Early Stages Of Extension Research Articles

Transfer zones and associated volcanic province in the eastern Valencia Basin: Evidence for a hot rifted margin?

The role of NW-SE transfer zones during the opening of the Northwestern Mediterranean Basin has long been evidenced, mostly from seismic-reflection and magnetic data. Each of these features corresponds to transverse basement steps and/or fractures controlling the progressive deepening from the Valencia Basin toward the center of the Liguro-Provençal Basin. The most important one, the North-Balearic-Fracture-Zone, accounts for the counterclockwise rotation of the Corsica-Sardinia block and separates the Valencia continental basin from the Liguro-Provençal oceanic basin. Another transfer zone, the Central Fracture Zone is the limit of the NE part of the Valencia Basin floored with volcanic basement. Further north, the Catalan Fracture Zone corresponds to a large scale transfer zone that limits between the eastern Pyrenees and the thinned crust of the Gulf of Lion. New observations on deep seismic lines show in detail the extent of a 1–3 km thick volcanic Province around the North-Balearic-Fracture-Zone. It includes the numerous volcanoes of the Valencia Basin mostly superimposed on the transverse fractures zones. The transfer zones are also characterized by narrow syn-rift grabens interpreted as transtensional pull-apart basins along the transfer faults. The transfer zones are associated at depth with domes uplifting the Moho, thinning locally the crust down to 3 km and exhuming deeper material. Deep reflections and thinning at the base of the crust draw extensional shear-zones and crustal boudins that, together with the large amount of syn-rift volcanism in the area, suggests a ductile behavior during early stage of extension. This ductile behavior has then evolved toward more brittle deformation and deposition of syn-rift sediments in asymmetric basins. The volcanic Province disappears in the distal prolongation of the Catalan Fracture Zone that we observe and characterize for the first time. Here a thin crust with flat top consists of westward-dipping reflections in the upper part, rooted in reflective layers. This crust passes laterally to the type II exhumed domain of the Liguro-Provençal Basin and to the oceanic domain to the south. Three crustal domains can thus be distinguished in the transition between the Valencia and Liguro-Provençal Basins illustrating the complex 3D deformation of the area.

Episodic heating of continental lower crust during extension: A thermal modeling investigation of the Ivrea-Verbano Zone

Rheology of the continental lower crust plays an integral role in governing the style of continental extension. Temperature-dependent creep deformation in the lower crust decreases lithospheric strength and promotes coupling of deformation in the brittle crust and upper mantle; however, few constraints exist concerning the thermal evolution of extending lower crust. Here, we present a high-temperature thermochronological investigation of the Ivrea-Verbano Zone—archetypal continental lower crust that was attenuated during opening of the Alpine Tethys oceanic basin. Rutile U–Pb dates collected from three samples spanning the bottom ∼2-km of the crustal section are between ∼160 and ∼180 Ma, and exhibit near-rim zonations in Zr, Hf, Nb and Ta. Thermal-kinematic modeling of these data, combined with existing rutile U–Pb dates, show that the base of the Ivrea-Verbano Zone experienced heating on two timescales: conductive heating over ∼10 Myr, associated with thinning of the lithospheric mantle, and advective heating over <0.1 Myr, associated with high-temperature infiltration of fluids during crustal exhumation. These constraints match the thermal predictions of geodynamic models that predict high-magnitude thinning of lithospheric mantle during the early stages of extension. Conductive heating of lower crust directly preceded mantle exhumation and crustal excision, suggesting that thermal weakening of the lithosphere promotes focusing of extensional strain.

Along strike behavior of the Tizi n’ Firest fault during the Lower Jurassic rifting (Central High Atlas Carbonate basin, Morocco)

The purpose of this study is to document the along-strike early syn-rift history of the Lower Jurassic Carbonate basin of the Central High Atlas (Morocco) by combining sedimentological observations and high-resolution biostratigraphy. Six sections, each from the Sinemurian to the Upper Pliensbachian, were investigated along a 75 km-long transect at the hanging wall of a major fault of the Lower Jurassic Basin (i.e. the Tizi n’ Firest fault). Depositional geometries of the early syn-rift deposits were reconstructed from the correlation between eight main timelines dated by biochronological markers for a time span covering about 6 Ma. Depocentre migration was examined and accommodation rates were calculated at the sub-zone timescale to discuss the along-strike-fault behavior of the Lower Jurassic basin formation. The early stages of extension are marked by contrasted along-strike variations in depositional geometry thickness, depocentre migration and accommodation rates, leading to the growth of three independent sub-basins (i.e. western, central, and eastern), ranging in size from 30 to 50 km, and displaying three contrasted tectono-sedimentary histories. Our results suggest that, during the early rifting phase, tectonic activity was not a continuous and progressive process evolving towards a rift climax stage, but rather a series of acceleration periods that alternated with periods of much reduced activity. The length of active fault segments is estimated at about 15–20 km, with a lifespan of a few ammonite sub-zones (> 2–3 Ma).

Reservoir quality variations within a conglomeratic fan-delta system in the Mahu sag, northwestern Junggar Basin: Characteristics and controlling factors

The conglomeratic reservoir in the Mahu depression, northwestern margin of the Junggar basin, Northwestern China has become an important exploration target. In this study, we use well and core data to investigate the characteristics and controlling factors of conglomeratic fan-delta reservoirs. The reservoir studied here developed in a retreating fan-delta complex that is dominated by conglomerate lithofacies and conglomeratic sandstone lithofacies. A petrological analysis shows that the reservoir exhibits poor compositional maturity and textural maturity. A physical property analysis shows that the reservoir exhibits extremely low porosity and extremely low permeability, as well as strong heterogeneity in the vertical and planar views. The depositional environment and compaction result in extremely low porosity and extremely low permeability in the reservoir. About 66% porosity was destroyed by the compaction. Fan-delta evolution and provenance control the macroscopic variations in reservoir quality. Muddy matrix content, grain size and dissolution control the microscopic variations in reservoir quality. About 52.5% porosity was induced by the dissolution. This reservoir study can be compared to fan-deltas that developed in marine-connected rift basins during early stages of extension and to nearshore subaqueous fans in East China. Additionally, the results of this study may provide a reference for such systems and provide a subsurface case for similar fan-delta outcrop studies and facies reservoir modelling.

Insoluble Residue Analysis of Limestone in Kolhan Group: Tectonic Implications

The data from the insoluble residue analysis of nearly ninety eight samples collected from different horizons fully corroborate the petrogenetic evidences obtained from the petrographic and field features of the Kolhan Limestone. High grade limestones containing nearly 10% of insolubles constitute about half of the limestone samples. Only in ten samples, the silt-clay portion dominates over the sand portion and such very high grade pockets (containing at times about 95% CaCO3) on chemical analysis should show almost equal distribution of SiO3 and Al2O3, both together totaling to 5 to 10%. In the remaining forty samples the sand fraction clearly outweighs the silt-clay fraction and these varieties of equally high grade pockets should analyze SiO2 content distinctly greater than the percentage of Al2O3 [1]. The sedimentation history in the Kolhans indicates a change from braided fluvial-ephemeral pattern to a fan delta lacustrine type. Repeated fault-controlled uplift of the source, followed by subsidence, generated multiple fining-upward cycles and a retrograding fan-delta system. The marked variations in thickness of the delta succession and the stacking pattern in different measured profiles reflect the overriding tectonic controls on fan-delta evolution. The accumulated fault displacement in active sectors created higher accommodation and thicker delta sequences. Intermittent uplift of fault blocks exposed fresh bedrock to mechanical weathering, generated a large amount of detritus, and resulted in forced closure of the land locked basin, repeatedly disrupting the fining upward pattern [2]. The controls of source rock lithology or climate were of secondary importance to tectonic effects. Such a retrograding fan delta are rarely reported and may be a stratigraphic response of connected rift basins at the early stage of extension.

Formation of Late Jurassic–Early Cretaceous metamorphic core complexes in Northeast China: insight from a structural study of the Yiwulüshan ductile detachment zone

ABSTRACTLate Jurassic to Early Cretaceous extensional structures are widespread in Northeast China and the eastern part of the Asian continent. The Yiwulüshan ductile detachment shear zone, the master ductile detachment zone of the Yiwulüshan metamorphic core complex (MCC), occurs in the northern North China Craton. This shear zone dips to the northwest on the western flank and to the southeast on the eastern flank of the MCC and consists of brittle faults, breccia, chloritized microbreccia, and ductile (mylonite) zones. The well-developed mylonitic foliation and stretching lineation are characterized by oriented minerals (quartz and feldspar) that exhibit ductile deformation, including undulose extinction, deformation bands, subgrain formations, and recrystallized grains. The shear indicators, such as asymmetric porphyroclasts, S-C fabrics, and shear bands, suggest consistent top-to-the-NW shear on both the western and eastern flanks of the MCC. Strain measurements using feldspar porphyroclasts show that the strain intensity (Es), Lode’s parameter (υ), and Flinn parameter (k) range from 0.46 to 1.03, −0.23 to 0.62, and 0.23 to 1.6, respectively. Kinematic vorticity (Wk) values, estimated by the Polar Mohr diagram and the C′ methods, range from 0.61 to 0.96, with an average of 0.80, for the mylonitic deformation, and from 0.24 to 0.53, with an average of 0.37, for extensional crenulation cleavage (ecc) domains. The kinematic vorticity values, together with the regional structural background, suggest the Yiwulüshan ductile detachment zone experienced simple-shear-dominated general shearing that formed the mylonitic fabric in the early stages of extension. Later in the deformation history, pure-shear-dominated general shearing dominated when ecc developed. Strain measurements and kinematic vorticity estimates indicate that the Yiwulüshan ductile detachment zone is a lengthening-thinning shear zone. It is proposed that simple-shear-dominated general shear developed during crustal extension and that pure-shear-dominated general shear is related to the upward flow of Precambrian basement and Jurassic plutons.

Tectono‐sedimentary development of early syn‐rift deposits: the Abura Graben, Suez Rift, Egypt

AbstractThe thickness and distribution of early syn‐rift deposits record the evolution of structures accommodating the earliest phases of continental extension. However, our understanding of the detailed tectono‐sedimentary evolution of these deposits is poor, because in the subsurface, they are often deeply buried and below seismic resolution and sparsely sampled by borehole data. Furthermore, early syn‐rift deposits are typically poorly exposed in the field, being buried beneath thick, late syn‐rift and post‐rift deposits. To improve our understanding of the tectono‐sedimentary development of early syn‐rift strata during the initial stages of rifting, we examined quasi‐3D exposures in the Abura Graben, Suez Rift, Egypt. During the earliest stage of extension, forced folding above blind normal fault segments, rather than half‐graben formation adjacent to surface‐breaking faults, controlled rift physiography, accommodation development and the stratigraphic architecture of non‐marine, early syn‐rift deposits. Fluvial systems incised into underlying pre‐rift deposits and were structurally focused in the axis of the embryonic depocentre, which, at this time, was characterized by a fold‐bound syncline rather than a fault‐bound half graben. During this earliest phase of extension, sediment was sourced from the rift shoulder some 3 km to the NE of the depocentre, rather than from the crests of the flanking, intra‐basin extensional forced folds. Fault‐driven subsidence, perhaps augmented by a eustatic sea‐level rise, resulted in basin deepening and the deposition of a series of fluvial‐dominated mouth bars, which, like the preceding fluvial systems, were structurally pinned within the axis of the growing depocentre, which was still bound by extensional forced folds rather than faults. The extensional forced folds were eventually locally breached by surface‐breaking faults, resulting in the establishment of a half graben, basin deepening and the deposition of shallow marine sandstone and fan‐delta conglomerates. Because growth folding and faulting were coeval along‐strike, syn‐rift stratal units deposited at this time show a highly variable along‐strike stratigraphic architecture, locally thinning towards the growth fold but, only a few kilometres along‐strike, thickening towards the surface‐breaking fault. Despite displaying the classic early syn‐rift stratigraphic motif recording net upward‐deepening, extensional forced folding rather than surface faulting played a key role in controlling basin physiography, accommodation development, and syn‐rift stratal architecture and facies development during the early stages of extension. This structural and stratigraphic observations required to make this interpretation are relatively subtle and may go unrecognized in low‐resolution subsurface data sets.

Final Gondwana breakup: The Paleogene South American native ungulates and the demise of the South America–Antarctica land connection

The biogeographic hypothesis more accepted today is that Antarctica (West Antarctica) and southern South America (Magellan region, Patagonia) were connected by a long and narrow causeway (Weddellian Isthmus) between the Antarctic Peninsula and South America since the Late Cretaceous (Campanian) until the Early Paleogene allowing terrestrial vertebrates to colonize new frontiers using this land bridge. Stratigraphically calibrated phylogenies including large, terrestrial native ungulates Litopterna and Astrapotheria taxa reveal long ghost lineages that extended into the Late Paleocene and provide evidence for the minimum times at which these “native ungulates” were present both on Antarctica and South America. Based on these results we estimate that the Weddellian Isthmus was functional as a land bridge until the Late Paleocene. Our data place the disconnection between Antarctica and South America in the Late Paleocene, indicating that the terrestrial faunistic isolation (Simpson's “splendid isolation”) in South America begun at the end of the Paleocene (~56 to 57m.y.). This faunistic isolation is documented to have occurred at least 25Ma before the existence of deep-water circulation conditions in Drake Passage (~30m.y.) based on the onset of seafloor spreading in the west Scotia Sea region. We hypothesize that in the early stages of extension (Late Paleocene, ~55m.y.) a wide and relatively shallow epicontinental sea developed between the Antarctic Peninsula and South America drowning the Weddellian Isthmus and preventing the faunal interchange for obligate cursorial terrestrial forms.

The role of disparity-mutagenesis model on tumor development with special reference to increased mutation rate

Human solid tumors are believed to have very high mutation rates at least in the early stage of extension. Irrespective ofwhether the increased mutation rate is a necessary condition for the tumor development or not, extremely high mutationrates such as in excess of the so-called “threshold” would before long result in the natural death of tumor cells. In reality,however, we are dying by cancer. Thus, it has been hypothesized that tumor cells should make a quick transition from thehigher mutation state to the lower one. According to our “disparity-mutagenesis model”, however, carcinogenesis couldcontinue without any incident even under a prolonged period of high mutation rates. Namely, if lagging-strand-biasedmutations far beyond the threshold of mutation rate are introduced in tumor cells, the tumor could progress to malignantextension without extinction. The results of evolution experiments using mutator microorganisms are discussed in terms ofcarcinogenesis.

Post-Orogenic Extension and Hydrothermal Ore Formation: High-Precision Geochronology of the Central Rhodopian Metamorphic Core Complex (Bulgaria-Greece)

The late Alpine evolution of the Rhodope Massif in southern Bulgaria and northern Greece involved postcollisional extension, which generated detachment faults, syndeformational sedimentary basins, and exhumation of a large metamorphic core complex composed of gneisses and marbles: the Central Rhodopian dome. Closely associated with this complex, subvolcanic rhyolite dikes and extrusive rocks were emplaced, shortly followed by major swarms of epithermal to mesothermal Pb-Zn veins and carbonate replacement orebodies. High-precision geochronology using complementary Ar-Ar, Rb-Sr, and U-Pb dating methods resolves how this process of tectonic denudation from deep crustal metamorphism to near-surface epithermal ore formation occurred within a period of about 12 m.y. After an early Alpine phase of accretion, eclogite-facies metamorphism, and orogenic nappe stacking, the late Alpine postcollisional evolution of the Central Rhodopian dome started with the intrusion of granitic bodies at about 42 to 41 Ma, probably marking the beginning of extension and core complex formation. The early stages of extension were characterized by normal faulting, rotation of fault blocks, and thinning that caused cooling of the hanging wall through ~300°C at about 40 to 38 Ma, as dated by Rb-Sr and Ar-Ar geochronology of metamorphic biotite. The main extensional phase occurred between 38 and 36 Ma and led to horizontal displacements of tens of kilometers in the hanging wall. In the footwall, high metamorphic temperatures and decompression persisted and resulted in partial melting and the formation of migmatites at 37 Ma and vuggy pegmatites at about 36 Ma. Cooling of the footwall below ~300°C occurred between 36 and 34 Ma, followed by emplacement of undeformed rhyolite porphyry dikes and the extrusion of volcanic products deposited onto the surface-exposed center of the dome at about 33 to 30 Ma. The hydrothermal ores were formed ca. 30.5 Ma in the south and ca. 29.3 Ma in the northern part of the dome during the last major event of focused heating to 270° to 330°C of near-surface rocks by hydrothermal fluid advection. Ore formation and localized, later fluid processes caused disturbance and younging of some Rb-Sr ages in the footwall of the dome. Field and geochronologic constraints indicate that the formation of the Pb-Zn deposits (~31–29 Ma) is up to 2 m.y. younger than the local rhyolitic magmatism, which is volumetrically minor in the mineralized core complex. This contrasts with ore formation related to calc-alkaline magmatism in the Eastern Rhodopes, where polymetallic Cu-Au-Ag-Pb-Zn mineralization was found to be coeval with the latest phases of igneous activity (~32 Ma). The chemically simpler but considerably larger metamorphic-hosted Pb-Zn deposits of the Central Rhodopian dome were generated by large-scale hydrothermal fluid circulation, driven by the high heat flow attending core complex formation, exhumation, and final fracturing of a rapidly thinned crust.

Fault architecture in the Main Ethiopian Rift and comparison with experimental models: Implications for rift evolution and Nubia–Somalia kinematics

The Main Ethiopian Rift (MER) offers a complete record of the time–space evolution of a continental rift. We have characterized the brittle deformation in different rift sectors through the statistical analysis of a new database of faults obtained from the integration between satellite images and digital elevation models, and implemented with field controls. This analysis has been compared with the results of lithospheric-scale analogue models reproducing the kinematical conditions of orthogonal and oblique rifting. Integration of these approaches suggests substantial differences in fault architecture in the different rift sectors that in turn reflect an along-axis variation of the rift development and southward decrease in rift evolution. The northernmost MER sector is in a mature stage of incipient continental rupture, with deformation localised within the rift floor along discrete tectono-magmatic segments and almost inactive boundary faults. The central MER sector records a transitional stage in which migration of deformation from boundary faults to faults internal to the rift valley is in an incipient phase. The southernmost MER sector is instead in an early continental stage, with the largest part of deformation being accommodated by boundary faults and almost absent internal faults. The MER thus records along its axis the typical evolution of continental rifting, from fault-dominated rift morphology in the early stages of extension toward magma-dominated extension during break-up. The extrapolation of modelling results suggests that a variable rift obliquity contributes to the observed along-axis variations in rift architecture and evolutionary stage, being oblique rifting conditions controlling the MER evolution since its birth in the Late Miocene in relation to a constant post ca. 11 Ma ~ N100°E Nubia–Somalia motion.

Crustal rheology of the Santorini–Amorgos zone: Implications for the nucleation depth and rupture extent of the 9 July 1956 Amorgos earthquake, southern Aegean

Abstract The 9 July 1956 Amorgos earthquake (Mw 7.6) was the largest event that hit Greece during the last century followed by a tsunami that inundated the coastal areas of the southern Aegean. This study investigates the rheological properties of the 1956 rupture zone between Amorgos and Santorini islands, in an effort to place some constraints on the nucleation depth and rupture extent of this large event. The seismic velocities inferred from tomographic and surface wave dispersion studies of the area are first correlated with laboratory determined velocities of known rock types. It is found that the lithosphere in the southern Aegean can be approximated by three layers representing the upper/lower crust and upper mantle consisting of quartzite, diabase and peridotite, respectively. Geotherms are calculated by using an analytical solution to the one-dimensional heat conduction equation, while Yield Strength Envelopes (YSEs) are produced after assuming laboratory estimated parameters of brittle and ductile deformation for each rock type. The depth frequency of earthquakes in the area, as well as other independent observations favour the YSE calculated for a wet upper crust/upper mantle, a dry lower crust and a geotherm corresponding to a surface heat flow of 62 mW m−2. In this YSE, the upper mantle exhibits maximum strength at 33 km becoming more ductile at greater depths. The lower crust retains significant strength and therefore cannot flow as it did during the early stages of extension, but it is relatively weaker than the upper mantle confirming the ‘jelly sandwich’ model previously proposed for the continental lithosphere. The downdip rupture width of the Amorgos event can be estimated from empirical relationships to be 26 km which means that its rupture may have extended from the depth of peak strength in the upper mantle (33 km) to 7 km upwards. Such a scenario agrees well with recent modelling results indicating that the Amorgos tsunami was probably caused by submarine landslides rather than coseismic rupture of the seafloor.

Evolution, pattern, and partitioning of deformation during oblique continental rifting: Inferences from lithospheric‐scale centrifuge models

Oblique rifting is investigated through centrifuge experiments that reproduce extension of a continental lithosphere containing a preexisting weakness zone. During extension, this weakness localizes deformation, and different rift obliquity is obtained by varying its trend with respect to the stretching direction. Model results show that deformation is mostly controlled by the obliquity angle α (defined as the angle between the orthogonal to the rift trend and the extension direction). For low obliquity (α &lt; 45°), rifting is initially characterized by activation of large, en echelon boundary faults bordering a subsiding rift depression, with no deformation affecting the rift floor. Increasing extension results in the abandonment of the boundary faults and the development of new faults within the rift depression. These faults are orthogonal to the direction of extension and arranged in two en echelon segments linked by a complex transfer zones, characterized by strike‐slip component of motion. In these models, a strong strain partitioning is observed between the rift margins, where the boundary fault systems have an oblique‐slip motion, and the valley floor that away from the transfer zones is affected by a pure extension. Moderate obliquity (α = 45°) still results in a two‐phase rift evolution, although boundary fault activity is strongly reduced, and deformation is soon transferred to the rift depression. The fault pattern is similar to that of low‐obliquity models, although internal faults become slightly oblique to the orthogonal to the direction of extension. Deformation partitioning between the rift margins and the valley floor is still observed but is less developed than for low‐obliquity rifting. For high obliquity (α &gt; 45°), no boundary faults form, and the extensional deformation affects the rift depression since early stages of extension. Dominance of the strike‐slip motion over extension leads to the development of oblique‐slip and nearly pure strike‐slip faults, oblique to both the rift trend and the orthogonal to the extension direction, with no strain partitioning between the margins and the rift floor. These results suggest that oblique reactivation of preexisting weaknesses plays a major role in controlling rift evolution, architecture, and strain partitioning, findings that have a significant relevance for natural oblique rifts.

Sequential development of interfering metamorphic core complexes: numerical experiments and comparison with the Cyclades, Greece

Abstract The mechanics of metamorphic core complex (MCC) development and the associated process of lower crustal flow have been the topic of several modelling studies. The model setup usually includes a local heterogeneity forcing deformation to localize at a given site, enabling only one MCC to develop. This paper presents numerical lithospheric-scale experiments in which deformation is not a priori localized in a specific place, in order to examine whether multiple MCCs could develop during extension, at which conditions, and how. Configurations with either a single MCC or several far-distant MCCs aligned in the section parallel to extension are obtained for a relatively wide range of initial conditions, the only firm requirement being that the lower crust and the sub-Moho mantle both have very low strengths. In contrast, only a narrow range of conditions leads to the development of closely spaced MCCs. In this case, the MCCs interfere with one another (the domes are partly superimposed or/and share a shear zone in common) and develop in sequence. This configuration is compared with the Cyclades archipelago, where closely spaced chains of MCCs have been described in the literature. A review of available data on the islands documents a good agreement with the experiments in terms of final depth of the Moho, geometry and kinematic pattern of the MCCs, and timing of exhumation of the metamorphic rocks. Based on this agreement, we tentatively deduce from the numerical results some of the conditions that prevailed at the initiation of, and during, post-orogenic MCC-type extension in the Cyclades. The most likely initial thickness of the crust is between 40 and 44 km. A thermal lithospheric thickness of only c . 60 km is also likely, which might be a condition at the onset of extension or may have been obtained during early stages of extension while the lithosphere was warmed up. Either a backarc subduction setting or a process of mantle delamination may account for this situation. The numerical results also suggest a boundary velocity of 2.0–2.3 cm/a, which should basically reflect the rate at which the South Hellenic subduction zone retreated. Considering c . 500 km as an upper bound for the amount of retreat balanced by Aegean extension and assuming that this retreat mostly occurred during MCC-type extension in the Cyclades, we find that the boundary velocity could have been as high as 2.1 cm/a if MCC-type extension lasted 24 Ma, starting at c . 30 Ma and finishing at c . 6 Ma, as suggested by available geochronological data. A velocity of 2.1 cm/a agrees well with the numerical results.

A retreating fan-delta system in the Neoproterozoic Chattisgarh rift basin, central India: Major controls on its evolution

A thick wedge of immature siliciclastics characterizes the basal part of the basin-fill succession in the eastern part of the Neoproterozoic Chattisgarh Basin. It comprises a mosaic of mass-flow–deposited conglomerate and pebbly sandstone, coarse-grained sandy braided stream deposits, wave-lag pebble sheets, high-energy, wave-dominated foreshore and upper shoreface facies, and wave- and tide-dominated lower shoreface facies. The succession is characterized by strong lateral and vertical facies variation. The facies associations and stacking pattern point to the development of an alluvial-fan to fan-delta complex, with outbuilding of delta lobes onto a high-energy shallow-marine shelf via a high-gradient, narrow delta plain. The delta-front deposits of coarse-grained sand interfinger and grade into prodelta mud in the seaward direction. The dominance of coarse-grained sandy detritus with a high content of fresh feldspar grains points to mechanical weathering of granites and gneisses of the cratonic basement and a semiarid climate. The sediment caliber and the climate exerted a major control on the facies and on processes of sediment transfer from the fan to the shelf. A critical balance between progradation, aggradation, and retrogradation controlled the evolution of delta successions within an overall transgressive regime related to the extension of a cratonic rift basin. Repeated fault-controlled uplift of the source, followed by subsidence and transgression, generated multiple fining-upward cycles and a retreating fan-delta system. The marked variations in thickness of the delta succession and the stacking pattern in different measured profiles reflect the overriding tectonic controls on delta evolution. The accumulated fault displacement in active sectors created higher accommodation and thicker delta sequences. Intermittent uplift of fault blocks exposed fresh bedrock to mechanical weathering, generated a large amount of detritus, and resulted in forced regressions, repeatedly disrupting the fining-upward pattern. The controls of source rock lithology or climate were of secondary importance to tectonic effects. Retreating fan deltas are rarely reported and may be a stratigraphic response of marine-connected rift basins at the early stage of extension. The delta stratigraphy is analogous to the submarine fan stratigraphy. In the case where basin shales act as hydrocarbon sources, the fan deltas are potential hydrocarbon reservoirs. The coarse-grained lobes can be excellent reservoirs, with the enclosing prodelta muds acting as seals to vertical migration of hydrocarbon. The coarse-grained deposits of the delta plain and proximal delta front, occurring as laterally coalesced, large sand bodies blanketed by transgressive prodelta mud, might also form good reservoirs.

THE COMPOSITE MANTLE EVOLUTION OF THE ERRO-TOBBIO PERIDOTITE (VOLTRI MASSIF, LIGURIA ALPS, ITALY)

THE COMPOSITE MANTLE EVOLUTION OF THE ERRO-TOBBIO PERIDOTITE (VOLTRI MASSIF, LIGURIA ALPS, ITALY)

U–Pb geochronology of the Leknes Group: an exotic Early Caledonian metasedimentary assemblage stranded on Lofoten basement, northern Norway

The Leknes Group is an amphibolite-facies metasedimentary assemblage thrust onto the Palaeoproterozoic basement of the Lofoten islands west of the Caledonian chain in northern Norway. Its genesis has been enigmatic, and both Sveconorwegian and Silurian ages of metamorphism have been proposed. U–Pb dating now shows that the main metamorphic and structural overprints occurred in the Mid-Ordovician, between the 469 ± 3 Ma age of a metatonalite and the 461 ± 1 Ma age of a titanite crystal co-planar with the main fabric. According to the prevalent structural interpretation this age also dates thrusting of the Leknes Group onto the Lofoten basement. Zircon and rutile in mica schists record this as well as a younger event of zircon growth at 412 ± 2 Ma, the latter coinciding with early stages of extension and enhanced fluid activity; in southern Lofoten this is also associated with the intrusion of 410 ± 3 Ma pegmatites. The Leknes Group shows an affinity with some terranes of peri-Baltican affinity in the Upper Allochthon of the Caledonides. It probably developed in a marginal basin close to Sveconorwegian basement, as shown by detrital zircon ages and published Nd isotopic data, before being thrust onto the Lofoten basement, which appears to be itself a late Caledonian allochthonous terrane.

Structural evolution of the Tethyan sedimentary sequence in the Annapurna area, central Nepal Himalaya

The Tethyan sedimentary sequence of central Nepal has been affected by three phases of folding (D 1, D 2, and D 4) with contrasting vergence, interspersed by two major extensional events (D 3 and D 5). The north-verging second phase folds (F 2) are the dominant deformation features preserved in the studied area. These D 2 structures indicate a vertical structural zonation that is interpreted to mark a preserved D 2 strain gradient increasing with depth, to a presumed north-verging décollement plane at an approximate depth of 10 km. Palinspastic bed-length restoration of the sequence implies a minimum 43% shortening, and a minimum 150% vertical thickening due to D 2 deformation. The D 4 event overprints the D 3 Annapurna detachment, and manifests itself as post-peak metamorphic south-verging kink folds and spaced crenulation cleavage. In the structurally lowest level observed, D 4 is associated with thrust-sense meter-scale fault zones. The youngest extensional event (D 5) marks the initiation and eventual southward propagation of the Thakkhola graben. A newly named northeast-dipping normal fault, the Lupra fault, is described. In the early stages of extension, the Lupra fault may have acted as a southern termination of the graben system, dropping down Cretaceous rocks and high-level structural features in its hanging wall. It is suggested that, as the extension progressed southward, the Lupra fault was itself down-faulted in the graben, and the extent of the Thakkhola graben reached the present position of the Annapurna detachment.

Neodymium isotopes in basalts of the southwest basin and range and lithospheric thinning during continental extension

The chemical compositions, ages, and Nd and Sr isotopic compositions of Miocene basaltic volcanic rocks from the southwestern Basin and Range Province were measured. The data presented, and other data obtained from the literature show that temporal variations in the Nd and Sr isotopic compositions are typically correlated with the timing of local crustal extension. The isotopic variations cannot be accounted for by crustal contamination, and are best interpreted as indicating the involvement of subcontinental lithospheric mantle as a major source of basalt magma before and during the early stages of extension, giving way to asthenospheric sources as extension proceeds. It is inferred that the basalt isotopic and age data provide information about the amount and timing of changes of the overall lithosphere thickness during extension. If this inference is valid, the inferred lithosphere thickness vs. time histories can be compared to the amount and timing of upper crustal extension to evaluate competing models for lithospheric strain during continental extension. The depth of the lithosphere–asthenospshere boundary is inferred primarily from the isotopic data, but can be refined by accounting for differences in the depth of origin of the basalts as suggested by differences in the degree of silica saturation (nepheline normative vs. hypersthene normative). The transition between silica saturated and undersaturated magmas is dependent on many factors, but for the conditions likely to apply to the Basin and Range Miocene volcanism, it corresponds to a depth of origin of 50±10 km. The data presented show strong support for the inferred mantle geochemical stratigraphy, and the inferred modern depths to the lithosphere–asthenosphere boundary are consistent with available seismic refraction and structural data. In the highly extended regions near the eastern and western margins of the southwestern Basin and Range, temporal changes in basalt isotopic compositions indicate that the subjacent lithospheric mantle thinned contemporaneously with the upper crust, as would be predicted for pure shear. These areas are also characterized by relatively high (although still low) basalt extrusion rates. In contrast, upper crustal extension in the Death Valley area was not accompanied by significant thinning of the subjacent lithospheric mantle, which implies that the location of lithospheric thinning was offset from the location of upper crustal extension as in a simple shear model. The total amount of lithospheric thinning is generally less than would be expected from the magnitude of upper crustal extension and is more evenly distributed; this suggests that there is also a component of distributed shear.

Role of fault interactions in controlling synrift sediment dispersal patterns: Miocene, Abu Alaqa Group, Suez Rift, Sinai, Egypt

Although fault growth is an important control on drainage development in modern rifts, such links are difficult to establish in ancient basins. To understand how the growth and interaction of normal fault segments controls stratigraphic patterns, we investigate the response of a coarse‐grained delta system to evolution of a fault array in a Miocene half‐graben basin, Suez rift. The early Miocene Alaqa delta complex comprises a vertically stacked set of footwall‐sourced Gilbert deltas located in the immediate hangingwall of the rift border fault, adjacent to a major intrabasinal relay zone. Sedimentological and stratigraphic studies, in combination with structural analysis of the basin‐bounding fault system, permit reconstruction of the architecture, dispersal patterns and evolution of proximal Gilbert delta systems in relation to the growth and interaction of normal fault segments. Structural geometries demonstrate that fault‐related folds developed along the basin margin above upward and laterally propagating normal faults during the early stages of extension. Palaeocurrent data indicate that the delta complex formed a point‐sourced depositional system developed at the intersection of two normal fault segments. Gilbert deltas prograded transverse into the basin and laterally parallel to faults. Development of the transverse delta complex is proposed to be a function of its location adjacent to an evolving zone of fault overlap, together with focusing of dispersal between adjacent fault segments growing towards each other. Growth strata onlap and converge onto the monoclinal fold limbs indicating that these structures formed evolving structural topography. During fold growth, Gilbert deltas prograded across the deforming fold surface, became progressively rotated and incorporated into fold limbs. Spatial variability of facies architecture is linked to along‐strike variation in the style of fault/fold growth, and in particular variation in rates of crestal uplift and fold limb rotation. Our results clearly show that the growth and linkage of fault segments during fault array evolution has a fundamental control on patterns of sediment dispersal in rift basins.