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1.
Geothermal exploration is typically limited to high-grade hydrothermal reservoirs that are usually found in the western United States, yet large areas with subsurface temperatures above 150°C at economic drilling depths can be found east of the Rocky Mountains. The object of this paper is to present new heat flow data and to evaluate the geothermal potential of Texas and adjacent areas. The new data show that, west of the Ouachita Thrust Belt, the heat flow values are lower than east of the fault zone. Basement heat flow values for the Palo Duro and Fort Worth Basins are below 50 mW/m2 while, in the frontal zone of the belt, they can exceed 60 mW/m2. Further east, along the Balcones fault system the heat flow is in general higher than 55 mW/m2. The eastern most heat flow sites are in Louisiana and they show very high heat flow (over 80 mW/m2), which is associated with the apparently highly radioactive basement of the Sabine uplift. The geothermal resource in this area is large and diverse, and can be divided in high grade (temperature above 150°C) convective systems, conductive based enhanced geothermal systems and geothermal/geopressured systems. One of the most attractive areas east of the cordillera extends from eastern Texas across Louisiana and Arkansas to western Mississippi. Here temperatures reach exploitation range at depths below 4 km, and tapping such a resource from shut in hydrocarbon fields is relatively easy. The initial costs of the development can be greatly reduced if existing hydrocarbon infrastructure is used, and therefore using shut-in hydrocarbon fields for geothermal purposes should not be neglected.  相似文献   

2.
Geothermal energy resources in North Dakota and South Dakota occur as low (T < 90°C) and intermediate (T < 150°C) temperature geothermal waters in regional-scale aquifers within the Williston and Kennedy Basins. The accessible resource base is approximately 21.25 exajoules (1018 J = 1 exajoule, 1018 J ~ 1015 Btu = 1 quad) in North Dakota and 12.25 exajoules in South Dakota. Resource temperatures range from 40°C at depths of about 700 m to 150°C at 4500 m in the Williston Basin in North Dakota. In South Dakota, resource temperatures range from 44°C at a depth of 550 m near Pierre to 100°C at a depth of 2500 m in the northwestern corner. This resource assessment raises the identified accessible resource base by 31% above the previous assessments and by 310% over an earlier assessment. The large increases in the identified accessible resource bases reported in this study result from including all potential geothermal aquifers and better understanding of the thermal regime of the region. These results imply that a reassessment of stratabound geothermal resources in the United States that includes all geothermal aquifers would increase significantly the identified accessible resource base. The Williston Basin in North Dakota is characterized by conductive heat flows ranging from 43 to 68 mW m–2 and averaging 55 mW m–2. Comparisons of calculated and bottomhole temperatures measured in oil fields over the Nesson Anticline and the Billings Nose show temperature differences which suggest that upward groundwater flow in fractures on the westward sides of the structures slightly perturbs the otherwise conductive thermal field. The maximum heat-flow disturbance is estimated to be of the order of 10 to 20 mW m–2. These thermal anomalies do not alter significantly the accessible geothermal resource base. Anomalous heat flow in south-central South Dakota is caused by heat advection in gravity-driven groundwater flow in regional aquifers. Heat flow is anomalously high (Q > 130 mW m–2) in the discharge area in south-central South Dakota and anomalously low (30 mW m2) in the recharge area near the Black Hills and along the western limb of the Kennedy Basin in western South Dakota. Heat-flow disturbances are the result of vertical groundwater flow through fractures in the discharge area of the regional flow system in South Dakota are minor and may be significant only in deeply incised stream valleys. An important factor that controls the temperature of the resource in both North Dakota and South Dakota is the insulating effect of a thick (500–2000 m) layer of low thermal-conductivity shales that overlie the region. The effective thermal conductivity of the shale layer is approximately 1.2 W m–1 K–1 in contrast to sandstones and carbonates, which have conductivities of 2.5 to 3.5 W m–1 K–1. This low conductivity leads to high geothermal gradients (dT/dz > 50°C km–1), even where heat flow has normal continental values, that is 40–60 mW m–2. Engineering studies show that geothermal space heating using even the lowest temperature geothermal aquifers (T 40 °C) in North Dakota and South Dakota is cost effective at present economic conditions. The Inyan Kara Formation of the Dakota Group (Cretaceous) is the preferred geothermal aquifer in terms of water quality and productivity. Total dissolved solids in the Inyan Kara Formation ranges from 3,000 to more than 20,000 mg L–1. Porosities normally are higher than 20%, and the optimum producing zones generally are thicker than 30 m. The estimated water productivity index of a productive well in the Inyan Kara Formation is 0.254179 l s–1 Mpa–1. Deeper formations have warmer waters, but, in general, are less permeable and have poorer water quality than the Inyan Kara.  相似文献   

3.
We performed a detailed analysis of the thermal state of the Cenozoic Roer Valley Graben, the north–western branch of the European Cenozoic Rift System, based on a new set of temperature data. We developed a numerical technique for correcting bottom hole temperatures, including an evaluation of the uncertainty of thermal parameters. Comparison with drill stem test temperatures indicated that the uncertainty in corrected bottom hole temperatures using a two‐component numerical model is approximately ± 4 °C, which is much more accurate than the up to 15 °C errors encountered in often‐used line‐source or Horner correction methods. The subsurface temperatures and the derived regional heat flow estimates of 53 ± 6 to 63 ± 6 mW m?2 show no significant difference between the central rift and the adjacent structural highs. The absence of an elevated heat flow is attributed to the low amount of lithospheric thinning during the Cenozoic rifting phase (β=1.06–1.15). A local thermal anomaly exceeding +10 °C was found in five wells in the north–western part of the rift basin at depths of 1000–1500 m, and is most likely caused by the upward flow of fluids along faults, whereas lower temperatures in the upper 1500 m in the southern part of the rift basin could indicate cooling by topography‐driven groundwater flow. Conflicting ideas exist on the active or passive rifting mechanisms responsible for the formation of the different rift basins of European Cenozoic Rift System. The low spatial variation in heat flow found in this study suggests that the mechanism responsible for forming the Roer Valley Graben is passive rifting.  相似文献   

4.
The stratigraphic, subsidence and structural history of Orphan Basin, offshore the island of Newfoundland, Canada, is described from well data and tied to a regional seismic grid. This large (400 by 400 km) rifted basin is part of the non‐volcanic rifted margin in the northwest Atlantic Ocean, which had a long and complex rift history spanning Middle Jurassic to Aptian time. The basin is underlain by variably thinned continental crust, locally <10‐km thick. Our work highlights the complex structure, with major upper crustal faults terminating in the mid‐crust, while lower crustal reflectivity suggests ductile flow, perhaps accommodating depth‐dependent extension. We describe three major stratigraphic horizons connected to breakup and the early post‐rift. An Aptian–Albian unconformity appears to mark the end of crustal rifting in the basin, and a second, more subdued Santonian unconformity was also noted atop basement highs and along the proximal margins of the basin. Only minor thermal subsidence occurred between development of these two horizons. The main phase of post‐rift subsidence was delayed until post‐Santonian time, with rapid subsidence culminating in the development of a major flooding surface in base Tertiary time. Conventional models of rifting events predict significant basin thermal subsidence immediately following continental lithospheric breakup. In the Orphan Basin, however, this subsidence was delayed for about 25–30 Myr and requires more thinning of the mantle lithosphere than the crust. Models of the subsidence history suggest that extreme thinning of the lithospheric mantle continued well into the post‐rift period. This is consistent with edge‐driven, small‐scale convective flow in the mantle, which may thin the lithosphere from below. A hot spot may also have been present below the region in Aptian–Albian time.  相似文献   

5.
Basin and petroleum systems are routinely modelled to provide qualitative and quantitative assessments of a hydrocarbon play. The importance of the rock thermal properties and heat flow density in thermal modelling the history of a basin are well-known, but little attention is paid to assumptions of the thermal conductivity, present-day heat flow density and thermal history of basins. Assumed values are often far from measured values when data are available to check parameters, and effective thermal conductivity models prescribed in many basin simulators require improvement. The reconstructed thermal history is often justified by a successful calibration to present-day temperature and vitrinite reflectance data. However, a successful calibration does not guarantee that the reconstruction history is correct. In this paper, we describe the pitfalls in setting the thermal conductivity and heat flow density in basin models and the typical uncertainties in these parameters, and we estimate the consequences by means of a one-dimensional model of the super-deep Tyumen SG-6 well area that benefits from large amounts of reliable input and calibration data. The results show that the entire approach to present-day heat flow evaluations needs to be reassessed. Unreliable heat flow density data along with a lack of measurements of rock thermal properties of cores can undermine the quality of basin and petroleum system modelling.  相似文献   

6.
The mechanism of lithospheric thinning of the North China Craton (NCC) remains controversial. To constrain the mechanism, this study investigated the upper-mantle deformation pattern of the craton by measuring shear wave splitting at the cratonic edge. The results, derived from data recorded at 47 stations, reveal a complex pattern of mantle deformation. Inside the eastern craton, the majority of fast direction trends SE–NW parallel to the tectonic extension direction accompanying with the lithospheric thinning. At the cratonic edge, 15 stations with only null splitting results indicate undetectable anisotropy beneath the stations. This may be due to upwelling or chaotic ascension of mantle flow. To the north, off the craton, large delay times and variation of splitting parameter with backazimuth are generated by the combination of lithospheric and asthenospheric anisotropy. Based on comparison of the splitting results and the predicted ones by the compelling models, it is likely that lithospheric delamination dominated the lithospheric thinning at the north edge of the NCC during the Mesozoic to Cenozoic.  相似文献   

7.
The attenuation of the continental crust during rifting and the subsequent filling of the rift‐related accommodation alter the long‐term thermal and mechanical state of the lithosphere. This is primarily because the Moho is shallowed due to density contrasts between the sediment fill and the crust, but also reflects the attenuation of the pre‐existing crustal heat production and its burial beneath the basin, as well the thermal properties of the basin fill. Moho shallowing and attenuation of pre‐existing heat production contribute to long‐term cooling of the Moho and thus lithospheric strengthening, as has been pointed out in many previous studies. In contrast, basin filling normally contributes to significant Moho heating allowing the possibility of long‐term lithospheric weakening, the magnitude of which is dependent on the thermal properties of the basin‐fill and the distribution of heat sources in the crust. This paper focuses on the thermal property structure of the crust and basin‐fill in effecting long‐term changes in lithospheric thermal regime, with particular emphasis on the distribution of heat producing elements in the crust. The parameter space appropriate to typical continental crust is explored using a formalism for the heat production distributions that makes no priori assumptions about the specific form of the distribution. The plausible parameter space allows a wide range in potential long‐term thermal responses. However, with the proviso that the accommodation created by the isostatic response to rifting is essentially filled, the long‐term thermal response to rift basin formation will generally increase average crustal thermal gradients beneath basins but cool the Moho due to its reduction in depth. The increase in the average crustal thermal gradient induces lateral heat flow that necessarily heats the Moho along basin margins, especially in narrow rift basins. Using coupled thermo‐mechanical models with temperature sensitive creep‐parameters, we show that such heating may be sufficient to localise subsequent deformation in the vicinity of major basin bounding structures, potentially explaining the offset observed in some stacked rift basin successions.  相似文献   

8.
A series of three‐dimensional models has been constructed for the structure of the crust and upper mantle over a large region spanning the NE Atlantic passive margin. These incorporate isostatic and flexural principles, together with gravity modelling and integration with seismic interpretations. An initial isostatic model was based on known bathymetric/topographic variations, an estimate of the thickness and density of the sedimentary cover, and upper mantle densities based on thermal modelling. The thickness of the crystalline crust in this model was adjusted to equalise the load at a compensation depth lying below the zone of lateral mantle density variations. Flexural backstripping was used to derive alternative models which tested the effect of varying the strength of the lithosphere during sediment loading. The models were analysed by comparing calculated and observed gravity fields and by calibrating the predicted geometries against independent (primarily seismic) evidence. Further models were generated in which the thickness of the sedimentary layer and the crystalline crust were modified in order to improve the fit to observed gravity anomalies. The potential effects of igneous underplating and variable upper mantle depletion were explored by a series of sensitivity trials. The results provide a new regional lithospheric framework for the margin and a means of setting more detailed, local investigations in their regional context. The flexural modelling suggests lateral variations in the strength of the lithosphere, with much of the margin being relatively weak but areas such as the Porcupine Basin and parts of the Rockall Basin having greater strength. Observed differences between the model Moho and seismic Moho along the continental margin can be interpreted in terms of underplating. A Moho discrepancy to the northwest of Scotland is ascribed to uplift caused by a region of upper mantle with anomalously low density, which may be associated with depletion or with a temperature anomaly.  相似文献   

9.
J.A. Nunn  G. Lin 《Basin Research》2002,14(2):129-145
ABSTRACT Sedimentary rocks rich in organic matter, such as coal and carbonaceous shales, are characterized by remarkably low thermal conductivities in the range of 0.2–1.0 W m?1 °C?1, lower by a factor of 2 or more than other common rock types. As a result of this natural insulating effect, temperature gradients in organic rich, fine‐grained sediments may become elevated even with a typical continental basal heat flow of 60 mW m?2. Underlying rocks will attain higher temperatures and higher thermal maturities than would otherwise occur. A two‐dimensional finite element model of fluid flow and heat transport has been used to study the insulating effect of low thermal conductivity carbonaceous sediments in an uplifted foreland basin. Topography‐driven recharge is assumed to be the major driving force for regional groundwater flow. Our model section cuts through the Arkoma Basin to Ozark Plateau and terminates near the Missouri River, west of St. Louis. Fluid inclusions, organic maturation, and fission track evidence show that large areas of upper Cambrian rocks in southern Missouri have experienced high temperatures (100–140 °C) at shallow depths (< 1.5 km). Low thermal conductivity sediments, such as coal and organic rich mudstone were deposited over the Arkoma Basin and Ozark Plateau, as well as most of the mid‐continent of North America, during the Late Palaeozoic. Much of these Late Palaeozoic sediments were subsequently removed by erosion. Our model results are consistent with high temperatures (100–130 °C) in the groundwater discharge region at shallow depths (< 1.5 km) even with a typical continental basal heat flow of 60 mW m?2. Higher heat energy retention in basin sediments and underlying basement rocks prior to basin‐scale fluid flow and higher rates of advective heat transport along basal aquifers owing to lower fluid viscosity (more efficient heat transport) contribute to higher temperatures in the discharge region. Thermal insulation by organic rich sediments which traps heat transported by upward fluid advection is the dominant mechanism for elevated temperatures in the discharge region. This suggests localized formation of ore deposits within a basin‐scale fluid flow system may be caused by the juxtaposition of upward fluid discharge with overlying areas of insulating organic rich sediments. The additional temperature increment contributed to underlying rocks by this insulating effect may help to explain anomalous thermal maturity of the Arkoma Basin and Ozark Plateau, reducing the need to call upon excessive burial or high basal heat flow (80–100 mW m?2) in the past. After subsequent uplift and erosion remove the insulating carbonaceous layer, the model slowly returns to a normal geothermal gradient of about 30 °C km?1.  相似文献   

10.
The McKenzie model proposed in 1978, which is widely used in calculating the thermal history of rift basins and other extensional basins, incorrectly assumes that all heat passing through the lithosphere originates below the lithosphere. In reality, heat from radiogenic sources within the lithosphere, especially in the upper crust, may represent more than half the heat flow at the top of basement. Thinning of the lithosphere during extension does indeed result in an increase of heat flowing from the asthenosphere, but this thinning also reduces the radiogenic heat from within the lithosphere. Because these two effects cancel to a large degree, the direct effects of lithospheric extension on heat flow at the top of basement are smaller than those predicted by the McKenzie model. Because of permanent loss of radiogenic material by lithospheric thinning, the heat flow at the top of basement long after rifting will be lower than the pre-rift heat flow.The McKenzie model predicts an instantaneous increase in heat flow during rifting. The Morgan model proposed in 1983, however, predicts a substantial time delay in the arrival of the higher heat flow from the asthenosphere at the top of basement or within sediments. Using the Morgan model, heat flow during the early stages of rifting will actually be lower than prior to rifting, because the time delay in the loss of radiogenic heat is less than the time delay in arrival of new heat from the asthenosphere.  相似文献   

11.
During the summer of 2010 ice concentration in the Eurasian Basin, Arctic Ocean was unusually low. This study examines the sea-ice reduction in the Eurasian Basin using ice-based autonomous buoy systems that collect temperature and salinity of seawater under the ice along the course of buoy drift. An array of GPS drifters was deployed with 10 miles radius around an ice-based profiler, enabling the quantitative discussion for mechanical ice divergence/convergence and its contribution to the sea-ice reduction. Oceanic heat fluxes to the ice estimated using buoy motion and mixed-layer (ML) temperature suggest significant spatial difference between fluxes under first-year and multi-year ice. In the former, the ML temperature reached 0.6 K above freezing temperature, providing >60–70 W m?2 of heat flux to the overlying ice, equivalent to about 1.5 m of ice melt over three months. In contrast, the multiyear ice region indicates nearly 40 W m?2 at most and cumulatively produced 0.8 m ice melt. The ice concentration was found to be reduced in association with an extensive low pressure system that persisted over the central Eurasian Basin. SSM/I indicates that ice concentration was reduced by 30–40% while the low pressure persisted. The low ice concentration persisted for 30 days even after the low dissipated. It appears that the wind-forced ice divergence led to enhanced absorption of incident solar energy in the expanded areas of open water and thus to increased ice melt.  相似文献   

12.
A two-layer lithospheric stretching model that includes the effects of decompression melting was used to estimate the deformation and thermal evolution of the Queen Charlotte Basin, British Columbia. The basin contains up to 6 km of Tertiary fill and is postulated to have been formed during a transtensional stage of Cenozoic plate motion between the Pacific and North American plates. Several models of basin formation have been proposed to explain the sediment distribution, contemporaneous volcanism and high present-day heat flow. We used bathymetry, Tertiary sediment thickness and crustal thickness to calculate the amount of stretching in the crust and lower lithosphere, and the volume of melt generated during advection of mantle rocks. A second set of calculations traced the thermal evolution of the sediments and lithosphere, and we show maps of estimated present-day heat flow and sediment maturity. This study differs significantly from previous work in the use of gridded data that provide coverage over a large region and permit lateral variations in lithospheric deformation and thermal properties to be clearly defined, a difficult quest in studies based on single-point or profile data. In addition, the use of crustal thickness, derived from a regional interpretation of gravity data and constrained by seismic refraction results, as an input allows reliable estimates of extension to be made despite recent deformation of sedimentary strata in Hecate Strait. We present results for a model which used a prerift crustal thickness of ≈34 km and a short rifting period from 25 to 20 Ma. This model infers that significant thinning occurred beneath south-western Hecate Strait and southern Queen Charlotte Sound, and several kilometres of igneous crust were added at these sites, without requiring elevated asthenospheric temperatures prior to extension. Net lithospheric extension is surprisingly uniform within the basin and averages 76%, or ≈50 km, across the margin. This amount is consistent with other estimates of extension and may provide information useful in refining models of plate motion along this margin.  相似文献   

13.
[Correction added after online publication 3 August 2010 ‐ ‘prelate’ has been changed to ‘pre‐late’ throughout the text]. Using apatite fission track and (U‐Th‐Sm)/He thermochronology, we report the low‐temperature thermal history of the Mesozoic Micang Shan Foreland Basin system, central China. This system, comprising the Hannan Dome hinterland, the northern Sichuan Foreland Basin and the intermediate frontal thrust belt (FB), shares a common boundary with three major tectonic terrains – Mesozoic Qinling‐Dabie Orogen, Mesozoic Sichuan Foreland Basin and Cenozoic elevated Tibetan Plateau. Results show: (1) a relatively rapid pre‐late Cretaceous cooling episode in the Hannan Dome; (2) a mid‐Cenozoic cooling phase (ca. 50°C at ca. 30 ± 5 Ma) within the northern Sichuan Basin; and (3) possible late Cenozoic cooling (ca. 25°C at ca. 16 ± 4 Ma) within the Hannan Dome‐FB, a phase which has also been reported previously from adjacent regions. The pre‐late Cretaceous cooling episode in the Hannan Dome is attributed to coeval tectonism in nearby regions. Mid‐Cenozoic cooling in the northern Sichuan Basin can possibly be attributed to either one of or a combination of shortening of the basin, onset of the Asian monsoon and drainage adjustment of the Yangtze River system, all of which are related to growth of the Tibetan Plateau. Possible late Cenozoic cooling in the hinterland and nearby regions is also probably related to the northeastward growth of the Tibetan Plateau. However, previous studies suggest a northeastward propagation for onset of cooling from the eastern Tibetan Plateau to western Qinling in response to northeastward lower crust flow from the central Tibetan Plateau. The timing of apparent late Cenozoic cooling in the Hannan Dome hinterland, at an intermediate locality, is not consistent with this trend, and supports a previous model suggesting northeastern growth of the Tibetan Plateau through reactivation of WE trending strike‐slip faults.  相似文献   

14.
Zones of anomalously high topography within continental interiors, distant from active plate boundaries, are interpreted as being either dynamically supported by viscous flow in the underlying mantle or influenced by plate tectonics. Constraining the models of their genesis requires accurate data on the timing and dimensions of such features. New apatite fission‐track and thermal maturity data from the Illizi Basin in Algeria quantify the magnitude and timing of kilometre–scale uplift and exhumation of the northern flank of the Hoggar swell in North Africa. The findings of this study, integrated with previously published thermochronological data, confirm that long‐wavelength regional uplift occurred during the Cenozoic extending over a distance in excess of 1500 km from north to south. The uplift, centred on the Hoggar Massif, significantly impacted the flanking Illizi and Tim Mersoï basins. The combination of thermal history modelling and regional stratigraphic observations indicates that the onset of exhumation of the Illizi Basin likely occurred during the Eocene, broadly coincident with magmatism on the Hoggar Massif to the south and the onset of tectonic shortening in the Atlasic belt to the north.  相似文献   

15.
The Limón back‐arc basin belongs to the southern Central American arc‐trench system and is situated at the east coast of Costa Rica. The basin‐fill consists of Late Cretaceous to Pleistocene sedimentary rocks. A northern and a southern sub‐basin can be defined, separated by the E–W‐trending Trans Isthmic Fault System. The North Limón Basin is nearly undeformed, whereas the South Limón Basin is characterized by a fold‐and‐thrust belt. Both sub‐basins have a very similar sedimentary fill and can act as a natural laboratory for reconstructing controlling factors of arc‐related sedimentary basins as well as the influence of deformation on a basin system. Modelling focused on burial history and temperature evolution. Two‐dimensional simulations were carried out with the software PetroMod®. The geohistory curve of the North Limón Basin is overall linear, indicating continuous subsidence. The South Limón Basin is also characterized by continuous subsidence, but rates strongly increased at the beginning of the Neogene. Despite a rapid Plio‐Pleistocene deformation of the fold‐and‐thrust belt, the present‐day temperature field is not disturbed in that area. The modelling results indicate a mean heat flow of 60 mW m?2 for the North Limón Basin and 41 mW m?2 for the South Limón Basin. These values are low compared with other back‐arc basins. The lower values are attributed to the following effects: (1) underlying basaltic crust, (2) the lack of an initial rift phase, (3) the low extension rates, (4) absence of volcanic activity and (5) insulation effects of a thick sediment pile. The reasons for the locally lower heat flow in the southern sub‐basin can be found in the low‐angle subduction of the Cocos Ridge. Owing to the low subduction angle, the cool fore‐arc mantle‐wedge below the island‐arc is pushed backwards increasing the cooled area.  相似文献   

16.
The southern South African continental margin documents a complex margin system that has undergone both continental rifting and transform processes in a manner that its present‐day architecture and geodynamic evolution can only be better understood through the application of a multidisciplinary and multi‐scale geo‐modelling procedure. In this study, we focus on the proximal section of the larger Bredasdorp sub‐basin (the westernmost of the five southern South African offshore Mesozoic sub‐basins), which is hereto referred as the Western Bredasdorp Basin. Integration of 1200 km of 2D seismic‐reflection profiles, well‐logs and cores yields a consistent 3D structural model of the Upper Jurassic‐Cenozoic sedimentary megasequence comprising six stratigraphic layers that represent the syn‐rift to post‐rift successions with geometric information and lithology‐depth‐dependent properties (porosities and densities). We subsequently applied a combined approach based on Airy's isostatic concept and 3D gravity modelling to predict the depth to the crust‐mantle boundary (Moho) as well as the density structure of the deep crust. The best‐fit 3D model with the measured gravity field is only achievable by considering a heterogeneous deep crustal domain, consisting of an uppermost less dense prerift meta‐sedimentary layer [ρ = 2600 kg m?3] with a series of structural domains. To reproduce the observed density variations for the Upper Cenomanian–Cenozoic sequence, our model predicts a cumulative eroded thickness of ca. 800–1200 m of Tertiary sediments, which may be related to the Late Miocene margin uplift. Analyses of the key features of the first crust‐scale 3D model of the basin, ranging from thickness distribution pattern, Moho shallowing trend, sub‐crustal thinning to shallow and deep crustal extensional regimes, suggest that basin initiation is typical of a mantle involvement deep‐seated pull‐apart setting that is associated with the development of the Agulhas‐Falkland dextral shear zone, and that the system is not in isostatic equilibrium at present day due to a mass excess in the eastern domain of the basin that may be linked to a compensating rise of the asthenospheric mantle during crustal extension. Further corroborating the strike‐slip setting is the variations of sedimentation rates through time. The estimated syn‐rift sedimentation rates are three to four times higher than the post‐rift sedimentation, thereby indicating that a rather fast and short‐lived subsidence during the syn‐rift phase is succeeded by a significantly poor passive margin development in the post‐rift phase. Moreover, the derived lithospheric stretching factors [β = 1.5–1.75] for the main basin axis do not conform to the weak post‐rift subsidence. This therefore suggests that a differential thinning of the crust and the mantle‐lithosphere typical for strike‐slip basins, rather than the classical uniform stretching model, may be applicable to the Western Bredasdorp Basin.  相似文献   

17.
Stratigraphic data from petroleum wells and seismic reflection analysis reveal two distinct episodes of subsidence in the southern New Caledonia Trough and deep‐water Taranaki Basin. Tectonic subsidence of ~2.5 km was related to Cretaceous rift faulting and post‐rift thermal subsidence, and ~1.5 km of anomalous passive tectonic subsidence occurred during Cenozoic time. Pure‐shear stretching by factors of up to 2 is estimated for the first phase of subsidence from the exponential decay of post‐rift subsidence. The second subsidence event occured ~40 Ma after rifting ceased, and was not associated with faulting in the upper crust. Eocene subsidence patterns indicate northward tilting of the basin, followed by rapid regional subsidence during the Oligocene and Early Miocene. The resulting basin is 300–500 km wide and over 2000 km long, includes part of Taranaki Basin, and is not easily explained by any classic model of lithosphere deformation or cooling. The spatial scale of the basin, paucity of Cenozoic crustal faulting, and magnitudes of subsidence suggest a regional process that acted from below, probably originating within the upper mantle. This process was likely associated with inception of nearby Australia‐Pacific plate convergence, which ultimately formed the Tonga‐Kermadec subduction zone. Our study demonstrates that shallow‐water environments persisted for longer and their associated sedimentary sequences are hence thicker than would be predicted by any rift basin model that produces such large values of subsidence and an equivalent water depth. We suggest that convective processes within the upper mantle can influence the sedimentary facies distribution and thermal architecture of deep‐water basins, and that not all deep‐water basins are simply the evolved products of the same processes that produce shallow‐water sedimentary basins. This may be particularly true during the inception of subduction zones, and we suggest the term ‘prearc’ basin to describe this tectonic setting.  相似文献   

18.
Summary. COCORP seismic reflection traverses of the U.S. Cordillera at 40°N and 48.5°N latitude reveal some fundamental similarities as well as significant differences in reflection patterns. On both traverses, autochthonous crust beneath thin-skinned thrust belts of the eastern part of the Cordillera is unreflective; immediately to the west the Cordilleran interior is very reflective above a flat, prominent reflection Moho. Mesozoic accreted terranes in the western part of the orogen are underlain on both traverses by very complex reflection patterns, in constrast to more easily deciphered patterns beneath areas of Cenozoic accretion. The prominent reflection Moho beneath the orogenic interior on both transects probably evolved through a combination of magmatic and deformational processes during Cenozoic extension. The main differences between the two traverses lie in the reflection patterns of the middle and lower crust in the Cordilleran interior; these differences are probably related to the way Cenozoic extension was accommodated at depth. Laminated middle and lower crust above the reflection Moho in the western Basin and Range (40°N) may be related to magmatism, ductile pure shear and large-scale transposition during Cenozoic extension. By contrast, beneath the eastern Basin and Range (40°N), and the orogenic interior in the NW United States (48.5°N), Cenozoic extension was probably accommodated along dipping deformation zones throughout the crust.  相似文献   

19.
Significant lateral variations in observed temperatures in the Beaufort‐Mackenzie Basin raise the question on the temperature‐controlling factors. Based on the structural configuration of the sediments and underlying crust in the area, we calculate the steady‐state 3D conductive thermal field. Integrated data include the base of the relic permafrost layer representing the 0 °C‐isotherm, public‐domain temperature data (from 227 wells) and thermal conductivity data. For >75% of the wells the predicted temperatures deviate by <10 K from the observed temperatures, which validates the overall model setup and adopted thermal properties. One important trend reproduced by the model is a decrease in temperatures from the western to the eastern basin. While in the west, a maximum temperature of 185 °C is reached at 5000 m below sea level, in the east the maximum temperature is 138 °C. The main cause for this pattern lies in lateral variations in thermal conductivity indicating differences in the shale and sand contents of the different juxtaposed sedimentary units. North‐to‐south temperature trends reveal the superposition of deep and shallow effects. At the southern margin, where the insulating effect of the low‐conductive sediments is missing, temperatures are lowest. Farther north, where the sub‐sedimentary continental crust is thick enough to produce considerable heat and a thick pile of sediments efficiently stores heat, temperatures tend to be highest. Temperatures decrease again towards the northernmost distal parts of the basin, where thinned continental and oceanic crust produce less radiogenic heat. Wells with larger deviations of the purely conductive model from the temperature observations (>15 K at 10% of the wells) and their basin‐wide pattern of misfit tendency (too cold vs. too warm temperature predictions) point to a locally restricted coupling of heat transport to groundwater flow.  相似文献   

20.
A Bouguer gravity anomaly map of the NW Himalayas and parts of the Kohistan/Hindukush region has been prepared using all available gravity data. Analysis of the gravity field has been carried out along a profile extending from Gujranwala (located near the edge of the Indian shield) to the Haramosh massif in a NNE–SSW direction. The gravity profile is located close to the DSS profile shot under the USSR–India scientific collaborative programme. Velocity information available along different parts of the profile has been used to infer values of crustal and upper mantle density.
The observed gravity field (Bouguer) has been interpreted in terms of Moho depth and density contrast between the crust and the mantle. The Moho depth is interpreted as increasing from nearly 35 km near the edge of the Indian shield to 75 km (below sea-level) underneath the Haramosh massif. A similar model is applicable to a profile passing to the west of Nanga Parbat massif, from Gujranwala to Ghizar, through the Kohistan region. However, along this profile high-density lower-crustal rocks appear to have been emplaced in the upper part along the main mantle thrust. The nature of isostatic compensation prevailing underneath the Himalayas has been discussed, as has the theory of lithospheric flexure proposed by Karner & Watts and Lyon-Caen & Molnar. It is felt that although these ideas explain the broad features of the Moho configuration as observed in the NW Himalayas, there are significant departures. The role of tectonic forces in shaping the Moho and causing changes in the density of the crust cannot be denied.  相似文献   

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