Depth-dependent Extension Research Articles

The spatio-temporal variation of exhumation and its impacts on the resource distribution in the Northern Carnarvon Basin, Australia

Deformation and rock fracture during exhumation can enhance rock porosity and permeability, thereby improving fluid migration through reservoirs and destroying seal integrity in caprocks. Therefore, analysing the spatio-temporal distribution of exhumation in connection to fluid migration models can be crucial for energy resource exploration and storage. Geological and geophysical evidence suggests that parts of the Northern Carnarvon Basin (NCB) were exhumed. However, the spatio-temporal distribution of this exhumation and its impacts on the region's resource distribution is yet to be established. We analysed porosity, sonic logs and paleotemperature data from 210 boreholes using compaction and vitrinite reflectance analysis techniques to constrain the spatio-temporal distribution of exhumation and its correlation to the region's resource distribution. We discovered that multiple exhumation phases occurred since the Late Triassic. The highest exhumation occurred in the Mid Jurassic, where approximately 1000 m of sediment thickness was eroded from the western and southwestern parts of the Exmouth Plateau. Exhumation at this time spatially correlates with magmatic intrusions in the NCB, and paleotemperature profiles indicate that sediments in the exhumed regions were partly heated by increased paleo-basal heat flow, suggesting that magmatic bodies through underplating and crustal intrusions were the primary causes of this exhumation. Another significant exhumation phase occurred in the Early Cretaceous, removing ∼800 m of sediments from the Exmouth, Barrow and Dampier Sub-basins. Depth-dependent extension events and magmatic intrusions possibly caused exhumation at this time. Significant exhumation ceased in the Early Cretaceous, and our study has established that the exhumation episodes in the NCB were part of large-scale regional events that affected the Australian Northwest Shelf. Combining results from different estimation techniques increased the spatio-temporal coverage of the analysis and improved the precision and accuracy of exhumation results.

Intraplate lithospheric extension revealed by seismic reflection profiling of South China

How lithospheric extension evolves in an intraplate setting remains uncertain due to the lack of reliable constraints on the lithospheric architecture. Here we present seismic reflection data across the Cretaceous extensional system of South China. Our results show that extension in magma-poor conditions was accommodated by localized normal faulting in the upper crust and distributed ductile stretching in the lower crust, followed by localized crustal necking and Moho uplift associated with mantle shear-zone formation. These vertically spaced crustal and mantle features appear to be kinematically linked as follows. First, lower-crustal stretching was accompanied by normal faulting and localized exhumation of extensional domes along low-angle detachments in the upper crust. Second, sub-horizontal lower-crustal stretching tends to compensate for upper-crustal heterogeneous thinning and distribute crustal strain evenly, enabling the crust to maintain an overall smooth and flat Moho. Third, mantle shear zones likely affected lithospheric extension by controlling localized Moho uplift and crustal necking. Our compilation of seismic observations suggests that the extensional modes vary laterally from magma-poor to magma-rich conditions, reflected in increased crustal melting, decreased crust-mantle decoupling, and the replacement of a two-layer (high-strength vs. low-strength) lithospheric mantle by a single-layer, low-strength lithospheric mantle. These findings reveal a first-order configuration of depth-dependent extension over ∼800 km, with vertical and lateral variations as a function of lithospheric strength, rheology, and temperature. This extension mechanism provides a basis for assessing modes of lithospheric extension in other tectonic settings.

Melt volume at Atlantic volcanic rifted margins controlled by depth-dependent extension and mantle temperature

Breakup volcanism along rifted passive margins is highly variable in time and space. The factors controlling magmatic activity during continental rifting and breakup are not resolved and controversial. Here we use numerical models to investigate melt generation at rifted margins with contrasting rifting styles corresponding to those observed in natural systems. Our results demonstrate a surprising correlation of enhanced magmatism with margin width. This relationship is explained by depth-dependent extension, during which the lithospheric mantle ruptures earlier than the crust, and is confirmed by a semi-analytical prediction of melt volume over margin width. The results presented here show that the effect of increased mantle temperature at wide volcanic margins is likely over-estimated, and demonstrate that the large volumes of magmatism at volcanic rifted margin can be explained by depth- dependent extension and very moderate excess mantle potential temperature in the order of 50–80 °C, significantly smaller than previously suggested.

Crustal structure and variation in the southwest continental margin of the South China Sea: Evidence from a wide-angle seismic profile

Abstract A 680-km long wide-angle seismic profile was acquired in the southwest continental margin (SWCM) of the South China Sea to better understand its crustal variation and continental extension. By using two-dimensional forward ray-tracing, and joint refraction and reflection travel time inversion methods, velocity structures of the SWCM are yielded. The model results show a hyper-stretched continental crust with a thickness of 10–18 km, which is variable under sedimentary basins and reef areas. Compared with βwc, βuc, and βlc derived from the crustal thickness, the degree of the whole crust extension appears to be larger than that of the upper crust but lower than that of the lower crust, except for the model of 560–610 km. Thus, we infer that depth-dependent extension of the crust has occurred in the SWCM. High velocity bodies with velocities larger than 7.0 km/s are sporadically distributed in the lower crust, which might be related to the remnant Mesozoic arc that can be attributed to the subduction of the paleo-Pacific plate. The obvious low velocity anomaly with the maximum thinning crust below the boundaries of the Nansha Trough and Palawan-Borneo thrust wedge (PBTW) is interpreted as the frontal thrust of the PBTW.

Seismic evidence for the crustal deformation and kinematic evolution of the Nansha Block, South China Sea

To understand the crustal deformation of the Nansha Block (NB), this study presents an interpretation of a recently acquired multichannel seismic profile that traverses the Southwest Subbasin (SWSB), the NB, and the Nansha Trough (NT). A total of six Cenozoic unconformities and three stratigraphic sequences are identified. On the basis of tectonic, structural and stratigraphic features, the NB is divided from northwest to southeast into two structural domains, namely, the Hyper-stretched and Forebulge Domains. Based on the interpretation of the seismic profile, the fault-derived and whole crustal extension factors (βf and βc, respectively) for the NB are calculated. The calculation results show that βc is greater than βf in a limited space of appr. 200 km, where the crust underwent depth-dependent extension. We suggest that the crustal extensional pattern of the NB may be a combination of distal limited depth-dependent extension and proximal uniform extension. The interpretation of the unconformities is also used to calculated the fault-derived crustal extension at various stages. The two main extension stages, the syn-rift and syn-drift, account for appr. 70% and 25% of the total extension, respectively. A comparison with the seismic profiles of the eastern NB shows that the crustal extension along the continental margin of the NB is heterogeneous, with more significant crustal deformation on the west side of the NB. The heterogeneity of the crustal extension along the depth and margin may be related to the thermal condition and rheological structure of the crust or lithosphere.

Numerical modeling of failed rifts in the northern South China Sea margin: Implications for continental rifting and breakup

Failed rifts record important information of continental extension and breakup process in the northern South China Sea (SCS) margin. The Tainan Southern Depression and the Baiyun Sag to the east are characterized with lower-crust high-velocity anomalies (LCHVA), and intracrust detachment faults, whereas the Xisha Trough to the west develops on a larger scale with crust-cutting normal faults and absence of LCHVA. These contrasts indicate different rifting processes between the northeastern and northwestern SCS. 2D numerical modeling is performed to understand the formation mechanism of these failed rifts. Two types of mechanisms are proposed: I) syn-rift competitive type and II) rift migration type with a half extension rate of 2 cm/yr and 1.5 cm/yr, respectively. In type I, two rifts develop initially on the shoulders of the weak zone, but they compete with each other during extension. One rift becomes dominant to furnish the final breakup, whereas the other one is abandoned. The crust structure of this type fits the observations in the Baiyun Sag and the Tainan Southern Depression. However, in type II, only one rift develops at the beginning. The initial rifting center will migrate and the final continental breakup will occur at a place far from the initial rifting location. In this type, normal faults cut through whole crust and wide extensional margins will form, such as observed in the Xisha Trough. Our results suggest that the depth-dependent extension of the SCS is strongly heterogeneous, resulting primarily from varying extensional rates.

Crustal structure and magmatic evolution in the Pearl River Delta of the Cathaysia Block: New constraints from receiver function modeling

We utilized receiver functions to study the crustal structure beneath the Pearl River Delta in the southwestern part of the Cathaysia Block, which was characterized by widespread tectono-magmatism during the late Mesozoic. Combination of waveform modeling and H-k stacking was used to derive crustal thickness, Vp/Vs ratio and average P-wave velocity. The results show that the crustal thickness ranges from 26 km in the center of the Pearl River Delta to 32 km in the periphery; the Vp/Vs ratio is between ~1.70 and 1.74. The average P-wave velocity ranges from 6.4 to 6.8 km/s and shows higher value beneath the islands along the Lianhuashan Fault zone. A weakly positive correlation between the crustal thickness and Vp/Vs ratio is observed in southwestern Pearl River Delta, indicating depth-dependent extension of the continental crust, that is, ductile lower crust was stretched to a much higher degree in comparison with the brittle felsic upper crust. The crust in the northeastern Pearl River Delta may be more basaltic owing to underplating caused by the subduction of the Paleo-Pacific Plate, and thus a relatively high Vp/Vs ratio is observed there. The overall low crustal Vp/Vs ratio and the relatively higher crustal P-wave velocities beneath the Lianhuashan Fault zone may imply a felsic-intermediate composition and high metamorphic grade of the crust along the fault. The Lianhuashan Fault zone may provide a pathway for rapid magmatic transportation from the underplating beneath the crust to the surface.

The influence of mechanically weak layers in controlling fault kinematics and graben configurations: Examples from analog experiments and the Norwegian continental margin

Fault systems in extensional basins commonly display geometries that vary with depth, reflecting depth- and lithology-dependent mechanical strength. Using an experimental approach, we investigate this relationship by deploying physical analog models with stratified sequences consisting of brittle–ductile (sand–silicone polymer) sequences subject to single and polyphase deformation. The experiments were used as analogs for a sandstone sequence interlayered by beds of evaporates or overpressured or unconsolidated mudstone in nature (the latter being representative of decollement horizons). Experiments (series 1 [S1]) using homogeneous and stratified quartz and feldspar sand produced asymmetric, composite single grabens with diverse fault frequencies and fault styles for the graben margin faults. For the mechanically stratified experiments with one decollement level (series 2), contrasting graben configurations were produced, in that the lowermost sequence was characterized by graben geometries of similar type to that of the S1 experiments, whereas the sequence above the decollement was characterized by large fault blocks, delineated by steepened or oversteepened faults. The experiments with two decollements (series 3) were displayed similarly but included graben geometries that widened upward, with each level being characterized by independent fault systems. The results can be used to explain strata-bound fault patterns and depth-dependent extension as seen in several places along the Norwegian continental margin and elsewhere.

Crustal stretching style variations in the northern margin of the South China Sea

Linking deep seismic profiles with regional-scale gravity inversion is a powerful tool to deduce the architecture of rifted margins and their structural evolution. Here we map upper and lower crustal thicknesses of the northern South China Sea (SCS) margin in order to investigate the occurrence of depth-dependent crustal extension from the proximal to the distal margin. By comparing upper and lower crustal stretching factors, we find that the northern margin of the SCS is segmented in three parts: (1) sedimentary basins where upper crust is stretched more than lower crust, (2) distal margin where lower crust is stretched more than upper crust, (3) mostly proximal margin regions where the two layers have similar stretching factors. Our results suggest that sedimentary basins and distal margin prominently feature depth-dependent extension, however accommodated by different processes. While differential thinning within sedimentary basins appears to be governed by lateral pressure variations inducing lower crustal flow, we suggest the distal margin to be affected by a combination of mantle flow-induced lower crustal shearing and sequential fault activity during crustal hyper-extension.

Lithospheric structure of the Southwest South China Sea: implications for rifting and extension

ABSTRACT The South China Sea (SCS) is an excellent site for studying the process of conjugate margin rifting, and the origin and evolution of oceanic basins. Compared with the well-defined northern margin of the SCS, the western and southern segments of the SCS margin have not been researched in significant detail. To investigate the regional structure of the southwestern SCS, a gravity model is constructed, along with the lithospheric thermal structure along a wide-angle seismic profile. The profile extends across the conjugate margins of the Southwest Sub-Basin (SWSB) of the SCS and is based on the latest multiple geophysical measurements (including heat flow and thermo-physical parameters). The results show that the average thicknesses of the crust and thermal lithosphere along the profile are about 15 km and 57 km, respectively. The overall amount of extension of continental crust and lithosphere is more than 200 km. Thermal structure of the lithosphere shows that the continental margins are in a warm thermal state. The southwest SCS is characterized by ultra-wide, thinned continental crust and lithosphere, high Moho heat flow, early syn-rift faulted basins, undeformed late syn-rifting, and high seismic velocities in the lower crust. These various pieces of evidence suggest that the break-up of the mantle lithosphere occurred before that of the continental crust favouring a depth-dependent extension of the southwestern SCS margin.

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.

The stratigraphic record of prebreakup geodynamics: Evidence from the Barrow Delta, offshore Northwest Australia

The structural and stratigraphic evolution of rift basins and passive margins has been widely studied, with many analyses demonstrating that delta systems can provide important records of postrift geodynamic processes. However, the apparent lack of ancient synbreakup delta systems and the paucity of seismic imaging across continent-ocean boundaries mean that the transition from continental rifting to oceanic spreading remains poorly understood. The Early Cretaceous Barrow Group of the North Carnarvon Basin, offshore NW Australia, was a major deltaic system that formed during the latter stages of continental rifting and represents a rich sedimentary archive, documenting uplift, subsidence, and erosion of the margin. We use a regional database of 2-D and 3-D seismic and well data to constrain the internal architecture of the Barrow Group. Our results highlight three major depocenters: the Exmouth and Barrow subbasins and southern Exmouth Plateau. Overcompaction of pre-Cretaceous sedimentary rocks in the South Carnarvon Basin, and pervasive reworking of Permian and Triassic palynomorphs in the offshore Barrow Group, suggests that the onshore South Carnarvon Basin originally contained a thicker sedimentary succession, which was uplifted and eroded prior to breakup. Backstripping of sedimentary successions encountered in wells in the Exmouth Plateau depocenter indicates that anomalously rapid tectonic subsidence (≤0.24 mm yr−1) accommodated Barrow Group deposition, despite evidence for minimal, contemporaneous upper crustal extension. Our results suggest that classic models of uniform extension cannot account for the observations of uplift and subsidence in the North Carnarvon Basin and may indicate a period of depth-dependent extension or dynamic topography preceding breakup.

Hyper-extended rift systems in the Xisha Trough, northwestern South China Sea: Implications for extreme crustal thinning ahead of a propagating ocean

From the apex of the NW Sub-basin of the South China Sea, we present new long-cable deep-penetration seismic profiles and deepwater drillings together with corrected seismic interpretations from previous studies. Structural mapping demonstrates that the Xisha Trough is characterized by a hyper-extended rift system that shows evidence of extremely thinned crust and rotated fault blocks. A series of NE, NEE and W-E striking listric normal faults along the Xisha Trough, which merge at depth into Moho reflection, record strongly extensional deformation. Based on the absence of an acoustic basement beneath a wedge-shaped rifted sequence, we document three diapiric magma structures exiting the Moho and penetrating into the extremely thinned continental crust, which is covered by a Cenozoic sequence. For the Xisha Trough we can demonstrate, based on seismic data and calculated fault slip rates, the faults bounding the Xisha Trough were initially formed by distributed rifting, and were reactivated and focused on faulting along the axis of the Xisha Trough. However, the peak faulting migrated along the margin strike from east to west and terminated at the apex of the Xisha Trough. One of the key processes, controlling the evolution of the Xisha Trough, is depth-dependent extension, which may arise from differential lithospheric stretching by regional dynamic lower crustal/mantle flow.

The Canada Basin compared to the southwest South China Sea: Two marginal ocean basins with hyper-extended continent-ocean transitions

Both the Canada Basin (a sub-basin within the Amerasia Basin) and southwest (SW) South China Sea preserve oceanic spreading centres and adjacent passive continental margins characterized by broad COT zones with hyper-extended continental crust. We have investigated strain accommodation in the regions immediately adjacent to the oceanic spreading centres in these two basins using 2-D backstripping subsidence reconstructions, coupled with forward modelling constrained by estimates of upper crustal extensional faulting. Modelling is better constrained in the SW South China Sea but our results for the Canada Basin are analogous. Depth-dependent extension is required to explain the great depth of both basins because only modest upper crustal faulting is observed. A weak lower crust in the presence of high heat flow and, accordingly, a lower crust that extends far more the upper crust are suggested for both basins. Extension in the COT may have continued even after seafloor spreading has ceased. The analogous results for the two basins considered are discussed in terms of (1) constraining the timing and distribution of crustal thinning along the respective continental margins, (2) defining the processes leading to hyper-extension of continental crust in the respective tectonic settings and (3) illuminating the processes that control hyper-extension in these basins and more generally.

Rifted continental margins: The case for depth-dependent extension

Even though many basic properties of non-volcanic rifted margins are predicted by uniform extension of the lithosphere, uniform extension fails to explain other important characteristics. Particularly significant discrepancies are observed at: 1) the Iberia–Newfoundland conjugate margins (Type I), where large tracts of continental mantle lithosphere are exposed at the seafloor, and at 2) ultra-wide central South Atlantic margins (Type II) where continental crust spans wide regions below which it appears that lower crust and mantle lithosphere were removed. Neither corresponds to uniform extension in which crust and mantle thin by the same factor. Instead, either the crust or mantle lithosphere has been preferentially removed during extension. We show that the Type I and II styles are respectively reproduced by dynamical numerical lithospheric stretching models (Models I-A/C and II-A/C) that undergo depth-dependent extension. In this notation A and C imply underplating of the rift zone during rifting by asthenosphere and lower cratonic lithosphere, respectively. We also present results for models with a weak upper crust and strong lower crust, Models III-A/C, to show that lower crust can also be removed from beneath the rift zone by horizontal advection with the mantle lithosphere. From the model results we infer that these Type I, II, and III margin styles are controlled by the strength of the mid/lower crust, which determines the amount of decoupling between upper and lower lithosphere during extension and the excision of crust or mantle. We also predict the styles of sedimentary basins that form on these margins as a test of the concepts presented.

The geodynamic province of transitional lithosphere adjacent to magma-poor continental margins

Abstract Two different types of ‘transitional lithosphere’ have been documented along magma-poor rifted margins. One consists of apparently subcontinental mantle that has been exhumed, brittlely deformed, and serpentinized during late stages of rifting. A second is thinned (<10 km) continental crust, which in some cases is known to have been supported near sea level at least early in the rift history and thus is interpreted to reflect depth-dependent extension. In both cases, it is typically assumed that oceanic crust forms at the time that the brittle continental crust is breached or soon thereafter, and thus that transitional lithosphere has relatively limited width. Here three representative cases of transitional lithosphere are examined: one in the Newfoundland–Iberia rift and one at Goban Spur (both exhumed mantle), and one off the Angola–Congo margin (thin continental crust flanked seaward by apparently exhumed lower continental crust±exhumed mantle). Considering the geological and geophysical evidence, it appears that depth-dependent extension (riftward flow of weak lower continental crust and/or upper mantle) may be a common phenomenon on magma-poor margins and that this can result in a much broader zone of transitional lithosphere than has hitherto been assumed. Transitional lithosphere in this wide zone may consist of subcontinental mantle, lower continental crust or some combination thereof, depending on the strength profile of the pre-rift continental lithosphere. Transitional lithosphere ceases to be emplaced (i.e. ‘final break-up’ occurs) only when emplacement of heat and melt from the rising asthenosphere becomes dominant over lateral flow of the weak lower lithosphere. This model implies a two-stage break-up: first, the rupture of the brittle continental crust; and, second, the eventual separation of the ductile subcontinental lithosphere which is coincident with emplacement of normal oceanic crust. Well defined magnetic anomalies can form in transitional lithosphere that consists of highly serpentinized, exhumed mantle, and such anomalies therefore are not diagnostic of oceanic crust. Where present, the anomalies can be helpful in interpreting and dating the rifting history.

Depth-dependent extension, two-stage breakup and cratonic underplating at rifted margins

Uniform lithospheric extension predicts basic properties of non-volcanic rifted margins but fails to explain other important characteristics. Significant discrepancies are observed at 'type I' margins (such as the Iberia-Newfoundland conjugates), where large tracts of continental mantle lithosphere are exposed at the sea floor, and 'type II' margins (such as some ultrawide central South Atlantic margins), where thin continental crust spans wide regions below which continental lower crust and mantle lithosphere have apparently been removed. Neither corresponds to uniform extension. Instead, either crust or mantle lithosphere has been preferentially removed. Using dynamical models, we demonstrate that these margins are opposite end members: in type I, depth-dependent extension results in crustal-necking breakup before mantle-lithosphere breakup and in type II, the converse is true. These two-layer, two-stage breakup behaviours explain the discrepancies and have implications for the styles of the associated sedimentary basins. Laterally flowing lower-mantle cratonic lithosphere may underplate some type II margins, thereby contributing to their anomalous characteristics.

The continent-ocean transition at the southeastern margin of the South China Sea

The South China Sea formed by magma-poor, or intermediate volcanic rifting in the Paleogene. We investigate the structure of the continent-ocean transition (COT) at its southern margin, off NW Palawan between the continental blocks of Reed Bank and the islands of Palawan and Calamian. Several surveys, recorded by the BGR from 1979 to 2008, established a comprehensive database of regional seismic lines, accompanied with magnetic and gravity profiles. We interpret two major rifted basins, extending in the NE direction across the shelf and slope, separated by a structural high of non volcanic origin. The continent-ocean transition is interpreted at the seaward limit of the continental crust, when magnetic spreading anomalies terminate some 80–100 km farther north. The area in between displays extensive volcanism - as manifest by extrusions that occasionally reach and cut the seafloor, by dykes, and by presumed basaltic lava flows - occurring after break-up. The COT is highly variable along the NW Palawan slope: One type shows a distinct outer ridge at the COT with a steep modern seafloor relief. The other type is characterised by rotated fault blocks, bounded by listric normal faults ramping down to a common detachment surface. Half-grabens developed above a strongly eroded pre-rift basement. The seafloor relief is smooth across this other type of COT. We suggest the pre-rift lithospheric configuration had major influence on the formation of the COT, besides transfer zones. Volcanic domains, confined to the north of competent crustal blocks correlate with the style of the COT. Gravity modelling revealed an extremely thinned crust across the shelf. We propose a depth-dependent extension model with crust being decoupled from mantle lithosphere, explaining the discrepancy of subsidence observed across the South China Sea region.

The structure, evolution and symmetry of the magma-poor rifted margins of the North and Central Atlantic: A synthesis

Magma-poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting, the serpentinization of the mantle beneath crust thinned to less than 8 ± 2 km, and the unroofing of a broad zone of mantle within the continent–ocean transition, accompanied by the development of detachment and other large-offset faults. These observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes. Although the paucity of magmatism may be explained by depth-dependent extension of the lithosphere, and by pre-depleted sub-crustal lithosphere, rifting above initially cool sub-lithospheric mantle also may explain the subsidence deficit observed at some margins: synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere. As it extends, thins and cools, ductile creeping layers in the mid- and deep crust become progressively more brittle, resulting in increased coupling between the upper and lower crust, and eventually the embrittlement of the entire crust, faults cutting from the surface across the Moho, bringing water into the mantle and causing its serpentinization. Increased coupling and the development of serpentine detachments predict the development of late-stage asymmetry once the entire crust is brittle. Such detachments are imaged on some margins and inferred on others; analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric. Similar analyses show that depth-dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning. Instead this “extension discrepancy” may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting, related to the changes in the rheological character and strength of the lithosphere as it is thinned. Complex polyphase faulting continues after complete crustal separation, resulting in the exhumation of broad expanses of peridotitic basement, the top of which is everywhere marked by an exhumed slip surface, similar to the corrugated surface observed at mid-ocean ridges. The similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the COT and true oceanic crust difficult and possibly moot.

Complex rifted continental margins explained by dynamical models of depth-dependent lithospheric extension

Observations from a number of rifted margins (e.g., South Atlantic salt basin, mid-Norwegian margin, Exmouth Plateau) reveal wide regions of extremely attenuated crust and depositional environments that indicate depth-dependent lithospheric extension. Although the one-dimensional thermal-kinematic consequences of depth-dependent extension are understood, no comprehensive process-based explanation for the complex style of these margins exists. Here, we present self-consistent numerical models of passive-margin formation that explain the depth-dependent extension, the width of the margin, its characteristic tripartite nature, and why such margins are prone to deposition of evaporites under appropriate climatic conditions. Some features that are important to reproducing the observed characteristics include decoupling between upper and lower parts of the lithosphere during stretching, contrasting wide and narrow extensional styles above and below the decoupling level, and progressive focusing of crustal extension toward the rift axis.