Extensional tectonic regimes in the Aegean basins during the Cenozoic

Abstract Kinematics of faults in the Northern Aegean show three extensional tectonic regimes the tensional directions of which trend (1) WNW-ESE, (2) NE-SW and (3) N-S. These were active during the Upper Miocene, Pliocene-Lower Pleistocene and Mid Pleistocene-Present day, respectively. The main characteristics of the stress patterns (1) and (2) on the overall Aegean is tentatively explained by variations of the horizontal lithospheric stress value σ_zz due to the slab push and of the vertical lithospheric stress value σ_zz due to mass heterogeneities. During the Mid Pleistocene-Present, due to the slab push, tectonics were compressional along the arc boundary: σ_zz was σ1. In the Aegean basins, tectonics were extensional, c_2Z was σ1 as a consequence of the thickness of the continental crust and, possibly of an updoming asthenosphere; thus σ_zz became σ2, allowing tension σ3 to be orthogonal to the compression along the arc, i.e. to be roughly parallel to the arc trend. During the Pliocene-Lower Pleistocene, the extensional regime was distinctly different. The tensional directions were roughly radial to the arc. It is suggested that σ_zz was weakly compressional, or eventually tensional, due a seaward migration of the slab so that σ_zz became σ3. In the Northern Aegean, the stress pattern has been also controlled by the westward push of the Anatolian landmass. During the Mid Pleistocene-Present day, this was typically extensional (al was vertical) and the right lateral strike-slip motion on the North Anatolian Fault transformed into a N-S-stretching, E-W-shortening of the Northern Aegean. Dextral strike-slip motions along the North Aegean Trough fault zone were possible on NE-SW-striking faults. During the Pliocene-Lower Pleistocene, normal fault components were higher; however, because the angle between the NE-SW trend of the tensional axis and the strike of the fault zone was acute, dextral strike-slip components were possible on all the faults striking NE-SW to E-W. A clockwise 15^o rotation of Limnos with respect to Samothraki, Thraki and Thassos, suggested by structural data, was probably associated with these dextral motions. The WNW-ESE trending tension during the Upper Miocene indicates that the dextral North Anatolian Fault had not yet merged into the North Aegean Trough fault zone at that time. We propose that the formation of Aegean basins during the Cenozoic was related to the activity of two major Hellenic arcs. The ‘Pelagonian-Pindic Arc’ resulted in the formation of the subsident Aegean basins of Middle Eocene-Lower Miocene age and of the older Northern Aegean orogenic volcanism. The ‘Aegean Arc’ resulted in the formation of the subsident Aegean basins of Middle Miocene to Present day age and of the Southern Aegean orogenic volcanism. Were these arcs associated with a unique subduction zone or with two such zones ? In the first case, the slab is no more than 16 Myr old, in the second it may be as old as 45–50 Myr. The answer depends on the accuracy of the seismic tomography profiles.     

Fault-proximal stratigraphic record of episodic extension and oblique inversion, Bisbee basin, southwestern New Mexico, USA

Tectonic inversion models predict that stratigraphic thickening and local facies patterns adjacent to reactivated fault systems should record at least two phases of basin development: (1) initial extension‐related subsidence and (2) subsequent shortening‐induced uplift. In the central Peloncillo Mountains of southwestern New Mexico, thickness trends, distribution, and provenance of two major stratigraphic intervals on opposite sides of a northwest‐striking reverse fault preserve a record of Early Cretaceous normal displacement and latest Cretaceous–Paleogene reverse displacement along the fault. The Aptian–Albian Bisbee Group thickens by a factor of three from the footwall to the hanging‐wall block, and the Late Cretaceous?–Eocene Bobcat Hill Formation is preserved only in the footwall block. An initial episode of normal faulting resulted in thickening of upper Aptian–middle Albian, mixed siliciclastic and carbonate deposits and an up section change from coarse‐grained deltas to shallow‐marine depositional conditions. A second episode of normal faulting caused abrupt thickening of upper Albian, quartzose coastal‐plain deposits across the fault. These faulting episodes record two events of extension that affected the northern rift shoulder of the Bisbee basin. The third faulting episode was oblique‐slip, reverse reactivation of the fault and other related, former normal faults. Alluvial and pyroclastic deposits of the Bobcat Hill Formation record inversion of the Bisbee basin and development of an intermontane basin directly adjacent to the former rift basin. Inversion was coeval with latest Cretaceous–Paleogene shortening and magmatism. This offset history offers significant insight into extensional basin tectonics in the Early Cretaceous and permits rejection of models of long‐term Mesozoic shortening and orogen migration during the Cretaceous. This paper also illustrates how episodes of fault reactivation modify, in very short distances (<10 km), regional patterns of subsidence, the distribution of sediment‐source areas, and sedimentary depositional systems.     

Trench-forearc interactions reflected in the sedimentary fill of Talara basin, northwest Peru

Exceptional exposure of the forearc region of NW Peru offers insight into evolving convergent margins. The sedimentary fill of the Talara basin spans the Cretaceous to the Eocene for an overall thickness of 9000 m and records within its stratigraphy the complicated history of plate interactions, subduction tectonics, terrane accretion, and Andean orogeny. By the early Tertiary, extensional tectonism was forming a complex horst and graben system that partitioned the basin into a series of localized depocentres. Eocene strata record temporal transitions from deltaic and fluvial to deep‐water depositional environments as a response to abrupt, tectonically controlled relative sea‐level changes across those depocentres. Stratigraphic and provenance data suggest a direct relationship between sedimentary packaging and regional tectonics, marked by changes in source terranes at major unconformities. A sharp shift is recognized at the onset of deepwater (bathyal) sedimentation of the Talara Formation, whose sediments reflect an increased influx of mafic material to the basin, likely related to the arc region. Although the modern topography of the Amotape Mountains partially isolates the Talara basin from the Lancones basin and the Andean Cordillera to the east, provenance data suggest that the Amotape Mountains were not always an obstacle for Cordilleran sediment dispersal. The mountain belt intermittently isolated the Talara basin from Andean‐related sediment throughout the early Tertiary, allowing arc‐related sediment to reach the basin only during periods of subsidence in the forearc region, probably related to plate rearrangement and/or seamounts colliding with the trench. Intraplate coupling and/or partial locking of subduction plates could be among the major causes behind shifts from contraction to extension (and enhanced subduction erosion) in the forearc region. Eventually, collisional tectonic and terrane accretion along the Ecuadorian margin forced a major late‐Eocene change in sediment dispersal.     

OLDLANDS: CHARACTERISTICS AND IMPLICATIONS BASED ON THE AUSTRALIAN EXPERIENCE

Oldlands are complex surfaces of low relief preserved on Precambrian shields and cratons and Paleozoic massifs. Interpretation of their character and age is difficult, but as a consequence of its particular location and of conceptual developments, much of the Australian Craton is now susceptible to analysis in terms of exhumation, etching, and multistage development. Exhumed surfaces of many ages are recorded. Long periods of weathering and erosion generated low relief, although recurrent block tectonics produced a differentiated topography and also resulted in regolithic veneers, some of them with mineral concentrations that later became duricrusts. The associated landforms are of various ages, but are mostly of Early and Middle Tertiary ages. Cretaceous and Early to Middle Tertiary etch forms are widely developed and preserved. Earlier Mesozoic (Jurassic, Triassic) surfaces are also represented or implied. Many cratonic landforms are related to the subsurface weathering and subsequent erosion to which oldlands have been subjected, to the exploitation of fractures in the basement rocks, to underprinting from fracture zones in the basement, and to deep erosion, causing rivers to breach alien structures. Multistage as well as two-stage forms are common, and pre-weathered detritus derived from regoliths was contributed to adjacent basins. [Key words: oldland, etch surface, underprinting, duricrust, paleosurface.]     

Numerical modelling of Cretaceous Pyrenean Rifting: The interaction between mantle exhumation and syn-rift salt tectonics

The preshortening Cretaceous Pyrenean Rift is an outstanding geological laboratory to investigate the effects of a pre-rift salt layer at the sedimentary base on lithospheric rifting. The occurrence of a pre-rift km-scale layer of evaporites and shales promoted the activation of syn-rift salt tectonics from the onset of rifting. The pre- and syn-rift sediments are locally affected by high-temperature metamorphism related to mantle ascent up to shallow depths during rifting. The thermo-mechanical interaction between décollement along the pre-existing salt layer and mantle ascent makes the Cretaceous Pyrenean Rifting drastically different from the type of rifting that shaped most Atlantic-type passive margins where salt deposition is syn-rift and gravity-driven salt tectonics has been postrift. To unravel the dynamic evolution of the Cretaceous Pyrenean Rift, we carried out a set of numerical models of lithosphere-scale extension, calibrated using the available geological constraints. Models are used to investigate the effects of a km-scale pre-rift salt layer, located at the sedimentary cover base, on the dynamics of rifting. Our results highlight the key role of the décollement layer at cover base that can alone explain both salt tectonics deformation style and high-temperature metamorphism of the pre-rift and syn-rift sedimentary cover. On the other hand, in the absence of décollement, our model predicts symmetric necking of the lithosphere devoid of any structure and related thermal regime geologically relevant to the Pyrenean case.     

Structural development of the Tertiary fold-and-thrust belt in east Oscar II Land, Spitsbergen

The Tertiary deformation in east Oscar II Land, Spitsbergen, is compressional and thin-skinned, and includes thrusts with ramp-flat geometry and associated fault-bend and fault-propagation folds. The thrust front in the Mediumfjellct-Lappdalen area consists of intensely deformed Paleozoic and Mesozoic rocks thrust on top of subhorizontal Mesozoic rocks to the east. The thrust front represents a complex frontal ramp duplex in which most of the eastward displacement is transferred from sole thrusts in the Permian and probably Carboniferous strata to roof thrusts in the Triassic sequence. The internal geometries in the thrust front suggest a complex kinematic development involving not only simple 'piggy-back', in-sequencc thrusting, but also overstep as well as out-of-sequence thrusting. The position of the thrust front and across-strike variation in structural character in east Oscar II Land is interpreted to be controlled by lithological (facies) variations and/or pre-existing structures, at depth, possibly cxtensional faults associated with the Carboniferous graben system.     

Shear-wave polarizations in the Peter the First Range indicating crack-induced anisotropy in a thrust-fault regime

Summary. Three-component seismograms of small local earthquakes recorded in the Peter the First Range of mountains near Garm, Tadzhikistan SSR, display shear-wave splitting similar to that previously observed near the North Anatolian Fault in Turkey. The Peter the First Range is in a region of compressional tectonics, whereas the North Anatolian Fault is a comparatively simple strike-slip fault. Detailed analysis of the Turkish records suggests that the splitting is diagnostic of crack-induced anisotropy caused by vertical microcracks aligned parallel to the direction of maximum compression. Preliminary examination of paper records from Garm shows that most shear waves arriving within the shear-wave window display shear-wave splitting, and that the polarizations of leading shear-waves are consistently aligned in a NE/SW direction. The area is complicated and the tectonics are not well-understood, but the NE/SW direction is approximately perpendicular to the compressional axis in many of the fault-plane mechanisms of the earthquakes. These earthquakes are usually at depths between 5 and 12 km, although there are some deeper events nearby.
Parallel shear-wave polarizations, such as those observed, are expected to indicate the strike of nearly vertical parallel microcracks, which would be aligned parallel to the direction of maximum compression. Thus the shear-wave polarizations in the Peter the First Range indicate that the directions of principal stress are reversed in the rock above the earthquake foci where thrust faulting is taking place.     

Controls on Late Jurassic seismic sequences, Inner Moray Firth, UK North Sea: a critical test of a key segment of Exxon's original global cycle chart

A new interpretation of a comprehensive seismic- and well-database has resulted in the subdivision of the Mesozoic into four, basin-wide, seismo-stratigraphic depositional megasequences in the Inner Moray Firth (IMF) basin. Regional mapping of the megasequences has led to the construction of a new model for Mesozoic-Recent basin development in the IMF. It now appears that extensional tectonics was the main control on the basin's evolution during the Mesozoic. Structural geometries suggest that both the Triassic (Tr) and Rhaetian-mid Oxfordian (Jl) megasequences were controlled by regional broad-based subsidence associated with local extensional fault activity prior to the onset of renewed rifting in the IMF. In contrast, the late Oxfordian-Ryazanian (Berriasian; J2) megasequence developed in response to active extension characterised by half-graben development. Subsequent Early Cretaceous (Kl) deposition appears to have occurred during a further period of broad regional (thermal) subsidence. It is evident that strike-slip movement on the Great Glen Fault played a negligible role in Mesozoic basin development and it appears only to have had a local control on structural styles during its reactivation in the Tertiary as it accommodated regional uplift and basin inversion. Further subdivision of the J2 megasequence was possible using biostratigraphically-controlled seismic reflector terminations and led to the definition of five regional seismo-stratigraphic sequences (J2.1–2.5). Their geometric, thickness and sedimentary facies variations imply that the onlap-defined sequence boundaries within the late Oxfordian-Ryazanian (Berriasian; J2) megasequence were caused by syn-sedimentary extensional tectonism in a fully marine domain, rather than by fluctuations in global sea-level in a basin that was relatively quiescent tectonically. The new interpretation has particular significance in view of the fact that the Late Jurassic of the IMF was used by Exxon workers to construct part of their chart demonstrating relative changes of coastal onlap and global eustatic sea levels. As they considered that data from the area showed ‘no evidence that tectonics caused the unconformities’, the new interpretation casts doubt on the global applicability of the Late Jurassic section of Exxon's original sea-level chart. Furthermore, the study demonstrates that reflector terminations within both tectonically active and/or fully marine sequences should be treated with extreme caution and not be used to define either periods of apparent low-stand or coastal onlap. Indeed, their appearance may sometimes only represent relatively local, auto- and allo-cyclic sedimentary processes such as submarine fan avulsion or channel switching, unrelated to changes in sea-level. Finally, the work shows that care must be taken in the selection of seismic lines used to establish and illustrate the nature of depositional sequences and their geometries if pitfalls are to be avoided.     

Cooling of the Sverdrup Basin during Tertiary basin inversion: implications for hydrocarbon exploration

ABSTRACT The regional thermal history of the north‐eastern Sverdrup Basin, Canadian Arctic Archipelago, has been assessed using apatite fission‐track thermochronology and vitrinite reflectance data. Fission‐track data for 27 samples from six wells through the Mesozoic section on Axel Heiberg and Ellesmere Islands reveal significant Palaeocene cooling associated with basin inversion during the Eurekan Orogeny. Fission‐track data for 29 outcrop samples, ranging in stratigraphic age from Cambrian to Tertiary, also reveal significant Palaeocene cooling. Vitrinite reflectance data from carbonaceous shales and coal seams in well and outcrop samples are consistent with these conclusions. The degree of Palaeocene cooling observed is greatest for well and outcrop samples in the cores of anticlines or the hanging walls of thrust faults, such as the Fosheim anticline, and faults, such as the Lake Hazen fault system, and the East Cape and Vesle Fiord thrust faults. Palaeocene cooling is largely attributed to the denudation of structures during the Eurekan Orogeny. At one locality on north‐western Ellesmere Island, which is on the northern flank of the Sverdrup Basin, the underlying Franklinian basement rocks yield Early Cretaceous fission track ages with relatively long mean track lengths. This indicates that this part of the basin was uplifted at this time and that subsequent sedimentation and subsidence in the Cretaceous and early Tertiary were modest. This locality thus appears to be on the rift shoulder, which developed along the flank of the Amerasia Basin in the Lower Cretaceous. At a locality on western Axel Heiberg Island, which is downflank from the rift shoulder, the Upper Jurassic Awingak sandstone has a Late Cretaceous fission track age. This is best explained by heating above the total annealing temperature for fission‐tracks in apatite by extensive Lower Cretaceous intrusions and subsequent heat dissipation and cooling in the Late Cretaceous followed by further substantial cooling due to Tertiary denudation. These results indicate that maximum burial temperatures occurred in the presently exposed Mesozoic section prior to basin inversion during the Eurekan Orogeny. It can therefore be inferred that peak hydrocarbon generation and primary migration predated the formation of structural traps during the Tertiary at shallow depths within the northern Sverdrup Basin.     

The nature of the Precambrian-Tertiary boundary at Renardodden, Bellsund, Svalbard

The previous literature on the occurrence of Tertiary strata at Renardodden provides contradictory information about the primary versus tectonic boundary with the Precambrian basement. Tertiary sandstones and shales overlie unconformably the basement rocks, which have been resedimented as boulder conglomerates prior to Caledonian metamorphism and weathered prior to the deposition of the Tertiary strata. The boundary relations are complicated by a repeatedly active fault system that may form part of the Inner Hornsund Fault Zone.     

Emergence of the Canadian Rockies and adjacent plains: A comparison of physiography between end-of-Laramide time and the present day

The landscape of the Canadian Rockies in southern Alberta is not a direct result of constructional processes; that is, the ridges and peaks have not been pushed into the positions in which we see them today. Tectonic activity provided original elevation but not mountains: at the end of Laramide time, what are now the front ranges and foothills of the Rockies comprised a high-elevation upland of relatively low relief. The present mountain physiography is the result of 55–60 million years of post-orogenic differential erosion, in which more resistant rocks have been left at higher elevations than less-resistant rocks.The Canadian Rockies and the foothills are developed in a thin-skinned, thrust-and-fold belt created during the Laramide Orogeny; the adjacent Interior Plains cut across foreland basin sediments derived from the mountains. The mountains currently consist of large parts of ridges of well-indurated Paleozoic and, locally, Proterozoic rock alternating with valleys developed in soft Mesozoic clastic rock. In the foothills, where the soft Mesozoic rock is at the surface, relief is subdued, but ridges of more-resistant sandstone rise above shaley lowlands. The plains are relatively flat but also contain erosional outliers of higher paleo-plains-surfaces.Numerous lines of evidence suggest that the mountains and foothills have lost several kilometers of overburden since the end of the Laramide Orogeny, while the western plains have lost at least 2 km, requiring that the local relief of the mountains and foothills that we see is erosional in origin. Local physiography is adjusted to lithology: the mountains have high relief because the exposed sub-Mesozoic rocks can hold up high, steep slopes, whereas the foothills have low relief because the underlying Cretaceous rocks cannot hold up high, steep slopes. The east-facing escarpment at the mountain front is a fault-line scarp along a low-angle thrust.Mesozoic rocks involved in the deformation originally extended all the way across the thrust and fold belt, and physiography of the belt at the end of Laramide time (60–55 Ma) depended mainly on whether Mesozoic or Paleozoic/Proterozoic rocks were exposed at the surface at that time. A reconstruction using critical-taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies: what are now the front ranges at the eastern edge of the Rocky Mountains were mostly or perhaps entirely covered with Mesozoic rocks and despite that high elevation had a hilly, not mountainous, character. The main ranges, in the central Rocky Mountains, were in part stripped of Mesozoic cover by then and more mountainous. Treeline was higher then, and the thrust belt may have been largely or entirely vegetated. Generation of modern relief in the front ranges, including the escarpment at the mountain front, had to await stripping of Mesozoic rocks and incision of rivers into harder substrates in post-Laramide time.The Interior Plains are an erosional surface that was cut 1 to 3 km below the aggradational top of the foreland basin sediments. Although some of the present low local relief of the plains results from weakness of underlying Cretaceous/Tertiary rocks, the low relief is probably largely related to the process of denudation.     

Giant submarine collapse of a carbonate platform at the Turonian–Coniacian transition: The Ayabacas Formation, southern Peru

The Ayabacas Formation of southern Peru is an impressive unit formed by the giant submarine collapse of the mid‐Cretaceous carbonate platform of the western Peru back‐arc basin (WPBAB), near the Turonian–Coniacian transition (～90–89 Ma). It extends along the southwestern edge of the Cordillera Oriental and throughout the Altiplano and Cordillera Occidental over >80 000 km² in map view, and represents a volume of displaced sediments of >10 000 km³. The collapse occurred down the basin slope, i.e. toward the SW. Six zones are characterised on the basis of deformational facies, and a seventh corresponds to the northeastern ‘stable’ area (Zone 0). Zones 1–3 display increasing fragmentation from NE to SW, and are composed of limestone rafts and sheets embedded in a matrix of mainly red, partly calcareous and locally sandy, mudstones to siltstones. In contrast, in Zones 4 and 5 the unit consists only of displaced and stacked limestone masses forming a ‘sedimentary thrust and fold system’, with sizes increasing to the southwest. In Zone 6, the upper part of the limestone succession consists of rafts and sheets stacked over the regularly bedded lower part. The triggering of this extremely large mass wasting clearly ensued from slope creation, oversteepening and seismicity produced by extensional tectonic activity, as demonstrated by the observation of synsedimentary normal faults and related thickness variations. Other factors, such as pore pressure increases or lithification contrasts probably facilitated sliding. The key role of tectonics is strengthened by the specific relationships between the basin and collapse histories and two major fault systems that cross the study area. The Ayabacas collapse occurred at a turning point in the Central Andean evolution. Before the event, the back‐arc basin had been essentially marine and deepened to the west, with little volcanic activity taking place at the arc. After the event, the back‐arc was occupied by continental to near‐continental environments, and was bounded to the southwest by a massive volcanic arc shedding debris and tuffs into the basin.     

Cretaceous evolution of the Andean margin between 36°S and 40°S latitude through a multi‐proxy provenance analysis of Neuquén Basin strata (Argentina)

During the Cretaceous, the Neuquén Basin transitioned from an extensional back‐arc to a retroarc foreland basin. We present a multi‐proxy provenance study of Aptian to Santonian (125–84 Ma) continental sedimentary rocks preserved in the Neuquén Basin used to resolve changes of sediment drainage pattern in response to the change in tectonic regime. Sandstone petrology and U–Pb detrital zircon geochronology constrain the source units delivering detritus to the basin; apatite U–Pb and fission track dating further resolve provenance and determine the age and patterns of exhumation of the source rocks. Sandstone provenance records a sharp change from a mixed orogenic source during Aptian time (ca. 125 Ma), to a magmatic arc provenance in the Cenomanian (ca. 100 Ma). We interpret this provenance change as the result of the drainage pattern reorganisation from divergent to convergent caused by tectonic basin inversion. During this inversion and early stages of contraction, a transient phase of uplift and basin erosion, possibly due to continental buckling, caused the pre‐Cenomanian unconformity dividing the Lower from Upper Cretaceous strata in the Neuquén Basin. This phase was followed by the development of a retroarc foreland basin characterised by a volcanic arc sediment provenance progressively shifting to a mixed continental basement provenance during Turonian‐Santonian (90–84). According to multi‐proxy provenance data and lag times derived from apatite fission track analysis, this trend is the result of a rapidly exhuming source within the Cordillera to the west, in response to active compressional tectonics along the western margin of South America, coupled with the increasing contribution of material from the stable craton to the east; this contribution is thought to be the result of the weak uplift and exhumation of the foreland due to eastward migration of the forebulge.     

The early Cretaceous marine transgression and its significance for landscape interpretation

Significant elements of the Australian landscape date from Mesozoic or earlier times. Australia did not separate completely from other Gondwanan components until Early Tertiary times and these Mesozoic and older elements can therefore be regarded as Gondwanan. During the separation and northern drift of the continent and particularly in Late Jurassic and earlier Cretaceous times the sea invaded and spread across much of the erstwhile landmass. The associated sediments not only covered and preserved much of the pre‐existing land surface, but they also augmented the effects of thalassostatic loading of the basins, causing further subsidence. Hinge lines developed near the coastal zones of the times, so that subsidence of the basins caused adjacent land masses to rise. Many old land surfaces have been re‐exposed at the former oceanic margins, but epigene forms are preserved high in the relief on the uplifted blocks. They survive partly because, as Crickmay (1976) suggested, rivers effectively erode at and near their channels; the divides remain untouched. A reinforcement effect also operates because the valleys are wet sites, the interfluves dry. Hence weathering and erosion proceed apace in the former while the latter are stable, allowing palaeoforms to survive.     

The stratigraphic and structural evolution of the Dzereg Basin, western Mongolia: clastic sedimentation, transpressional faulting and basin destruction in an intraplate, intracontinental setting

The Dzereg Basin is an actively evolving intracontinental basin in the Altai region of western Mongolia. The basin is sandwiched between two transpressional ranges, which occur at the termination zones of two regional‐scale dextral strike‐slip fault systems. The basin contains distinct Upper Mesozoic and Cenozoic stratigraphic sequences that are separated by an angular unconformity, which represents a regionally correlative peneplanation surface. Mesozoic strata are characterized by northwest and south–southeast‐derived thick clast‐supported conglomerates (Jurassic) overlain by fine‐grained lacustrine and alluvial deposits containing few fluvial channels (Cretaceous). Cenozoic deposits consist of dominantly alluvial fan and fluvial sediments shed from adjacent mountain ranges during the Oligocene–Holocene. The basin is still receiving sediment today, but is actively deforming and closing. Outwardly propagating thrust faults bound the ranges, whereas within the basin, active folding and thrusting occurs within two marginal deforming belts. Consequently, active fan deposition has shifted towards the basin centre with time, and previously deposited sediment has been uplifted, eroded and redeposited, leading to complex facies architecture. The geometry of folds and faults within the basin and the distribution of Mesozoic sediments suggest that the basin formed as a series of extensional half‐grabens in the Jurassic–Cretaceous which have been transpressionally reactivated by normal fault inversion in the Tertiary. Other clastic basins in the region may therefore also be inherited Mesozoic depocentres. The Dzereg Basin is a world class laboratory for studying competing processes of uplift, deformation, erosion, sedimentation and depocentre migration in an actively forming intracontinental transpressional basin.     

Tertiary structures in the platform cover strata of Nordenskiold Land, Svalbard

Air photo interpretation along with limited field work is the basis of a compilation map of Tertiary structures in the Upper Paleozoic through Mesozoic platform cover strata of Nordenskiold Land. Permian Kapp Starostin Formation strata form a continuous marker horizon delineating both a large NE-verging fold complex, which involves the basement (Hecla Hoek sequence) through basal Tertiary strata, and somewhat smaller scale folds, some of which may have formed in association with detachments and thrusts within the platform cover sequence. The map pattern is both a function of local structural plunge and changes in fold geometry along strike. Regional considerations suggest that subsurface basement-involved thrusts exist. In S Nordenskiold Land, to the E of folded Kapp Starostin Formation strata, a 3.5 km wide zone of folding and thrusting in Triassic and Jurassic strata above a subhorizontal decollement is inferred to occur. Further E is the W limb of the central Tertiary basin syncline.     

Basin tectonic history and paleo‐physiography of the pelagian platform,northern Tunisia,using vitrinite reflectance data

Constraining the thermal, burial and uplift/exhumation history of sedimentary basins is crucial in the understanding of upper crustal strain evolution and also has implications for understanding the nature and timing of hydrocarbon maturation and migration. In this study, we use Vitrinite Reflectance (VR) data to elucidate the paleo‐physiography and thermal history of an inverted basin in the foreland of the Atlasic orogeny in Northern Tunisia. In doing so, it is the primary aim of this study to demonstrate how VR techniques may be applied to unravel basin subsidence/uplift history of structural domains and provide valuable insights into the kinematic evolution of sedimentary basins. VR measurements of both the onshore Pelagian Platform and the Tunisian Furrow in Northern Tunisia are used to impose constraints on the deformation history of a long‐lived structural feature in the studied region, namely the Zaghouan Fault. Previous work has shown that this fault was active as an extensional structure in Lower Jurassic to Aptian times, before subsequently being inverted during the Late Cretaceous Eocene Atlas I tectonic event and Upper Miocene Atlas II tectonic event. Quantifying and constraining this latter inversion stage, and shedding light on the roles of structural inheritance and the basin thermal history, are secondary aims of this study. The results of this study show that the Atlas II WNW‐ESE compressive event deformed both the Pelagian Platform and the Tunisian Furrow during Tortonian‐Messinian times. Maximum burial depth for the Pelagian Platform was reached during the Middle to Upper Miocene, i.e. prior to the Atlas II folding event. VR measurements indicate that the Cretaceous to Ypresian section of the Pelagian Platform was buried to a maximum burial depth of ~3 km, using a geothermal gradient of 30°C/km. Cretaceous rock samples VR values show that the hanging wall of the Zaghouan Fault was buried to a maximum depth of <2 km. This suggests that a vertical km‐scale throw along the Zaghouan Fault pre‐dated the Atlas II shortening, and also proves that the fault controlled the subsidence of the Pelagian Platform during the Oligo‐Miocene. Mean exhumation rates of the Pelagian Platform throughout the Messinian to Quaternary were in the order of 0.3 mm/year. However, when the additional effect of Tortonian‐Messinian folding is accounted for, exhumation rates could have reached 0.6–0.7 mm/year.     

Palaeomagnetism of dykes and tuffs from the Mesudiye region and rotation of Turkey

Summary. The pole positions obtained from Upper Cretaceous and Eocene tuffs and dykes of the Mesudiye region, which is located between the north of the North Anatolian Fault Zone and eastern Black Sea coast, are at 75.3° N, 275.4° E and 41.7° N, 138.6° E respectively. These results, taken together with the results of previous studies of Turkish rocks, suggest that rotational movements of Turkey have been 45–50° counterclockwise to Europe since the Upper Cretaceous.     

Impact of normal faulting and pre‐rift salt tectonics on the structural style of salt‐influenced rifts: the Late Jurassic Norwegian Central Graben,North Sea

Studies of salt‐influenced rift basins have focused on individual or basin‐scale fault system and/or salt‐related structure. In contrast, the large‐scale rift structure, namely rift segments and rift accommodation zones and the role of pre‐rift tectonics in controlling structural style and syn‐rift basin evolution have received less attention. The Norwegian Central Graben, comprises a complex network of sub‐salt normal faults and pre‐rift salt‐related structures that together influenced the structural style and evolution of the Late Jurassic rift. Beneath the halite‐rich, Permian Zechstein Supergroup, the rift can be divided into two major rift segments, each comprising rift margin and rift axis domains, separated by a rift‐wide accommodation zone – the Steinbit Accommodation Zone. Sub‐salt normal faults in the rift segments are generally larger, in terms of fault throw, length and spacing, than those in the accommodation zone. The pre‐rift structure varies laterally from sheet‐like units, with limited salt tectonics, through domains characterised by isolated salt diapirs, to a network of elongate salt walls with intervening minibasins. Analysis of the interactions between the sub‐salt normal fault network and the pre‐rift salt‐related structures reveals six types of syn‐rift depocentres. Increasing the throw and spacing of sub‐salt normal faults from rift segment to rift accommodation zone generally leads to simpler half‐graben geometries and an increase in the size and thickness of syn‐rift depocentres. In contrast, more complex pre‐rift salt tectonics increases the mechanical heterogeneity of the pre‐rift, leading to increased complexity of structural style. Along the rift margin, syn‐rift depocentres occur as interpods above salt walls and are generally unrelated to the relatively minor sub‐salt normal faults in this structural domain. Along the rift axis, deformation associated with large sub‐salt normal faults created coupled and decoupled supra‐salt faults. Tilting of the hanging wall associated with growth of the large normal faults along the rift axis also promoted a thin‐skinned, gravity‐driven deformation leading to a range of extensional and compressional structures affecting the syn‐rift interval. The Steinbit Accommodation Zone contains rift‐related structural styles that encompass elements seen along both the rift margin and axis. The wide variability in structural style and evolution of syn‐rift depocentres recognised in this study has implications for the geomorphological evolution of rifts, sediment routing systems and stratigraphic evolution in rifts that contain pre‐rift salt units.     

Shear-wave anisotropy and the stress field from borehole recordings at 2.5 km depth at Cajon Pass

53 local earthquakes recorded at 2.5 km depth in the Cajon Pass scientific borehole are analysed for shear-wave splitting. The time delays between the split shear waves can be positively identified for 32 of the events. Modelling these observations of polarizations and time delays using genetic algorithms suggests that the anisotropic structure near Cajon Pass has orthorhombic symmetry. The polarization of the shear waves and the inferred strike of the stress-aligned fluid-filled intergranular microcracks and pores suggests that the maximum horizontal compressional stress direction is approximately N13°W. This is consistent with previous results from earthquake source mechanisms and the right-lateral strike-slip motion on the nearby San Andreas Fault, but not with stresses measured within the uppermost 3 km of the borehole. This study suggests that the San Andreas Fault is driven by deeper tectonic stresses and the present understanding of a weak and frictionless San Andreas Fault may need to be modified. The active secondary faulting and folding close to the fault are probably driven by the relatively shallow stress as measured in the 3.5 km deep borehole.