. In contrast, there is a significant decrease in tectonic activity intensity during third‐member to first‐member of Dongying Formation deposition. The tectonic activity intensity tends to cease by the end of Dongying Formation.

Main controlling factors and depositional model of source rock of Paleogene Shahejie Formation in Liaodong Bay Depression

In the Paleogene Shahejie Formation of Liaodong Bay Depression, two sets of main source rocks are developed, namely the Es1 and the Es3 Formation. In order to determine the characteristics, main controlling factors and patterns of source rock development, firstly, we comprehensively use source rock geochemical and logging data to establish TOC logging prediction models for source rocks in two formations; secondly, we predict the TOC plane distribution of the two layers separately according to the prediction model; then, we use the tectonic subsidence history and sedimentary background data of the study area to analyze its relationship with the source rock development and determine the main controlling factors for the source rock development; finally, the source rock deposition models are established according to the main controlling factors of source rock development. The results show the source rock of the Es1 has high abundance of organic matter, the average TOC of the sample is as high as 2.41%, but the thickness of the source rock does not exceed 130 m; the abundance of source rocks in the Es3 is slightly worse than that of the Es1, the average TOC of the samples is 1.75%, the thickness of source rocks is generally larger, mainly 200-350 m; the rate of tectonic subsidence controls the thickness of the source rock, the amount of fracture extension controls the distribution shape of the source rock, and the degree of water stratification controls the degree of source rock enrichment; in the Es1 Formation, the source rock deposition model is the deposition model of the salinity layered water body in the saline lake basin; in the Es3 Formation, the source rock deposition model is the deposition model of deep water temperature stratification in the separated lake basin. To determine the main controlling factors and depositional patterns of source rock development provides a direction for subsequent exploration of favorable areas.

The influence of mantle flow on intracontinental basins: Three examples from Australia

AbstractDuring the Paleozoic, sedimentary basins developed within Gondwana without evolving to diverging plate boundaries. Such intracontinental basins present long subsidence histories with multiple phases of accelerated subsidence that are not always easily explained by far‐field tectonic forces, and may be driven by processes other than rifting and thermal subsidence. Here we investigate the subsidence of Paleozoic Australian intracontinental basins by comparing one‐dimensional backstripped tectonic subsidence histories from the western Australian Canning and Southern Carnarvon Basins and the central Australian Cooper Basin to forward subsidence models for pure shear lithospheric thinning. We make the hypothesis that differences between observed and model subsidence may be explained by mantle‐flow driven topography, in addition to tectonic forces. To test this hypothesis, we compute dynamic topography from the first geodynamic models of mantle flow spanning the entire Phanerozoic Eon, and we analyse the relationship between dynamic topography and anomalous basin subsidence to dynamic topography and mantle flow. Although reconstructions of mantle flow in deep geological times are uncertain, our results suggest that long‐wavelength dynamic topography could explain aspects of the complex tectonic histories intracontinental basins. In the presented reconstruction of mantle flow, topographic rebound following the sinking of a Cambrian aged slab resulted in a minor phase of dynamic uplift in the Cooper Basin in middle Permian times. Throughout Carboniferous‐Triassic times Australia was positioned above a mantle upwelling driven by a hot structure at the base of the mantle. Structural uplift in the Canning and Southern Carnarvon basins during the Triassic‐Jurassic interval was augmented by dynamic uplift produced by that large‐scale upwelling, and possibly augmented by a focused active mantle plume during the Permo‐Triassic. In Late Jurassic‐Cretaceous times, Australia drifted east away from the mantle upwelling, resulting in a period of subsidence in the Canning and Southern Carnarvon basins. During the Cretaceous the Cooper Basin moved over a downwelling produced by long‐lived subduction along the east Australian margin, resulting in a period of accelerated subsidence.

Ediacaran-Paleozoic subsidence history of the Volyn-Podillya-Moldavia Basin (W and SW Ukraine, Moldova, NE Romania)

Tectonic subsidence history was analysed for the Ediacaran-Paleozoic Volyn-Podillya-Moldavia Basin, by performing 1D backstripping for 21 boreholes located in western and SW Ukraine, Moldova and NE Romania. Tectonic subsidence history is coherent across the area studied. Development of the basin commenced with an Ediacaran phase of extension, initiated with the emplacement of rift-related volcanic rocks and associated with rapid syn-rift tectonic subsidence. During this event, tectonic subsidence increased towards the SW, i.e. towards the edge of the East European Craton, where the Ediacaran rift zone was located. At that time, a rift developed along the whole SW margin of the East European Craton from Scandinavia to the Black Sea. Development of this large extensional basin was related to the latest stages of break-up of the Precambrian supercontinent Rodinia/Pannotia and ultimately the formation of the Tornquist Ocean. The latest Ediacaran to Late Ordovician tectonic subsidence pattern was characteristic of the post-rift thermal sag stage of extensional basins. The SW margin of the newly formed Baltica, including the area studied, became a passive continental margin. The late Cambrian uplift and erosion was presumably related to a far-field effect of contractional events or intra-plate stresses. Since the Late Ordovician, a gradual change to a collisional tectonic setting is observed across the SW margin of Baltica. In the study area, this is indicated by a systematic increase in subsidence rate from Wenlock to Early Devonian time, creating subsidence curves with convex shapes typical of foreland basin development. The Silurian to Early Devonian Volyn-Podillya-Moldavia Basin is interpreted here as a flexural foredeep related to a Caledonian collision zone located further to the SW. The prominent diachroneity in the initiation of the foredeep basin development at a scale of the whole SW margin of Baltica is coherent with a model of oblique collision of Avalonia and Baltica. During the Pragian-Emsian, the basin was part of a system of post-collisional Old Red basins, with subsidence driven presumably by lithospheric isostatic imbalance resulting from the Caledonian collision and development of an accretionary wedge. Middle to Late Devonian short-term phases of rapid subsidence in small depocentres might be regarded as an indication of a transtensional tectonic regime

Non-uniform subsidence and its control on the temporal-spatial evolution of the black shale of the Early Silurian Longmaxi Formation in the western Yangtze Block, South China

Abstract The Ordovician-Silurian transition was a significant period when an obvious variation occurred worldwide in terms of tectonics, climate, oceans, paleontology, and sedimentology. Here we use stratigraphic forward modeling constrained by paleobiota, paleobathymetry, sedimentology, and geochemistry to study the tectonic subsidence history of the Early Silurian in the western Yangtze Block, South China. Numerical modeling suggests that the tectonic subsidence was not uniform during the deposition of the Early Silurian black shale of the Longmaxi Formation, with acceleration of tectonic subsidence during the later regression period. Furthermore, the subsidence rate at this time was at least three times that during the early transgression period. This acceleration resulted from the collision between the Yangtze Block and the southeastern Cathaysia Block, which affected the tectonic subsidence and the heterogeneity of the black shale. The results may provide quantitative implications for the reconstruction of the paleoenvironment, the evolution of paleobiota, the temporal-spatial distribution of the black shale, and the mechanism of block assemblage in South China.

Tectono-thermal evolution of the Liwan Sag, deepwater area in the Zhujiang River Mouth Basin, northern South China Sea

The Liwan Sag, with an area of 4 000 km2, is one of the deepwater sags in the Zhujiang River (Pearl River) Mouth Basin, northern South China Sea. Inspired by the exploration success in oil and gas resources in the deepwater sags worldwide, we conducted the thermal modeling to investigate the tectono-thermal history of the Liwan Sag, which has been widely thought to be important to understand tectonic activities as well as hydrocarbon potential of a basin. Using the multi-stage finite stretching model, the tectonic subsidence history and the thermal history have been obtained for 12 artificial wells, which were constructed on basis of one seismic profile newly acquired in the study area. Two stages of rifting during the time periods of 49–33.9 Ma and 33.9–23 Ma can be recognized from the tectonic subsidence pattern, and there are two phases of heating processes corresponding to the rifting. The reconstructed average basal paleo-heat flow values at the end of the rifting events are ~70.5 and ~94.2 mW/m2 respectively. Following the heating periods, the study area has undergone a persistent thermal attenuation phase since 23 Ma and the basal heat flow cooled down to ~71.8–82.5 mW/m2 at present.

Neogene residual subsidence and its response to a sinking slab in the deep mantle of eastern China

Mantle convection could have a significant effect on basin evolution; however, research quantifying this relationship is controversial. To understand the formation mechanism and evolution of the Cenozoic rift basins in eastern China, we applied the back-stripping technique and strain rate inversion modeling to 119 wells from Sangjiang Basin, Bohai Bay Basin, North Yellow Sea Basin, South Yellow Sea Basin, East China Sea Shelf Basin, and four basins within the northern South China Sea margin. The modeled results can be used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of eastern China. Residual subsidence is defined as the difference between the theoretical and observed tectonic subsidences. Our results show that the residual subsidence since 20Ma in eastern China generally increases from ∼100 to 300m in terrestrial areas to ∼1.2–1.8km onthe continental shelf. Our observed residual subsidence is generally consistent with the present-day dynamic topography that is predicted from mantle flow models, and it is associated with a stagnant slab within the mantle that has been observed by seismic tomography. The migration pathway of the residual subsidence since 20Ma is consistent with the movement direction of the Pacific and Philippine plates. Therefore, we suggest that the residual subsidence might be a dynamic subsidence induced by a negative buoyancy of the sinking slab in the deep mantle beneath eastern China.

Tectonic subsidence of the Zhu 1 Sub-basin in the Pearl River Mouth Basin, northern South China Sea

The Pearl River Mouth Basin, which is situated on the northern margin of the South China Sea, has attracted great attention not only because of its tectonic setting but also because of its abundant hydrocarbon resources. We have analyzed the Cenozoic tectonic subsidence history of 4 drilled wells and 43 artificial wells from the Zhu 1 Sub-basin of the Pearl River Mouth Basin by back-stripping, using newly interpreted seismic profiles. We also calculated the average tectonic subsidence rates of the four sags in the Zhu 1 Sub-basin. The rifting and post-rifting stages are separated by abrupt changes in the tectonic subsidence curves and average subsidence rates. In the eastern sags of the Zhu 1 Sub-basin, tectonic subsidence started to slow at ca. 30 Ma, compared with ca. 23.8 Ma in the western sags. This probably corresponds to the timing of break-up and suggests that rifting in the Pearl River Mouth Basin ended earlier in the eastern sags than in the western sags. Anomalously accelerated tectonic subsidence occurred at 17.5–16.4 Ma during the post-rifting stage, with average subsidence rates as high as 301.9 m/Myr. This distinguishes the Pearl River Mouth Basin from classical Atlantic passive continental marginal basins, which demonstrate exponentially decaying post-rift tectonic subsidence.

Present-Day Heat Flow, Thermal History, and Tectonic Subsidenceof the Jianghan Basin

Thermal gradient distribution of the Jianghan Basin is obtained based on systematic steady-state and oil-test temperature data. The present geothermal gradient and terrestrial heat flow of 18 wells are calculated in the Jianghan Basin. A total of 9 measured terrestrial heat flow values are reported based on detailed thermal conductivity data and systematic steady-state temperature data. These values vary from 41.9 to 60.9 mW/m2, with a mean value of 52.3 ± 6.3 mW/m2. However, thermal history analyses, derived from vitrinite reflectance (RO) and apatite fission track (AFT) data, indicate that the Jianghan Basin was at a stable stage before the Indosinian movement. The heat flow increased and reached its maximum value of ∼68 mW/m2 during the Late Indosinian to Early Yanshan period. Since the Late Himalayan period, rapidly decreasing basement heat flow made the Jianghan Basin cool down. Furthermore, geohistory analysis reveals that the tectonic subsidence of the Jianghan Basin from the Cretaceous to Early Miocene was characterized by initial synrift subsidence followed by thermal subsidence. The thermal and tectonic subsidence history have directly effects on petroleum source rock maturation, so their analyses are of great significance forpetroleum exploration and hydrocarbon source assessment in the Jianghan Basin.

Cretaceous anomalous subsidence and its response to dynamic topography in the Songliao Basin, Northeast China

To understand the formation mechanisms and evolution of the Songliao Basin, we applied backstripping and strain rate inversion methods to 20 wells and reinterpret two seismic sections in the Songliao Basin. The obtained data were used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of this area. The predicted post-rift subsidence based on the uniform stretching model that followed earlier lithospheric thinning events was much lower than the subsidence provided by backstripping. The anomalous subsidence during the post-rift stage was between 200m and 800m in the Songliao Basin, and the depocenter of anomalous subsidence moved eastward between ca. 110 and 80Ma before returning to the west between ca. 80 and 65Ma. Regional variations in the subsidence rates suggest a possible deficit in the negative buoyancy (mantle loading) that is induced by the sinking slab beneath the Songliao Basin. In addition, the anomalous post-rift subsidence was primarily driven by downward drag pressure from the subducting western Izanagi slab and asthenospheric mantle flow beneath the Songliao Basin. The basement rifting in the Songliao Basin during the early Cretaceous and the normal faulting that occurred during the post-rift stage resulted in a secondary contribution to the anomalous post-rift subsidence. Unlike the Songliao Basin, several early Cretaceous rift basins south of the Yanshan Belt were only filled during the syn-rift subsidence, and the anomalous post-rift uplift of the Yanshan belt was driven by deep mantle flow.

Discussion on Several Problems in Tectono‐Thermal Modeling of Rift Basins

AbstractTectono‐thermal modeling of rift basin is to calculate the thermal and subsidence history of a rift basin during its formation and evolution on a lithosphere scale, which links tectonic and thermal developments perfectly. However, most extension models have the following problems: assuming the lithosphere to be 125 km thick initially, ignoring heat convection in the asthenosphere, fixing the upper boundary and rarely including thermal effects of sediments when calculating rift flank uplifts. Some effects on the tectono‐thermal modeling are discussed, including the initial crustal and lithosphere thicknesses, the convection of asthenosphere and the upper boundary of the model. We also calculate the differences of rift flank uplift between water and sediment loaded models.The tectono‐thermal modeling is based on a two‐dimensional non‐instantaneous extension model using the finite‐element method in the Lagrangian system. By solving the heat conduction equation, the thermal and tectonic subsidence histories of the basin are modeled simultaneously.The results are as follows: (1) Initial crustal and lithospheric thicknesses have great influence on tectonothermal modeling, especially on the tectonic subsidence. (2) Compared to the model ignoring heat convection in the asthenosphere, this model predicts slower tectonic subsidence and thermal decay rates, and significantly higher temperature from the lower crust to the bottom of the model. (3) The fixed or moving upper boundary during the modeling influences the calculated heat flow and tectonic subsidence in the thermal subsidence phase, and has remarkable effects on the temperature field. The effect increases with the increasing of stretching factor. (4) Rift flank uplift of the water‐loaded model would disappear within millions of years while the uplift would be stable at a certain value for the sediment‐loaded model.In cases that the initial crustal and lithospheric thicknesses are uncertain, the optimum initial thicknesses for crust and lithosphere can be determined when the calculated heat flow, Moho depth and lithospheric thickness fit observed values best. Pure heat conduction and pure heat convection in the asthenosphere are two limiting cases for extension models. Within the rheological boundary layer, heat is transferred by both conduction and convection. For strongly extensional areas, using the fixed upper boundary model would generate large calculation errors, especially in the temperature field. The thermal effect of sediment cannot be ignored when calculating the rift flank uplift.

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.

Constraining the age and origin of the seamount province in the Northeast Indian Ocean using geophysical techniques

The breakup of western margin of Australia from Greater India started around 155 Ma and progressed southwards. After the separation, the interceding intraplate region experienced large volumes of submarine volcanism, extending over 100 Myrs. The Christmas Island Seamount Province (CHRISP, as it has been dubbed) lies south of the Java-Sunda Trench, and contains numerous submerged volcanic seamounts, and two sub-aerially exposed island groups—Cocos (Keeling) Islands, and Christmas Island. While recent geochronological investigations have shed light on the diverse eruption ages of the volcanics of this region, some islands/seamounts have demonstrated protracted volcanic histories, and it is not clear how the volcanic loading, tectonic subsidence, and subsequent emergence history of the islands relates to these discrete volcanic episodes. This study utilises a number of geophysical techniques to determine the crustal structure, loading and subsidence history, and last sub-aerial exposure age for the CHRISP. The study shows that flexural and subsidence modelling are reliable techniques in constraining the age of the seamounts when geochronological techniques are not possible. Utilising regional gravity signatures, we model the crustal structure underneath the Cocos (Keeling) Island, and constrain the thickness of the limestone cover between 900 and 2,100 m. Using age-depth subsidence curves for oceanic lithosphere the time since these seamounts were exposed above sea-level was determined, and a trend in exposure ages that youngs towards the west is observed. Two episodes of volcanism have been recorded at Christmas Island and they are of different origin. The younger phase in the Pliocene is a manifestation of flexure induced cracks produced in the lithosphere as it rides the subduction fore-bulge, whereas a low velocity seismic zone rising from the lower mantle, and tectonic reorganization, may be associated with the older Eocene volcanic phase, as well as much of the rest of the province. Our modelling also supports the existence of an older, undated volcanic core to Christmas Island, based on the loading ages from flexural modelling.

Stretching factor estimation for the long-duration and multi-stage continental extensional tectonics: Application to the Baiyun Sag in the northern margin of the South China Sea

Conventional subsidence analysis methods are generally based on the assumption of instantaneous stretching, which totally ignores the heat loss during continental rifting. For long-duration or multi-episodic extensional tectonics, such assumption is no longer valid and could cause significant deviation from real subsidence. Here I extend original finite stretching model to a multi-stage scenario in terms of geothermal and structural successions between two adjacent rifting events. A procedure is designed to estimate stretching factor by fitting the whole tectonic subsidence history in a self-consistent manner on the basis of the extended model. I apply this procedure to the Baiyun Sag in the continent slope of the South China Sea. Subsidence analysis shows that the sag is characterized by two episodes of rifting. The first stage (49 to 23.8 Ma) resulted in stretching factors in the range of 1.3 to 4.0 with the greatest stretching at the sag center, while the second one (17.5 to 10.5 Ma) yielded minor stretching factors of 1.1 to 1.2. These estimates agree well with that determined from crustal thinning for the northern and central potions of the sag, but significant discrepancy occurs between the southern edge of the sag and the present continent–ocean boundary, where the crustal thickness is less than 20 km and potentially extensive magmatic intrusion was well imaged in the seismic profiles. I suggest that the magma-assisted rheologically weakening and possible lower crust ductile flow contributed to the extension discrepancy. The margin north of the Baiyun Sag, having a relatively thick crust and vacant of magmatic activity, was basically stretched in a depth-uniform style. It is inferred that preexisting weakness in the lithosphere and abundant sediment supply were responsible for the extremely thick Cenozoic sediment developed in the Baiyun Sag.

Subsidence history, crustal structure, and evolution of the Somaliland‐Yemen conjugate margin

We have used biostratigraphic data from deep exploration wells to determine the tectonic subsidence history of the Somaliland (northwestern Somalia)‐Yemen conjugate margin, a poorly known margin in the central part of the Gulf of Aden. Bathymetry and magnetic anomaly data suggest the Gulf of Aden is a young feature that formed following the rifting apart and breakup of the African and Arabian plates ~32 Ma. Our tectonic subsidence data suggest, however, that the present‐day Gulf of Aden developed on an earlier Mesozoic rift system. The oldest episode of rifting initiated at ~156 Ma and lasted for ~10 Ma and had a NW‐SE trend. We interpret the rift as a late stage event associated with the breakup of Gondwana and the separation of Africa and Madagascar. At ~80 Ma, there is evidence of an intermediate rift event which correlates with a rapid increase in spreading rate on the ridges separating the African and Indian and African and Antarctica plates and a contemporaneous slowing down of Africa's plate motion. The combined effect of all three rifting events has been to thin the crust and upper mantle by up to a factor of 2. The amount of thinning deduced from the wells is in accord with the crustal structure inferred from available seismic refraction data and process‐oriented gravity and flexure modeling. The margin is asymmetric with a steeper gradient in the Moho on the Yemen side than the Somaliland side. The main discrepancy is on the Yemen side where the gravity‐derived Moho is 10 km deeper than the well‐derived Moho. We attribute the discrepancy to the addition of material at the base of the crust since rifting, possibly magma sourced from the Afar plume.

The Analysis on the Tectonic Subsidence of Linnan Sag Located in Huimin Sag during the Depositional Stage of Es<sub>4</sub>

On the basis of back-stripping analysis of 30 wells in Linnan sag，in this paper we calculate the tectonic subsidence as well as the total subsidence about the upper and lower sub-section of Es4 in Linnan sag, then we obtain the graph of the tectonic subsidence and analyze the tectonic subsidence history of Es4, and get a result of the basin’s geological structure moments strength and the tectonic geology, as well as the degree of development of the basin in order to know more about the tectonic history and the law of hydrocarbon accumulation in this area.

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.

Tectonic subsidence history and source-rock maturation in the Campos Basin, Brazil

The Campos Basin is a petroleum-productive, marginal sag basin along the Brazilian margin. It contains a stratigraphic sequence recording lithospheric extension and rift tectonics developing to a fully evolved post-break-up setting. We present a combined approach using subsidence analysis and basin history inversion models to make predictions on source-rock maturation. The classical uniform stretching model does not account for the observed tectonic subsidence , and so we consider the thermal and subsidence implications of incorporating different events of magmatic underplating. The post-rift is characterized by one major phase of post-rift thermal subsidence: changes in sediment supply and load, as well as halokinetic movements, cause deviations from normal thermal subsidence and relaxation. The dimensions of these events have been examined and quantified. Our best-fit forward model is tested against measured borehole temperature (BHT) data. Results show a systematic overestimation of present-day temperatures in the shallow-water wells. Including the effects of hydrothermal convection by raising the conductivity of the top part of the stratigraphic sequence results in an almost perfect fit between predicted and measured BHTs in the shallow-water wells. Based on these forward models, we predict that the lacustrine shales of the syn-rift Lagoa Feia Fm. could be mature to generate oil and/or gas over a larger area than previously expected. Furthermore, our results suggest that the early post-rift Macaé Fm. is locally mature to generate oil, and that the middle/late post-rift Carapebus/Ubatuba Fm. shales are likely to generate oil as well, if organic-rich facies are indeed present. In light of the most recent pre- and post-salt discoveries made near established oil fields in the Campos Basin, our results support the idea that many more such accumulations may be found.

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.