Late Oligocene To Early Miocene Time Research Articles

Reconciling along-strike disparity in slip displacement of the San Andreas fault, central California, USA

Abstract The correlation of the ca. 23 Ma Pinnacles and Neenach volcanic complexes provides the most robust estimate on the timing and magnitude of Neogene right-lateral displacement on the San Andreas strike-slip fault system (California, United States). Displacement of ∼315 km has been applied rigorously along the plate margin to guide reconstruction of offset paleogeographic features. We present new detrital zircon U-Pb geochronology from the La Honda and western San Joaquin basins to document sediment provenance and reevaluate compositional constraints on a hypothesized key cross-fault tie (i.e., Castle Rock–Recruit Pass submarine fan system). Whereas the Upper Oligocene–Lower Miocene Vaqueros Formation of the La Honda basin was likely recycled from or shared a similar southern Sierra Nevada–western Mojave source with the underlying Eocene stratigraphy, we found that the Temblor Formation of the central Temblor Range (e.g., Recruit Pass submarine fan) was derived directly from Late Cretaceous northern Salinian basement. Furthermore, the Carneros Sandstone of the northern Temblor Range had a central Sierra Nevada batholith source that was likely recycled during early Miocene unroofing of the underlying stratigraphy. Conversely, strata of the southwest San Joaquin basin have provenance characteristics that match more closely with those of the La Honda basin. Our data preclude a contiguous Castle Rock–Recruit Pass submarine fan system across the San Andreas fault. These relationships are resolved by restoring the ca. 105–100 Ma basement of the northernmost Salinian block an additional ∼45 km or greater farther south relative to the Sierra Nevada batholith during late Oligocene–early Miocene time. Inconsistency in displacement along the San Andreas fault with the coeval correlation of the Pinnacles–Neenach volcanic complex is reconciled by postdepositional Miocene–Quaternary off-fault NW-SE structural shortening via major thrusts and/or transrotation of the Tehachapi block, in combination with extension of the northern Salinian block. This additional displacement reduces the need for pre–28 Ma slip on the San Andreas or predecessor faults to resolve Cretaceous through Eocene cross-fault relationships and reconciles an early Miocene discrepancy with Pacific–North America relative plate motion. This study highlights the fact that displacement histories of major strike-slip faults are divergent across changing structural domains, and recognition of slip disparities can constrain the magnitude of deformation.

Late Mesozoic and Cenozoic thermotectonic history of eastern, central and southern Mexico as determined through integrated thermochronology, with implications for sediment delivery to the Gulf of Mexico

AbstractA database of 134 apatite fission track (AFT), and apatite and zircon (U–Th)/He analyses has been assembled for eastern Mexico. Most of these samples have reset ages and track lengths reflecting rapid cooling. Time–temperature histories were modelled for 99 localities, and were converted to depth using a constant gradient of 30°C km−1. Maps of these results reveal smooth temperature patterns in space and time, indicating that heating was due to regional burial rather than hydrothermal circulation. Cooling began by 90 Ma in the west and 50 Ma along the eastern edge of the Sierra Madre Oriental. These ages mimic the duration of the Mexican Orogeny, which verifies that most of these AFT ages have event significance. The elongate Mayrán Basin, a part of the Mexican foreland basin system, formed and grew across and above the eastern toe of the active Sierra Madre Oriental. This basin subsided between at least 70 and c. 40 Ma, and reached a minimum depth of 6 km. It was a both a catchment and routing system for sediment from US and Mexican sources. The shape of the basin suggests that early outflow was directed through the Burgos Basin into the Gulf of Mexico (GoM). By 50 Ma, some outflow potentially routed southwards through the Tampico Misantla Basin area. The Mayrán Basin subsided until 40 Ma, and then began to uplift and erode. This inversion mobilized the stored sediment and redeposited it into the GoM, filling the offshore Bravo Trough. Volcanism swept eastwards between 90 and 40 Ma, driven by northeastward-directed flat-slab subduction, which may also have driven the contraction. Local subsidence during contraction suggests there was dynamic pull-down created by the underriding flat slab. Subsidence ceased at c. 40 Ma, as volcanism swept back westward and asthenosphere replaced the flat slab. The crust rebounded, creating an ensuing period of massive erosion which peaked around 20 Ma. Southern Mexico was relatively quiet until rapid uplift began in Oaxaca in late Oligocene–early Miocene time. Uplift progressed eastwards to the Chiapas Massif in the late Miocene, commensurate with the eastward translation of the Chortis Block.

Structural evolution and Cenozoic tectonostratigraphy of the Cairo-Suez district, north Eastern Desert of Egypt: Field-structural data from Gebel Qattamiya-Gebel Um Reheiat area

Detailed field mapping reveals that continental rifting is strongly deforming the Gebel Qattamiya-Gebel Um Reheiat area and the entire Cairo-Suez district, in north Eastern Desert of Egypt. Rift-related structures are predominantly represented by E to WNW, NNW and NW oriented faults. The E to WNW oriented faults are small and build up the Gebel Qattamiya en echelon fault belt, whereas the faults trending NNW and NW establish a pervasive horst and graben structural style involving some rhomb-shape horsts as Gebel Qattamiya (GQRH), Gebel Um Reheiat (GURRH) and south Gebel Um Reheiat (SGURRH). Rock units of the Eocene succession and Oligocene sediments are well exposed and highly controlled by rift-related structures. Rifting was developed through two rift-phases; initial and major ones. The initial phase (a newly recognized phase in this contribution) has been occurred in Late Eocene (Priabonian), while the main phase was prevailing during Late Oligocene-Early Miocene time and is characterized by hydrothermal veins and basaltic eruptions. Continental transtension in the Cairo-Suez district, including the study area, was probably synchronous with a major tectonic stage (Pyrenean-Atlasic movement) of continental collision between African–Arabian and Eurasian plates in Late Eocene–Oligocene time.Field investigation suggests that the transfer of displacement (slip) from the Gulf of Suez proto-rift into the E–W oriented faults ‘‘relays’’ is an important mechanism, which helps to explain the current structural framework and tectonic evolution of the Cairo-Suez district. Reactivation of such faults with right-lateral divergent wrenching with NE–SW oriented extension deformed the Cairo-Suez district with several E–W oriented en echelon fault belts (e.g. Gebel Qattamiya fault belt in the study area). Thus the Cairo-Suez district represents an accommodation or transfer zone in northeastern Egypt, intercepting the ‘‘far-field stresses’’ from the Arabian–Nubian Shield, the Red Sea-Gulf of Suez rift and the Maghrebian Shield.

Extensional strain and displacement distribution due to mesoscale normal faults in Late Miocene-Pliocene sedimentary rocks along the northwestern side of the Red Sea, Egypt

Field observations are presented on the NW-SE mesoscale, dip-slip, normal faults in the Late Miocene–Pliocene sedimentary rocks, along the northwestern part of the SW side of the Red Sea, Egypt. These faults were initiated parallel to the Red Sea, and were originated by the NE-SW extension associated with the Red Sea opening in the Late Oligocene-Early Miocene time. About 100 mesoscale normal faults were measured in the Late Miocene-Pliocene sedimentary rocks along seven scan-lines. The extensional strain determined in five scan-lines ranges from 2.6393 to 5.12% with an average of 3.53145%. The other two scan-lines have anomalous values of 6.2988 and 15.53%. The represented data demonstrate that the extensional strain varies significantly from profile to profile and even along the fault because of several surficial factors. The first factor is a difference in lithology. The second and third factors are the local stress and the difference between perpendicular to the direction of maximum lateral extension of area and strike of faults. The L-D (Length-Displacement) diagrams along twelve selected faults reveal three patterns. These patterns include a cone-shaped (C-type), meso-shaped (M-type), and a zigzag-shaped (Z-type). The remarkable variation of displacement (D) along the fault plane (L) is a result of the difference in lithology, and/or the overlapping fault segments, as well as the local stress along the faults.

Pressure–temperature–deformation–time of the ductile Alpine shearing in Corsica: From orogenic construction to collapse

Definition of the Tertiary tectono-metamorphic history of Alpine Corsica is a key task to decipher the space–time linkage between the Alpine and Apennine subduction systems in the Mediterranean region. Alpine Corsica exposes a nappe stack of oceanic- and continental-derived units, structurally juxtaposed onto the former European continental margin (Hercynian Corsica). Still uncertain is the timing of involvement of the continental-derived units in orogenic construction and shift to regional extension. This paper focuses on reconstruction of the pressure–temperature–deformation–time evolution of selected ductile shear zones activated during transition from the tectonic underplating to the extensional reworking stages. New Rb–Sr mineral age data, integrated with structural and thermobarometric investigations constrain the waning stages of the high-pressure (from blueschist to greenschist facies metamorphic conditions) top-to-the-W thrusting of the HP, oceanic-derived realm (Schistes Lustrés Complex) onto the Hercynian Corsica along the East Tenda Shear Zone in the early Oligocene (from ~32 to ~27Ma). This early compressional evolution is overprinted by a major phase of retrogressive, syn-greenschist top-to-the-E extensional shearing in the Schistes Lustrés Complex with the last episode of deformation-related ductile recrystallization recorded during the early Miocene at ~20–21Ma, in a continuum transition from ductile to brittle shearing. The same early Miocene Rb–Sr deformation ages are recovered from the ductile-to-brittle top-to-the-E reactivation domains within the East Tenda Shear Zone, documenting that transition from compression to extension in Alpine Corsica occurred during the late Oligocene–early Miocene time lapse. Implications of these data are discussed in the broader context of the Tertiary geodynamic evolution of the Central Mediterranean region.

Petrography and geochemistry of Cretaceous to quaternary siliciclastic rocks in the Tarfaya basin, SW Morocco: implications for tectonic setting, weathering, and provenance

The petrography, heavy mineral analysis, major element geochemical compositions and mineral chemistry of Early Cretaceous to Miocene-Pliocene rocks, and recent sediments of the Tarfaya basin, SW Morocco, have been studied to reveal their depositional tectonic setting, weathering history, and provenance. Bulk sediment compositional and mineral chemical data suggest that these rocks were derived from heterogeneous sources in the Reguibat Shield (West African Craton) including the Mauritanides and the western Anti-Atlas, which likely form the basement in this area. The Early Cretaceous sandstones are subarkosic in composition, while the Miocene-Pliocene sandstones and the recent sediments from Wadis are generally carbonate-rich feldspathic or lithic arenites, which is also reflected in their major element geochemical compositions. The studied samples are characterized by moderate SiO2 contents and variable abundances of Al2O3, K2O, Na2O, and ferromagnesian elements. Binary tectonic discrimination diagrams demonstrate that most samples can be characterized as passive continental marginal deposits. Al2O3/Na2O ratios indicate more intense chemical weathering during the Early Cretaceous and a variable intensity of weathering during the Late Cretaceous, Early Eocene, Oligocene-Early Miocene, Miocene-Pliocene and recent times. Moreover, weathered marls of the Late Cretaceous and Miocene-Pliocene horizons also exhibit relatively low but variable intensity of chemical weathering. Our results indicate that siliciclastics of the Early Cretaceous were primarily derived from the Reguibat Shield and the Mauritanides, in the SW of the basin, whereas those of the Miocene-Pliocene had varying sources that probably included western Anti-Atlas (NE part of the basin) in addition to the Reguibat Shield and the Mauritanides.

Structural and thermochronological evidence for Paleogene basement-involved shortening in the axial Eastern Cordillera, Colombia

Although most recent studies regard the northern Andes primarily as a low-shortening inversion orogen, new mapping and thermochronology along the paired basement-involved Floresta massif and Floresta basin in the axial Eastern Cordillera of Colombia suggest major Paleogene shortening in a ramp-flat fold-thrust belt. Field mapping indicates that the hanging wall of the east-directed Soápaga fault system contains a series of upright thrust sheets with flat-on-flat cutoff relationships and a deformed footwall characterized by a complex triangle zone. These geometries necessitate roughly east–west shortening exceeding that of a previously mapped overturned hanging wall anticline and disharmonic footwall folds. Zircon (U–Th)/He (ZHe) ages indicate exhumation-induced cooling of the Soápaga hanging wall through the ∼180 °C closure temperature at 31–25 Ma. This cooling postdated documented shortening to the west and predated shortening to the east, suggesting an eastward progression of Paleogene deformation. Synorogenic Oligocene footwall strata of the Floresta basin contain distal fine-grained sediments and lack growth strata or Oligocene detrital ZHe ages, suggesting relatively high heave along the Soápaga fault system. These results are consistent with a rapidly eastward-propagating, basement-involved fold-thrust belt with ramp-flat structures that accommodated tens of km of shortening. Long-term stasis of the deformation front on the eastern and western flanks of the Eastern Cordillera due to localization of Neogene shortening along Mesozoic rift-bounding normal faults indicates a shift in deformational style in late Oligocene–early Miocene time. This geometric and temporal framework implies: 1) a total shortening in the northern Andes exceeding most current estimates; 2) Paleogene deformation in the Eastern Cordillera marked by rapid advances of the deformation front along a ramp-flat thrust system; and 3) focused Neogene reactivation (inversion) of master rift-bounding faults achieved by earlier crustal thickening related to ramp-flat thrust deformation.

A tectonically-induced Eocene sedimentary mélange in the West Ligurian Alps, Italy

The southern Alpine Foreland basin succession (Late Eocene–Early Oligocene?) lies over the Mesozoic condensed carbonate succession of the Provençal Dauphinois Domain, ends with a chaotic complex, and is overthrust by more internal Alpine units (San Remo–Monte Saccarello Ligurian unit) emplaced in Late Oligocene–Early Miocene times. The chaotic complex consists of debris flow paraconglomerates, breccias, and megablocks, up to km-sized. Despite the post-Eocene Alpine tectonics overprint, particularly intense in its uppermost part, the sedimentary origin of the chaotic complex is proved by the interbedding of paraconglomerates and breccias with fine-grained turbidites. Megablocks consist both of formations buried below the chaotic deposits (Mesozoic Provençal Dauphinois succession, Eocene Alpine Foreland basin succession) both of exotic units (Ligurian Helminthoides Flysch). Paraconglomerate and breccia clasts were sourced by the penecontemporaneous turbidite sands and muds as well as the same lithologies as megablocks. All these features suggest the activity of early Alpine Eocene strike-slip or transtensive fault systems that juxtaposed and exhumed the Helminthoides Flysch unit and the Provençal Dauphinois succession. Submarine ridges were generated and favored rock fall phenomena that involved both small, mm- to dm-sized, clasts and huge slabs detached from the main rock masses. These morphostructural highs were flanked by oversteepened slopes affected by failures that gave rise to debris flows, involving hemipelagic muds and turbidite sands and lithified fragments of older formations, which resulted in strongly polygenic paraconglomerates. The studied mélange is thus fully due to sedimentary processes that were, however, completely controlled by early Alpine tectonics.

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.

Geology of the Cenozoic Indus Basin sedimentary rocks: Paleoenvironmental interpretation of sedimentation from the western Himalaya during the early phases of India-Eurasia collision

This study reassesses the stratigraphy, sedimentology, and provenance of the Indus Basin sedimentary rocks, deposited within the Indus Tsangpo Suture Zone (ITSZ) during the early phases of India-Eurasia collision. Using field observations, biostratigraphy, and petrographic and isotopic analyses we create a paleodepositional reconstruction within the paleotectonic setting of the early phases of India-Eurasia collision. We then re-examine existing constraints to the timing of India-Eurasia collision previously interpreted from the earliest occurrence of mixed Indian- and Eurasian-derived detritus in the succession. From mid-Cretaceous to early Paleocene times the Jurutze and Sumda Formations were deposited within an arc-bounded marine basin between the Dras and Kohistan-Ladakh Island arcs. The <51 Ma aged deltaic Chogdo Formation then filled the basin until deposition of the 50.8–49.4 Ma aged Nummulitic Limestone during a marine incursion, before continental facies developed in an evolving intermountain basin with the deposition of the Paleogene Indus Group. Within these systems, sediment was sourced from the Eurasian margin to the north and was transported southward into the suture zone. In this section, we see no unequivocal evidence of Indian Plate input to the sedimentary succession (and thus no evidence of mixed Indian-Eurasian-derived detritus indicative of India-Asia collision) until the upper stratigraphic horizons of the Indus Group, when facies are representative of an axial, northwesterly flowing river system. We suggest that the paleo-Indus River was initiated within the ITSZ during late Oligocene-early Miocene times. Sedimentation of the Indus Group continued until the late Miocene.

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.

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).

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 (

Unidirectional shift in leaf morphology of coexisting species — a possible indicator of palaeoclimatic differences

The pre-Neogene basement of the Pannonian domain is built up by a collage of terranes deriving from different parts of the Tethys, which finally accreted in the Late Oligocene–Early Miocene times. Combined stratigraphical, sedimentological, palaeomagnetic and tectonic investigations have revealed the palaeogeographic position of the Palaeogene Basin, including the Buda and Bükk tectonic units However, despite floristic similarities, the Buda and Bükk regions also show important physiognomic differences. In the vicinity of Budapest (Buda unit), the large size and width of leaves are conspicuous, whereas in Eger-Kiseged (Bükk unit), leaves are definitely smaller and narrower. In order to define the differences quantitatively, morphometrical measurements were adopted. The shape and size of leaf fossils were compared by means of a circular grid ( Hill, 1980). Leaves of 4 dominant species ( Sloanea elliptica (Andreánszky) Z. Kvaček & Hably, Engelhardia orsbergensis (Wessel & Weber) Jähnichen, Mai & Walther, Eotrigonobalanus furcinervis (Rossmässler) Walther & Kvaček, and Zizyphus zizyphoides (Unger) Weyland were the target of our study. The average leaf area is 1.4–2.3 times larger in the Buda tectonic unit (Budapest locality) than in the Bükk tectonic unit (Eger-Kiseged locality). Statistical tests proved highly significant differences between these values. Regarding the asymmetry of the leaf lamina, the four species differed considerably. Nevertheless, a significant difference between the two localities was shown only in one of them ( Sloanea elliptica). Consequently, a significant alteration of measures of asymmetry as a function of habitat conditions was not indicated. Smaller leaf area and significantly narrower leaf blade are possible indicators of drier climatic conditions in the Bükk unit.

Transtensional origin of the NE–SW Simitli basin along the Strouma (Strymon) Lineament, SW Bulgaria

Several depocentres have developed along the NNW–SSE-striking Strouma (Strymon) Lineament since the Miocene, forming the Strouma/Strymon graben system. Some of these basins, including the Dzherman and Simitli, strike at high angles to the lineament. The rhomboid-shaped Simitli basin is limited in the south by the NE–SW-striking, seismically active Kroupnik fault and contains terrestrial clastic sediments, which have been deposited since the Sarmatian. Its formation is attributed to ‘wrench-dominated transtension’ (D B event) which activated the basin9s NE–SW-striking boundary faults as left-lateral strike-slip structures during the Early–Middle Miocene. This deformation was associated with NNE–SSW contraction and WNW–ESE extension and succeeded the NNE–SSW contraction caused by ‘pure shear-dominated transpression’ (D A event), which governed the region in Late Oligocene–Early Miocene times. During the Middle–Late Miocene, the D B event was followed by WNW–ESE ‘pure shear-dominated extension’ (D 1 event), which caused further widening of the basin. The transtensional origin of the Simitli basin is explained by lateral extrusion of the crustal mass squeezed between the Apulian–Adriatic microplate and the European foreland towards the North Aegean Sea. The subduction retreat of the Hellenic orogen possibly enhanced this lateral extrusion, and from the Late Miocene onwards the NE–SW-oriented back-arc extension balanced the continuing retreat.

Mesozoic-Cenozoic Deformation of the Central Structure Belt in the Turpan-Hami Basin, Northwest China: Tectonic Evolution of an Intracontinental Basin, Central Asia

The central structure belt (CSB) in the Turpan-Hami Basin is composed of the Huoyanshan structure and the Qiketai structure, formed in the Late Triassic—Early Jurassic, and is characterized by extensional tectonics. The thickness of strata in the hanging wall of the fault is clearly larger than that in the footwall, and a deposition center evolved in the Taibei sag where the hanging wall of the fault is located. In the Late Jurassic, collision between the Lhasa block and the Eurasia continent resulted in the transformation of the Turpan-Hami Basin from an extensional into a compressional structure, and consequently in the tectonic inversion of the CSB from an extensional normal fault in the earlier stage to a compressive thrust fault in the later stage. Tertiary collision between the Indian and Eurasian plates occurred around 55 Ma, and this Himalayan orogenic event played a profound role in shaping the Tianshan area. However, the effect of the collision on this area was delayed and culminated approximately in Late Oligocene—Early Miocene time. The CSB was strongly deformed and thrust upward as a result of this tectonic event.

Granitoid rocks of the southern Menderes Massif (southwestern Turkey): field evidence for Tertiary magmatism in an extensional shear zone

Recent field campaign in the southern Menderes Massif in southwestern Turkey revealed that the so-called ‘core of the massif’ comprises two distinct types of granitoid rocks: an orthogneiss (traditionally known as augen gneisses) and leucocratic metagranite, where the latter is intrusive into the former and the structurally overlying ‘cover’ schists. These differ from one another in intensity of deformation, degree of metamorphism and kinematics. The orthogneiss display penetrative top-to-the-N–NNE fabrics formed under upper-amphibolite facies conditions during the Eocene main Menderes metamorphism (MMM), whereas foliation and stretching lineation exists in the leucocratic metagranites but are not strongly developed. The leucocratic metagranites show evidence of syn- to post-emplacement deformation in a series of weakly developed top-to-the-S–SSW fabrics formed under lower greenschist-facies (?) conditions. Leucocratic metagranite bodies occur all along the augen gneiss–schist contact in the southern Menderes Massif; they are emplaced as sheet-like bodies into country rocks (previously deformed and metamorphosed during a top-to-the-N–NNE Alpine orogeny) along a ductile extensional shear zone, located between orthogneisses and metasediments, which was possibly active during emplacement. The data presently available indicate that emplacement and associated ductile extensional deformation occurred during Late Oligocene–Early Miocene time. These results confirm previous contentions that there are Tertiary granites in this part of the Menderes Massif.

Tectonic development of Central Thailand: new evidences from airborne geophysical data

The N-S oriented central floodplain of Thailand is situated between the NE-SW-trending strike-slip Uttaradit Fault Zone (UTFZ) to the north, the northern shoreline of the Gulf of Thailand to the south, and irregular paths of normal faults to the east and west. Major structures of the area include the NW-SE-trending sinistral strike-slip Mae Ping and Three Pagoda Fault Zones (MPFZ and TPFZ) located to the west and a N-S-oriented Central Volcanic Belt (CVE) located to the east of the plain. Tertiary basins located within the Central Plain have geometries indicating that they were formed due to the reversed movement from sinistral to dextral ofthe major NW-SE-trending strike-slip fault zones during Late Oligocene-Early Miocene times. Geophysical data interpretation results (magnetic, gamma-radiometric, digital terrain data and regional gravity data) show the N-S-trending CVE, outline the known major fault zones, discover the Chao Phraya Fault Zone (CPFZ) and other subsurface lineaments and magnetic bodies within the Central Plain, as well as suggest a new fault path for the TPFZ. The findings indicate that the tectonic evolution of the Central Plain was controlled by the movement along the CVE, which may represent an ancient suture zone, and strike-slip faults bounding small sub­ continental blocks. These movements include: extension (oblique) along the old sutures and volcanic arcs, reversed sinistral-to-dextral movement on the NW-SE-trending MPFZ, TPFZ and CPFZ, and reversed dextral-to-sinistral UTFZ. Indochina's southeast extrusion tectonic setting associated with the collision of the Indian and Eurasian plates may have reactivated these lithospheric weak zones and caused the opening of the Central Plain and the Gulf of Thailand since Late Oligocene time.

Miocene-Pleistocene volcanism and tectonics in southern Korea and their relationship to the opening of the Japan Sea

Miocene-Pleistocene volcanism and tectonics in southern Korea are discussed. The Miocene volcanism and related tectonics are represented by the Eoil Orogeny, which occurred in the Yangnam and Pohang basins in the southeastern corner of the Korean Peninsula. The Yangnam and Pohang basins represent the uplifted central part of the Pohang-Ulsan Graben which was rifted between the Proto-Korean Peninsula and Paleo-Ulleung Land, probably in Late Oligocene-Early Miocene times. The Eoil Orogeny is correlated with the Green Tuff Orogeny in Japan. Evidence of Pliocene volcanism is observed on Ulleung Island located in the western part of the central Japan Sea. Volcanic and tectonic events represented on Ulleung Island are, in chronological order: (1) volcanic eruption; (2) subsidence and marine transgression; (3) dome-like uplift; and (4) volcanic eruption. Evidence of Pleistocene volcanism is present on Cheju Island located at the southwestern entrance to the Korea-Tsushima Strait. Volcanic and tectonic events represented on Cheju Island are, in chronological order: (1) volcanic eruption; (2) subsidence and marine transgression; (3) uplift and volcanic eruption; and (4) doming and volcanic eruption. Three grabens, namely the Pohang-Ulsan Graben, the Ulleung Basin and the Yamato Basin, were formed in the southern half of the Japan Sea, which was above sea level in Early Miocene times. Paleo-Ulleung Land was present between the Pohang-Ulsan Graben and the Ulleung Basin in Miocene-Pliocene times. These three grabens are active rifts, and the tectonic events that occurred in this region originated from the action of rising magma. The extension and deepening of the southern half of the Japan Sea resulted from the tectonic movements of these active rifts and their surrounding regions. Thus, it is suggested that the opening of the Japan Sea progressed by diapiric tectogenesis.

Late Oligocene-Early Miocene compressional tectosedimentary episode and associated land-mammal faunas in the Andes of central Chile and adjacent Argentina (32–37°s)

A reassessment of the geologic and land-mammal fossil evidence used in attribution of a tectosedimentary episode in the Andes between 32 and 37°S to the Middle Eocene “Incaic tectonic phase” of Peru indicates that the episode occurred during Late Oligocene-Early Miocene times(~ 27-20 Ma). From west to east, three structural domains are recognized for this time span in the study area: a volcanic arc (Chile); a thin-skinned, E-verging fold-thrust belt (Cordillera Principal, Chile-Argentina border strip); and a foreland basin (Argentina). Initiation of thrusting in the Cordillera Principal fold-thrust belt produced the coeval initiation of sedimentation in the foreland basin of adjacent Argentina. This onset of foreland deposition postdates strata bearing a Divisaderan Land Mammal Age fauna (i.e. ~ 35-30 Ma) and is marked at ~ 36°30′S by the base of the “Rodados Lustrosos” conglomerates, which are conformably overlain by sedimentary rocks containing a Deseadan Land Mammal Age fauna (i.e. ~ 29-21 Ma). Geologic relationships between the thick volcanic Abanico (Coya-Machalí) and Farellones formations also demonstrate that this tectosedimentary episode practically ended at ~ 20 Ma at least in the volcanic arc, and was therefore roughly coeval with the major tectonic crisis (~ 27-19 Ma) known in northwestern Andean Bolivia some 1500 km to the north. This strongly suggests that a long, outstanding tectonic upheaval affected at least an extended 12–37°S segment of the Andean margin of South America during Late Oligocene and Early Miocene times.