Subsidence Curves Research Articles

Seismic Stratigraphy and Stratigraphic Modelling of the South-Eastern German Molasse Basin: ABSTRACT

Although the German Molasse Basin can be regarded as a mature hydrocarbon province, no regional sequence stratigraphic analysis has been carried out so far. We have studied seismic lines and well data from the region between the Isar and Inn rivers (SE Germany) that have been generously supplied by German oil companies (DEE, BEB, Mobil, RWE-DEA and Wintershall). Initial work indicates that five major seismic sequences within three main depositional cycles are developed. The Alpine thrust belt to the south serves as the primary sediment source in the foreland basin. However, sedimentary infill mainly took place parallel to the basin axis. Our analysis suggests that the stratigraphic development of the Molasse Basin was mainly controlled by eustatic sea-level changes which caused the shoreline to shift in the W-E direction. The shifting of the depocenter axis in a N-S direction was controlled by the tectonic evolution of the thrust belt. The sea-level curve determined by seismic stratigraphy and well-derived subsidence curves have been used as input parameters for stratigraphic modelling, using the programs GeoMOD and PHIL. Basin-fill simulations with PHIL are in good agreement with the main features of the sequence stratigraphy and the distribution of the systems tracts observed inmore » the study area. The qualitative sea-level curve for the German Molasse Basin derived from the seismostratigraphic analysis could be quantified by the stratigraphic exercises.« less

Evolution of the Cauvery basin, India from subsidence modelling

Backstripping and sediment unloading have been carried out on stratigraphic data from wells in the various sub-basins of Cauvery Basin to obtain tectonic subsidence curves. Analysis of the tectonic subsidence curves reveals that cessation of rifting/extension occurred earlier on the western side and rifting persisted for a longer time on the eastern side. Subsidence was more or less continuous through most of Cretaceous time. A non-uniform (or two-layer) extension model is used to determine extension parameters from the tectonic subsidence curves. The values of the crustal (β) and subcrustal (δ) stretching parameters obtained in the various sub-basins show the trends of evolution of the different sub-basins. The analysis suggests that the tectonic subsidence curves conform more to those in an intracratonic basin than those on a passive margin. Geological evidence corroborating this hypothesis are presented, necessitating the classification of Cauvery Basin as an intracratonic failed rift basin as opposed to the earlier classification as a pericratonic basin.

INFLUENCE OF PLATE TECTONIC RE-ORGANISATIONS AND TECTONIC SUBSIDENCE ON THE MESOZOIC STRATIGRAPHY OF NORTHWESTERN AND SOUTHEASTERN AUSTRALIA: IMPLICATIONS FOR SEQUENCE STRATIGRAPHIC ANALYSIS

Determining and predicting the interplay of plate tectonic events, subsidence, flexure and depositional systems is important in frontier exploration, play concept development, and maturation modelling. A circum-Australian plate tectonic model is here tied to an internally consistent global plate tectonic model to determine the timing and orientation of changes in the lithospheric stress regime induced by plate tectonic changes. One-and three-dimensional geohistory results for the Otway Basin and North West Shelf/Exmouth Plateau are presented, based on an integrated sequence stratigraphic framework. These geohistory results compare the timing and types of changes in subsidence rate to the changes in lithospheric stress due to plate tectonic changes. Changes in tectonic subsidence rate appear to be discrete events related to plate tectonic changes; subsidence events bound major transgressive-regressive facies cycle packages ('supersequences') in a basin. The recognition of sequence system tracts and especially system tract boundaries within a 'supersequence' is enhanced or diminished by processes occurring only during certain phases of the supersequences. Recognition of lowstand systems tracts and sequence boundaries is improved due to erosion during the regressive phase of the supersequence. Conversely, during the transgressive phase of the supersequence, transgressive and highstand system tracts are emphasised and recognition of flooding surfaces improved. Good reservoir sands form during enhanced lowstands, while good source and sealing shales form during enhanced transgressions.In the southeastern Australian Otway Basin, every perturbation of the tectonic subsidence rate during the Late Cretaceous can be correlated directly to a local and/or global plate tectonic event, and each supersequence is bounded by tectonic events. In the North West Shelf/Exmouth Plateau region of Western Australia, the situation is complicated during the Berriasian by uncompensated f lexural load effects related to the rapid formation and filling of multiple Barrow Delta depocentres. Two supersequences correlate to tectonic events, while one supersequence is bounded by a f lexural subsidence event unrelated to regional or global plate tectonic events. Hence not all perturbations of the tectonic subsidence curve are related to tectonic events, and not all supersequences are bound by tectonic events. Without three-dimensional geohistory techniques, it is impossible to isolate the flexural load effects from the effects of plate tectonic events.

Establishing resolution limits for tectonic subsidence curves by forward basin modelling : Marine & Petroleum geology, 11(3), 1994, pp 400–411

Establishing resolution limits for tectonic subsidence curves by forward basin modelling : Marine & Petroleum geology, 11(3), 1994, pp 400–411

BSAS: A basic program for two-dimensional subsidence analysis in sedimentary basins

Subsidence analysis, including geohistory analysis and backstripping, now is a usual approach in basin analysis to investigate quantitatively the basin formation and evolution. Its results are used widely in basin simulation as framework for other reconstructions such as stratigraphic and thermal modeling. These analyses may have been restricted to individual wells from which the necessary data are available. This restriction limits the two- and three-dimensional basin analysis and modeling. A BASIC computer program is presented for handling this problem. The program, BSAS, uses a key well as a control point to obtain the basic data for subsidence analysis, and extrapolates the data set to a cross section throughout the entire basin. In this way, geohistory analysis and backstripping can be carried out at any site along the section in order to generate the corresponding tectonic and total subsidence curves. Furthermore, the two-dimensional tectonic and total subsidence profiles for selected geological time slices can be plotted.

Onset of thermal convection in fluids with temperature‐dependent viscosity: Application to the oceanic mantle

Heat flow measurements through old seafloor demonstrate that the oceanic lithosphere is heated from below away from hot spot tracks. We reevaluate the hypothesis of small‐scale convection beneath the lithosphere with laboratory experiments in fluids whose viscosity depends strongly on temperature. Rayleigh numbers were between 106 and 108 and viscosity contrasts were up to 106. A layer of fluid was impulsively cooled from above, and a cold boundary layer grew at the top of the fluid layer. After a finite time, convective instabilities developed in the lowermost part of the boundary layer, while the upper part remained stagnant. The variation of surface heat flow as a function of time reflects the three‐dimensional nature of the flow and the presence of a thick lid. At viscosity contrasts greater than 103, this variation is very similar to what is observed on the oceanic lithosphere. For small times, heat flow follows the behavior of a half‐space cooled from above by conduction. Some time after the onset of convection, it deviates from the conductive evolution and settles to a value which seems almost constant over a length of time equal to a few multiples of the onset time. The occurrence of small‐scale convection is difficult to detect in global data sets of seafloor depths. The onset of convection is marked by a small “trough” in the local subsidence curve but does not occur at the same time everywhere because of the probabilistic nature of the instability process. Later instabilities occur independently of each other and, at any given age, involve a region of small horizontal extent below a thick lid. The characteristics of the instability depend on the function describing the variation of viscosity with temperature. Scaling laws are derived for the onset time and for the surface heat flow. The requirement that small‐scale convection supplies 45 mW m−2 to the oceanic lithosphere provides a relationship between the activation enthalpy for creep and the asthenosphere viscosity. For a range of activation enthalpy of 250 to 600 kJ mol−1, the asthenosphere viscosity must be between 3×1018 and 4×1017 Pa s. The thickness of the stagnant lid and the temperature difference driving small‐scale convection are predicted to be about 80 km and 200°C, respectively.

Flexural and stratigraphic development of the west Indian continental margin

Subsidence curves for 27 wells from the western continental margin of India show a characteristic late Oligocene to early Miocene (∼24±5 Ma) rapid increase in subsidence rate superposed on the long‐lived, slow subsidence typical of the thermal subsidence phase of passive continental margins. By subtracting a best fit negative exponential subsidence from the observed subsidence curves, we obtain an estimate of the distribution and magnitude of the “excess” subsidence affecting the Neogene development of the west Indian margin. The magnitude of this excess subsidence increases seaward from the coast, ranging from a few meters to >2000 m near the shelf edge. We examine the following hypotheses to explain the distribution and timing of this excess subsidence: (1) modification of basin stratigraphy due to the effects of lithospheric in‐plane compression, (2) creation of accommodation space on the margin by flexural effects associated with Indus fan loading, and (3) rapid growth of the continental margin and associated flexural effects. Of the three hypotheses tested, the least important mechanism to account for the observed excess subsidence is that of variations of lithospheric in‐plane force, principally because maximum in‐plane compression within the Indo‐Australian plate was only achieved in the late Miocene. Because Indus fan sediment deposition began in the late Oligocene to early Miocene, we investigated three‐dimensional flexural effects associated with fan loading as a cause of the excess subsidence beginning at ∼24 Ma. The distribution and magnitude of modeled flexural deflection, however, are not consistent with the observed excess subsidence. Interpretation of seismic reflection data indicates that the margin has aggraded and prograded by ≈100 km basin ward since the Oligocene. Therefore, we evaluate the flexural effects of this margin growth by estimating the amount of space infilled by margin progradation and aggradation since 24 Ma and computing the resulting deflection. This deflection matches the distribution and magnitude of observed excess subsidence along the margin. In addition, the distribution of the flexural bulge predicted from the combined deflections due to Indus fan and margin loading is spatially coincident with the distribution of exposed marine terraces and drainage divides in the Saurastra Peninsula and the regions surrounding the gulfs of Cambay and Kutch, respectively. Available gravity, seismic reflection, refraction, and well data are consistent with our prediction of a 4000 to 5000 m thick sediment load developed during the Neogene along the outer margin. We propose that flexural deformation due to sedimentary loading provides a potential tectonic feedback mechanism that affects coastal and fluvial depositional processes. As regions in close proximity to the load are depressed, regions farther from the load experience uplift (i.e., the peripheral bulge), which is sufficient to cause subaerial exposure of large portions of the shelf and to modify existing drainage networks. This feedback represents a mechanism for inducing relative sea level changes without invoking glacial eustasy.

Spatial variations in tectonic subsidence during Tippecanoe I in the Michigan Basin

AbstractEpisodic tectonic subsidence can be seen from all backstripped wells from the Michigan Basin. These episodes are observed as periods of relatively rapid subsidence, contrasting with the relatively linear subsidence. Steady tectonic subsidence curves have been explained as being due to a decaying thermal anomaly in the underlying uplifted asthenosphere. Rapid episodes of subsidence require another explanation and prompt us to examine the tectonic history of surrounding areas for correlative events.The Taconic orogeny has been inferred to have influenced the development of the Michigian Basin. To analyse the extent of coupling between the Appalachian foredeep and the Michigan Basin we have used Ordovician formation tops correlated among 172 wells distributed over the whole of Michigan's Lower Peninsula. The spatial variation of tectonic subsidence over this interval supports this suggestion.Contour maps of decompacted sediment thickness and subsidence rate show substantial changes in form and magnitude during the development of the Tippecanoe I sequence. Early basin‐centred subsidence, during deposition of the Shakopee, St. Peter and Glenwood formations, was followed by tilting toward the Appalachian orogen concurrent with the early stages of the Taconic orogeny. This tilting continued, declining gradually, during deposition of the Black River, Trenton and Utica formations. Deposition during Tippecanoe I in the Michigan Basin may indicate modulation of continuing thermally driven subsidence by flexure of the lithosphere due to Taconic thrust loading, but the distinctive spatial distribution of subsidence over each interval may indicate that the important mechanisms making space available in the basin operated independently, not concurrently, invalidating the flexural model for the coupling and suggesting other regional, tectonic possibilities for models of basin initiation, evolution and filling.

Establishing resolution limits for tectonic subsidence curves by forward basin modelling

A forward model was used to study the relationship between gravitational compaction and tectonic subsidence in an idealized, one-dimensional sedimentary basin. Gravitational compaction was modelled by a non-linear, partial differential equation that gives the sediment porosity as a function of time and burial depth. The sediments were described in terms of soil mechanics laws for mechanical compressibility and permeability. The sedimentation rate appeared as a boundary condition at the depositional surface. Tectonic subsidence was coupled to the compaction model via an isostatic balance based on Airy's hypothesis. Parameter studies showed that changes in the sedimentation rate and in the sediment compressibility can lead to variations from 454 to 781 m in the subsidence for sediment thicknesses in the range 6.9–7.9 km. The forward model was used to assess how well standard backstripping methods reconstructed subsidence history from present day porosity versus depth data. Standard backstripping can bias the tectonic subsidence by 100–200 m, comparable with inferred eustatic variations. This bias results because the porosity depends not only on burial depth but also on time. By using deterministic models to describe gravitational compaction, mechanical properties can be used to better constrain compaction history, thus improving the accuracy of reconstructed tectonic subsidence curves.

Tectonic modelling in the Bjørnøya West Basin of the Western Barents Sea1

AbstractThe Bjøirnøya West Basin lies between latitudes 73° and 74°, longitudes 16°E and 18°E, contains at least 8 km of sediments deposited from the Late Jurassic, and is of considerable interest for hydrocarbon exploration. The Cenozoic extensional tectonics in the basin can be clearly seen from seismic data with normal faulting and from subsidence curves with rapid subsidence. The extension occurred during the Late Palaeocene with active extension lasting about 6 million years (m.y.) followed by thermal cooling. The tectonic subsidence within the study area shows a three‐phase development: phase 1, synrift (58–52 Ma (million years before the present day)), is characterized by rapid subsidence; phase 2, postrift (52–5 Ma), by slow subsidence with occasional uplift; and phase 3 (5–0 Ma), by rapid subsidence. An adaptive finite‐element model, with consideration of the radiogenic heat production in the lithosphere, has been used to model the subsidence and heat flow. The modelling of subsidence shows the β‐factor distribution varying from 1.9 to 3.5 with an average of 2.4 for the uniform lithospheric extension. The heat‐flow modelling predicts a rapid increase of heat flow during the Early Palaeocene. The maximum heat flow at about 52 Ma, which could be as much as 3.0 hfu (10−6 cal/cm2/s), was followed by a decrease in heat flow. A plate‐weakening model has been proposed to explain the rapid subsidence for the last 5 m.y. by flexure of the elastic lithosphere which is weakened by a decrease in elastic thickness caused by an increase of the temperature gradient in the lithosphere. The plate‐weakening model predicts a heat‐flow increase at 5 Ma of up to 2.0 hfu. Our study, using quantitative modelling of the tectonic subsidence, provides a partial (if not a full) understanding of the tectonic development and thermal evolution of the Bjønøya West Basin.

Magnetostratigraphic Chronology of Cretaceous-to-Eocene Thrust Belt Evolution, Central Utah, USA

The Axhandle basin in central Utah is a piggyback basin that formed above the Gunnison thrust system. The basin contains ~1 km of upper Cretaceous-to-Eocene strata of the North Horn and lower Flagstaff Limestone formations. A significantly more exact chronology, based on magnetic polarity stratigraphy, has been developed for the basin-filling strata. The new data provide a detailed temporal framework for the interpretation of the depositional and tectonic evolution of the basin during chrons 33-21 (~75-49 Ma). Tectonic subsidence curves and stratigraphie relations along the eastern basin margin define two episodes of motion on the Gunnison thrust system. The majority of tectonic shortening on the system occurred between 83-75 Ma and caused widespread non-deposition within the Axhandle basin. A second episode of motion, in detail comprising of four separate periods of backthrusting, occurred between 70-60 Ma. The Canyon Range, Pavant, and Nebo thrust sheets were emplaced 30-50 km to the west of the Axhandle basin between 97 and 74 Ma. The initial episode of motion on the Gunnison thrust system was thus partly coeval with the emplacement of these thrust sheets. Uplift and erosion above the San Rafael Swell, a basement flexure located ~100 km east of the Axhandle basin, occurred at the same time-between 70 and ~60 Ma-as the second episode of motion on the Gunnison thrust system. These age constraints clearly demonstrate coeval motion on multiple contractional structures. Episodes of thrust wedge reorganization with durations of 5-10 m.y. appear to be separated by quiescent periods of up to 5 m.y. Evolution of the thrust belt in NE Utah and Wyoming was remarkably similar to that in central Utah 300 km to their south. This similarity suggests that the tectonic stresses driving the thrust wedge were relatively uniform along strike.

Subsidence Curves and Modeling of Some Indonesian Tertiary Basins: ABSTRACT

Subsidence Curves and Modeling of Some Indonesian Tertiary Basins: ABSTRACT

Three-dimensional controls on subsidence of a foreland basin associated with a thrust-belt recess: Black Warrior basin, Alabama and Mississippi

Tectonic transport from different directions and convergence of separate thrust belts at structural recesses impart three-dimensional variations to the pattern of foreland-basin subsidence. Assuming approximately uniform flexural rigidity of the lithosphere, subsidence is directly controlled by the geometry and density of the separate thrust loads, and flexural deformations caused by the loads interfere within the recess. For example, the Black Warrior foreland basin, which is located in Mississippi and Alabama, within a recess in the Appalachian-Ouachita orogen, formed in response to two separate thrust loads: (1) the Ouachita accretionary prism along the southwest side of the basin, and (2) thrust-imbricated passive-margin rocks in the Appalachian thrust belt along the southeast side of the basin. The subsidence history differs along two profiles, each perpendicular to the Appalachian and Ouachita thrust fronts. Subsidence of the foreland basin began in middle Mississippian (Meramecian) time in response to northeast-directed translation of the Ouachita accretionary prism. Subsequently, northwest-directed Appalachian thrusting began in middle Early Pennsylvanian (Morrowan) time. Subsidence curves from the profile perpendicular to the Ouachita prism show a clear progression of increasing subsidence rates and amounts toward the thrust front, whereas curves from the profile perpendicular to the Appalachian belt indicate out-of-plane effects from the Ouachita thrust front and a later increase in subsidence toward the Appalachian thrust front. The major implication of this work is that the three-dimensional geometry of a foreland basin within a thrust-belt recess includes diachronous interfering elements.

Anorogenic granites, magmatic underplating and the origin of intracratonic basins in a non-extensional setting

We present an active rift model for cratonic basin formation with crustal kinematics governed by basaltic underplating. A case study of a deeply eroded continental rift belt in south Finland shows close association of anorogenic intrusives and remnants of cratonic basins. Lower-crustal thinning took place during anorogenic magmatism, whereas the upper crust experienced vertical block faulting in a laterally neutral deformation facies. Combined analyses of field data, petrological constraints, and geophysical evidence allow the reconstruction of the kinematic scenario of crustal thinning. This model involves the formation of an incipient crustal dome by asthenospheric upwelling, associated with basaltic underplating, and migration of induced lower-crustal partial melts to upper-crustal levels. In this model, light lower-crustal “diorite” is replaced by a denser mafic layer. The consequences for the post-magmatic crustal history are evaluated by numerical modelling of the effects of crustal densification. Subsidence occurs in the absence of crustal extension, suggesting that the magmatic underplate is the key element for the formation of an intracratonic basin. Subsidence curves for this model show an incipient, nearly linear fast downwarp related to crystallization of gabbro at the Moho, followed by slow contraction induced by solid state cooling. The predicted subsidence is similar to the pattern of the vertical movement induced by passive extension. In the proposed model of continental rifting, the effects of extension and underplating largely cancel out. Concurrent crustal extension will cause the removal of a magmatic underplate by promoting basaltic extrusion. The model provides a consistent explanation for a number of key aspects of cratonic basins, including their circular shape, crustal configuration and evolution, and their spatial distribution on continents.

Subsidence analyses from the North Sea ‘triple-junction’

When compared with theoretical subsidence curves calculated from the uniform stretching model, water-loaded subsidence data from the ‘triple-junction’ region of the North Sea suggest that the Jurassic–Cretaceous rifting event caused lithospheric thinning by a factor of c. 2.0. Although somewhat larger than the stretching factors found elsewhere in the North Sea, this amount of thinning is anticipated from the overall geometry of the three-graben system and is consistent with the observed volume and elemental and isotopic composition of the Jurassic Forties volcanic province. Apart from in the southern North Sea, Permo-Triassic extension is thought to have been relatively minor in comparison to the later Jurassic-Cretaceous phase. The anomalously small amount of Late Jurassic syn-rift subsidence in those wells where local fault-controlled effects are minimal, supports the well-known idea of localized relative uplift or ‘doming’ in the triple-junction area ( c . 10 4 km 2 ) prior to and during the early stages of the Jurassic–Cretaceous rift phase. A time-dependent differential stretching model, in which the lithospheric mantle is initially stretched by a greater amount than the crust is stretched might provide an explanation. Such a model would require the total amount of stretching integrated over space and time to be the same for the lithospheric mantle and for the crust in order to avoid space problems. Alternatively, the same data could be explained by invoking a small transient thermal anomaly in the asthenosphere.

High-Resolution Stratigraphic Forward Modeling: A Case Study of the Lower-Middle San Andres Formation, Permian Basin

This study has attempted to calibrate Shell's two-dimensional (2-D) basin modeling program as an exploration tool by simulating the stratigraphy of a mixed carbonate/clastic third-order depositional sequence. The lower-middle San Andres Formation was selected because available log, core, and outcrop data from the Northwest Shelf area, Permian basin, provided an excellent calibration set. A regional stratigraphic cross section from the Cato-Chaveroo to the Wasson fields was constructed delineating lithology and porosity distribution. Approximately 10 shoaling-upward depositional cycles were interpreted. A higher frequency, five-in-one cyclicity was also interpreted based on core and outcrop data. The observed stratigraphy was simulated using a composite eustasy consisting of third-order (2,000,000 yr), fourth-order (100,000 yr), and fifth-order (20,000 yr) sinusoids each at five-meter amplitudes. Subsidence input was constrained by back-stripped tectonic subsidence curves calculated from well data. Sedimentation parameters were interactively derived. New empirically based algorithms were used to model Dunham lithofacies, environmental facies, and sabkha anhydrite distribution. Synthetic log and 2-D synthetic seismic profiles were constructed from the simulation output. The simulation results suggest that (1) relative sea level is the dominant control on the observed depositional cyclicity, (2) the distribution of regional seal facies (anhydrite) reflects falling sea level and prolonged exposure, (3)more » limestone-dolomite trends on the shelf are grossly related to environment and (4) the distribution of grainstones and packstones (potential reservoirs) occurs as fourth- and fifth-order offlapping and aggradational pods. The synthetic log signatures compared to [open quotes]real[close quotes] logs substantiate the interpreted depositional cyclicity, but also point out the difficulty in interpreting high-order cycles based on log data alone.« less

Correlation between Plate Motions and Tectonic Subsidence of Sedimentary Basins in Africa

From the early Mesozoic until the Holocene, the African continent was generally in a state of extension, based on plate tectonic reconstructions and sedimentary basin subsidence studies. Beginning with the breakup of Gondwana in the Permian-Triassic, this resulted in the formation of the present-day African continental margins and a series of intracontinental rift basins, located mainly on older (late Proterozoic) shear zones. Numerous wells from marginal, as well as intracontinental rift basins, have been backstripped to elucidate their Mesozoic and Tertiary tectonic histories. They show a generally consistent patterns of subsidence and uplift phases in all basins. During the evolution of these basins, the direction of African plate motion changed several times. This was related to the differential opening of the central and south Atlantic oceans, changes in spreading rates in both the Atlantic and Indian oceans, and the collision between Africa and Europe. Episodes of compressional deformation related to these plate tectonic changes are revealed in backstripped tectonic subsidence curves.

Measuring lithospheric stretching on Tethyan passive margins

Preliminary estimates of lithospheric stretching have been made by examining 63 stratigraphic sections, from the northern and southern Tethyan margins (southeast France to Bulgaria). The study was centred on the Italian Southern Alps (the Dolomites), a portion of the southern margin within the external zones of the Alpine orogen. Sections were decompacted and backstripped using the standard procedure, allowing for errors in palaeowater-depth estimates. Theoretical subsidence curves based on the finite-duration stretching model were then fitted to the resultant water-loaded subsidence curves in an attempt to clarify the pre-compressional tectonic history of the region. According to our results, stretching started in the early to mid-Triassic and lasted 30–40 Ma. This conclusion is broadly in agreement with some previously published estimates. The stretching factor, β, is generally small (1.1–1.3), especially in southeast France and northern Italy. Larger values of β occur in the Balkans ( c. 1.5). To our knowledge, the finite-duration stretching model has not yet been applied to water-loaded stratigraphic sections from the Tethyan belt. Theory and observation appear to agree well.

Subsidence analysis and modelling of the Roer Valley Graben (SE Netherlands)

The Roer Valley Graben (southeastern Netherlands) forms the most northern branch of the European Cenozoic Rift System. Sedimentary backstripping of exploration wells provides control on vertical tectonic motions during the complex post-Paleozoic geodynamic evolution of the area. The Roer Valley Graben evolved by repeated reactivation of Permo-Carboniferous fracture systems. At least three episodes of extension-related subsidence are recorded. These occurred during the Late Permian-Early Triassic (> 255-240 Ma), the Middle Jurassic (168-165 Ma) and the late Cenozoic (36-0 Ma). A Late Jurassic-Early Cretaceous stretching phase is weakly inferred from incomplete data. Crustal stretching values calculated from the tectonic subsidence curves can account for the reflection seismic controlled crustal configuration of the Roer Valley Graben area. Results demonstrate a close relationship between the Roer Valley Graben and the basins in the North Sea during the Mesozoic. In contrast, the Cenozoic evolution of the Roer Valley Graben is independent from that of the North Sea and is related to the development of the Rhine Graben.

A note on the subsidence history of the northern margin of the Alboran Basin

Backstripping of four marine wells on the northern margin of the Alboran Basin reveals a tectonic subsidence history that varies laterally along the margin from east to west. The well on the eastern margin (AndA1) records a fairly steady, continuous subsidence for most of its history, whereas wells on the western margin (AndG1 and AlbA1) show two distinct phases in their subsidence curves. The form of the subsidence curve for DSDP well 121 reflects its position on a structural high. Beta values for the tectonic subsidence curves range between 1.3 and 1.6.