Flexural Cantilever Model Research Articles

Lithospheric stretching-style variations and anomalous post-rift subsidence in the deep water sub-basins of the Pearl River Mouth Basin, northern South China Sea

The stretching and subsidence characteristics of passive continental margins have been investigated for decades but are still controversial. We focused our study on the deep water sub-basins of the Pearl River Mouth Basin in the northern South China Sea, the only well-preserved passive margin in the South China Sea. Inverse modeling and forward modeling based on a flexural cantilever model were employed. The stretching factors of the upper crust, lower crust, whole crust, and mantle lithosphere were mapped, and the anomalous post-rift subsidence was isolated from the McKenzie model. The Liwan Sub-basin showed highly depth-dependent stretching that significantly increased with depth. The stretching factor of the lower crust in the center of the Baiyun Sub-basin was higher than that of the mantle lithosphere. Two significant anomalous rapid subsidence events were revealed. The 23.03-19.8 Ma event was characterized by anomalous subsidence at the center, located in the southern Baiyun Sub-basin, with a maximum value in excess of 1200 m. Strong negative P-wave abnormal and magma addition may indicated an upwelling mantle under the Baiyun Sub-basin around 23 Ma. We suggest that the 23.6 Ma southward ridge jump in the South China Sea could have weakened or eliminated the mantle upwelling under the Baiyun Sub-basin and caused rapid subsidence at 23.03-19.8 Ma. The 14.3-11.9 Ma event was characterized by a subsidence zone orientated northeast along the Baiyun Sub-basin, with a maximum value in excess of 600 m. We propose that rapid depositional sediment during this period would have driven the lower crustal flow away from the Baiyun Sub-basin center, causing rapid vertical subsidence and leading to the stretching factor of the lower crust being higher than that of the mantle lithosphere.

Differential extension and dynamic model of the deep-water area of the Pearl River Mouth Basin, northern South China Sea

Based on the flexural-cantilever model and flexural isostasy model, three independent quantitative methods have been used to calculate the stretching factors of the upper crust, whole crust, and whole lithosphere in the deep-water area of the Pearl River Mouth Basin, northern South China Sea. These results demonstrate that depth-dependent stretching has occurred within the lithosphere of the study area. The lithospheric extension shows lateral differences between the Baiyun Sag and the Kaiping–Shunde Sags. The broad forearc pre-rifting basement and hot thinned lithosphere tend to generate a structural style of wide half-graben in the Baiyun Sag, while the volcanic arc basement and normal or thickened lithosphere form a structural style of narrow half-graben in the Kaiping–Shunde Sags. In line with the lithospheric deformational features and tectonic evolution stages, we propose three various dynamic mechanisms at different tectonic stages, and they are probably uniform, composite, and depth-dependent extension models, respectively.

Non‐uniform hyper‐extension in advance of seafloor spreading on the vietnam continental margin and the SW South China Sea

AbstractThe SW South China Sea preserves a propagating oceanic spreading centre and associated continent–ocean transition (COT) that characterizes the continental margin offshore SE Vietnam. We investigated the nature of strain accommodation in the region immediately in front of the propagating rift using a combination of 1‐ and 2‐D backstripping subsidence reconstructions, coupled with forward modelling based on the measured upper crustal extensional faulting applied to a flexural cantilever model. Major normal faulting ceases after an inversion event at ca. 16 Ma, although moderate extension was also noted at 22–23 Ma, representing the end of an extensional phase that initiated around 28 Ma. 1‐D subsidence models indicate rapid ‘syn‐rift’ subsidence, possibly lasting until 10 Ma, despite the lack of observed extensional faulting. We show that depth‐dependent extension is required to explain the great depth of the basins despite the modest observed upper crustal faulting. Brittle faulting could not have extended much deeper than 10 km, suggestive of weak crust in the presence of high heatflow. The regional topographic slope on the basement suggests very low mid‐crustal viscosities of 1019–1020 Pa.s., consistent with the idea that flow in the ductile mid and lower crust was responsible for much of the subsidence prior to, and possibly after, seafloor spreading, which extended ca. 300 km from the tip of the mid ocean ridge. Flow is inferred to be dominant towards the spreading centre prior to 16 Ma. Extension in the COT postdates seafloor spreading and further supports the idea of this crust being very weak, albeit with more coherent, less extended crustal fragments, now forming banks offshore the Sunda continental shelf and surrounded by hyperextended crust of the COT.

Numerical modeling of the anomalous post-rift subsidence in the Baiyun Sag, Pearl River Mouth Basin

The Baiyun Sag is the deepest sag in the Pearl River Mouth Basin in northern continental margin of South China Sea, with the maximum sediment thickness over 12.5 km above the basement including >6.5 km sediments above the 30 Ma breakup unconformity. According to the theoretical models for the rifted basins, the post-rift subsidence is driven solely by the thermal contraction and can be calculated as the function of the lithospheric stretching factor. A method combining the forward modeling and reverse backstripping was designed to estimate lithospheric stretching factor. Using the 2D forward modeling based on the flexural cantilever model, we simulated the multi-rifting process of the Baiyun Sag with constrain of the backstripped profiles. By doing this the lithospheric stretching factor was obtained, and then the theoretical post-rift subsidence was calculated. The calculated theoretical subsidence was much smaller than the observed subsidence given by backstripping. Along the 1530 line in the Baiyun Sag, the anomalous post-rift subsidence is over 2 km in the sag center, and varies slightly to the north and south edges of the sag. This suggests that the anomalous post-rift subsidence continues beyond the sag both in the continental shelf to the north and in the continental slope to the south. The sensitivity tests in the forward modeling process indicate that only the use of low-angle faults (⩽13°) can we simulate the shape of the backstripped profile.

Cenozoic Post‐Rift Unconformity and the Accelerated Subsidence Events of the Jiyang Depression, Bohai Bay Basin and Preliminary Analyses on their Original Mechanism

AbstractTo investigate the events of the post‐rift unconformity and the following accelerated subsidence in the Cenozoic Jiyang Depression, Bohai Bay basin, and discuss on their original mechanism, a forward model of flexural cantilever model and a reverse model of 2D flexural backstripping model were employed to rebuild the Cenozoic tectonic evolution of two seismic lines trending in NS of the Jiyang Depression. The forward modeling showed that, to rebuild the geometrical structures of the two lines at 14 Ma, uplift events, producing syn‐rift and post‐rift unconformities, should be included in the model, and the uplift amplitude in the northeast is higher than in the southwest. While the reverse modeling showed that, the amount of the thermal subsidence, based on the stretching factors deduced from the faults, is not enough to restore the palaeo‐water depth at 14 Ma. Excess subsidence should be added, and the amplitude of the excess subsidence in the northeast should be larger than in the southwest. The results suggest that during the development of the Jiyang Depression, the lithosphere might be thinned not only passively by horizontal extension, but also thinned actively by some vertical factors; and the accelerated subsidence since 14 Ma has a trend of propagating from the northeast to the southwest. The analyses also indicate that, in addition to the passive factor of horizontal extension, by which the asthenosphere was perturbated thermally and then experienced quick thermal decay, some other active factors, such as lithosphere delamination and mantle lithosphere metasomatism, can be candidate mechanism for the post‐rift unconformity and the following accelerated subsidence.

The formation of the south-eastern part of the Dniepr–Donets Basin: 2-D forward and reverse modelling taking into account post-rift redeposition of syn-rift salt

Forward and reverse modelling of structure and stratigraphy has been used to investigate the syn-rift (Late Devonian) and early post-rift (Carboniferous) evolution of the south-eastern part of the Dniepr–Donets Basin (DDB). Modelling was carried out with and without taking into consideration the withdrawal and surface extrusion of Devonian salt during the formation of salt diapirs. The great thickness of Carboniferous deposits can be explained by the superimposed actions of three processes: post-rift thermal subsidence, withdrawal of Devonian salt from the mother layer during phases of salt diapir activity, and regional subsidence of the East European Platform. The effects of other tectonic and/or non-tectonic processes are not required. Forward syn-rift modelling using the flexural cantilever model of sedimentary basin formation predicts the total syn-rift extension across the southeastern DDB to be approximately 65 km with a maximum β stretching factor of 2.4. Shallowing of the Moho during the syn-rift phase is estimated to be 15 km. The present-day Moho, after thermal subsidence and basin fill, is predicted to be 4–6 km shallower than surrounding regions. In the axial zone of the south-eastern DDB the thickness of the Devonian syn-rift sequence may have reached 7.5 km by the end of the rift stage. This is 3–3.5 km more than at present. The thicknesses reduction is due to the outflow of Devonian salt during post-rift periods of halokinetic activity in the early Visean, the middle Serpukhovian, and in the Early Permian. The withdrawal of salt from the mother layer produced additional accommodation space and up to 1.5–1.7 km of the total eventual thickness of the Carboniferous sedimentary succession can be explained as a result of this.

Evidence of low flexural rigidity and low viscosity lower continental crust during continental break-up in the South China Sea

The South China Sea was formed by seafloor spreading in the Late Oligocene at ∼30 Ma following a series of extensional events within crust formed by Mesozoic continental arc. In this study, we interpreted faults along seismic reflection profiles from both the northern and southern conjugate margins of the South China Sea, and forward modeled these using a flexural cantilever model to predict modern basin geometries. When compared with the observed structure, the models based on upper crustal faulting consistently underpredicted the amount of subsidence, especially towards the continent–ocean transition (COT). We interpret this to indicate preferential extension of the continental lower crust along the COT on both margins, extending up to ∼80 km landward from COT. The regional slope of the South China continental shelf indicates lower crustal viscosities of 10 19–10 18 Pa s, representing an offshore continuation of the weak crust documented onshore on the eastern flanks of the Tibetan Plateau. Only in the region of Hainan Island in the western South China Shelf does lower crustal viscosity increase (10 21–10 22 Pa s) and the preferential loss of lower crust become less pronounced and limited to <40 km from COT. This western area represents a rigid block analogous to the Sichuan Basin onshore. Forward models based on upper crustal faulting support the idea of a very weak continental crust because models where the effective elastic thickness of the plate ( T e) exceeds 5 km fail to reproduce the geometry of the sub-basins within the Pearl River Mouth Basin (PRMB) of the South China Margin. The observed basins are too deep and narrow to be consistent with models invoking high flexural rigidity in the upper crust or mantle lithosphere. The fact that rifting and seafloor spreading seem to co-exist for ∼5 my. adjacent to the PRMB is consistent with very weak continental crust during break-up.

Structural modelling of regional depth profiles in the Vøring Basin: implications for the structural and stratigraphic development of the Norwegian passive margin

Interpretation of key regional seismic lines across the Vøring Basin has identified a number of seismic megasequences that have provided a framework for further basin analysis. These megasequences have been classified as pre-, syn- and post-rift with respect to the late Cretaceous-early Tertiary phase of extension. Forward and reverse structural and stratigraphic modelling of two of these regional lines has been performed in an attempt to test and validate the structural and stratigraphic interpretations of the syn and post-rift megasequences. Reverse thermal subsidence modelling (flexural backstripping) of these lines to the base Tertiary was performed using a laterally varying beta factor across the sections. A number of features on each of the depth profiles are considered to be regional subaerial datums for the backstripped sections. The backstripped templates were then forward modelled using the flexural cantilever model. The forward models calculate a laterally varying beta profile along their length which can be compared to the reverse models, thereby ensuring an internally consistent structural model between both the reverse and forward modelling techniques. The modelling work has highlighted the existence of a significant discrepancy between the extension measured from the forward models (fault heaves) and the extension needed to produce geologically-acceptable backstripped profiles. This observation is of particular importance as it may place a significant constraint on the mode of lithospheric extension that can be used to explain the evolution of the Norwegian continental margin.

Flank uplift and topography at the central Baikal Rift (SE Siberia): A test of kinematic models for continental extension

Uplifted flanks at intracontinental rifts are supported by flexural isostasy, as shown by the pattern of isostatic residual gravity anomalies associated with them. Models for flexural rift flank uplift differ in their kinematic description of extension, with respect to the asymmetry of rifting and the importance of brittle versus elastic upper crustal deformation. In this paper, I test different kinematic models of continental extension by comparing their predictions of rift flank topography and crustal structure with observations from the Baikal rift (SE Siberia). The rift is characterized by prominent flank topography on both sides of lake Baikal. The flanks reach similar elevations but differ in their structure: the tilt of the footwall flank is away from the basin, whereas the basinward part of the hanging wall flank tilts toward the basin center. Fission track data indicate that very little erosion affected the flanks since rifting started; geomorphological and sedimentological observations suggest that prerift relief was minor. Thus, the present‐day topography reflects rift‐related tectonic uplift. Pure‐shear “necking” and pure‐shear/simple‐shear “detachment” models of extension predict the topographic and Bouguer gravity anomaly patterns observed along a profile across the central Baikal rift equally well. They do not permit to discriminate between different scenarios that have been proposed for the central Baikal rift; that is, half‐graben versus full graben development; rifting at a continuous rate since the Oligocene versus a large increase in extension rate since the Pliocene. The models predict that the kinematics of rifting in Baikal are controlled by a midcrustal (20 km) depth of necking and/or a mid to lower crustal (20–30 km) detachment level; best‐fit elastic thicknesses are in the range 30–50 km. These predictions are in agreement with results from coherence studies of Bouguer gravity and topography, as well as with the rheology of the lithosphere underneath Baikal as inferred from heat flow, seismic refraction and seismological observations. In contrast, a “flexural cantilever” model with low (&lt; 10 km) elastic thickness predicts topographic patterns which are very different from those observed, for a wide range of rifting scenarios. Significant (&gt; 3 km) erosion of the footwall flank is required to fit the topography if a flexural cantilever model is applied; this is incompatible with the fission track data. Thus, the kinematics of extension at deep and narrow intra‐continental rifts such as Baikal appear to be controlled by a strong elastic lithosphere and require significant brittle deformation of the upper crust, as suggested by dynamic models for continental rifting.

Syn-rift evolution of the Pripyat Trough: constraints from structural and stratigraphic modelling

The Pripyat Trough is a Late Devonian rift basin. It forms part of the larger Pripyat-Dniepr-Donets rift system which has a length in excess of 800 km and separates the Ukrainian Shield from the Voronezh Massif. The Pripyat Trough contains a well resolved stratigraphic sub-division within the syn-rift basin fill sequence which allows the duration and rate of rifting to be determined using 2-D forward and reverse structural and stratigraphic modelling. The analysis shows that rifting was extremely rapid. Sequential decompaction and flattening of 2-D cross-sections has been applied to six syn-rift time horizons between top Upper Devonian (364 Ma) and top Middle Devonian (377 Ma) and used to quantify the syn-rift development of basin cross-sectional area in time. The evolution of basin cross-sectional area shows that some initial rifting had occurred prior to the Middle Frasnian (369 Ma); however, most rifting occurred in the Famennian (367-364 Ma). Forward syn-rift modelling using the flexural cantilever model of rift basin formation has also been applied to quantify the magnitude of extension within the Pripyat Trough in time. Forward syn-rift models are constrained by the intra syn-rift flattened and decompacted cross-sections and the observed evolution of the cross-sectional area. Most rapid rift basin formation is shown to have occurred in the Famennian with over 66% of basin cross-sectional area forming in less than 5 Myr, and with approximately 80% or more of total extension occurring in less than 3 Myr. This period of most rapid extension during the Famennian coincides with the most active period of volcanicity. Total Devonian extension across the Pripyat Trough is estimated to be of the order of 11–14 km with a maximum β stretching factor of approximately 1.12. Extensional strain rates are estimated to be of the order of 0.8 × 10 −15 s −1.

The formation of the northwestern Dniepr-Donets Basin: 2-D forward and reverse syn-rift and post-rift modelling

Forward and reverse modelling of structure and stratigraphy have been used to investigate the syn-rift (Devonian) and early post-rift (Carboniferous) evolution of the northwestern Dniepr-Donets Basin. Modelling shows that basin formation is consistent with Frasnian-Famennian rifting followed by post-rift subsidence starting in the early Tournaisian. Forward syn-rift modelling, using the flexural cantilever model of rift basin formation, satisfactorily models the observed syn-rift stratigraphic thicknesses and structure within the basin using total syn-rift extension of approximately 15 km in the region studied with a maximum β stretching factor of 1.3. Forward structural and stratigraphic modelling suggests that the Devonian rifting, which formed the northwestern Dniepr-Donets Basin, was accompanied by regional uplift of the order of 300 m. Both forward and reverse (flexural backstripping) modelling of post-rift stratigraphy through the Carboniferous suggest that post-rift thermal subsidence, which commenced in the Tournaisian following Devonian rifting, was augmented by additional regional subsidence of the order of 300 m in middle Carboniferous times. It is suggested that this transient regional uplift event, which accompanied rifting and decayed in middle Carboniferous times, was generated by a mantle plume.

Discussion of potential errors in fault heave methods for extension estimates in rifts, with particular reference to fractal fault populations and inherited fabrics

Abstract Fault heave methods for estimating extension (including balanced cross-sections and the flexural cantilever model) are important because they are the only methods that are universally applicable to estimating extension from the syn-rift stratigraphic section. One of the main criticisms of the methods when based on reflection seismic data is the failure to account for crustal extension along sesimically invisible faults (displacements up to a few tens of metres). The fractal distribution of several fault populations has been cited as evidence for the existance of such minor faults. It has been suggested by several workers that the discrepancies between extension estimates for the North Sea rifts calculated from the syn-rift section and by backstripping (using the McKenzie model) can be explained by a substantial amount of crustal extension having taken place on seismically invisible faults. This paper examines whether it is reasonable to extrapolate such conclusions to other rifts. Estimates for the contribution of minor faults to crustal extension based on fractal distributions of faults need to be treated with caution for several reasons: (1) in some rifts large boundary faults (several kilometres displacement) may account for 80–95% of the total extension amount. In such rifts minor faults may not contribute significantly to crustal extension (less than 5%). (2) The different distribution of fault size and spacing between different zones within rifts suggests that a single fractal distribution along a regional rift traverse is commonly unrealistic. (3) Fractal fault populations measured in syn-rift sedimentary sequences may be measuring faults involved in gravity tectonics and compaction, not just those faults that contribute to crustal extension. Regions with very complex pre-rift structural histories (e.g. North Sea) appear to have significant amounts of extension on minor faults. This contrasts with rifts developed on crusts with a relatively stable history (e.g. East African rift), where much of the extension is accommodated on a few large faults. The differences between these two rifts (despite both rifts having undergone similar amounts of extension) illustrate that pre-existing fabrics are extremely significant for understanding fault populations, fault orientations, partitioning of extension between different sizes of fault, the types of linkage between faults (transfer zones), syn-rift basin subsidence and fault lengths. An appreciation of this variability should lead to more accurate prediction of the contribution of seismically invisible faults to extension on balanced cross-sections.

Forward and reverse stratigraphic modelling of Cretaceous-Tertiary post-rift subsidence and Paleogene uplift in the Outer Moray Firth Basin, central North Sea

Abstract The Paleogene stratigraphy in the Outer Moray Firth is part of the North Sea post-rift sedimentary sequence that was deposited in a thermally subsiding basin following Late Jurassic rifting. Previous studies have drawn attention to anomalous uplift and departure from the McKenzie (1978, Earth Planetary Science Letters , 40 , 25–32) post-rift subsidence trend in the early Paleocene followed by an accelerated phase of basin subsidence in the early Eocene before returning to the normal post-rift subsidence. 2D forward and reverse stratigraphic modelling incorporating first-order global sea-level variations have been used to determine the timing and magnitude of departures from McKenzie post-rift thermal subsidence in the early Paleogene for the Outer Moray Firth. Reverse post-rift modelling, consisting of flexural backstripping, decompaction and reverse thermal subsidence, has been used to produce restored post-rift sections from present-day stratigraphy, which have been constrained by palaeobathymetric data. Forward syn-rift and post-rift structural and stratigraphic modelling uses the flexural cantilever model of rift basin formation and has been constrained by syn-rift structural data, present-day stratigraphy and palaeobathymetric markers. Forward and reverse modelling show that observed Cretaceous and Tertiary stratigraphy of the Outer Moray Firth was generated by the combined effects of inherited Late Jurassic syn-rift accommodation space, post-Jurassic rift thermal subsidence, sediment supply, long-term eustasy and an additional transient uplift event in the Paleocene. The much thicker Tertiary compared with Cretaceous can be explained by sediment infilling of starved Cretaceous palaeobathymetry. Reverse and forward post-rift modelling predict regional Paleocene uplift in the Outer Moray Firth of the order of 375–390 m (375 m reverse modelling, 390 m forward modelling), followed by c. 160 m of rapid Eocene subsidence, both superimposed on post-rift thermal subsidence following Late Jurassic rifting. Regional Paleocene uplift in the Outer Moray Firth is attributed to dynamic uplift associated with the development of the Iceland plume.

Palaeocene uplift and Eocene subsidence in the northern North Sea Basin from 2D forward and reverse stratigraphic modelling

Structural and stratigraphic basin modelling has been used to examine quantitatively the Cretaceous and Tertiary post-rift subsidence history of the northern North Sea Basin following Late Jurassic extension. 2D forward and reverse, syn-rift and post-rift modelling has been used to determine the magnitude and timing of departures from the McKenzie post-rift thermal subsidence trend for regional stratigraphic profiles in the Outer Moray Firth, the South Viking Graben and the North Viking Graben. Observed stratigraphy has been reverse post-rift modelled using flexural backstripping combined with decompaction and reverse thermal subsidence calculations, and forward modelled through the syn-rift and post-rift period using the flexural cantilever model of continental rift basin formation. Long-term global eustasy has been included in the analysis. Only reliable palaeobathymetric markers, such as erosion surfaces and coals, have been used to constrain subsidence history. Both forward and reverse 2D modelling show that a Palaeocene regional uplift followed by a rapid Eocene subsidence must be superimposed on McKenzie post-rift Cretaceous and Tertiary basement subsidence in order to generate the stratigraphy observed. The magnitude of Palaeocene uplift deduced from modelling was of the order of 375 m in the Outer Moray Firth (central North Sea) increasing to 525 m in the North Viking Graben (northern North Sea). Rapid Early Eocene subsidence was of the order of 160–310 m. The most likely mechanism for regional Palaeocene uplift and rapid Eocene subsidence in the northern North Sea was the Palaeocene development of the Iceland plume situated at a distance of 700–900 km from the study area in the Early Tertiary. Forward and reverse modelling predict axial bathymetries in the graben of up to 1300 m for the Early Cretaceous and 900 m for the Upper Cretaceous. The large palaeobathymetries at the end of the Cretaceous provided much of the accommodation space for Tertiary sedimentation.

Cenozoic extension in northern Kenya: a quantitative model of rift basin development in the Turkana region

Recent geological and geophysical studies in the Turkana region of the northern Kenya rift have highlighted the presence of a number of major sedimentary depocentres (Morley et al., 1992) overlying anomalously thin crust (Mechie et al., 1994). The region overlies a putative structure linking the Mesozoic-Palaeogene rifts of south Sudan and eastern Kenya (Ibrahim et al., 1991; Bosworth, 1992). The flexural cantilever model of continental extension (Kusznir et al., 1991) has been used to describe the rift evolution of the Turkana region, constrained by seismic reflection data and surface geology. Sections showing crustal structure, basin geometry and β stretching factor profiles have been modelled for end Palaeogene (25 Ma), end Miocene (5 Ma) and end Pliocene (2 Ma). We also show that, whilst extension estimates giving a β stretching factor estimate of 1.55–1.65 are consistent with geophysical data showing that the Moho has been shallowed to 20 km by extension, a significant portion of that extension may pre-date the Neogene. We propose that across the Turkana region during the Palaeogene a series of basins formed, linking known loci of extension in southern Sudan with extension in the Anza/Kaisut system and South Kerio basins in northern and central Kenya. These basins have no surface outcrop expression, but are defined by gravity anomalies and a number of pieces of indirect evidence. Continued extension on the later East African rift system resulted in reactivation of some of these structures in Neogene-Recent times, and the abandonment of others. Modelling of the rift basin geometry in the Turkana region using the flexural cantilever model gives a value of effective elastic thickness ( T e ) of 3.5 km. Flexural bending of the lithosphere associated with extensional fault block rotations is shown (following Buck, 1988) to generate large bending stresses which give rise to substantial upper lithosphere brittle failure and plastic deformation, so producing low values of lithosphere flexural strength and T e . The low values of T e estimated locally for the rift region by the flexural cantilever model differ from the much larger values of T e determined using gravity-topography coherence techniques (Ebinger et al., 1989) which sample regional lithosphere strength including that of strong continental shield outside the rift. The maximum β stretching factor of 1.55–1.65 defined by the model is insufficient to generate the observed volcanics using published quantitative models of melt generation (McKenzie and Bickle, 1988). An anomalously hot mantle temperature is required to generate any melt at such low stretching factors. Whilst previous authors (Karson and Curtis, 1989; Latin et al., 1993) have proposed that large volumes of melt have been intruded into and ponded within the lithosphere, we show that crustal underplating and intrusion do not significantly alter estimates of crustal thickness and extension across the rift system.

Rifting, erosion, and uplift history of the Recôncavo‐Tucano‐Jatobá Rift, northeast Brazil

The Recôncavo‐Tucano‐Jatobá (RTJ) Rift and many other smaller sedimentary basins in northeast Brazil formed during South Atlantic rifting and were subsequently uplifted and exhumed so that Albian marine sediments are now located up to 800 m above sea level some 400 km inland from the Atlantic margin. Local erosion caused by footwall uplift and regional erosion, probably resulting from magmatic underplating and uplift, has removed a large part of the thermal sag phase sediments from the RTJ Rift. The flexural cantilever model, incorporating the flexural isostatic response to simple‐shear faulting in the upper crust and pure‐shear necking of the lower crust and upper mantle, can explain the main geometrical features observed in the RTJ Rift without resorting to lithosphere scale detachments. The model has also been used to estimate the amount of uplift and erosion of the faulted rift flanks. Rift flank erosion can produce uplift and erosion across the whole RTJ Rift, thus explaining the postrift unconformity which preceded deposition of the Aptian age Marizal Formation. The largest uplift is predicted to occur at the edges of the basin and may explain the anomalously shallow depth to onset of oil generation at the basin margins deduced from vitrinite reflectance data. The model also predicts that maximum burial of much of the basin fill in the RTJ Rift occurred at the end of rifting and not during the postrift infill, as is usually the case. The amount of observed coarse conglomeratic detritus in the Recôncavo subbasin suggests that about 25% of the eroded footwall detritus was deposited and preserved in the adjacent hanging wall half‐graben, with the rest being transported more distally as finer material.

Jurassic extension estimates for the North Sea ‘triple junction’ from flexural backstripping: implications for decompression melting models

The Rattray volcanics lie at the ‘triple junction’ between the three arms of the North Sea rift system. The mildly undersaturated volcanics in this province were erupted during the early syn-rift phase of a Middle-Late Jurassic rifting event, and are thought to be the products of decompression-induced partial melting of the mantle, resulting from lithospheric attenuation during rifting. Situated as they are in one of the most intensively studied rift basins in the world, the Rattray volcanics provide an excellent opportunity to evaluate the relationship between extension and melting described by McKenzie and Bickle [1]. This study uses a modified version of the backstripping technique, which takes into account the flexural strength of the lithosphere, to constrain the stretching factor, β, for the Jurassic rift phase. Two-dimensional interpreted sections are flexurally backstripped to the base post-rift to give an initial estimate of β. β is then further constrained by forward modelling to the base post-rift basin geometry generated by backstripping. Forward modelling uses the flexural cantilever model, a combined simple-shear/pure-shear model for continental extension which takes into account the thermal, rheological and flexural isostatic consequences of extension. This is an approach that is different from that of previous studies [2–5], which assume local, rather than regional, isostatic compensation to allow the backstripping of individual wells. The results obtained in this new study give maximum Jurassic β-factors on each of the three rift arms close to the ‘triple junction’ of 1.2–1.35. This suggests a maximum Jurassic β-factor in the ‘triple junction’ itself of 1.35–1.75. This is considerably lower than previous estimates, and is due to the breakdown of the assumption used in these previous studies that Airy isostasy can approximate the response of a low flexural rigidity lithosphere in subsidence analysis. Whilst such a β-factor is insufficient to generate melt by decompression alone, an anomalously high mantle potential temperature (θ) and the presence of volatiles as well as a complex Mesozoic and Late Palaeozoic rifting history are likely to have played a role in the generation of melt.

The mechanics of continental extension and sedimentary basin formation: A simple-shear/pure-shear flexural cantilever model

Abstract Deep seismic reflection data and earthquake seismology show the fundamental importance of major crustal faults during extension of continental lithosphere controlling the development of rifted sedimentary basins. Major basement faults are generally planar and extend down to 10–15 km depth corresponding to the base of the seismogenic layer. Beneath this depth deformation gives way to distributed ductile shear in the lower crust and lithospheric mantle. A mathematical model is described of the geometric, thermal and flexural isostatic response of the lithosphere to extension by planar faulting of the upper crust and plastic distributed deformation in the lower crust and mantle. During faulting, upper crustal footwall and hanging-wall blocks behave as two mutually self supporting flexural cantilevers; their response to isostatic forces induced by extension generates footwall uplift and hanging wall collapse. For extension on multiple faults, interference of footwall uplift and hanging wall collapse gives rise to the familiar block rotations of rift tectonics. The flexural cantilever model is able to predict crustal structure and sedimentary basin geometry for faults of arbitrary spacing, horizontal displacement and polarity. The effects of fault spacing, fault polarity and the amount of extension, as well as erosion of footwall uplift on crustal structure and basin geometry during both syn-rift and post-rift stages of basin evolution are explored. Models of “passive” rifting, involving lithosphere extension in response to the build-up of far field tectonic forces and decompression induced partial melting of the upper asthenosphere and lower lithosphere, appear to conform to the main geological observations. The role of “active” rifting, driven by locally derived deviatoric tensional forces generated by mantle plumes remains uncertain.

The Formation of Lake Tanganyika, Western Rift, East Africa: Application of a Simple-Shear/Pure-Shear Flexural Cantilever Model of Continental Lithosphere Extension

The Formation of Lake Tanganyika, Western Rift, East Africa: Application of a Simple-Shear/Pure-Shear Flexural Cantilever Model of Continental Lithosphere Extension