Stress dynamics associated with the Nyasa / Malawi rift: Implication for the present-day East African Rift System dynamics

The Nyasa/Malawi rift (NMR), known for its poor magma and notable seismic activity, has sparked a debate regarding its stress kinematics. It is on one hand viewed as a transform fault, while on other hand as a rift structure characterized by normal faulting. In order to address this controversy, we conducted paleostress analysis that involved collecting fault slip data along the central to southern region of the rift. We integrated our findings with published kinematic data on focal mechanisms in the rift. Our results reveal that the central part of the rift experiences radial or sub-radial extension, while the southern half is subject to oblique NNE-SSW transtensive tectonic forces. The minimum horizontal principal stress axis aligns with an orientation of 020°. As we move further south, the extension direction changes by approximately 25°, resulting in a predominantly north-south opening with a minimum horizontal stress axis direction of 175° (Shmin = 175°). The degree of structural penetration and intensity of faulting indicate that the north-south opening is more significant and pronounced in the southern region compared to the northern region. Additionally, we observed that faults dipping to the east and trending NW-SE exhibit sinistral (left-lateral) movement, while faults dipping to the southwestern side display dextral (right-lateral) movement. This suggests that, regionally, the NMR primarily experiences a normal faulting regime, albeit with a significant strike-slip component, which accounts for the oblique kinematics observed. The tectonic regimes identified through our fault slip data encompass the crust and upper mantle, spanning a lithospheric scale.

The role of gravitational body forces in the development of metamorphic core complexes

Within extreme continental extension areas, ductile middle crust is exhumed at the surface as metamorphic core complexes. Sophisticated quantitative models of extreme extension predicted upward transport of ductile middle-lower crust through time. Here we develop a general model for metamorphic core complexes formation and demonstrate that they result from the collapse of a mountain belt supported by a thickened crustal root. We show that gravitational body forces generated by topography and crustal root cause an upward flow pattern of the ductile lower-middle crust, facilitated by a detachment surface evolving into low-angle normal fault. This detachment surface acquires large amounts of finite strain, consistent with thick mylonite zones found in metamorphic core complexes. Isostatic rebound exposes the detachment in a domed upwarp, while the final Moho discontinuity across the extended region relaxes to a flat geometry. This work suggests that belts of metamorphic core complexes are a fossil signature of collapsed highlands.

A long-standing controversy surrounds low-angle nature of observed detachment faults within metamorphic core complexes. Here, the authors show that post-orogenic collapse of mountain belts can create a low-angle detachment, resolving the controversy.

Surface Displacements Mechanism of the Dobi Graben from ASAR Time-Series Analysis of InSAR: Implications for the Tectonic Setting in the Central Afar Depression, Ethiopia

The Dobi graben is a Quaternary, NW-trending continental rift found within the East-Central Block (ECB) of the Afar Depression (AD) in Ethiopia. The AD might be the only place where three active rifts meet on land. This diffused, Rift–Rift–Rift (RRR) triple junction in the ECB comprises the overlap zone between the Red Sea and the Gulf of Aden propagators. Rifting is ongoing in the Dobi graben as evidenced by the August 1989 earthquakes (of magnitude 5.7 < MW < 6.2). This study carried out a surface displacement time-series analysis to examine the kinematics of the Dobi graben and the surrounding area using 18 ascending orbit scenes (between May 2005 and March 2010) along tract 257 and 15 along the descending orbit (tract 006) of the Advanced Synthetic Aperture Radar (ASAR), C-band (λ = 5.6 cm) acquired by the ENVIronmental SATellite (ENVISAT). We utilized the Small Baseline Algorithm (SBA) techniques of the distributed scatterer, which were implemented independently to generate Line of Sight (LOS) displacement maps. These LOS displacement surface movements, identified in both geometries, can be interpreted as ± signs of predominantly vertical movement in both geometries: positive for uplifting and negative for subsidence. Additionally, opposite signs of ± horizontal movement in both geometries indicate that the movement is from East to West (or vice versa). Results from the velocity and displacement maps and time series analysis suggest that creeping is associated mainly with normal faulting and could be the primary mechanism for strain distribution for the Southeastern part of the Dobi graben. The anomalous, continuous uplifting exhibited at the rift shoulder and in the horst area might be linked to the presence of temporary reactivation of normal faulting in the region. The oblique, positive LOS signals observed in different parts of the Dobi graben might serve as a proxy for understanding how strain is accommodated as normal faulting and is distributed in a distinct northeast direction. This explanation supports both the arguments for the Northeast migration of the triple junction and the transfer of strain from the southernmost Red Sea Rift (RSR) to the Central AD.

Sensitivity of rift tectonics to global variability in the efficiency of river erosion

The efficiency of erosion in leveling relief mainly depends on climate and strength of exposed rocks. However, whether erosion is sufficiently efficient to influence the architecture of a tectonic plate boundary remains a topic of debate. Here, we analyze continental rift landscapes reworked by river incision to assess a globally representative range of fluvial erosion efficiency. We then simulate crustal extension exposed to surface processes acting within this documented range. We find that more efficient erosion favors the growth of half-grabens over horsts, which can explain contrasting tectonic styles across the Basin and Range province and the East African Rift. This suggests that variability in Earth’s geological structures partly reflects variability in hydrological conditions and associated surface processes.

Erosion and sedimentation constantly rework topography created by tectonics but also modulate stresses in the underlying crust by redistributing surficial loads. Decades of numerical modeling further suggest that surface processes help focus deformation onto fewer, longer-lived faults at tectonic plate boundaries. However, because the surface evolution parameters used in these models are not quantitatively calibrated against real landscapes and because the history of fault activity can be difficult to infer from the geological record, the sensitivity of tectonic deformation to a realistic range of erosional efficiency remains unknown. Here, we model the growth of half-grabens, where slip on a master normal fault shapes an adjacent mountain range as it accommodates crustal stretching. We subject our simulations to fluvial incision acting at rates assessed by morphometric analysis of rivers draining natural rift systems. Increasing erosional efficiency within the geologically documented range alleviates the energy cost of topographic growth and increases the total extension that can be accommodated by half-graben master faults by as much as ∼50%. Efficient erosion favors an eventual basin-ward relocalization of strain, preventing the development of horst structures. This behavior is consistent with structural and morphometric observations across 12 normal fault-bounded ranges, suggesting that surface erodibility and climatic conditions have a measurable impact on the tectonic makeup of Earth’s plate boundaries.

Cretaceous basin evolution in northeast Asia: tectonic responses to the paleo-Pacific plate subduction

Cretaceous rift basin evolution was an important part of the tectonic history of northeast Asia in the late Mesozoic. Three types of rift basins are identified-active, passive and wide rift basins-and they developed in different regions. Passive rift basins in the eastern North China craton are thought to be the consequence of crustal stretching and passive asthenospheric upwelling. Wide rift basins in the eastern Central Asian orogen are assumed to originate from gravitational collapse of the thickened and heated orogenic crust. Active rift basins in the northern North China craton are attributed to uprising of asthenospheric materials along a lithospheric-scale tear fault. Slab tearing of the subducting paleo-Pacific plate is postulated and well explains the spatial distribution of different types of rift basins and the eastward shifting of magmatism in the northern North China craton. The Late Cretaceous witnessed a period of mild deformation and weak magmatism, which was possibly due to kinematic variation of the paleo-Pacific plate.

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.

Rifted margins classification and forcing parameters

Rifted margins are the result of the successful process of thinning and breakup of the continental lithosphere leading to the formation of new oceanic lithosphere. Observations on rifted margins are now integrating an increasing amount of multi-channel seismic data and drilling of several Continent-Ocean Transitions. Based on large scale geometries and domains observed on high-quality multi-channel seismic data, this article proposes a classification reflecting the mechanical behavior of the crust from localized to diffuse deformation (strong/coupled to weak/decoupled mechanical behaviors) and magmatic intensity leading to breakup from magma-rich to magma-poor margins. We illustrate a simple classification based on mechanical behavior and magmatic production with examples of rifted margins. We propose a non-exhaustive list of forcing parameters that can control the initial rifting conditions but also their evolution through time. Therefore, rifted margins are not divided into opposing types, but described as a combination and continuum that can evolve through time and space.

South China Sea documents the transition from wide continental rift to continental break up

During extension, the continental lithosphere thins and breaks up, forming either wide or narrow rifts depending on the thermo-mechanical state of the extending lithosphere. Wide continental rifts, which can reach 1,000 km across, have been extensively studied in the North American Cordillera and in the Aegean domain. Yet, the evolutionary process from wide continental rift to continental breakup remains enigmatic due to the lack of seismically resolvable data on the distal passive margin and an absence of onshore natural exposures. Here, we show that Eocene extension across the northern margin of the South China Sea records the transition between a wide continental rift and highly extended (<15 km) continental margin. On the basis of high-resolution seismic data, we document the presence of dome structures, a corrugated and grooved detachment fault, and subdetachment deformation involving crustal-scale nappe folds and magmatic intrusions, which are coeval with supradetachment basins. The thermal and mechanical weakening of this broad continental domain allowed for the formation of metamorphic core complexes, boudinage of the upper crust and exhumation of middle/lower crust through detachment faulting. The structural architecture of the northern South China Sea continental margin is strikingly similar to the broad continental rifts in the North American Cordillera and in the Aegean domain, and reflects the transition from wide rift to continental breakup.

The transition from wide continental rift to continental break-up remains enigmatic. Here, the authors show that northern margin of the South China Sea records the transition between wide continental rift to a highly extended continental margin, with strikingly similar structures and metamorphic core complexes to those described from the North American Cordillera and the Aegean.

Aborted propagation of the Ethiopian rift caused by linkage with the Kenyan rift

Continental rift systems form by propagation of isolated rift segments that interact, and eventually evolve into continuous zones of deformation. This process impacts many aspects of rifting including rift morphology at breakup, and eventual ocean-ridge segmentation. Yet, rift segment growth and interaction remain enigmatic. Here we present geological data from the poorly documented Ririba rift (South Ethiopia) that reveals how two major sectors of the East African rift, the Kenyan and Ethiopian rifts, interact. We show that the Ririba rift formed from the southward propagation of the Ethiopian rift during the Pliocene but this propagation was short-lived and aborted close to the Pliocene-Pleistocene boundary. Seismicity data support the abandonment of laterally offset, overlapping tips of the Ethiopian and Kenyan rifts. Integration with new numerical models indicates that rift abandonment resulted from progressive focusing of the tectonic and magmatic activity into an oblique, throughgoing rift zone of near pure extension directly connecting the rift sectors.

Continuous continental rift zones evolve from enigmatic interactions between individual propagating rift segments. Here, the authors document progressive focusing of tectonic and magmatic activity caused by interactions between the Kenyan and Ethiopian rift segments of the East African Rift.

Chapter 4 Cenozoic rifting, passive margin development and strike-slip faulting in the Andaman Sea: a discussion of established v. new tectonic models

The Andaman Sea evolved from near-pure extension (WNW–ESE) during the Late Palaeogene, to highly oblique extension (NNW–SSE) during the Neogene, to strike-slip-dominated deformation (Late Miocene–Recent). These changes in extension direction and deformation style probably reflect the switch from slab rollback-driven extension to India coupling with Myanmar and driving oblique extension/dextral strike-slip. The East Andaman, Mergui–North Sumatra and Martaban Shelf basins, along with the Alcock and Sewell rises and Central Andaman Basin (CAB), were all involved with this deformation which became increasingly focused on the CAB and the rises with time. Possible revisions to traditional models for the Andaman Sea include: (1) the Alcock and Sewell rises are hyper-extended continental or island arc crust, not Miocene oceanic crust; (2) the East Andaman Basin is predominantly underlain by strongly necked to hyper-extended continental crust, not oceanic crust; or (3) CAB oceanic crust is of Miocene, not Pliocene–Recent age. At present a number of major issues can be addressed but not fully resolved, including: (1) the distribution, timing, volume and origin of magmatism in the basins; (2) the causes and significance of strong crustal reflections imaged on 2D and 3D seismic data; (3) implications for crustal thinning geometries, upper crustal extensional patterns and distribution of igneous intrusions for current models of passive margin development (i.e. volcanic v. non-volcanic margins), and how the back-arc setting modifies these models. Elements of both volcanic and non-volcanic margins are present in the East Andaman Sea, with well-developed necking of continental crust (perhaps due to dry mafic, granulite facies lower crust) and extensive igneous intrusions in the lower and middle crust.

The importance of structural softening for the evolution and architecture of passive margins

Lithospheric extension can generate passive margins that bound oceans worldwide. Detailed geological and geophysical studies in present and fossil passive margins have highlighted the complexity of their architecture and their multi-stage deformation history. Previous modeling studies have shown the significant impact of coarse mechanical layering of the lithosphere (2 to 4 layer crust and mantle) on passive margin formation. We built upon these studies and design high-resolution (~100–300 m) thermo-mechanical numerical models that incorporate finer mechanical layering (kilometer scale) mimicking tectonically inherited heterogeneities. During lithospheric extension a variety of extensional structures arises naturally due to (1) structural softening caused by necking of mechanically strong layers and (2) the establishment of a network of weak layers across the deforming multi-layered lithosphere. We argue that structural softening in a multi-layered lithosphere is the main cause for the observed multi-stage evolution and architecture of magma-poor passive margins.

Lasting mantle scars lead to perennial plate tectonics

Mid-ocean ridges, transform faults, subduction and continental collisions form the conventional theory of plate tectonics to explain non-rigid behaviour at plate boundaries. However, the theory does not explain directly the processes involved in intraplate deformation and seismicity. Recently, damage structures in the lithosphere have been linked to the origin of plate tectonics. Despite seismological imaging suggesting that inherited mantle lithosphere heterogeneities are ubiquitous, their plate tectonic role is rarely considered. Here we show that deep lithospheric anomalies can dominate shallow geological features in activating tectonics in plate interiors. In numerical experiments, we found that structures frozen into the mantle lithosphere through plate tectonic processes can behave as quasi-plate boundaries reactivated under far-field compressional forcing. Intraplate locations where proto-lithospheric plates have been scarred by earlier suturing could be regions where latent plate boundaries remain, and where plate tectonics processes are expressed as a ‘perennial' phenomenon.

The causes of intraplate deformation remain poorly constrained. Heron et al. use numerical models to show that ancient plate tectonic processes produce mantle lithosphere structures that may be reactivated to generate intraplate deformation.

Structure and evolution of the lunar Procellarum region as revealed by GRAIL gravity data

The Procellarum region is a broad area on the nearside of the Moon that is characterized by low elevations, thin crust, and high surface concentrations of the heat-producing elements uranium, thorium, and potassium. The region has been interpreted as an ancient impact basin approximately 3,200 kilometres in diameter, although supporting evidence at the surface would have been largely obscured as a result of the great antiquity and poor preservation of any diagnostic features. Here we use data from the Gravity Recovery and Interior Laboratory (GRAIL) mission to examine the subsurface structure of Procellarum. The Bouguer gravity anomalies and gravity gradients reveal a pattern of narrow linear anomalies that border Procellarum and are interpreted to be the frozen remnants of lava-filled rifts and the underlying feeder dykes that served as the magma plumbing system for much of the nearside mare volcanism. The discontinuous surface structures that were earlier interpreted as remnants of an impact basin rim are shown in GRAIL data to be a part of this continuous set of border structures in a quasi-rectangular pattern with angular intersections, contrary to the expected circular or elliptical shape of an impact basin. The spatial pattern of magmatic-tectonic structures bounding Procellarum is consistent with their formation in response to thermal stresses produced by the differential cooling of the province relative to its surroundings, coupled with magmatic activity driven by the greater-than-average heat flux in the region.

Rift migration explains continental margin asymmetry and crustal hyper-extension

When continents break apart, continental crust and lithosphere are thinned until break-up is achieved and an oceanic basin is formed. The most remarkable and least understood structures associated with this process are up to 200 km wide areas of hyper-extended continental crust, which are partitioned between conjugate margins with pronounced asymmetry. Here we show, using high-resolution thermo-mechanical modelling, that hyper-extended crust and margin asymmetry are produced by steady state rift migration. We demonstrate that rift migration is accomplished by sequential, oceanward-younging, upper crustal faults, and is balanced through lower crustal flow. Constraining our model with a new South Atlantic plate reconstruction, we demonstrate that larger extension velocities may account for southward increasing width and asymmetry of these conjugate magma-poor margins. Our model challenges conventional ideas of rifted margin evolution, as it implies that during rift migration large amounts of material are transferred from one side of the rift zone to the other.

Subduction, convergence and the mode of backarc extension in the Mediterranean region

30-35 Ma ago a major change occurred in the Mediterranean region, from a regionally compressional subduction coeval with the formation of Alpine mountain belts, to extensional subduction and backarc rifting. Backarc extension was accompanied by gravitational spreading of the mountain belts formed before this Oligocene revolution. Syn-rift basins formed during this process above detachments and low-angle normal faults. Parameters that control the formation and the kinematics of such flat-lying detachments are still poorly understood. From the Aegean Sea to the Tyrrhenian Sea and the Alboran Sea, we have analysed onshore the deformation and P-T-t evolution of the ductile crust exhumed by extension, and the transition from ductile to brittle conditions as well as the relations between deep deformation and basin formation. We show that the sense of shear along crustal-scale detachments is toward the trench when subduction proceeds with little or no convergence (northern Tyrrhenian and Alboran after 20 Ma) and away from the trench in the case of true convergence (Aegean). We tentatively propose a scheme explaining how interactions between the subducting slab and the mantle control the basal shear below the upper plate and the geometry and distribution of detachments and associated sedimentary basins. We propose that ablative subduction below the Aegean is responsible for the observed kinematics on detachments (i.e. away from the trench). The example of the Betic Cordillera and the Rif orogen, where the directions of stretching were different in the lower and the upper crust and changed through time, is also discussed following this hypothesis.

Continental lithospheric thinning and breakup in response to upwelling divergent mantle flow: application to the Woodlark, Newfoundland and Iberia margins

Depth-uniform stretching is not the dominant deformation process for thinning continental lithosphere leading to breakup; it cannot explain the observed depth-dependent lithosphere stretching and mantle exhumation at rifted continental margins. Depth-dependent lithosphere thinning, in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust, has been observed at many non-volcanic and volcanic rifted continental margins including conjugate margin pairs. Passive continental margins show a paucity of brittle deformation in the upper crust during continental lithosphere thinning leading to breakup and sea-floor spreading initiation. A new model of rifted continental margin formation has been developed that assumes that deformation and thinning of continental lithosphere leading to breakup occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere, and that this deformation evolves into sea-floor spreading. The new model successfully predicts depth-dependent stretching of continental margin lithosphere for both non-volcanic and volcanic margins and mantle exhumation at non-volcanic margins, both of which are observed, but are not explained, by existing depth-uniform continental lithosphere stretching models. The new model provides a balance of extensional strain, supplies an explanation for the paucity of synrift brittle deformation, and offers a simple transition from prebreakup lithosphere thinning to sea-floor spreading. The observed diversity of rifted continental margin structure and width of the ocean–continent transition can be explained by variability in the form of the upwelling divergent flow field. The new upwelling divergent flow model of continental lithosphere thinning leading to continental breakup successfully predicts the observed bathymetry and margin geometry for the most recent segment of sea-floor spreading initiation in the Woodlark Basin in the western Pacific, and the observed bathymetry and free air gravity anomaly for the Newfoundland and Iberian margins.

Timing and magnitude of depth-dependent lithosphere stretching on the southern Lofoten and northern Vøring continental margins offshore mid-Norway: implications for subsidence and hydrocarbon maturation at volcanic rifted margins

Subsidence analysis on the southern Lofoten and northern Vøring segments of the Norwegian rifted margin shows depth-dependent stretching of continental margin lithosphere in which lithosphere stretching and thinning at continental break-up at ~ 54 Ma greatly exceeds that of the upper crust within 100 km landward of the COB. For the southern Lofoten margin lithosphere β stretching factors approaching infinity are required at 54 Ma west of the Utrøst Ridge to restore the top Basalt (inner lava flow) reflectors and top Tare formation (54 Ma) to presumed sub-aerial depositional environments, while for the northern Vøring margin lithosphere β values of 2.5 are required. In contrast, upper crustal extension by faulting shows little stretching with β < 1.1 at break-up or immediately preceding break-up in the Paleocene and Late Cretaceous. The presence of lithosphere depth-dependent stretching and the absence of significant Paleocene and Late Cretaceous upper crustal extension imply that depth-dependent stretching of the southern Lofoten and northern Vøring margins occurred during sea-floor spreading initiation rather than during pre-break-up intra-continental rifting. Depth-dependent stretching, where upper-crustal extension is significantly smaller than whole-crustal or whole-lithosphere extension within 75–150 km of the COB, has been observed worldwide for both volcanic and non-volcanic rifted continental margins. Temperature and hydrocarbon maturation modelling show that the inclusion of depth-dependent stretching has an important effect on temperature and hydrocarbon maturation evolution in depth and time. Failure to include the large β factors for the lower crust and lithospheric mantle (below the less stretched upper crust) leads to a serious under-prediction of temperature and hydrocarbon maturation. While the effect of emplacing thick sill intrusions or magmatic underplating at continental break-up has an important effect on predicted temperature and %VR, their effects can be small in comparison with that of depth-dependent stretching.

Late Mesozoic–Cenozoic structural and stratigraphic correlations between the conjugate mid-Norway and NE Greenland continental margins

Late Mesozoic–Cenozoic tectonostratigraphic correlations between the conjugate mid-Norway and NE Greenland continental margins are provided, based on the recently refined structural and stratigraphic framework off mid-Norway, new early opening plate reconstructions of Eurasia versus Greenland between the Jan Mayen and Senja fracture zones, and a sparse regional grid of seismic reflection profiles off NE Greenland. The Norwegian margin exhibits a distinct along-strike margin segmentation governed by across-margin transfer systems. In particular, the Bivrost Fracture Zone and its landward transfer zone prolongation are well-defined features. Corresponding features are here interpreted on the conjugate NE Greenland margin. Together, these conjugate transfer/fracture zones represent a first-order, across-margin tectonomagmatic boundary of prolonged structural inheritance. Regional transects across both margins reveal important vertical and lateral variations in crustal configuration and composition resulting from a complex history of rifting prior to and during the last rift episode in Late Cretaceous–Early Tertiary time, leading to break-up and volcanic passive margin formation. Although the composite Late Jurassic–earliest Cretaceous rifting was the dominant tectonic episode, one also observes structural and stratigraphic relations that indicate an Aptian–?Albian rift phase, probably co-eval with similar events elsewhere on the NE Atlantic margins. Late Cretaceous rifting, with onset in middle Campanian time, was characterized by low-angle detachment faulting, culminating in regional uplift, intrusive igneous activity and subsequent erosion towards the end of the Paleocene. Thick seaward-dipping reflector sequences indicate massive eruptions of lavas during break-up at the Paleocene–Eocene transition. The post-break-up passive margin development was characterized by the transport and deposition of large amounts of sediment in response to margin subsidence and continental uplift, particularly during two distinct phases of outbuilding in Oligocene?/Miocene and Plio-Pleistocene times. Of special interest are a number of mid-Cenozoic intra-basin inversion features recognized both off Norway and off Greenland, revealing a regional compressive regime.

Patterns of extension and magmatism along the continent-ocean boundary, South China margin

Early Oligocene sea-floor spreading in the South China Sea was preceded by at least two episodes (in Maastrichtian and Mid-Eocene time) of continental extension that generated a series of rift basins on the South China margin, which are separated from the continent-ocean boundary (COB) by an outer structural high. Regional multichannel seismic profiles showing faulting of the pre-rift basement allow the amount of extension in the upper crust to be measured. The total subsidence across the South China margin is far in excess of that predicted using a forward flexural-cantilever model of extension and the degree of faulting measured seismically in the upper crust. This mismatch suggests preferential extension of the lower crust, increasing towards the COB to account for the subsidence. The same feature is seen in the Nam Con Som Basin, which is located close to the southwest end of an extinct propagating spreading ridge offshore from Vietnam. However, in the Beibu Gulf Basin, which is not adjacent to the COB, subsidence is approximately compatible with uniform extension in the upper and lower crust across the entire basin, if not at all locations. We predict that extension of the lower crust exceeds that in the lithospheric mantle along the COB. Heat-flow measurements at Ocean Drilling Program (ODP) sites on the Chinese continental slope and on the conjugate Dangerous Grounds margin yield values consistent with, or slightly higher than, those predicted by models of uniform extension in the lithosphere. Although there is no magmatism comparable with the seaward-dipping volcanic rocks of rifted volcanic margins, there is seismic evidence of rift-related volcanic rocks spanning a width ofc.25 km landward of the COB. Simple adiabatic melting models do not predict magmatism, and we suggest that the presence of water in the mantle lithosphere, together with residual pre-rift heat, may instead be responsible for increasing melting here. Deep-water syn-rift sediments recovered by the ODP near the COB indicate that volcanism was submarine and that rifting culminated in a mass wasting event that marks a break-up unconformity. The average extension in South China Sea is much less than that seen in the extreme ‘non-volcanic’ Iberian margin. The South China margin may represent an intermediary form of continental extension between the end member extremes of the Iberia-type non-volcanic and the Greenland-type volcanic margin.

Timing of Extension on the Büyük Menderes Graben, Western Turkey, and Its Tectonic Implications

The Büyük Menderes Graben is one of the most prominent structures of western Anatolia (Turkey) and borders the Aegean. New structural and stratigraphic evidence demonstrates that the (?)Miocene fluvio-lacustrine, coal-bearing red clastic sediments exposed along the northern margin of the graben are northward back-tilted, locally folded and overlain unconformably by horizontal terraced Pliocene-Pleistocene sediments. Also, there is no evidence that these red clastics at the base of the Neogene sequence were deposited during neotectonic extension. It is suggested here that these sediments cannot be regarded as passive neotectonic graben-fill deposits.

This new evidence further indicates that the age of the modern Büyük Menderes Graben is Pliocene, younger than previously considered (Early-Middle Miocene) and that initiation of north-south neotectonic extensional tectonics in the graben, and thus in western Anatolia, is unlikely to have resulted from orogenic collapse. The Pliocene estimate of the start of extension is in close agreement with the start of slip on the North Anatolian Fault Zone. The north-south extensional tectonics, and associated east-west faulting and basin formation, commenced during the Pliocene due to the effect of westward tectonic escape of the Anatolian block along the North and East Anatolian Faults. New mammal evidence also constrains the start of slip on the younger faults which bound the present-day graben floor to c. 1 Ma.

The Büyük Menderes Graben has experienced a two-stage extension. An initial extension (latest Oligocene-Early Miocene) along initially moderately, steeply dipping normal faults was superseded by movement on steeper normal faults during the (?)Pliocene. The two phases of deformation appear to reflect significant changes in the tectonic setting of western Anatolia and are attributed to orogenic collapse followed by tectonic escape.