Basal Miocene Research Articles

Oligocene-early Miocene rapid exhumation along the Xianshuihe fault system: Implications for the growth of the Southeastern Tibetan Plateau

• 24 new thermochronometric ages from the bend section of the Xianshuihe fault system are reported. • New data indicate migration of the locus of fast exhumation towards the fault zone from ∼25 to ∼10 Ma. • Oligocene-early Miocene fast exhumation along the fault system resulted from crustal shortening. • southeastern Tibetan Plateau; Xianshuihe fault; low-temperature thermochronology; • Oligocene-Early Miocene; rapid exhumation; Plateau growt. Large-scale strike-slip faults in the southeastern Tibetan Plateau are considered to have accommodated significant crustal extrusion and deformation during the plateau growth. However, the long-term deformation and exhumation history along the large-scale strike-slip faults is still unclear. Here, we present new thermochronological datasets from the near-field of the bend section of the Xianshuihe fault system, they reveal migration of the loci which show rapid exhumation between ∼25 Ma and ∼10 Ma towards the fault zone. Our new results clearly show a phase of rapid Oligocene-early Miocene basement exhumation, indicating contemporaneous crustal shortening along the fault. Through synthesizing the evidence of Oligocene-early Miocene fast exhumation along the Xianshuihe fault system, and synchronous deformation along adjacent strike-slip and thrust faults in the region, we propose that the southeastern Tibetan Plateau may have reached a remarkable high-elevation topography during this period. This was accompanied by significant crustal shortening under a contractional regime, with northeastward advance of the eastern Himalayan syntaxis and southeastward crustal extrusion originated from the plateau hinterland. Our findings are generally in agreement with sedimentary records in the marginal sea basins and paleo-altimetry studies.

Controls on Early Miocene carbonate and siliciclastic deposition in eastern Java and south Makassar Straits, Indonesia

The Early Miocene stratigraphy of NE Java and the southern Makassar Straits is examined in the context of siliciclastic sediment supply and subsidence (creation of accommodation space), as indicated by biohermal carbonate growth. Stratigraphic variation is examined geographically and assessed quantitatively, using geohistory analysis. During the Oligocene through to basal Miocene times, basin development had been dominated by extension with local transtension. This changed within the Early Miocene to regional compression, increasing in magnitude with time. Three tectonic events, at about 24 Ma (base of section studied), at 21 Ma, and roughly 16 Ma are described in terms of the influence they had on stratigraphy. Most significant was the concentration of siliciclastics on the proximal side of the Oligo-Miocene axis of subsidence, which caused isostatic loading and nearby pinnacle reefs to achieve high relief, but ultimately an acceleration in siliciclastic input at about 16 Ma caused simultaneous reef extinction. Carbonates of Early Miocene age are important hydrocarbon reservoirs, and the siliciclastics are a geographically variable part of the overburden and seal, as well as causing the thermal maturation of hydrocarbon source rocks. The stratigraphic framework described here will have implications for facies and exploration risk prediction into under-explored regions around the study area. The tectono-stratigraphic framework is carefully constrained for age and facies inputs, and the resulting palaeogeographic history indicates a new model of basin development. This dynamic framework correlates to coeval tectonic processes over a large region and has important implications for Sundaland plate tectonics.

Reef-rim structure and building history, Rangiroa, an uplifted Atoll, French Polynesia: The role of morphotectonics and extreme marine hazard events

Reconstructing the responses of low-lying, atoll-rim islands to future environmental changes due to global warming requires a robust knowledge of the building history of reef rims through time. In the northwest of Tuamotu Archipelago, atolls have experienced opposite vertical motions (uplift versus subsidence) in response to differential tectonic constraints undergone by the underlying oceanic crust. As a result, during Pleistocene high sea levels, some atolls remained emerged whereas others remained flooded. Based on two electrical resistivity tomography profiles across the northern rim-island of the uplifted Rangiroa Atoll and on U/Th dating of corals clasts collected from unconsolidated sand-rubble deposits within two wells through the same island area, the internal structure of the atoll-rim and its mode and timing of building are tentatively reconstructed. The irregularly shaped, upper boundary of the basal resistive unit is interpreted as residual, pinnacle-shaped reliefs of Miocene age. At the seaward rim side, the uppermost highly resistive layers, an about 5 m-thick, were assumed to be reef-related, partly consolidated material emplaced during the Holocene. Thicker resistive layers that infill cavities in the Miocene basement, in the central part of the atoll rim, refer to the fresh water lense in porous, more or less consolidated material (phosphorites?) presumably emplaced during the Mio-Pliocene. In northern Rangiroa, the age of the oldest coral clasts trapped into the atoll-rim islands ranges between about 11,000 and 8000 calendar years when sea level was approximately 60 to 20 m below its present position. This suggests that coral-clast supply from the adjacent forereef slopes was triggered by extreme wave-surge events (presumably, tsunamis generated by island flank failures). The earliest island initiation phase is thought to start by approximately 6000 cal. yr and to be achieved by about 3000 cal. yr. By constrast, on the nearby subsiding Takapoto Atoll, the rim island began to initiate not earlier than 2500 cal. yr. At Rangiroa, morphotectonics is assumed to have played a major role in controlling the internal rim architecture and, associated to extreme marine hazard events, to have promoted the earlier settlement of low-lying, rim islands through an earlier infilling of accommodation space as compared to nearby subsiding atolls.

Rappel historique et aspects stratigraphiques et taphonomiques du gisement pléistocène supérieur d'Arenal de son Servera (Majorque, îles Baléares)

The upper Pleistocene deposit of Arenal de son Servera is reviewed using data from earlier works and new considerations based on its stratigraphy and taphonomical aspects of the fossils. In the present study, five different kinds of facies are identified: [1] palaeosols with pebbles from the Miocene basement, [2] aeolianites, [3] foreshore deposits with thermophilous molluscan fossil fauna, [4] palaeosols resulting from the pedogenesis of the beach units, and [5] bioclastic channelled deposits eroding the underlaying units. The detailed taphonomical analysis revealed that the fossils enclosed in these rocks remained on the sea bottom for a significant amount of time, but they were not bioeroded, perhaps because a submerged sand bar repeatedly buried and exhumed them. In addition, the fact that Persististrombus latus shells are in their position of maximum stability allows to infer that they were washed up on the shore during moderately energetic events, possibly comparable to present-day ordinary storms. These two last points could be of interest to the field of coastal management, as they provide insight on the scope of physical changes these systems could undergo in the present warming of the Mediterranean.

Miocene basement exhumation in the Central Alps recorded by detrital garnet geochemistry in foreland basin deposits

Abstract. The Neogene evolution of the European Alps was characterized by the exhumation of crystalline basement, the so-called external crystalline massifs. Their exhumation presumably controlled the evolution of relief, distribution of drainage networks, and generation of sediment in the Central Alps. However, due to the absence of suitable proxies, the timing of their surficial exposure and thus the initiation of sediment supply from these areas are poorly constrained. The northern Alpine foreland basin preserves the Oligocene to Miocene sedimentary record of tectonic and climatic adjustments in the hinterland. This contribution analyses the provenance of 25 to 14 Myr old alluvial fan deposits by means of detrital garnet chemistry. Unusually grossular- and spessartine-rich garnet is found (1) to be a unique proxy for identifying detritus from the external crystalline massifs and (2) to occur abundantly in ca. 14 Myr old deposits of the foreland basin. In contrast to previous assumptions, we therefore propose that the external massifs were already exposed to the surface ca. 14 Myr ago.

Geology of the Yeşilyuva and Kocabaş region and its geotectonic significance

Southwestern Turkey experienced a transition from crustal shortening to extension during Late Cenozoic, and evidence of this was recorded in different basin types in the east of Denizli region. The study area comprises a heterogeneous suite of Mesozoic to Recent igneous, metamorphic, and sedimentary rocks. Two main rock groups are distinguished in the area: the basement rocks of pre-Neogene age and the sedimentary cover of Neogene-Quaternary age. In the region, several Late Cenozoic structural basins are filled with terrestrial clastic and volcanic rocks. These basins are different in orientation, geometry, and nature of basin infills. In the studied area, the exposed basal units of the ophiolitic melange correspond to the Lycian Nappe complex. The first sedimentary cover unit, unconformably overlying the nappes, is represented by an E-W-trending basin that is filled with the Oligocene-Lower Miocene shallow marine-terrestrial deposits. The infill of the studied basin consists of the Caykavustu and the Akcay Formations which were deposited during the Late Oligocene and the Early Miocene, respectively. These basins developed as a piggy-back basin along the fault plane on the hanging wall blocks of normal faults. A graben basin (Yesilyuva-Kocabas) started to occur along with NE-SW-trending normal faults which were formed as a result of E-W extensional regime during the Late Miocene. The coarse clastics of the Kizilburun Formation of the Late Miocene age were deposited within an alluvial fan derived from the Elmadag Horst in the east of the basin. Whereas the coarse clastics of the Late Miocene basement were accumulated in front of the hanging fault blocks, the fine-grained clastics of the Saraykoy Formation overlie conformably the Kizilburun Formation. The sequence continues with the lacustrine limestones of the Aktepe Formation. Alkaline magmatics of Early-Late Pliocene cut all successions. E-W-trending fault systems formed with the N-S extensional regime affected the basin, and the recent E-W grabens (Curuksu and Acigol Graben) occurred during the Early Pliocene. The recent morphology of the studied area developed under the control of the Asagidagdere oblique fault zones which are located in the south of the Curuksu Graben.

Seismic velocity structure of the Jakarta Basin, Indonesia, using trans-dimensional Bayesian inversion of horizontal-to-vertical spectral ratios

Characterizing the interior structure of the Jakarta Basin, Indonesia, is important for the improvement of seismic hazard assessment there. A dense-portable seismic broad-band network, comprising 96 stations, has been operated between October 2013 and February 2014 covering the city of Jakarta. The seismic network sampled broad-band seismic noise mostly originating from ocean waves and anthropogenic activity. We used horizontal-to-vertical spectral ratio (HVSR) measurements of the ambient seismic noise to estimate fundamental-mode Rayleigh wave ellipticity curves, which were used to infer the seismic velocity structure of the Jakarta Basin. By mapping and modelling the spatial variation of low-frequency (0.124–0.249 Hz) HVSR peaks, this study reveals variations in the depth to the Miocene basement. These variations include a sudden change of basement depth from 500 to 1000 m along N–S profile through the centre of the city, with an otherwise gentle increase in basin depth from south to north. Higher frequency (2–4 Hz) HVSR peaks appear to reflect complicated structure in the top 100 m of the soil profile, possibly related to the sediment compaction and transitions among different sedimentary sequences. In order to map these velocity profiles of unknown complexity, we employ a trans-dimensional Bayesian framework for the inversion of HVSR curves for 1-D profiles of velocity and density beneath each station. Results show that very low-velocity sediments (<240 m s−1) up to 100 m in depth cover the city in the northern to central part, where alluvial fan material is deposited. These low seismic velocities and the very thick sediments in the Jakarta Basin will potentially contribute to seismic amplification and basin resonance, especially during giant megathrust earthquakes or large earthquakes with epicentres close to Jakarta. Results have shown good correlation with previous ambient seismic noise tomography and microtremor studies. We use the 1-D profiles to create a pseudo-3-D model of the basin structure which can be used for earthquake hazard analyses of Jakarta, a megacity in which highly variable construction practices may give rise to high vulnerability. The methodology discussed can be applied to any other populated city situated in a thick sedimentary basin.

Quantification of proportions of different water sources in a mining operation

The water drained in mining operations (galleries, shafts, open pits) usually comes from different sources. Evaluating the contribution of these sources is very often necessary for water management. To determine mixing ratios, a conventional mass balance is often used. However, the presence of more than two sources creates uncertainties in mass balance applications. Moreover, the composition of the end-members is not commonly known with certainty and/or can vary in space and time. In this paper, we propose a powerful tool for solving such problems and managing groundwater in mining sites based on multivariate statistical analysis. This approach was applied to the Cobre Las Cruces mining complex, the largest copper mine in Europe. There, the open pit water is a mixture of three end-members: runoff (RO), basal Miocene (Mb) and Paleozoic (PZ) groundwater. The volume of water drained from the Miocene base aquifer must be determined and compensated via artificial recharging to comply with current regulations. Through multivariate statistical analysis of samples from a regional field campaign, the compositions of PZ and Mb end-members were firstly estimated, and then used for mixing calculations at the open pit scale. The runoff end-member was directly determined from samples collected in interception trenches inside the open pit. The application of multivariate statistical methods allowed the estimation of mixing ratios for the hydrological years 2014–2015 and 2015–2016. Open pit water proportions have changed from 15% to 7%, 41% to 36%, and 44% to 57% for runoff, Mb and PZ end-members, respectively. An independent estimation of runoff based on the curve method yielded comparable results.

Evolución del relieve de un frente montañoso y respuesta del sistema erosivo fluvial ante el forzamiento causado por la actividad tectónica (Andes centrales, provincia de San Juan, Argentina)

Fil: Ocana, Raul Emmanuel. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Fisicas y Naturales. Instituto de Geologia Dr. Emiliano Aparicio; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Fisicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; Argentina

Neogene-Quaternary evolution of the Tefenni basin on the Fethiye-Burdur fault zone, SW Anatolia-Turkey

The Fethiye-Burdur fault zone (FBFZ) is a complex belt of major break in the southwestern Anatolia. A number of basins occur within the FBFZ. The Tefenni basin is one of the NE-SW trending basins located in the central part of the FBFZ. The basin is 10–20 km wide and 60 km long. It contains two infills of fluvial, lacustrine and alluvial fan deposits from late Miocene to Recent. The older and folded infill rests on the pre-middle Miocene basement rocks with an angular unconformity and consists of fluvial and lacustrine sediments. The younger and undeformed Plio-Quaternary basin fill unconformably overlies the older basin fill and is composed predominantly of conglomerate, mudstone, silt, clay and recent basin floor sediments. The Tefenni basin is controlled by a series of NE-SW trending left lateral oblique-slip normal faults along its margins. The Tefenni and Mürseller faults bound the northwestern margin of the basin and the Kemer fault bounds the southeastern margin of the basin. The basin is also cut by NE-SW striking major and NW-SE, N-S and E-W striking small scale normal faults. Structural analyses in the basin show that NE-SW-trending contraction stress regime ended by Pliocene and was followed by NE-SW-trending extension from Pliocene onward.

Correction: A New Late Miocene Odobenid (Mammalia: Carnivora) from Hokkaido, Japan Suggests Rapid Diversification of Basal Miocene Odobenids

Correction: A New Late Miocene Odobenid (Mammalia: Carnivora) from Hokkaido, Japan Suggests Rapid Diversification of Basal Miocene Odobenids

A New Late Miocene Odobenid (Mammalia: Carnivora) from Hokkaido, Japan Suggests Rapid Diversification of Basal Miocene Odobenids.

The modern walrus, Odobenus rosmarus, is specialized and only extant member of the family Odobenidae. They were much more diversified in the past, and at least 16 genera and 20 species of fossil walruses have been known. Although their diversity increased in the late Miocene and Pliocene (around 8–2 Million years ago), older records are poorly known. A new genus and species of archaic odobenid, Archaeodobenus akamatsui, gen. et sp. nov. from the late Miocene (ca. 10.0–9.5 Ma) top of the Ichibangawa Formation, Hokkaido, northern Japan, suggests rapid diversification of basal Miocene walruses. Archaeodobenus akamatsui is the contemporaneous Pseudotaria muramotoi from the same formation, but they are distinguishable from each other in size and shape of the occipital condyle, foramen magnum and mastoid process of the cranium, and other postcranial features. Based on our phylogenetic analysis, A. akamatsui might have split from P. muramotoi at the late Miocene in the western North Pacific. This rapid diversification of the archaic odobenids occurred with a combination of marine regression and transgression, which provided geological isolation among the common ancestors of extinct odobenids.

Structure of regional dykes and local cone sheets in the Midhyrna-Lysuskard area, Snaefellsnes Peninsula (NW Iceland)

This paper provides the first detailed structural description of 48 vertical dykes, 384 inclined sheets and two large intrusions and the geometry (strike, dip direction and dip) of 1116 fractures in the central area of the Snaefellsnes peninsula, NW Iceland. Our data show a more complex setting than that depicted by the WNW-ESE en-echelon trend of the volcanic structures at the surface. In the Miocene basement lavas, dykes dominantly strike N50–100°E whereas other directions are also present with a higher dispersion. Two main swarms of centrally dipping sheets have also been recognized, focussing towards two areas. Sheet dips range from 2 to 75° with the higher frequency between 10 and 45°. In section view, there is no systematic variation of sheet dip with distance from the focus area. Gabbro and granophyre laccoliths are present in the studied area but cross-cutting relations indicate that most of the inclined sheets are younger. Comparison with regional tectonics suggests that the N50–80°E-striking dykes are coherent with emplacement under the stress field of the pre-6 Ma Snaefellsnes Rift dominated by a NNW-SSE-directed least principal stress (σ3). The N80–100°E dykes and the late Quaternary WNW-trending sub-aerial volcanic features are instead consistent with the development of a more recent E-W, right-lateral shear zone affecting the Snaefellsnes peninsula. Coherent sets of fractures have also been found. Within the inclined sheet swarms, the stress tensor rotated in response to an excess magma pressure linked to two underlying magma chambers of lobate shape, located at an estimated depth of about 400 and 500 m below sea level. This local magmatic stress also produced the centrally inclined fracture swarms that have been found in this area.

Early Miocene basement clasts in ANDRILL AND-2A core and their implications for paleoenvironmental changes in the McMurdo Sound region (western Ross Sea, Antarctica)

A detailed provenance study of gravel-size clasts in the lowermost 438 m of the ANDRILL AND-2A core reveals a pattern of dynamic provenance variations which can be discussed for its implications for Early Miocene paleogeographical reconstructions and glaciological models in the McMurdo Sound region (Ross Sea). Clast distribution patterns and distribution of diagnostic basement clast assemblages indicative of three different provenance areas (Mulock–Skelton glacier, Carlyon–Darwin glacier and Koettlitz–Blue glacier regions) can be interpreted in terms of two distinctive ice dynamic scenarios: i) local fluctuations of outlet Transantarctic Mountains glaciers with dominant flows from W to E to the coast (documented in the lowermost core section, at ca. 20.2–20.1 Ma), and ii) larger volume of ice grounded at the regional scale in the Ross Embayment with flow lines running N–S close to the TAM front (in two periods, at ca. 19.7–19.7 Ma and at ca. 17.8–17.4 Ma). Moreover, in core sections including facies typical of open-marine to iceberg-influenced depositional environments, ice-rafted debris compositions reveal that during periods of glacial-minima setting sites of active calving processes dominantly occurred in areas including the present-day Blue–Koettlitz glacier coast, and subordinately in glacier tongues located further south in the Skelton–Mulock glacier area. Consistently with results of numerical modelling the reconstructed glaciological scenarios provide further constraints to paleogeographical reconstructions and glaciological models at the south-western tip of the Ross Sea during significant steps in Antarctic ice sheet evolution through the Early Miocene climatic events.

The Izu Peninsula, Japan: Zircon geochronology reveals a record of intra-oceanic rear-arc magmatism in an accreted block of Izu–Bonin upper crust

The Izu Peninsula, central Japan, is situated in a zone where the active intra-oceanic Izu–Bonin arc has been colliding end-on with the mainland Honshu arc for the past 15million years. As a result of this arc–arc collision, parts of the submarine Izu–Bonin upper crustal sequences have been accreted and uplifted to form the Izu Peninsula, exposing seafloor volcaniclastic deposits, associated lava flows, and coeval intrusive bodies. Parts of this sequence have been subjected to extensive hydrothermal alteration, and these altered rocks have previously been interpreted as representative of hypothetical widespread Middle Miocene basement that presumably underlay northern Izu–Bonin arc volcanoes. New zircon U–Pb ages presented here, however, show that both fresh and altered volcanic sequences exposed in Izu Peninsula are broadly contemporaneous and were products of the same Late Miocene to Pleistocene magmatism. Geochemical characteristics of these sequences show them to have formed in the Izu–Bonin rear-arc environment, providing an unusual opportunity to investigate in detail the growth and architecture of a rear-arc region in an active intra-oceanic arc. Moreover, zircon ages from altered basal units of Kozushima and Niijima, Quaternary volcanic islands in the northern Izu–Bonin rear-arc, show that these islands rest on units only slightly older (<1Ma) than the main body of these subaerial edifices, not, as previously believed, part of a regional older Miocene basement. The near-continuum growth of these arc volcanoes and their underlying successions, plus the absence of a distinctly older basement underlying the Izu Peninsula and northern Izu–Bonin arc, provide new insight into upper crust development in an intra-oceanic, convergent margin environment.

Ichnofacies and microbial build-ups on Late Miocene rocky shores from Menorca (Balearic Islands), Spain

Angular unconformites between Jurassic and Miocene strata are exposed in the sea cliffs of Cala Cigonya on the northwest coast of Menorca in the Balearic Islands of Spain. The geological discontinuities represent rocky shores on opposite sides of a former headland with 15 m of topographic relief. On the east flank, Jurassic dolomite is overlain by Upper Miocene (Tortonian to Messinian) breccia and laminated limestone. Here, a partially exhumed dolomite surface records Miocene bivalve borings of the ichnospecies Gastrochaenolites torpedo and G. lapidicus that achieved a density of >1,000 borings/m2. Other associated traces include sponge borings (Entobia isp.) and polychaete borings (Caulostrepsis isp.). A breccia deposit 0.8 m thick was derived from the underlying dolomite and angular clasts still retain evidence of bivalve borings. Above follows a succession of laminated limestone beds more than 5 m thick, including some levels with dome-shaped stromatolites and other horizons with reworked dolomite clasts. Thin-section analysis of the laminated limestone reveals dark and light couplets 0.2 mm thick consistent with microbial origins. In contrast, the west flank was buried by coarse sandstone and laminated sediments. Here, dwelling structures of regular echinoids (Circolites kotoncensis) are the dominant traces preserved on the dolomite surface, reaching a maximum density of 66 borings/m2. Associated borings include Entobia geometrica as well as rare traces of Gastrochaenolites isp. and Trypanites isp. Notable for the absence of a basal Miocene breccia, the west flank is interpreted as a sheltered rocky shore coeval with an exposed rocky shore on the east flank. Today, heavy surf on the north coast of Menorca is related to the Tramontana winds that blow out of Spain during winter months. Similar atmospheric circulation patterns must have prevailed during the Late Miocene, but the replacement of ichnofacies by microbial build-ups resulted from increased salinity during the Messinian crisis.

Basement Imaging Using Sp Converted Phases from a Dense Strong-Motion Array in Lan-Yang Plain, Taiwan

Abstract We have collected a large number of accelerograms recorded by the Taiwan Strong Motion Instrumentation Program (TSMIP) stations to study the thickness variations of Quaternary alluviums beneath Lan-Yang Plain, Taiwan, using an Sp converted wave. The estimated thicknesses of the Quaternary sediments inferred by the travel-time difference of S and Sp waves are between 200 and 1400 m and become thicker toward the northeast. In general, our resulting features of the time difference of arrivals between Sp and S waves are consistent with the previous studies on thickness variations of the Quaternary alluviums beneath Lan-Yang Plain assuming the converting point is at the unconsolidated Quaternary alluvial sediments–Miocene basement interface. Our study suggests that this technique of using P - S converted phases could be applied to the other populated basins or plains in the Taiwan region based on its dense coverage of the TSMIP stations and high seismic activity. This technique is simple and time effective and can be used to determine the general characteristics of velocity/thickness structure of a study area.

Structures and sedimentary deposition in the Sunda Strait, Indonesia

The Sunda Strait lies between the islands of Java and Sumatra and separates the nearly orthogonal subduction along the south of Java and oblique subduction beneath Sumatra. The Sumatra fault accommodating most of the trench parallel shear of the oblique subduction initiates in the Sunda Strait. This setting implies that the Sunda Strait is an attractive area to study the geodynamic evolution of the western Indonesian region. Thirteen multichannel seismic lines acquired in the Sunda Strait have allowed a detailed study of the structure, sedimentation patterns and volcanic activities that occurred in the Sunda Strait. The Sunda Strait evolution has been strongly tied with the development of the Sumatra fault in this area. The opening of the Sunda Strait initiated in the early Late Miocene following the commencement of the Sumatra fault system in the Middle Miocene. Since then, three major graben systems have been evolved, which are referred to as the West and East Semangko Grabens, and the Krakatau Graben. These graben systems can be regarded as pull-apart basins developed within the releasing fault overstep area in between the main strand of the Sumatra fault and its relay faults south off Ujung Kulon. Prior to the Late Miocene most of the Sunda Strait and the surroundings probably developed in a non-marine environment. In Late Miocene the Sunda Strait started to open and volcanism began and has been the main source of clastics. Three major sedimentary units can be identified which represent Late Miocene, Pliocene and Pleistocene sediments respectively. The Late Miocene unit has been developed in the shelfal area on top of the Middle Miocene unconformity following a relative rapid sea level rise, and a subsequent highstand prior to the sea level fall at the end of Miocene. Its lower part has syn-structurally been deposited into the Late Miocene basement block faulting. The early deposition of the Pliocene unit was in response to the Pliocene sea level rise and abundant clastic supply. The deposition became very rapid due to graben formation combined with stronger volcanism. However, its lateral distribution was confined by uplift of ridges within the grabens.

Giant hyracoid from basal Middle Miocene deposits at Gebel Zelten, Libya

Previous records of the presence of the hyracoid Prohyrax sp. at Gebel Zelten are unsubstantiated and the original material seems to have been lost. A distal metapodial of a large hyracoid collected by R. Savage at locality W, Gebel Zelten, is similar in morphology and dimensions to a fossil from Nova, South Africa provisionally attributed to Parapliohyrax. Large hyracoids were thus present in Libya during the early part of the Middle Miocene.

Regional restoration across the Kwanza Basin, Angola: Salt tectonics triggered by repeated uplift of a metastable passive margin

Restoration of a 375-km (230-mi)-long section across the Kwanza Basin, Angola, shows three stages of deformation detaching on Aptian salt, each caused by basement tectonics. First, tilting related to postrift thermal subsidence initiated early Albian deformation, shortly after salt deposition ended. Deformation waned in the late Albian, probably because of thinning of salt lubricant beneath the extensional province. The second phase of deformation was triggered by hitherto unrecognized crustal uplift beneath the continental rise around 75 Ma (Campanian). Uplift led to salt extrusion and seaward advance of the Angola salt nappe over the abyssal plain. Exposure of the nappe toe removed the buttress provided by abyssal-plain cover, which rejuvenated seaward translation. Third, Miocene basement uplift below the shelf steepened the bathymetric slope and greatly accelerated downslope translation. This deformation is now slowing because accelerated sedimentation on the abyssal plain reduced the relief of the system and blocked salt-nappe advance.