Gravitational Tectonics Research Articles

Active detachment faulting controls folding and faulting in western Borneo, SE Asia

The origin of active deformation and structural evolution in large areas of Western Borneo has been highly debated, with two contrasting views involving gravitational tectonics and plate tectonics. The scarcity of field data on land has significantly hampered our understanding of the onshore structures and their relationship with those of the offshore regions. The Baram Delta province is one of the best examples in SE Asia, where debates on the origin and evolution of active tectonic versus gravitational tectonic structures are broadly associated with the complexity of faults and folds in Brunei and Sarawak (Malaysia). In this paper, we present the results of the first large-scale satellite-based structural mapping and the detailed outcrop-based structural mapping in Brunei Darussalam. The results of the satellite-based mapping reveal a major ∼NE-SW trending detachment fault in Brunei and adjacent regions, which is termed the Tutoh fault, with numerous associated secondary faults and folds. The fault acts as a detachment structure onto which several NW-SE trending faults ramp and have asymmetrical folds showing an en echelon fold-fault system. The topographic expression and the recent strike-slip faulting event on the Tutoh fault system suggest that the fault remains active, challenging the discourse that gravitational tectonics is the only cause of active deformation in Borneo.

INTERPRETAÇÃO SÍSMICA E MODELAGEM 3D ESTRUTURAL NA BACIA DE BARREIRINHAS (MARANHÃO, BRASIL)

Discoveries of hydrocarbons in the basins of the African Equatorial Margin and Guinea Gulf stimulated the exploratory interest in the basins of the Brazilian Equatorial Margin, for being together before the Continental Drift. This interest emerges because both African and South American equatorial margin are considered analogous. The Barreirinhas Basin is a member of the Brazilian equatorial margin. The objective of this work is to present the results obtained through the seismic interpretation and structural 3D modeling, in the context of gravitational tectonics, in an area covered by 3D seismic data, in the Barreirinhas Basin. The compressional domain of an extensive-compressive system was mapped. In this study, were identified reverse faults, thrust faults and fault-related folding like fault-bend and fault-propagation fold that can be accompanied by backthrust features in deep to ultra-deep waters. The 3D structural model allowed the representation of the geometric variations present in the study area. The new information will be important for the identification and evaluation of structures with greater potential for hydrocarbon accumulations and can help in future studies to characterize the reservoir, contributing to the evolution of knowledge of the equatorial margin, especially in the Barreirinhas Basin.

State-of-Art and Remarks on Some Open Questions About Dsgsds: Hints from a Review of the Scientific Literature on Related Topics

Deep-Seated Gravitational Slope Deformations (DSGSDs) are complex, slope-scale, gravity-driven phenomena, which are placed between landslide movements s.s. and gravitational tectonics. These processes, which develop over very long time periods, are considerably relevant both for the remarkable influence on the evolution of hillslope morphology and for risk associated with the induced deformations and the possible evolution into catastrophic landslides. A large number of studies on DSGSDs was produced in the scientific literature; the lack of a widely recognized classification, together with the great variability of shapes and characteristics, have led to the development of numerous terminologies and definitions, which make the approach to these phenomena particularly complex. Despite this, it is possible to identify some fundamental characteristics, both in terms of kinematic mechanisms and induced morphological elements and of controlling factors and possible evolution. In more recent years, some studies highlighted the relevance of the elevated spatial and temporal scales that feature DSGSDs, with specific reference to the time-dependent rheology regulating the deformation of rock masses as a prominent distinctive feature of such processes. This study describes the current state of the art, starting from both the numerous works present in the literature and specialized review articles aimed at individual aspects of these phenomena. The work is developed in order to describe both the history of scientific progress in this field and all the main aspects that characterize the phenomena in question, both in terms of intrinsic characteristics and in relation to the control factors and possible evolution. Through the analyzes of literature data, indications are given on the space-time scale and on the characteristics that allow to distinguish DSGSDs from other gravitational phenomena.

Enormous Mass Movements, and Gravitational Tectonics Model of the North Serayu Mountains, Karangkobar Area, Banjarnegara Regency, Central Java, Indonesia

North Serayu Mountains in the Central Java province, Indonesia, stretches with west - east axis, bordered by Slamet volcano to the Bogor Mountains in the west, and delimited by Ungaran volcano to the Kendeng Mountains in the east. In the north of these mountains there is coastal alluvial plain of Java, and in the south there is a depression zone of River Serayu. Overall geomorphostructures of the North Serayu Mountains form a faulted anticlinorium, which one of its flank relatively dipping to the south. In the bottom part of this ranges are such plastic, clastic, clayey sedimentary rocks, Eocene to Miocene aged. While at the upper portion there is a group of elastic, brittle, massive volcanic rocks, andesitic to basaltic composition, Pliocene to Pleistocene aged. A volcano, called Rogo Jembangan stands over the top of the North Serayu anticlinorium, with two of its eccentric cones, namely Mount Telagalele and Mount Pawinihan, situated in the Karangkobar District. North Serayu anticlinorium with a plastic bedrock, which is overlain by elastic, hard, and heavy rock has created such a tectonic model influenced by gravity. Parts of volcano’s body and volcanic rocks blocks in the Banjarnegara Regency area generally move slowly but surely southward over a giant slip plane in the form of orographic fields. Locally, this gravitational tectonic is manifested as mass slides, glides, and creeps, occur any time in the study area. Orogenetics of the North Serayu Mountains is still on going in line with the active tectonism of the Java island, caused by subduction of Indian-Australian plate beneath the Eurasian plate. As long as that is the case, gravitational tectonic will continue to run, and mass movements in the research area will regenerate to happen.

Interaction between sea-level changes and depositional tectonics: Implications for hydrocarbon prospectivity in the western coastal swamp depobelt, Niger Delta Basin, Nigeria

This paper presents the potential depositional framework of sandstones and unconsolidated sand reservoirs, seals, and traps in the western part of the coastal swamp depobelt of the Niger Delta Basin, Nigeria, through integration of sequence stratigraphy and seismic imaging. The integration of well logs and biostratigraphic (foraminifera and palynological) data with three-dimensional seismic volume established the sequence stratigraphic framework, which has improved our understanding of the impact of sea-level changes in the distribution of sand facies. Four third-order seismic depositional sequences (sequences 1–4) of Serravallian deposits were identified based on the Vail stratigraphic concept. Depositional processes, structures, thickness, and seismic signatures vary in each sequence. Sequence 1, dominated by mud and very fine-grained sand facies, represents the outer shelf–shelf margin setting and has experienced shale tectonics (ductile deformation). Sequences 2 and 3 consist of alternating sand–shale facies and have experienced gravitational tectonics. Although sequence 2 represents middle shelf–inner shelf setting, sequence 3 represents inner shelf–transitional setting. Sequence 4 consists of thick fluvial–coastal plain sand facies and represents deltaic plain or continental sediments. These deposits reflect postgravitational tectonics. Within the third-order sequences are fourth-order sequences that recorded the interaction between small-scale transgressive–regressive cycles and depositional tectonics. The potential reservoirs within these sequences are stacked channel sandstones, shoreface sandstones, and heteroliths. The juxtaposition of the potential reservoirs against transgressive shales provides potential seals and traps. Structural interpretation shows synthetic and antithetic faults. In addition, event maps at various ages document structural closures that indicate the existence of possible hydrocarbon leads and prospects.

Overprinting translational domains in passive margin salt basins: insights from analogue modelling

Abstract. Current models of gravitational tectonics on the structural styles of salt-influenced passive margins typically depict domains of upslope extension and corresponding downslope contraction separated by a mid-slope domain of translation that is rather undeformed. However, an undeformed translational domain is rarely observed in natural systems as extensional and contractional structures tend to interfere in the mid-slope area. In this study, we use sandbox analogue modelling analysed by digital image correlation (DIC) to investigate some of the factors that control the structural evolution of translational domains. As in nature, experimental deformation is driven by slowly increasing gravitational forces associated with continuous basal tilting. The results show that a translational domain persists throughout the basin evolution when the pre-kinematic layer is evenly distributed. However, a thin (1 mm in the experiment, 100 m in nature) pre-kinematic layer can render the translational domain relatively narrow compared to settings with a thicker (5 mm) pre-kinematic layer. In contrast, early differential sedimentary loading in the mid-slope area creates minibasins separated by salt diapirs overprinting the translational domain. Similarly, very low sedimentation rate (1 mm per day in the experiment, < 17 m Ma−1 in nature) in the early stage of the experiment results in a translational domain quickly overprinted by downslope migration of the extensional domain and upslope migration of the contractional domain. Our study suggests that the architecture of passive margin salt basins is closely linked to the pre- and syn-kinematic cover thickness. The translational domain, as an undeformed region in the supra-salt cover, is a transient feature and overprinted in passive margins with either low sedimentation rate or a heterogeneous sedimentation pattern.

Mass Transport Deposits and geo-hazard assessment in the Bradano Foredeep (Southern Apennines, Ionian Sea)

Seafloor bathymetry, combined with multi-scale seismic reflection profiles, were used to describe the morphostructural setting of the Bradano Foredeep (Gulf of Taranto, northern Ionian Sea), where a submerged portion of the Southern Apennines is facing the Apulia Platform in the Calabrian Arc convergent margin. In this complex area, marine geophysical data highlight the presence of two mega-slide deposits at the shelf-slope transition, which are most likely the largest ever described in the region. These slid masses, named the Bradano Basento MegaSlide (BBMS) and Bradano Basento MegaSlide 1 (BBMS1), form a Mass Transport Deposit Complex (MTDC) affecting and eroding the topmost portion of the outer Apennines deformation front and the Apulian Foreland Ramp. It was emplaced in late Pleistocene times inside the narrow (about 10 km wide) Bradano Foredeep basin, a Plio-Pleistocene submarine trough developing at the chain front. The youngest of these deposits (BBMS1) likely predates the Last Glacial Maximum. Location of the MTDC within the subduction complex suggests that active tectonics and seismic shaking might represent the main triggering mechanisms for gravitative instability in this area, although gravitational tectonics and sediment creeping mechanisms cannot be excluded. On the other hand, size and distribution of the MTDC in the sedimentary record of the Bradano Foredeep suggest the need of re-evaluating the potential for large earthquakes/tsunamis, and more in general geological hazard related to submarine sliding masses, along the coast of this highly populated area.

Comment on: Coexistent pre-existing extensional and subsequent compressional tectonic deformation in the Kashmir basin, NW Himalaya by Akhtar et al., 2017 (article in press)

Kashmir basin is a piggyback basin that has developed at the last stage of the ongoing collision between India and Eurasia, and it is ~4 Ma old and contains 1300 m thick blanket of Plio-Pleistocene to Holocene deposits. These unconsolidated sediments are locally known as Karewas, which are deformed and displaced by a number of normal faults. These young normal faults are previously linked to the tectonic collision, however, here we question such consensus on the tectonic origin of the faults by demonstrating that the widely reported field evidence for active normal faulting in Kashmir is actually non-tectonic because the faults have formed by gravitational tectonics. This was achieved by reevaluation of all of the major studies that have reported the field data on active normal faulting in the region. We supplement our results with the structural, geological, geodetic and earthquakes centroid moment tensor data, which clearly consolidate our interpretations and demonstrate that active normal faults in Kashmir basin are not tectonically formed, and instead the lithological contrast between the Upper and Lower Karewas have created suitable conditions for structural failure. So, the faults are active, and would be expected to remain so as long as the vertical load favors the slip on the faults but these faults will not produce major earthquakes as are expected on the major Himalayan fault systems, and therefore the seismic hazard associated with such faults is minimum, which needs to be quantified and implemented in the future earthquake scenario maps.

Structural Restoration of Carbonate Platform in the Southern Part of Central Luconia, Malaysia

Carbonates of Central Luconia, Malaysia, had been serving as hydrocarbon reservoirs for more than 25 years. However, the relationship and impacts of extensive tectonic events to the growth of Miocene carbonates in Central Luconia have not been revealed sufficiently. In this work, two carbonates platforms in the southern part of Central Luconia were used for detailed interpretation and seismic based structural restoration. This work had provided new insight for the interpretation of carbonate growth in Central Luconia and its association with the surrounding tectonic. This work suggested three possible tectonic evolutions from Late Oligocene to Pliocene are responsible for the growth of carbonates in Central Luconia. These stages are equivalent with pre-carbonate stage (Late Oligocene–Early Miocene), syn-carbonate stage (Middle–Late Miocene) and post-carbonate stage (Pliocene). Rifting of the South China Sea and subduction of proto-South China Sea are believed to be responsible for the development of faulting during pre-carbonate stage, while movement of the ancient Baram Line is thought to control the parallel striking direction of normal faults during syncarbonate stage. Finally, subsidence and compaction due to the overburden clastic materials from the prograding deltas is considered as the main reason for the impacts of gravitational tectonics in this area, which corresponding to the post-carbonate stage.

Assessing the Long-Term Behaviour of the Industrial Bentonites Employed in a Repository for Radioactive Wastes by Studying Natural Bentonites in the Field

Bentonite makes an important contribution to the performance of the engineered barriers in most radioactive waste repository designs. The choice of bentonite results from its favourable properties for waste isolation and its stability in relevant geological environments. However, the longevity of bentonite (especially the resistance to waste container sinking) has been little studied. Modelling results suggest significant bentonite deformation and associated canister sinking is unlikely and, here, long-term natural system data are used as a reality check on model predictions. Results indicate that bentonite from the investigated site shows no significant deviation in bulk physical parameters from repository bentonite. However, micro-scale shear planes can be seen throughout the sampled cores. The presence of multi-directional S- and C-type shears suggests they originate from loading from the overlying limestone, not gravitational tectonics. The plastic limits and angles of shearing resistance for natural and repository bentonites suggest both are susceptible to shearing. The impact of bentonite shear under load could be minimised by appropriate design, but existing lower activity waste container designs do not consider the potentially high external stresses from the bentonite backfill and this should be addressed in future.

Ophiolitic detritus in Kimmeridgian resedimented limestones and its provenance from an eroded obducted ophiolitic nappe stack south of the Northern Calcareous Alps (Austria)

Abstract The causes for the Middle to Late Jurassic tectonic processes in the Northern Calcareous Alps are still controversially discussed. There are several contrasting models for these processes, formerly designated “Jurassic gravitational tectonics”. Whereas in the Dinarides or the Western Carpathians Jurassic ophiolite obduction and a Jurassic mountain building process with nappe thrusting is widely accepted, equivalent processes are still questioned for the Eastern Alps. For the Northern Calcareous Alps, an Early Cretaceous nappe thrusting process is widely favoured instead of a Jurassic one, obviously all other Jurassic features are nearly identical in the Northern Calcareous Alps, the Western Carpathians and the Dinarides. In contrast, the Jurassic basin evolutionary processes, as best documented in the Northern Calcareous Alps, were in recent times adopted to explain the Jurassic tectonic processes in the Carpathians and Dinarides. Whereas in the Western Carpathians Neotethys oceanic material is incorporated in the mélanges and in the Dinarides huge ophiolite nappes are preserved above the Jurassic basin fills and mélanges, Jurassic ophiolites or ophiolitic remains are not clearly documented in the Northern Calcareous Alps. Here we present chrome spinel analyses of ophiolitic detritic material from Kimmeridgian allodapic limestones in the central Northern Calcareous Alps. The Kimmeridgian age is proven by the occurrence of the benthic foraminifera Protopeneroplis striata and Labyrinthina mirabilis, the dasycladalean algae Salpingoporella pygmea, and the alga incertae sedis Pseudolithocodium carpathicum. From the geochemical composition the analysed spinels are pleonastes and show a dominance of Al-chromites (Fe3+–Cr3+–Al3+ diagram). In the Mg/(Mg+ Fe2+) vs. Cr/(Cr+ Al) diagram they can be classified as type II ophiolites and in the TiO2 vs. Al2O3 diagram they plot into the SSZ peridotite field. All together this points to a harzburgite provenance of the analysed spinels as known from the Jurassic suprasubduction ophiolites well preserved in the Dinarides/Albanides. These data clearly indicate Late Jurassic erosion of obducted ophiolites before their final sealing by the Late Jurassic–earliest Cretaceous carbonate platform pattern.

Jurassic raft tectonics in the northeastern Gulf of Mexico

The northeastern Gulf of Mexico is dominated by the 900–1800-m Florida Escarpment, which forms the bathymetric expression of the Cretaceous carbonate shelf edge. Outboard of the escarpment lies a region of salt-detached raft blocks, which are closely analogous to type examples in the Kwanza Basin, Angola, in terms of structural style, scale, and amount of extension. We undertook the first detailed structural interpretation of an emerging petroleum exploration province. The rafts detached and translated basinward by gravity gliding on the autochthonous Louann salt in the late Jurassic to early Cretaceous. The Upper Jurassic source rock (lime mudstones) of the Smackover Formation and eolian sandstone reservoir intervals of the Norphlet Formation are structurally segmented and entirely contained within the raft blocks. The rafts are separated by salt ridges and/or extensional fault gaps containing expanded uppermost Jurassic and lower Cretaceous strata of the Cotton Valley Group. The main episode of rafting occurred after deposition of the Smackover and Haynesville Formations and broke the Jurassic carbonate platform into raft blocks 2–40 km in length, which were then translated 25–40 km basinward from their original position. Map-view restoration of the raft blocks suggested a minimum extension of 100%, with basinward transport directions indicating a radial divergence of rafts. In the north of the study area, the transport direction was westerly, whereas in the south, translation was southerly. This pattern, which mimics the Florida Escarpment, suggested that the morphology of the Jurassic slope controlled the style of gravitational tectonics and the location of subsequent Cretaceous carbonate buildups. As with other linked systems on mobile substrates, the observed extension and translation must be balanced by downdip contraction. In the case of the northeastern Gulf of Mexico, the contraction is largely cryptic, being accommodated by salt evacuation, compression of salt walls/stocks, and possibly open-toed canopy advance.

Misinterpreting by localism: transposing European geology and tectonics onto Jamaica and the Antilles

Abstract ‘Localism’ occurs when geologists are most influenced by data with which they are familiar, applying it to new, but not necessarily appropriate, areas of study. Jamaica has been particularly prone to localist interpretations. Henry T. De la Beche (1796–1855) mapped eastern Jamaica in 1823–1824, and made lithostratigraphic comparisons between Jamaica and Europe that were the first attempt at intercontinental correlation. He correlated the Jamaican succession on the basis of lithological similarity to rocks in Europe. Charles A. Matley (1866–1947) is best remembered for his mapping in North Wales. He was geologist to the second geological survey of Jamaica in 1921 and developed the Basal Complex hypothesis which envisaged a geological structure analogous to that of North Wales, where the deformed Mona Complex underlies the Lower Palaeozoic succession. Matley thus provided a ‘factual’ basis for theories that the Antillean islands were the peaks of a foundered continent. Charles T. Trechmann (1885–1964) was the principal opponent of the Basal Complex hypothesis, not recognising evidence for old basement in Jamaica. His ‘answer’ was the Theory of Mountain Uplift, based on gravitational tectonics, but Trechmann's theory was comparable to the earlier fixist theories of tectonics that he learnt as a student.

Present‐day stress orientations and tectonic provinces of the NW Borneo collisional margin

Borehole failure observed on image and dipmeter logs from 55 petroleum wells across the NW Borneo collisional margin were used to determine maximum horizontal stress (σH) orientations; combined with seismic and outcrop data, they define seven tectonic provinces. The Baram Delta–Deepwater Fold‐Thrust Belt exhibits three tectonic provinces: its inner shelf inverted province (σH is NW‐SE, margin‐normal), its outer shelf extension province (σH is NE‐SW, margin‐parallel), and its slope to basin floor compression province (σH is NW‐SE, margin‐normal). In the inverted province, σH reflects inversion of deltaic normal faults. The σH orientations in the extension and compression provinces reflect deltaic gravitational tectonics. The shale and minibasin provinces have been recognized in offshore Sabah. In the shale province, σH is N010°E, which aligns around the boundary of a massif of mobile shale. Currently, no data are available to determine σH in the minibasin province. In the Balingian province, σH is ESE‐WNW, reflecting ESE absolute Sunda plate motions due to the absence of a thick detachment seen elsewhere in NW Borneo. The Central Luconia province demonstrates poorly constrained and variable σH orientations. These seven provinces result from the heterogeneous structural and stratigraphic development of the NW Borneo margin and formed due to complex collisional tectonics and the varied distribution and thicknesses of stratigraphic packages.

Passive margin evolution and its controls on natural gas leakage in the southern Orange Basin, blocks 3/4, offshore South Africa

Using a 2D seismic dataset that covers part of the southern Orange Basin offshore South Africa, we reconstructed the geological evolution of the basin. This evolutionary model was then used to investigate the occurrence of natural gas within the sedimentary column and the distribution of gas leakage features in relation to the observed sedimentary and tectonic structures developed in the post-rift succession since the Early Cretaceous. The Cretaceous succession has been subdivided into five seismic units. The highest sedimentation rates occur within the Barremian/Aptian (unit C1) and the Turonian/Coniacian (unit C3). Two Cenozoic units (T1 and T2) have been distinguished. These show a sudden decrease in sedimentation rate for the whole of the Cenozoic. Three phases of gravitational tectonics, with two Late Cretaceous phases of mass movement in the northern study area and Cenozoic slumping in the southern study area, have been related to sedimentation rates, sea-level changes, paleoenvironmental evolution and regional tectonics. The occurrence of natural gas leakage follows a coast-parallel distribution within the study area. In the near shore part at water depths shallower than 400 m, massive gas chimneys penetrate through the sediment layers and reach the (near-) surface. Within an intermediate narrow band, between 300 and <500 m water depth, the gas migrates more diffusely through sub-vertical faulted Cretaceous sediments, while in the outer part of the basin, through the Cretaceous and Cenozoic gravitational wedges, only very few signs of gas accumulation and migration can be seen along the faults. A conceptual model has been established with the Aptian source rock generating gas in the outer part of the basin. This source rock underlies the Cenozoic wedge in the south and the thick Cretaceous wedge in the north and is a postulated source for the natural gas within the sedimentary column. This thermogenically generated gas does not migrate directly through the gravitational faults and the above lying sediments, but moves buoyancy driven up-dip along stratigraphic layers, to escape through the sediments to the sea-floor in the inner shelf area.

Balancing deformation in NW Borneo: Quantifying plate-scale vs. gravitational tectonics in a delta and deepwater fold-thrust belt system

Recent GPS measurements demonstrate that NW Borneo undergoes 4–6 mm of plate-scale shortening a year, which is not accommodated by plate-scale structures. The only geological structure in NW Borneo described to accommodate on-going shortening is the Baram Delta System located on the outer shelf to basin floor. Delta toe fold-thrust belts are commonly thought to be caused by margin-normal compressional stresses generated by margin-parallel upslope gravitational extension. The Baram Delta System is divided into three neotectonic provinces: 1) an inactive onshore and inner shelf region of inversion superimposed on an older extensional deltaic province, 2) an outer shelf region of present-day deltaic extension, and; 3) a compressional delta toe. However, it is uncertain whether compression in the delta toe area is purely driven by gravity (i.e. shortening is less than upslope extension) or if there is a component of regional shortening involved, which can only be demonstrated if shortening is greater than upslope extension. In order to quantify the balance between the shortening related to the upslope gravitational extension and the shortening related to the regional geodynamic framework in the Baram Delta System, we have used a geomechanical code based on the Finite Element Method and conservation of mass and momentum, Dynel 2D. This enabled us to reconstruct the tectonic evolution of the delta and to demonstrate for the first time that the total shortening observed in the delta toe does not balance against the active extension in the delta top; with the system exhibiting >1.8% shortening overall (or ∼2.0 km). This additional shortening is therefore attributed to plate-scale shortening across NW Borneo produced by far-field compression. Using a convergence rate of 4 mm y −1, demonstrated by GPS measurements, the delta toe has accommodated far-field compression only in the last 0.5 My.

Cenozoic crustal deformation of the offshore Burgos basin region (NE Gulf of Mexico). A new interpretation of deep penetration multichannel seismic reflection lines

Abstract Along northeastern Mexico close to the Texas-Mexico border, the Burgos basin and its extension offshore was developed and deformed from the Paleocene up to Present time. This is a key triple junction between the sub meridian dextral transtensive coastal plain of the Gulf of Mexico extending far to the south in Mexico, the NE Corsair fault zone offshore and the sinistral Rio Bravo fault zone, a reactivated segment of the Texas lineament. Offshore NE Mexico, in the main study area covered by available seismic profiles, we have evidenced below the main well known gravitational décollement level (5 to 7 s twtt → 6 to 8 km) a Cenozoic deep-rooted deformation outlined by a N010° W trending deep-seated reverse fault zone and crustal folding down to the Moho (11 s twtt → ~ 20 km). Based on extensive offshore 2D industrial multi-channel seismic reflection surveys, deep exploration wells and gravimetric data, we focus our study on the deep crustal fabric and its effects on the gravitational tectonics in the upper sedimentary layers: submeridian crustal transtensional normal faults and open folding of the identified Mesozoic basement were interpreted as Cenozoic buckling of the crust during a major phase of oblique crustal extension. This deformation has probably enhanced gravity sliding along N030° growth-faults related to salt withdrawal and halokinesis in the offshore Burgos basin. We have tentatively made a link between this crustal deformation episode and the Neogene tectonic inversion of the Laramide foredeep basin of the Sierra Madre Oriental. The latter is still affected by crustal strike slip faulting associated with basaltic volcanism observed into the gulf coastal plain. This study favours a dominant crustal Cenozoic tectonic activity along the gulf margin without any clear evidence of Mesozoic tectonic reactivation. We propose that the large gravity collapse of the gulf margin was triggered by subsequent crustal deformation.

The Transylvanian Basin (Romania) and its relation to the Carpathian fold and thrust belt: Insights in gravitational salt tectonics

Interpretation of regional seismic profiles, stratigraphic and sedimentologic data improved insights in the evolution of the Transylvanian Basin. The basin evolution was coeval with the post-Mid-Cretaceous to recent deformation of the Carpathian Mts. Four tectonostratigraphic megasequences are differentiated: Upper Cretaceous (rift), Paleogene (sag), Lower Miocene (flexural basin) and Middle to Upper Miocene (backarc sequence dominated by gravitational tectonics). The Mid-Miocene continental collision in the Eastern Carpathians is associated with the rising Carpathians. This uplift enhanced the differential load, which, together with the high heat flow induced by Late Miocene to Pliocene arc volcanism, triggered large-scale Mio-Pliocene gravity spreading of the salt overburden. This ‘mega-slide’ comprises three structural domains, as follows: extensional weld (upslope), contractional folds (central) and contractional toe thrust (downslope). The diapirs in the east indicate a pre-shortening reactive/passive growth stage. The central folds are mostly the result of late shortening. Basement involved thrusting uplifted the toe thrust domain by the Late Pliocene. The Late Neogene to recent Carpathians uplift, backarc volcanism and gravity spreading are largely coeval.

South Atlantic continental margins of Africa: A comparison of the tectonic vs climate interplay on the evolution of equatorial west Africa and SW Africa margins

Africa displays a variety of continental margin structures, tectonic styles and sedimentary records. The comparative review of two representative segments: the equatorial western Africa and the SW Africa margins, helps in analysing the main controlling factors on the development of these margins. Early Cretaceous active rifting south of the Walvis Ridge resulted in the formation of the SW Africa volcanic margin that displays thick and wide intermediate igneous crust, adjacent to a thick unstretched continental crust. The non-volcanic mode of rifting north of the Walvis ridge, led to the formation of the equatorial western Africa margin, characterised by a wide zone of crustal stretching and thinning, and thick, extensive, syn-rift basins. Contrasting lithologies of the early post-rift (salt vs shale) determined the style of gravitational deformation, whilst periods of activity of the decollements were controlled by sedimentation rates. Regressive erosion across the prominent shoulder uplift of SW Africa accounts for high clastic sedimentation rate during Late Cretaceous to Eocene, while dominant carbonate production on equatorial western Africa shelf suggests very little erosion of a low hinterland. The early Oligocene long-term climate change had contrasted response in both margins. Emplacement of the voluminous terrigenous Congo deep-sea fan reflects increased erosion in equatorial Africa, under the influence of wet climate, whereas establishment of an arid climate over SW Africa induced a drastic decrease of denudation rate, and thus reduced sedimentation on the margin. Neogene emplacement of the African superswell beneath southern Africa was responsible for renewed onshore uplift on both margins, but it accelerated erosion only in the Congo catchment, due to wetter climatic conditions. Neogene high sedimentation rate reactivated gravitational tectonics that had remained quiescent since late Cretaceous.

A TRANSECT IN SOUTHERN TUSCANY, FROM THE BACCINELLO BASIN TO THE CETONA RIDGE

The Northern Apennines (NA) is a NE-verging thrust and fold belt (Fig. 1) produced by the Cenozoic collision between the Corso-Sardinian block and the Adria Plate (Boccaletti and Guazzone, 1972; 1974; Principi and Treves, 1984; Malinverno and Ryan, 1986, among others). The NA thrust-nappe pile consists of sequences deposited on the Adria Plate continental margin (Tuscan and Umbria-Marche Units) which were overthrust by the Ligurian Units during continental collision. The Ligurian Units, which consist of ophiolites and their sedimentary covers, represent the remnants of the Ligurian-Piedmont paleo-ocean (Abbate et al., 1970). The NA evolution, after the Oligocene continental collisional phase, has been referred to two different models. In the first one, the development of an external thrust belt shifting eastwards, coupled with an internal extensional area is envisaged. In this process both the crust and the lithosphere would be involved (Merla, 1951; Boccaletti and Guazzone, 1974; Elter et al., 1975; Carmignani et al., 1980; Boccaletti et al., 1990; Patacca et al., 1990). In front of the migrating chain a foredeep basin developed during Late Oligocene-Miocene, whose deposits were progressively annexed to the chain (Ricci Lucchi, 1986; Boccaletti et al., 1990). In the second model, the accretionary prism, thickened by crustal collision, would collapse to recover its equilibrium conditions (Carmignani and Kligfield, 1990). As a consequence of the gravitational collapse, the development of core complex structures and of extensional tectonics has taken place since Burdigalian-Langhian (Carmignani et al., 1994). In this model, the Northern Apennines thrust belt is explained as due to gravitational tectonics active since the Early Miocene (Decandia et al., 1993; Carmignani et al., 1995). Starting in late Tortonian, the internal area (hinterland) of NA was characterized by the development of continental and marine basins, mostly striking from NW-SE to N-S, and subparallel to the main thrust fronts of the chain. Their formation has been commonly referred to the extensional processes related to the opening of the Tyrrhenian Basin, either in a back-arc regime (Boccaletti and Guazzone, 1972; 1974; Malinverno and Ryan, 1986; Royden et al., 1987; Boccaletti et al., 1990; Patacca et al., 1990), or as due to late orogenic gravity collapse (Carmignani and Kligfield, 1990; Cameli et al., 1993; Carmignani et al., 1994; Keller et al., 1994). The eastward younging trend of the hinterland basins has been related to the eastward shifting of the extensional front, which followed the migration of the thrust front (Merla 1951; Elter et al., 1975). However, in the last two decades structural studies have shown that the deposits of the hinterland basins have been affected by widespread compressional deformations and by major angular unconformities (Boccaletti et al., 1995, and references therein) that allow to subdivide the entire succession into five Unconformity Bounded Stratigraphic Units (UBSUs, Salvador, 1987).) since the unconformities that limited each UBSU have been correlated on a regional scale, (Boccaletti et al., 1994; 1995). The deformations affecting the basin fill, their architecture, the presence of regional unconformities strongly suggest that they have mostly developed under a compressional tectonic regime, tied to reactivation of thrust faults affecting the pre-Neogene substrate (Boccaletti et al., 1995; 1997; Bonini and Moratti, 1995; Landi et al., 1995). The objective of this excursion, whose road log is reported in Fig. 2, is to illustrate the relationships between tectonics and sedimentation in the hinterland basins along a transect across Southern Tuscany including the Baccinello-Cinigiano, Velona and Siena-Radicofani Basins. In this area, basin development has been mainly related to the activity of two crustal thrusts: the Mid-Tuscany Metamorphic Ridge and the Cetona Mountain thrusts (Fig. 3).