Early Miocene Time Research Articles

Oligocene-Miocene Kailas basin, southwestern Tibet: Record of postcollisional upper-plate extension in the Indus-Yarlung suture zone

Research Article| July 01, 2011 Oligocene–Miocene Kailas basin, southwestern Tibet: Record of postcollisional upper-plate extension in the Indus-Yarlung suture zone P.G. DeCelles; P.G. DeCelles † Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA †E-mail: decelles@email.arizona.edu Search for other works by this author on: GSW Google Scholar P. Kapp; P. Kapp Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar J. Quade; J. Quade Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar G.E. Gehrels G.E. Gehrels Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2011) 123 (7-8): 1337–1362. https://doi.org/10.1130/B30258.1 Article history received: 25 Jan 2010 rev-recd: 26 May 2010 accepted: 24 Jun 2010 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation P.G. DeCelles, P. Kapp, J. Quade, G.E. Gehrels; Oligocene–Miocene Kailas basin, southwestern Tibet: Record of postcollisional upper-plate extension in the Indus-Yarlung suture zone. GSA Bulletin 2011;; 123 (7-8): 1337–1362. doi: https://doi.org/10.1130/B30258.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Kailas basin developed during late Oligocene–early Miocene time along the Indus-Yarlung suture zone in southwestern Tibet. The >2.5-km-thick basin-filling Kailas Formation consists of a lower coarse-grained proximal conglomerate and more distal fluvial sandstone member, a lacustrine shale and sandstone member, and an upper red-bed clastic member. Felsic tuffs and trachyandesite layers are locally present. Detrital and igneous zircon U-Pb ages indicate deposition of most of the Kailas Formation between ca. 26 and 24 Ma. The Kailas Formation was deposited by alluvial-fan, low-sinuosity fluvial, and deep lacustrine depositional systems in buttress unconformity upon andesitic volcanic (ca. 67 Ma) and granitoid (ca. 55 Ma) rocks of the Gangdese magmatic arc. Abundant organic material, fish and amphibian fossils, and sparse palynomorphs suggest that Kailas lakes developed in a warm tropical climate, quite different from coeval basins in central Tibet, which formed at high elevation in a dry climate. Provenance and paleocurrent data indicate that the bulk of the Kailas Formation was derived from the northerly Gangdese magmatic arc (Kailas magmatic complex). Only during the latest stages of basin filling was abundant sediment derived from the southerly Tethyan Himalayan thrust belt in the hanging wall of the Great Counter thrust. Kailas basin stratigraphy resembles a classic lacustrine sandwich and is most consistent with deposition in an extensional or transtensional rift that developed along the suture zone some 30 m.y. after the onset of Indo-Eurasian intercontinental collision. Correlative coarse-grained syntectonic strata similar to the Kailas Formation crop out along a >1300 km length of the Indus-Yarlung suture zone, suggesting that the basin-forming mechanism recorded by the Kailas Formation was of regional significance and not exclusively related to local kinematics near the southeastern end of the Karakoram fault. We propose that extension of the southern edge of the Eurasian plate was caused by southward rollback of underthrusting Indian continental lithosphere, followed by slab break-off. Alternating episodes of hard and soft collision, associated with regional contraction and extension, respectively, in the Tibetan-Himalayan orogenic system may have been related to changing dynamics of the subducting/underthrusting Indian plate. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Registro de Moridae (Teleostei: Gadiformes) en la Formación Chenque (Oligoceno Tardío-Mioceno Temprano) de la provincia de Chubut, Argentina

Record of Moridae (Teleostei: Gadiformes) from the Chenque Formation (Late Oligocene - Early Miocene) from the Chubut province, Argentina. A nearly complete specimen belonging to the family Moridae is described in this paper. The material has been collected in outcrops referable to the Chenque Formation (Oligocene-Miocene) from the Chubut province, Argentina. The specimen constitutes the first fossil record for the Gadiformes in Argentina and one of the first osteological occurrences for Moridae. The morphology of the specimen suggests affinities with the genera Physiculus and Salilota. The presence of Moridae in the Early Miocene of Patagonia and New Zealand is in agreement with previous hypotheses suggesting marine transantarctic connections between both landmasses during Late Oligocene - Early Miocene times.

Magnetostratigraphy of the Apalós Formation (early Pleistocene): Evidence for pulsed uplift of Cyprus

[1] Landscape formation on Cyprus is controlled by processes directly linked to the uplift and unroofing of the Troodos ophiolite complex since mainly early Miocene times. Understanding the island's geology and dating individual tectonic events will help in differentiating between tectonically controlled uplift and eustatic sea level or climatic changes. In order to improve the timing of these events, a magnetostratigraphic study was carried out on two terraces in the Mesaoria Basin of central Cyprus. At Vlokkaria, southwest of Nicosia, an artificial cliff exposes the sedimentary Apalos Formation of early Pleistocene age. A nearby section situated on a terrace, probably postdating the Vlokkaria section, was sampled at Kokkinos. Remanence-carrying minerals are end-member magnetite and maghemite formed by exsolution and oxidation from ophiolitic titanomagnetite. Hematite as a late alteration product is also present. Alternating field demagnetized paleomagnetic samples yield predominantly reversed polarities interpreted to have been acquired as early detrital remanent magnetization during the Matuyama chron. Two zones of normal polarity within the Apalos Formation are interpreted to correlate to the Olduvai and Jaramillo subchrons. Pre-Apalos marine sediments at the bottom of the Vlokkaria section show transitional polarity behavior and therefore might correlate with the onset of the Reunion event. Results from the Kokkinos section are exclusively of normal polarity, which has been acquired during the Brunhes chron. Sedimentation rates derived from these magnetostratigraphic results are in the order of 3–6 cm/kyr with a marked increase to 50 cm/kyr just below the Matuyama-Jaramillo polarity transition, which is interpreted to reflect increased uplift of the source area and supports the hypothesis of pulsed uplift of the Troodos ophiolite complex.

First diatomyid rodent from the Early Miocene of Arabia

The Asian family Diatomyidae is known from the Early Oligocene to the present. Among living rodents, this group comprises only the recently discovered Laonastes aenigmamus from Laos. Fossil diatomyids are known from only a few sites, in which they are often rare. The discovery of Pierremus explorator gen. nov. sp. nov. in the Lower Miocene of As-Sarrar (Saudi Arabia) raises to ten the number of extinct diatomyid species recognized. Pierremus explorator is the first record of a diatomyid from the Afro-Arabian plate. This discovery provides evidence that, together with other rodents (ctenodactylids, zapodids…), the diatomyids took advantage of the corridor that was established between Afro-Arabia and Eurasia in Early Miocene times.

A revision ofAceratherium blanfordiLydekker, 1884 (Mammalia: Rhinocerotidae) from the Early Miocene of Pakistan: postcranials as a key

Rhinocerotids are particularly abundant and diversified in Neogene deposits of the Indian subcontinent, but their systematics is far from being well defined. Based on the revision of old collections and new findings from the Early Miocene of the Bugti Hills and Zinda Pir, Pakistan, ‘Aceratherium blanfordi Lydekker, 1884’ is a chimera, consisting of two dentally convergent but postcranially distinct rhinocerotid taxa: Pleuroceros blanfordi and Mesaceratherium welcommi sp. nov. Postcranial features appear to be much more diagnostic than craniodental morphology in this case. A phylogenetic analysis based on 282 morphological characters scored for 28 taxa (four outgroups and ingroup including both taxa of interest and a ‘branching group’) strengthens this statement and supports Pleuroceros and Mesaceratherium as monophyletic genera within Rhinocerotinae. Both genera are recognized for the first time outside Europe. In the Bugti Hills, P. blanfordi and M. welcommi are part of an exceptionally diversified rhinocerotid fauna, with up to nine species associated in the same locality (Kumbi 4f). This rhinocerotid assemblage confirms the earliest Miocene age (Agenian/Aquitanian) of the upper member of the Chitarwata Formation as a whole. Coeval homotaxic rhinocerotid faunas from Europe (France, Czech Republic) and East Africa (Uganda, Kenya) support broad and sustainable rhinocerotid interchanges amongst South Asia, Europe, and Africa under compatible environmental conditions throughout earliest Miocene times. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 160, 139–194.

Reconciling the geological history of western Turkey with plate circuits and mantle tomography

We place the geological history since Cretaceous times in western Turkey in a context of convergence, subduction, collision and slab break-off. To this end, we compare the west Anatolian geological history with amounts of Africa–Europe convergence calculated from the Atlantic plate circuit, and the seismic tomography images of the west Anatolian mantle structure. Western Turkish geology reflects the convergence between the Sakarya continent (here treated as Eurasia) in the north and Africa in the south, with the Anatolide–Tauride Block (ATB) between two strands of the Neotethyan ocean. Convergence between the Sakarya and the ATB started at least ~ 95–90 Myr ago, marked by ages of metamorphic soles of ophiolites that form the highest structural unit below Sakarya. These are underlain by high-pressure, low-temperature metamorphic rocks of the Tavşanlı and Afyon zones, and the Ören Unit, which in turn are underlain by the Menderes Massif derived from the ATB. Underthrusting of the ATB below Sakarya was since ~ 50 Ma, associated with high-temperature metamorphism and widespread granitic magmatism. Thrusting in the Menderes Massif continued until 35 Ma, after which there is no record of accretion in western Turkey. Plate circuits show that since 90 Ma, ~ 1400 km of Africa–Europe convergence occurred, of which ~ 700 km since 50 Ma and ~ 450 km since 35 Ma. Seismic tomography shows that the African slab under western Turkey is decoupled from the African Plate. This detached slab is a single, coherent body, representing the lithosphere consumed since 90 Ma. There was no subduction re-initiation after slab break-off. ATB collision with Europe therefore did not immediately lead to slab break-off but instead to delamination of subducting lithospheric mantle from accreting ATB crust, while staying attached to the African Plate. This led to asthenospheric inflow below the ATB crust, high-temperature metamorphism and felsic magmatism. Slab break-off in western Turkey probably occurred ~ 15 Myr ago, after which overriding plate compression and rotation accommodated ongoing Africa–Europe convergence. Slab break-off was accommodated along a vertical NE trending subduction transform edge propagator (STEP) fault zone, accelerating southwestward slab retreat of the Aegean slab. The SE Aegean slab edge may have existed already since early Miocene times or before, but started to rapidly roll back along the southeastern Aegean STEP in middle Miocene times, penetrating the Aegean region in the Pliocene.

Timing of thrust activity in the High Zagros fold-thrust belt, Iran, from (U-Th)/He thermochronometry

[1] Apatite and zircon (U-Th)/He cooling ages are used to quantify the timing of exhumation associated with thrust faulting in the High Zagros fold-thrust belt. Single-grain cooling age data were collected from (1) Cambrian sandstone in various thrust sheets, (2) sandstone and basement clasts derived from structurally controlled salt plugs or fault-bounded slices, and (3) syntectonic Neogene siliciclastics strata. In the northwestern (Kuhrang) and central (Kuh-e Lajin) High Zagros, apatite (U-Th)/He (AHe) ages range from ∼26.7 to ∼0.38 Ma. Most cooling and exhumation occurred in the early to middle Miocene, constrained by AHe ages ∼19–15 Ma from the High Zagros thrust sheet, localized faults, and reset cooling ages from Bakhtiyari deposits. In the southeastern High Zagros (Kuh-e Dinar), cooling occurred later with AHe ages ranging from ∼16.5 to ∼0.79 Ma. Here most cooling and exhumation occurred in the late Miocene, constrained by AHe ages ∼12–8 Ma from the High Zagros fault, and exhumed Paleozoic clasts in synorogenic strata. Zircon (U-Th)/He (ZHe) ages from bedrock samples across the High Zagros are reflective of the precollisional thermal history. The preservation of precollisional ZHe ages limits the pre-Miocene maximum burial temperature of the exhumed strata to < 180°C, and indicate < 7–9 km of maximum exhumation in the central Zagros. This study shows that thrust activity in the High Zagros and continental suturing along the Zagros suture was underway by at least 19 Ma, and initiated no later than latest Oligocene to early Miocene time (∼23 Ma).

A new species of Cythere (Ostracoda) from the Middle Eocene McIntosh Formation, Doty Hills, Washington State, USA, and its significance for the evolutionary history of the genus

Cythere ikeyanoriyukii n. sp., an extant phytal ostracode genus, was obtained from the Middle Eocene McIntosh Formation in the Doty Hills, western Washington State, USA. It was associated with eleven taxa, which are extant phytal and shelfal genera such as Loxocorniculum, Xestoleberis, Ambostracon, Coquimba, and Acanthocythereis. The presence of Cythere in this assemblage is surprising and indicates that the first appearance of this genus was middle Eocene time at the latest, or at least 20 Ma earlier than previously thought. Cythere did not originate in the Pacific Ocean around Japan as previously thought, but instead must have migrated from the northeastern Pacific to the northwestern Pacific between middle Eocene and early Miocene time.

A new species ofCythere(Ostracoda) from the Middle Eocene McIntosh Formation, Doty Hills, Washington State, USA, and its significance for the evolutionary history of the genus

Cythere ikeyanoriyukiin. sp., an extant phytal ostracode genus, was obtained from the Middle Eocene McIntosh Formation in the Doty Hills, western Washington State, USA. It was associated with eleven taxa, which are extant phytal and shelfal genera such asLoxocorniculum,Xestoleberis,Ambostracon,Coquimba, andAcanthocythereis.The presence ofCytherein this assemblage is surprising and indicates that the first appearance of this genus was middle Eocene time at the latest, or at least 20 Ma earlier than previously thought.Cytheredid not originate in the Pacific Ocean around Japan as previously thought, but instead must have migrated from the northeastern Pacific to the northwestern Pacific between middle Eocene and early Miocene time.

Oligocene-Miocene basin evolution in the northern Altiplano, Bolivia: Implications for evolution of the central Andean backthrust belt and high plateau

The upper Oligocene to lower Miocene Penas and Aranjuez formations are exposed in north-northwest–trending outcrop belts of the central Andean backthrust belt situated within the central Andean plateau along the boundary between the northern Altiplano and the Eastern Cordillera of Bolivia. Sedimentary lithofacies analyses indicate that these coarse-grained siliciclastic formations were deposited primarily in alluvial fan to braided fluvial environments. An upsection change from principally fine-grained sandstone to cobble conglomerate is consistent with increased proximity to the sediment source with time. Paleocurrent analyses reveal that flow was predominantly directed toward the west-southwest away from Cordillera Real, the elevated core of the Eastern Cordillera. Provenance data from conglomerate clast compositions and sandstone petrofacies suggest derivation from recycled quartz-rich metasedimentary and sedimentary rocks from the Paleozoic section in the Eastern Cordillera. The paleoflow orientations, sediment provenance, and increased proximity of the sediment source suggest that deposition of the Penas and Aranjuez formations was related to surface uplift of the Eastern Cordillera relative to the Altiplano. Growth strata observed in the Aranjuez Formation further indicate that shortening was synchronous with deposition, probably in a hinterland basin. New 40 Ar/ 39 Ar ages from a lowermost exposed igneous unit and interbedded ash-fall tuff beds in the Aranjuez and Penas formations show that synorogenic sedimentation and fold-thrust deformation in the frontal (west-southwestern) zone of the central Andean backthrust belt was concentrated during late Oligocene–early Miocene time. These age results are consistent with previous studies of east-derived sedimentation in the Altiplano and indicate regional uplift of the Eastern Cordillera at this time. Upsection trends in provenance data further suggest a progressively greater contribution from younger Paleozoic strata, possibly due to activation of new thrust faults during west-southwestward propagation of the backthrust belt toward the Altiplano. Such a progression of late Oligocene–early Miocene shortening along the Altiplano–Eastern Cordillera boundary likely reflects significant crustal thickening, potential isostatic uplift, and initial topographic expression of the eastern margin of the central Andean plateau.

Dating of fault zone activity by apatite fission track and apatite (U-Th)/He thermochronometry: a case study from the Lavanttal fault system (Eastern Alps)

IntroductionFaulting in the upper crust may leadto thermal anomalies caused by fric-tional heating (e.g., Maddock, 1983;dAlessio et al., 2003) and⁄or infiltra-tion of hydrothermal fluids (Yamadaet al., 2007). Heating effects are ex-pected to be localised along faultzones and restricted to zones a fewcentimetres adjacent to the main faultsurface (dAlessio et al., 2003).In this study, we attempted to datethermal anomalies within fault zonesby comparison of cooling ages at thetransition from fault-related rocks tothe undeformed host rocks. For thispurpose, we chose the Lavanttal faultsystem. Due to underground excava-tion and cored drillings related togeological site investigations for amajor railway project in Austria, it isone of the best exposed fault zones inthe Eastern Alps. We used a combi-nation of fission track (AFT) and(U–Th)⁄He analysis on apatites. Thesensitivity intervals of these thermo-chronometers are often referred to aspartial annealing and retention zones(APAZ, HePRZ) and range from 120to 60 C and 80 to 40 C, respectively(Green et al., 1986; Wolf et al., 1996),assuming holding times over geo-logical time-scales (Wagner, 1979;Ketcham et al., 1999). The low-temperature sensitivity of these ther-mochronometers allows thermalanomalies along surfaces of fault slipor fluid pathways to be detected. Forsuch applications, however, annealingand retention depend not only onheating temperature but also on heat-ing duration (Laslett et al., 1987;Murakami et al., 2006). A combina-tion of forward and inverse modellingon apatite age, track length and Hedata allows quantitative analysis ofthe cooling behaviour of the hostrocks.Our new results underline theimportance of using multiple low-temperature thermochronometersin elucidating details of the thermalhistory of a fault zone.Geological outlineThe NNW-trending Lavanttal faultsystem, one of the major strike–slipfaults in the Eastern Alps (Fig. 1),separates the basement complexes ofthe Kor- and Saualm by dextral offsetof approximately 10 km (Reinecker,2000). It is assumed to have beenactive since Early Miocene times withpeaks in activity between 18–16 and14–12 Ma (Reinecker, 2000) as indi-cated by the evolution of adjacentsedimentary basins and the regionalstructural framework (e.g., Pischingeret al., 2008). Near its southern termi-nation, the Lavanttal fault offsets thePeriadriatic fault by about 20 km.Despite some models at the scale ofthe Eastern Alps describing the gen-eral evolution of confining fault zonesduring Miocene times (e.g., Peressonand Decker, 1997; Frisch et al., 1998,2000; Neubauer et al., 2000), littleremains known about the evolutionof the Lavanttal fault system. How-ever, these models include a relation-ship between the Lavanttal faultsystem and the uplift of the KoralmComplex (Neubauer and Genser,1990). Previous thermochronologicalstudies documented that the Koralmcomplex was already near the surfaceduring the whole Cenozoic period(Hejl, 1997). The timing of its finaluplift, however, is less constrained.Sample descriptionParts of the Lavanttal fault system arecharacterised by cataclasites and faultgouges up to several tens of metresthick. Thermochronological analyseswere carried out on drill core samples(diameter 10 cm) taken during thegeological investigations for theKoralm railway tunnel. DrillingABSTRACT

Role of sedimentation during basin inversion in analogue modelling

Sand-box experiments were designed to reproduce the effect of sedimentation during extensional basin development and tectonic inversion. These experiments were motivated by growing evidence for tectonic inversion in early Miocene times of an extensional basin (Abanico Basin) in the central Chilean Andes with contemporaneous intense volcanic activity. The experiments perform an extension followed by compression. The first series of experiments were performed without sedimentation in the developing basin; the second series were performed with partial filling of the basin, and the third series with total filling of the extensional basin. The effect of sedimentation on the sides of the basin was also controlled. Compression experiments reproducing tectonic inversion of the previously developed extensional basin were also performed with and without coeval sedimentation (series with null, partial or total fill, and also external sedimentation). The analogue experiments show that high amounts of sediments accumulated during the evolution of an extensional basin enhance subsidence and lead to the development of shortcuts in the footwall of the basin bounding faults during tectonic inversion. During extension the earliest appearance of normal faults occurs in the compartment with the greater sedimentary fill, and this causes asymmetric basin development. The post-extension compressional models show that the greater burden caused by a thicker accumulation of sediments within the basin inhibits tectonic inversion, causing the development of thrust structures rooted in the basin bounding faults. The accumulated sedimentary load affects both the extension and compression phases of the experiments, but has greater influence in the case of sedimentation during extension. A higher burden on the outer edges of the basin compared to the interior enhances the reactivation of normal faults and hampers the development of new thrust structures. The thrust fault(s) originating from the most loaded basin compartment develops as a footwall shortcut from the reactivated fault. With increasing sedimentation the shortcut is developed more superficially. The shortcut absorbs less deformation than the associated main fault.

Complex tectonic and tectonostratigraphic evolution of an Alpine foreland basin: The western Duero Basin and the related Tertiary depressions of the NW Iberian Peninsula

The tectonic and tectonostratigaphic evolution of foreland basins and related Tertiary depressions are the key to investigate deformation history and the uplifting of the continental lithosphere of the Alpine–Pyrenean Orogeny. The northern part of the Duero basin is the foreland basin of the Cantabrian Mountains, which are, in turn, the western part of the Pyrenean Orogen. We have studied the western sharp end of the Duero foreland basin, and its relation to the Tertiary deposits of the NW Iberian Peninsula and the topography evolution. In order to propose a coherent tectonic and tectonosedimentary model that could explain all Tertiary deposits, we have analysed the depositional environment, stratigraphic sequences, paleocurrents and established a correlation of the main outcrops. Besides, a detailed structural mapping of the Alpine structures that limit and affect the main Tertiary outcrops has been carried out. The Tertiary deposits of the NW Iberian Peninsula depressions are affected and fragmented by Alpine structures that limit their extensions and locations. The stratigraphic succession is similar in the NW Tertiary outcrops; they are mainly terrigenous and carbonated continental deposits formed by assemblage of alluvial fans developed at the mountains front, in arid or semiarid conditions. Three formations can be identified in the main depressions: Toral Fm, Santalla Fm and Médulas Fm. The NW Tertiary outcrops were the western deposits of the Duero foreland basin that surrounded the lateral termination of the Pyrenean Orogen. These deposits were fragmented and eroded by the subsequent uplift of the Galaico–Leoneses Mountains and the NE–SW strike-slip faults activity (broken foreland basin). Only the latest stages of some of these outcrops can be considered as intramontane basins as traditionally have been interpreted. The sedimentation started in the northeast (Oviedo–Infiesto) during the Eocene and migrated to the west (As Pontes) during the Late Oligocene–Early Miocene times. Meanwhile, southern areas were far away from the tectonic activity (El Bierzo, O Barco, Monforte, Sarria depressions).

The age of immigration of the vertebrate faunas found at Gargano (Apulia, Italy) and Scontrone (l’Aquila, Italy)

Recently a discussion is taking place about the Scontrone (l’Aquila) and Gargano (Apulia, Italy) mammal faunas and the age of their immigration. Mazza and Rustioni (2008) dated the Scontrone mammal fossils as Tortonian on the basis of their position in the Lithothamnium Limestone and came to the conclusion that some elements of the Scontrone and Gargano faunas must have colonised the area in Oligocene or Early Miocene times. Van den Hoek Ostende et al. (2009) disagreed with this interpretation and suggested a Late Miocene (10 Ma) age for the time of immigration. We think the arguments to place Scontrone in the Tortonian are not convincing. An analysis of the potential ancestors of each of the Gargano faunal components shows that a Messinian age for the immigration is fully compatible with the distribution of these ancestors in the European Miocene.

Alkaline series related to Early-Middle Miocene intra-continental rifting in a collision zone: An example from Polatlı, Central Anatolia, Turkey

Abstract A large volcanic area (∼7600 km2), the Galatean Volcanic Province (GVP), developed in northwest Central Anatolia during the Miocene along the Neo-Tethys Ocean suture zone possibly by post-collisional processes. The GVP mainly comprises 20–14 My old acid to intermediate volcanites with a geochemical signature indicating a mantle source modified by earlier (Late Cretaceous) subduction-related events. 100 km south of the GVP, near Polatli, Ankara, basaltic rocks that cover large areas are intercalated with the Miocene deposits of the Beypazari basin, an intra-continental subsidence zone at the southwest of the GVP. Field observations, geochemistry and K–Ar age dating of the Polatli volcanites show that they are Early (19.9 Ma) to mid (14.1 Ma) Miocene in age, covering an area as large as 215 km2. Variations in lava thickness and the thickness of the underlying silicified/baked zones suggest that the basaltic lavas erupted from a southern source, possibly from the Eskisehir fault zone, and flowed northwards. Most Polatli samples have chemical compositions that indicate derivation from a mantle source with crustal contamination during ascent. They do not display any characteristic to suggest a subductional component. Although the GVP and Polatli lavas formed close in time and space, they were derived from different mantle sources. Considering the positions of these two magmatic regions with regard to the Tethyan suture zone, we propose that the mantle beneath the GVP and near the suture zone memorised the earlier subduction while the mantle beneath Polatli that is located about 100 km further from the suture zone remained apparently unchanged. After a significant volume of magma was consumed in the GVP, a later (∼10 My) and last activity (Guvem activity) has produced quantitatively much less basaltic rocks where this subductional signature seems to completely disappear. Considering that the western Anatolian crust is proposed to undergo extension since the Late Oligocene–Early Miocene times, the Early Miocene intra-plate Polatli activity may have developed within this extensional tectonic regime. Combined with regional data, Polatli data also provide broad estimations on how long a subductional event continues to modify the mantle after the subduction ceased (at least ∼20 My), how long the subductional signature is preserved during significant magmatism (between 6 and 10 My) and how far the subductional effect disappears laterally on the mantle with respect to the collision zone (

Wrench structural deformation in Ras Al Hilal-Al Athrun area, NE Libya: a new contribution in Northern Al Jabal Al Akhdar Belt

Al Jabal Al Akhdar is a NE/SW- to ENE/WSW-trending mobile part in Northern Cyrenaica province and is considered a large sedimentary belt in northeast Libya. Ras Al Hilal-Al Athrun area is situated in the northern part of this belt and is covered by Upper Cretaceous–Tertiary sedimentary successions with small outcrops of Quaternary deposits. Unmappable and very restricted thin layers of Palaeocene rocks are also encountered, but still under debate whether they are formed in situ or represent allochthonous remnants of Palaeocene age. The Upper Cretaceous rocks form low-lying to unmappable exposures and occupy the core of a major WSW-plunging anticline. To the west, south, and southeast, they are flanked by high-relief Eocene, Oligocene, and Lower Miocene rocks. Detailed structural analyses indicated structural inversion during Late Cretaceous–Miocene times in response to a right lateral compressional shear. The structural pattern is themed by the development of an E–W major shear zone that confines inside a system of wrench tectonics proceeded elsewhere by transpression. The deformation within this system revealed three phases of consistent ductile and brittle structures (D1, D2, and D3) conformable with three main tectonic stages during Late Cretaceous, Eocene, and Oligocene–Early Miocene times. Quaternary deposits, however, showed at a local scale some of brittle structures accommodated with such deformation and thus reflect the continuity of wrenching post-the Miocene. D1 deformation is manifested, in Late Cretaceous, via pure wrenching to convergent wrenching and formation of common E- to ENE-plunging folds. These folds are minor, tight, overturned, upright, and recumbent. They are accompanied with WNW–ESE to E–W dextral and N–S sinistral strike-slip faults, reverse to thrust faults and pop-up or flower structures. D2 deformation initiated at the end of Lutetian (Middle Eocene) by wrenching and elsewhere transpression then enhanced by the development of minor ENE–WSW to E–W asymmetric, close, and, rarely, recumbent folds as well as rejuvenation of the Late Cretaceous strike-slip faults and formation of minor NNW–SSE normal faults. At the end of Eocene, D2 led to localization of the movement within E–W major shear zone, formation of the early stage of the WSW-plunging Ras Al Hilal major anticline, preservation of the contemporaneity (at a major scale) between the synthetic WNW–ESE to E–W and ENE–WSW strike-slip faults and antithetic N–S strike-slip faults, and continuity of the NW–SE normal faults. D3 deformation is continued, during the Oligocene-Early Miocene, with the appearance of a spectacular feature of the major anticline and reactivation along the E–W shear zone and the preexisting faults. Estimating stress directions assumed an acted principal horizontal stress from the NNW (N33°W) direction.

Fluvial stacking due to plate collision and uplift during the Early Pleistocene in Cyprus

AbstractSouthern Cyprus is situated within a mosaic terrane that has been fragmented between the northward drifting African and Arabian plates and the Eurasian plate. Enormous uplift of the earth mantle in the Tróodos Mountains is explained by two models. The subduction model explains subduction along the Cyprean arc to be the driving force for uplift whereas after the restraining bend model westward squeezing of Cyprus along strike-slip faulting is responsible for the enormous uplift at restraining bends. Since its emergence as an island in early Miocene times, landscape formation on Cyprus has been strongly controlled by this uplift. Until the Plio-Pleistocene, a strait belt separated the southern unroofed ophiolitic core region-the Tróodos Mountains-from the folded Kyrenia range to the north. This former sea basin, nowadays the Mesaoría Basin, is linked with the Tróodos Mountains by a dissected glacis with a thick cover of river deposits. The highest and oldest river deposits (Apalós Formation) were studied in the Vlokkariá hill southwest of Lefkosía. The 45.5 m thick Apalós Formation of Early Pleistocene age exhibits 24 sedimentary units (Fluviatile Series). Their magnetostratigraphical characters align with the Matuyama chron including the Olduvai and Jaramillo subchrons thus comprising about 1.15 Ma within the Early Pleistocene. This fluvial stack indicates a very flat and deeply lying river environment flowing from a slowly uplifting Tróodos hinterland. It happened during the end of Early Pleistocene when the enhanced Tróodos uplift started the dissection of the stacked river plain.

Tectonic evolution of the central Levant domain (Lebanon) since Mesozoic time

Abstract The tectonic history of the central part of the Levant domain (Lebanon) is re-evaluated. Examination of the tectonic structures and mechanical analysis of the meso-scale brittle deformation indicate that Lebanon has experienced four major tectonic events since Late Mesozoic time. The first was an Early Cretaceous extensional phase orientated north–south to NNE–SSW. It produced WSW–ENE to WNW–ESE normal faults with offsets up to several hundreds of meters and led to the development of an approximately WNW–ESE-trending basin. A second extension, with similar driving stresses, occurred during Eocene time and persisted, perhaps until Oligocene times. The Early Neogene period marked a dramatic change in the structural evolution of Lebanon after which strike–slip and reverse faulting and folding dominated. During Early Miocene times, an east–west compression produced moderate folding and faulting. A second, but much more severe, folding event occurred during Late Miocene time owing to a NNW–SSE compression. This new tectonic history allows the discussion of several aspects of the Eastern Mediterranean basin development and the later deformation of its continental margin and surroundings, in particular: (1) the driving mechanisms of the Levant basin opening; (2) the inversion of its adjacent margin; and (3) the age, origin, and evolution of the restraining bend of the Dead Sea Transform in Lebanon.

Tectonic significance of synextensional ductile shear zones within the Early Miocene Alaçamdağ granites, northwestern Turkey

AbstractSynextensional granitoids may have significant structural features leading to the understanding of the evolution of extended orogenic belts. One of the highly extended regions, the Aegean region, includes a number of metamorphic core complexes and synextensional granitoids that developed following the Alpine collisional events. The Alaçamdağ area in northwestern Turkey is one of the key areas where Miocene granites crop out along the boundary of various tectonic units. Structural data from the Early Miocene Alaçamdağ granites demonstrated two different deformation patterns that may provide insights into the development of granitic intrusions and metamorphic core complexes. (1) Steeply dipping ductile shear zones caused emplacement of syn-tectonic granite stocks; they include kinematic indicators of a sinistral top-to-the-SW displacement. This zone has also juxtaposed the İzmir–Ankara Zone and the Menderes Massif in the west and east, respectively. (2) Gently dipping ductile shear zones have developed within the granitic stocks that intruded the schists of the Menderes Massif on the structurally lower parts. Kinematic data from the foliated granites indicate a top-to-the-NE displacement, which can be correlated with the direction of the hanging-wall movement documented from the Simav and Kazdağ metamorphic core complexes. The gently dipping shear zones indicate the presence of a detachment fault between the Menderes Massif and the structurally overlying İzmir–Ankara Zone. Mesoscopic- to map-scale folds in the shallow-dipping shear zones of the Alaçamdağ area were interpreted to have been caused by coupling between NE–SW stretching and the accompanying NW–SE shortening of ductilely deformed crust during Early Miocene times. One of the NE-trending shear zones fed by granitic magmas was interpreted to form the northeastern part of a sinistral wrench corridor which caused differential stretching between the Cycladic and the Menderes massifs. This crustal-scale wrench corridor, the İzmir–Balıkesir transfer zone, may have controlled the asymmetrical and symmetrical extensions in the orogenic domains. The combination of the retreat of the Aegean subduction zone and the lateral slab segmentation leading to the sinistral oblique-slip tearing within the Eurasian upper plate appears to be a plausible mechanism for the development of such extensive NE-trending shear zones in the Aegean region.

Paleomagnetic evidence for a pre-early Eocene (∼50Ma) bending of the Patagonian orocline (Tierra del Fuego, Argentina): Paleogeographic and tectonic implications

Abstract The southernmost segment of the Andes of southern Patagonia and Tierra del Fuego forms a ∼ 700 km long orogenic re-entrant with an interlimb angle of ∼ 90° known as Patagonian orocline. No reliable paleomagnetic evidence has been gathered so far to assess whether this great orogenic bend is a primary arc formed over an articulated paleomargin, or is due to bending of a previously less curved (or rectilinear) chain. Here we report on an extensive paleomagnetic and anisotropy of magnetic susceptibility (AMS) study carried out on 22 sites (298 oriented cores), predominantly sampled in Eocene marine clays from the external Magallanes belt of Tierra del Fuego. Five sites (out of six giving reliable paleomagnetic results) containing magnetite and subordinate iron sulphides yield a positive fold test at the 99% significance level, and document no significant rotation since ∼ 50 Ma. Thus, the Patagonian orocline is either a primary bend, or an orocline formed after Cretaceous–earliest Tertiary rotations. Our data imply that the opening of the Drake Passage between South America and Antarctica (probably causing the onset of Antarctica glaciation and global climate cooling), was definitely not related to the formation of the Patagonian orocline, but was likely the sole consequence of the 32 ± 2 Ma Scotia plate spreading. Well-defined magnetic lineations gathered at 18 sites from the Magallanes belt are sub-parallel to (mostly E–W) local fold axes, while they trend randomly at two sites from the Magallanes foreland. Our and previous AMS data consistently show that the Fuegian Andes were characterized by a N–S compression and northward displacing fold–thrust sheets during Eocene–early Miocene times (50–20 Ma), an unexpected kinematics considering coeval South America–Antarctica relative motion. Both paleomagnetic and AMS data suggest no significant influence from the E–W left-lateral Magallanes–Fagnano strike–slip fault system (MFFS), running a few kilometres south of our sampling sites. We thus speculate that strike–slip fault offset in the Fuegian Andes may range in the lower bound values (∼ 20 km) among those proposed so far. In any case our data exclude any influence of strike–slip tectonics on the genesis of the great orogenic bend called Patagonian orocline.