Synorogenic Sediments Research Articles

Thick- and thin-skinned contractional styles and the tectonic evolution of the northern Sangre de Cristo Mountains, Colorado, USA

The Sangre de Cristo Mountains of southern Colorado and northern New Mexico, USA, contain an unusual combination of thick- and thin-skinned contractional structures involving both basement and cover rocks in the Laramide Rocky Mountain foreland. These structures are truncated by down-faulted extensional basins to the east and west. Together with synorogenic sediments, these structures preserve a record of the rise of the Ancestral Rocky Mountains, the Laramide orogeny, and Rio Grande rifting. Laramide structures within the mountains provide clues to processes that link the three events and to necessary conditions for thin-skinned and thick-skinned contractional structures to form together in continental interiors. To examine the full variety of structural styles, a portion of the northern Sangre de Cristo fold-and-thrust belt in Colorado was described and interpreted using geologic maps and structural cross-sections. Stratigraphic relations of the Ancestral Rocky Mountain highlands and basin fill were reconstructed from existing maps. These relations allow identification of faults inherited from the Ancestral Rocky Mountains, differentiation of thrust sheets, and in some cases, estimation of the magnitude of displacement. To examine relations between Laramide thrusts and Rio Grande rifting, kinematic data were collected from a thrust fault adjacent to rift faults. Three thrust fault styles were recognized: thin-skinned basement, thin- skinned cover rocks, and thick-skinned basement. Thin-skinned thrusts arising from a hinterland beneath the present San Luis Valley carried sheets of Proterozoic basement rocks northeast over a Laramide foreland. These basement thrusts are interpreted to be faults of the Ancestral Rocky Mountains that reactivated during the Laramide orogeny. The Laramide foreland consists of thin-skinned thrusts and folds in sedimentary cover rocks as young as 49 Ma. Both thin-skinned thrusts in basement and cover rocks are bounded by thick-skinned basement thrusts that moved intermittently throughout the Laramide orogeny. We infer that thin-skinned thrusts form in continental interiors where deformation is focused in weak strata of thick basin fill and in fluid-reaction weakened preexisting faults in basement rocks. Both conditions are met in the Sangre de Cristo Mountains. Basement thrusts adjacent to the San Luis Valley contain evidence of plastic contractional microstructures overprinted by extensional microstructures that may record the transition from Laramide contraction to Rio Grande extension of the crust.

Identifying and characterizing missing source orogens for syn-orogenic basins based on detrital accessory mineral U-Pb geochronology and trace element geochemistry

Abstract Identifying source orogens for syn-orogenic sediments in dispersed fragments of a supercontinent provides key information on the tectonic evolution of the orogens involved in supercontinent assembly and on paleogeographic reconstruction. An approach using U-Pb geochronology and trace elements of multiple detrital minerals including zircon, rutile, apatite, and monazite has major advantages over detrital zircon studies alone. Application of this multi-pronged approach to Ediacaran and Cambrian clastic sedimentary rocks from the West Cathaysia terrane of the South China block demonstrates that these rocks were deposited in two distinct, but overlapping, syn-orogenic basins related to the assembly of eastern Gondwana. The Ediacaran rocks have detritus likely sourced from the Paterson-Petermann orogen (PPO) in Australia, and the dominant 650–600 Ma populations and trace element compositions of detrital rutile and apatite indicate a significant Ediacaran thermal and high-pressure metamorphic event in the orogen. The appearance of Cambrian and Tonian detrital rutile and apatite populations in the Cambrian rocks indicates derivation from a different source orogen characterized by multiple thermal events, most likely the Kuunga-Pinjarra orogen (KPO). Our data suggest that West Cathaysia was located adjacent to both the PPO and the KPO and support the hypothesis that the PPO was a collisional, rather than a long-lived post-Grenvillian intraplate, orogen. Our data also suggest a poly-metamorphic history for the poorly preserved northern part of the KPO and a larger expanse of Greater India.

Multiple Exhumation Stages During the Cenozoic Evolution of the Northeast Tibetan Plateau

AbstractThe Cenozoic growth history of the northeast (NE) Tibetan Plateau has been strongly debated in the past few years with three deformation models being proposed: progressive northeastward propagation, out‐of‐sequence deformation, and episodic deformation. Reconstruction of the long‐term deformation and exhumation history of the different blocks can help elucidate the growth pattern and tectonic processes involved in the formation of the Plateau. Both the Qaidam and Jiuquan basins—the two largest basins of the NE Tibetan Plateau—contain continuous and well‐exposed successions of synorogenic sediments that span the entire Cenozoic. We used apatite fission‐track thermochronology, sedimentary facies, and structural and provenance analyses of these successions to determine the exhumation history of the Tibetan Plateau. Five distinct fast exhumation events were recognized and dated: 65–54, 43–39, 34–29, 24–21, and 16–15 Ma. Comparison with existing morphotectonic information enabled us to reconstruct a multiple‐stage growth scenario for the NE Tibetan Plateau in the context of the surface uplift phases across the Himalayan‐Tibetan orogen during the Cenozoic. Overall, our findings support the episodic deformation model and emphasizes that the current relief of the NE Tibetan Plateau is largely derived from these five of stage of exhumation.

Cenozoic source-to-sink relations between the West Kunlun Mountains and SW Tarim Basin: Evidence from an integrated provenance analysis

The tectonic evolution of West Kunlun Mountains (WKM), which is located in the NW margin of the Tibetan Plateau, plays an essential role in understanding the source-to-sink relationship between this orogenic belt and SW Tarim Basin. In this study, we conduct an integrated research incorporating detrital zircon UPb geochronology, petrography, and heavy mineral analysis of the Cenozoic synorogenic sediments from Keliyang section, SW Tarim Basin, together with the published data in the WKM forelands, and thus we can discuss the Cenozoic tectonic evolution in this activate tectonic domain. The results suggest that the NW Tibetan Plateau has experienced a four-stage evolution during the Cenozoic, characterized by three notable provenance changes occurring in the late Eocene (∼40 Ma), late Oligocene (∼26 Ma) and latest Miocene-early Pliocene, respectively. In the early Middle Eocene, the Tarim-Tajik basin was still occupied by an epicontinental sea belonging to the Neotethys Sea. Sediments in the forelands were mainly originated from erosion of the residual relief of WKM. By ∼40 Ma, the new additions of sediments from the uplifted Tianshuihai and/or South Kunlun terranes occurred, which can be attributed to the uplift of central Tibet due to the ongoing Indian northward indentation. This scenario was roughly synchronous with the final open marine seawater retreat from the Tarim Basin. We propose that the Cenozoic uplift of the Tianshuihai/South Kunlun terranes may be initiated prior to ∼40 Ma, indicating that the collision-induced deformation has propagated to the southern part of WKM by that time. Since ∼26 Ma, distal deposits increased abruptly, which can be attributed to the outward growth of the Tibetan Plateau and/or regional climate change. Finally, the shift in provenance since the latest Miocene-early Pliocene was related to the intensive uplift of WKM.

Retro‐Foredeep Basin Evolution in Taiwan: Zircon U‐Pb and Hf Isotope Constraints From the Coastal Range

AbstractThe evolution of the Taiwan orogen, an active arc–continent collision zone between the North Luzon Arc (NLA) and the Eurasian continent, is recorded in arc volcanic and sedimentary rocks from the Taiyuan retro‐foredeep basin of the Coastal Range, eastern Taiwan. Arc volcanism in the northern NLA experienced a magmatic culmination at 9–7 Ma and a cessation at 5.3–4.3 Ma, based on new U‐Pb zircon ages. Young zircon U‐Pb ages (1.74, 1.58, and <1.2 Ma) and low Hf isotope signatures also indicate that three biotite‐bearing lapilli tuff layers deposited in the retro‐foredeep basin are associated with eruptions of the Lutao volcano in the southern NLA. NLA volcanism terminated progressively from ∼5.3 to <1.2 Ma in response to a southward‐propagating collision zone that followed the uplift of the orogenic wedge by <3 myr. The detrital zircon U‐Pb age spectra suggest no significant shift in provenance during the Plio‐Pleistocene period and indicate that the synorogenic sediments in the Taiyuan Basin were mainly derived from the Yuli Belt, a Miocene accretionary complex in the eastern Taiwan orogen. The accumulation rate of synorogenic deposits varies from 0.53 to 5.14 km/myr, with an abrupt increase at ∼2 Ma. This new rate is higher than the previously proposed rates and that of the pro‐foreland basin to the west, suggesting asymmetric and rapid uplift of the Taiwan orogen.

Plio-Pleistocene fluvial dynamics in the pro-foreland basins of Taiwan: Thermochronological constraints and tectonic implications from the syn-orogenic deposits

In order to tackle the relationship between the exhumed Taiwan Orogeny and the associated sediment routing systems, new results from zircon fission track (ZFT) dating, calcareous nannofossil biostratigraphy, and the occurrence of the diagnostic magnetic mineral pyrrhotite in modern and ancient syn-orogenic sediments from pro-foreland basins in western Taiwan are reported. The ZFT age spectra of modern river sediments correspond well to the thermochronometric ages of exposed bedrock and indicate a rapidly exhumed source with a young ZFT peak age of 1.7 Myr in central Taiwan, which has not been widely reported. ZFT results from the Pleistocene successions show diachronous deposition of totally-reset zircons from south to north, accompanied by a trend of younging up-section. The progression of these ZFT signals in conjunction with changes in petrography and pyrrhotite occurrence suggest transient developments of sediment routing systems in central Taiwan and the importance of paleo sediment routing systems. In the Dadu River, totally-reset zircons did not appear until 1–0.45 Myr, when the flux of thermally mature material became prominent. The course of the Choshui river migrated during the past 1 Myr and the reorganization of the river reflects activity of frontal thrusts in Pleistocene times. Exhumation rates recorded by detrital sediments of the Choshui River catchment have been consistently higher than those of the Dadu River, suggesting inherited heterogeneity of exhumation in the orogen.

Paleogeographic reconstruction of Mississippian to Middle Pennsylvanian basins in Nevada, southwestern Laurentia

Compilation of stratigraphic, sedimentological, and structural data on palinspastically restored maps reveals the Mississippian-Pennsylvanian paleogeography of Nevada in unprecedented detail, and has important implications for the development of the southwestern Laurentian margin. In contrast to previous interpretations of latest Devonian-earliest Mississippian tectonism, the maps show the major tectonic events to have been in late Middle Mississippian and Middle Pennsylvanian time. Regionally extensive unconformities mark both, with the deepest erosion in the northwest. Sub-unconformity structures record primarily NW-SE shortening, and synorogenic sediments were shed toward the east, southeast and south. These observations are consistent with protracted sinistral-oblique convergence in Nevada along the southwest Laurentian margin, rather than the W-E convergence invoked in previous interpretations. Terminology for late Paleozoic orogenies in Nevada must be revised accordingly. • Southwestern Laurentia experienced NW-SE shortening in the late Paleozoic. • Major tectonic evens in late Middle Mississippian and Middle Pennsylvanian. • Data consistent with sinistral-oblique margin throughout the late Paleozoic. • Data presented here are not consistent with E -W convergence in late Paleozoic.

Contractional salt tectonics and role of pre-existing diapiric structures in the Southern Pyrenean foreland fold–thrust belt (Montsec and Serres Marginals)

Triassic Keuper evaporites have long been recognized as the main detachment level for thrusting in the Pyrenean fold–thrust belts. The deformed Late Cretaceous–Eocene foreland basin of the Southern Pyrenees has structures and stratal geometries that can be interpreted as related to salt tectonics (e.g. unconformities, rapid thickness variations, long-lived growth fans and overturned flaps), although they have been overprinted by shortening and thrusting. Based on field observations and published maps, we build new structural cross-sections reinterpreting two classic transects of the Southern Pyrenees (Noguera Ribagorçana and Noguera Pallaresa river transects). The sequential restoration of the sections explores the variations in structural style, addressing the role of halokinesis in the tectonic and sedimentary development. In the Serres Marginals area, we propose that salt pillows and diapirs started developing locally during the Mesozoic pre-orogenic episode, evolving into a system of salt ridges and intervening synclines filled with early synorogenic sediments. Rapid amplification of folds recorded by widespread latest Cretaceous–Paleocene growth strata is taken as marking the onset of contractional folding in the area. During Pyrenean compression, folding mechanisms transitioned from dominantly halokinetic to a combination of buckling and differential sedimentary loading. Squeezing of salt diapirs and thrust welding occurred as salt ridges were unroofed. We provide new field observations that lead to a reinterpretation of the regional structural development and contribute to the debate about the role of salt tectonics in the Pyrenees. Supplementary material: Table S1, giving the thickness of the main stratigraphic units, is available at https://doi.org/10.6084/m9.figshare.c.5287737

Early Cenozoic activated deformation in the Qilian Shan, northeastern Tibetan Plateau: Insights from detrital apatite fission‐track analysis

AbstractThe evolution of the tectonic deformation of the northeastern Tibetan Plateau (TP) in the Cenozoic is significant for understanding plateau growth during India‐Asia convergence. However, when deformation began and how it has developed in this pivotal region remain controversial. We focus on the temporal progress of Cenozoic deformation in the Qilian Shan, a major tectonic belt of the northeastern TP. In the present study, detrital apatite fission‐track (AFT) thermochronological analysis was performed on Oligocene‐Quaternary synorogenic sediments in the northern Qaidam Basin, where detritus is sourced from the Qilian Shan. Age components of buried but unannealed detrital AFT samples reveal two static peaks (i.e., peak ages that are consistent upsection) at ca. 60–50 Ma and ca. 40–36 Ma and a moving peak (i.e., peak ages that are younger upsection) with increased lag time during ca. 30–8 Ma. These new detrital AFT ages, integrated with the analysis of sedimentary provenance and data from previously published studies, indicate that Cenozoic tectonic deformation began in the Qilian Shan in the late Paleocene‐early Eocene. Furthermore, the Qilian Shan experienced a subsequent episodic deformation event in the late Eocene and the deformation or erosion of some terranes in the Qilian Shan decelerated during the Oligocene‐Miocene. Our results suggest that the northeastern TP responded to the India‐Asia collision almost instantaneously.

Role of the Down-Bending Plate as a Detrital Source in Convergent Systems Revealed by U–Pb Dating of Zircon Grains: Insights from the Southern Andes and Western Italian Alps

In convergent zones, several parts of the geodynamic system (e.g., continental margins, back-arc regions) can be deformed, uplifted, and eroded through time, each of them potentially delivering clastic sediments to neighboring basins. Tectonically driven events are mostly recorded in syntectonic clastic systems accumulated into different kinds of basins: trench, fore-arc, and back-arc basins in subduction zones and foredeep, thrust-top, and episutural basins in collisional settings. The most widely used tools for provenance analysis of synorogenic sediments and for unraveling the tectonic evolution of convergent zones are sandstone petrography and U–Pb dating of detrital zircon. In this paper, we present a comparison of previously published data discussing how these techniques are used to constrain provenance reconstructions and contribute to a better understanding of the tectonic evolution of (i) the Cretaceous transition from extensional to compressional regimes in the back-arc region of the southern Andean system; and (ii) the involvement of the passive European continental margin in the Western Alps subduction system during impending Alpine collision. In both cases, sediments delivered from the down-bending continental block are significantly involved. Our findings highlight its role as a detrital source, which is generally underestimated or even ignored in current tectonic models.

From hydroplastic to brittle deformation: Controls on fluid flow in fold and thrust belts. Insights from the Lower Pedraforca thrust sheet (SE Pyrenees)

We present a multidisciplinary study to decipher the controls of deformation on fluid flow regime in fold and thrust belts using the Lower Pedraforca thrust sheet in the SE Pyrenees as an example. We integrate field-based and petrographic observations and geochemical and geochronological data to differentiate seven types of fractures, eight types of calcite cement (Cc1 to Cc8) and two sets of stylolites during the deformation stretching the studied thrust sheet. During syn-sedimentary hydroplastic normal faulting affecting poorly consolidated Upper Cretaceous and Eocene syn-orogenic sediments, calcite cement did not precipitate. During burial, bed-parallel stylolites formed and Cc1 and Cc2 precipitated from formation waters in a closed palaeohydrological system. During the layer-parallel shortening, Cc3 precipitated from formation waters (~+5.4‰ VSMOW) with 87Sr/86Sr ratios of 0.707922 and at ~70 °C. Cc4 precipitated from formation waters recording different burial conditions, as the up to 4‰ dispersion in δ18O of this cement suggests. Contrarily, during folding and thrusting, Cc5 to Cc7 precipitated in an open palaeohydrological system. Cc6 precipitated from formation waters (~+5‰ VSMOW), with 87Sr/86Sr ratios of 0.707817 and at ~75 °C. These fluids carried hydrocarbons and probably interacted with Upper Triassic evaporites. An 87Sr/86Sr ratio of 0.708230 for Cc5 indicates that formation waters also interacted with clays within continental deposits. During this period, stylolites formed in relation to faulting, and previous hydroplastic normal faults reactivated as reverse and strike-slip faults allowing fluid flow. Cc7 precipitated after Cc6, also from fluids in isotopic disequilibrium with their adjacent host rock. The fluid system continued open during the Oligocene, when Cc8 precipitated in normal faults affecting syn-orogenic conglomerates deposited during the reactivation of the Lower Pedraforca thrust sheet. The influence of deformation on fluid flow observed in the Lower Pedraforca thrust sheet is similar to that observed in other fractured areas worldwide.

Constraining the onset of flexural subsidence and peripheral bulge extension in the Miocene foreland of the southern Apennines (Italy) by Sr-isotope stratigraphy

In fold and thrust belts developing at convergent margins, the migration of the advancing wedge is accompanied by bulging of the downgoing plate, followed by the development of a foredeep basin filled by a thick succession of syn-orogenic sediments. The transition from forebulge to foredeep marks a key moment in the evolution of the orogenic system. In deep water environments, the record of this transition is typically complete and progressive. Conversely, in the shallow-water/continental environment of many collisional systems, the uplift of the forebulge area can imply emersion and erosion, obliterating the stratigraphic record of key steps of the evolution of the orogenic system. The southern Apennines constitute one of these collisional fold and thrust belts where the development of the forebulge has implied emersion and erosion, with the development of a Miocene forebulge erosional unconformity, accompanied by extensional deformation associated with the bending of the lithosphere during the forebulge stage. In this paper, we use strontium isotope stratigraphy to constrain with unprecedented time-resolution the age of the forebulge unconformity in areas presently incorporated in the northern sector of the southern Apennines fold and thrust belt. Integration of our results and those of previous studies indicates, at the regional scale, a younging toward the foreland of the forebulge unconformity across the belt. Our high-resolution ages also reveal a diachronous onset of the flexural subsidence over short distances, associated with the occurrence of horst and graben structures, possibly resulting from inherited paleotopography along with forebulge extension. This work highlights how high-resolution dating is critical to unravel the evolution of foreland basin systems at different scales.

Detrital zircon geochronology analysis of the Late Mesozoic deposition in the Turpan‐Hami basin: Implications for the uplift history of the Eastern Tian Shan, north‐western China

AbstractAnalysing the provenance changes of synorogenic sediments in the Turpan‐Hami basin by detrital zircon geochronology is an efficient tool to examine the uplift and erosion history of the easternmost Tian Shan. We present detrital zircon U‐Pb analysis from nine samples that were collected within marginal lacustrine Middle‐Late Jurassic and aeolian‐fluvial Early Cretaceous strata in the basin. Middle‐Early Jurassic (159–172 Ma) zircons deriving from the southern Junggar dominated the Middle Jurassic sample from the western Turpan‐Hami basin, whereas Permian‐Carboniferous (270–330 Ma) zircons from the Bogda mountains were dominant in the Late Jurassic to Early Cretaceous samples. Devonian‐Silurian (400–420 Ma) and Triassic (235–259 Ma) zircons from the Jueluotage and Harlik mountains constituted the subordinate age groups in the Late Jurassic and Early Cretaceous samples from the eastern basin respectively. These provenance transitions provide evidence for uplift of the Bogda mountains in the Late Jurassic and the Harlik mountains since the Early Cretaceous.

Early Pleistocene uplift of the northeastern Tibetan Plateau: Evidence from the Dunhuang Basin, NW China

The Cenozoic uplift of the Tibetan Plateau plays an important role in global climate change, basin-mountain geomorphology formation and desertification in western China. As an important component, the Early Pleistocene uplift process and its influence on the northeastern Tibetan Plateau has been controversial for many years. Quaternary synorogenic sediments within the Dunhuang Basin on the northeastern Tibetan Plateau record the details of plateau uplift. Detailed grain size analysis, rock magnetic properties, mudstone 36Cl dating and 40Ar/39Ar detrital mica dating were conducted on the Quaternary alluvial-fluvial-lacustrine-aeolian sediments of the Dunhuang Basin. The Aksay section in the southern Dunhuang Basin is dominated by conglomerate, sandstone, siltstone and mudstone. Detrital mica 40Ar/39Ar ages show major age populations of 200–160 Ma, 400–200 Ma and 510–430 Ma from the northeastern Altyn Mountains. The results of 36Cl ages and rock magnetic properties show that the Altyn Mountains rapidly uplifted at 1.16 Ma. From 1.16 to 0.80 Ma, the sedimentary environment was stable with two small-scale uplift pulses at ca. 1.03 Ma and 0.98 Ma. After 0.80 Ma, the climate became arid, and the Kumtag Desert formed in the Dunhuang Basin. These multistage uplifts are also widely found in the northeastern Tibetan Plateau. The Early Pleistocene uplift of the Altyn Mountains, corresponding to the Kunhuang Movement, resulted in the northeastern margin of the Tibetan Plateau reaching its present height and climate changeing towards drought in this region.

Geodynamic evolution of the San Vitero basin, a foreland-type basin developed in the hinterland of the Variscan Orogen (Zamora, NW Spain)

espanolLa region de Alcanices se situa en las zonas internas del Orogeno Varisco de la Peninsula Iberica y esta constituida por la sucesion Paleozoica autoctona de la Zona Centroiberica (CIZ), la sucesion aloctona del Dominio Esquistoso de la Zona de Galicia-Tras-Os-Montes (GTOMZ) y depositos sinorogenicos discordantes de tipo fysch (Formacion San Vitero). La realizacion de una cartografia geologica detallada y el analisis estructural a todas las escalas han permitido establecer las relaciones entre estas sucesiones y especialmente con los sedimentos sinorogenicos, que dada su localizacion en las partes internas del orogeno proporcionan una importante informacion acerca de la evolucion tectonica. La restauracion parcial de las estructuras variscas permite establecer la existencia de un sistema imbricado de cabalgamientos asociado con una cuenca de antepais. La reconstruccion de esta cuenca permite estimar su anchura en unos 100 km, con una profundidad aproximada de 1500 m en su depocentro. La cuenca sinorogenica comienza a desarrollarse durante el Carbonifero temprano en relacion con el emplazamiento de grandes unidades aloctonas adscritas a la fase de deformacion varisca D2. La evolucion de la cuenca termina durante la tercera fase de deormacion varisca (D3) con la formacion de cabalgamientos fuera de secuencia (que involucran al basamento) y pliegues asociados. EnglishThe Alcanices region, located in the internal zones of the Variscan Orogen of the Iberian Peninsula, is composed of the autochthonous Paleozoic succession of the Central Iberian Zone (CIZ), the allocthonous Schistose Domain of the Galicia-Tras-OsMontes Zone (GTOMZ) and the unconformable synorogenic fysch-type deposits. A new detailed geological map and the analysis of the structure at all scales contribute to establishing the geological relationships between the diferent paleo-geographic units, and in particular of the synorogenic sediments, that given their location in the hinterlands provide further insights into the tectonic evolution of the orogen. The partial restoration of the Variscan structures suggests a model of imbricate thrust-system and an associated foreland-type basin. A reconstruction of the basin allows estimating its width in c.100 km, with a depth of c. 1500 m in its depocenter. During the Early Carboniferous the basin developed ahead in relation with the emplacement of the large allochthonous units linked to the D2 deformation event. The evolution of the basin ends with the formation of out-of-sequence thrusts involving their basement and associated folds during the third Variscan deformation event (D3).

Middle-Late Jurassic sedimentary mélange formation related to ophiolite obduction in the Alpine-Carpathian-Dinaridic Mountain Range

The Middle-Late Jurassic mountain building process in the Western Tethyan realm was triggered by west- to northwestward-directed ophiolite obduction onto the wider Adriatic shelf. This southeastern to eastern Adriatic shelf was the former passive continental margin of the Neo-Tethys, which started to open in the Middle Triassic. Its western parts closed from around the Early/Middle Jurassic boundary with the onset of east-dipping intra-oceanic subduction. Ongoing contraction led to ophiolite obduction onto the former continental margin since the Bajocian. Trench-like basins formed concomitantly within the evolving thin-skinned orogen in a lower plate situation. Deep-water basins formed in sequence with the northwest-/westward propagating nappe fronts, which served as source areas of the basin fills. Basin deposition was characterized by coarsening-upward cycles, i.e. sedimentary mélanges as synorogenic sediments. The basin fills became sheared successively by ongoing contractional tectonics with features of typical mélanges. Analyses of ancient Neo-Tethys mélanges along the Eastern Mediterranean mountain ranges allow both, a facies reconstruction of the outer western passive margin of the Neo-Tethys and conclusions on the processes and timing of Jurassic orogenesis. Comparison of mélanges identical in age and component spectrum in different mountain belts figured out one Neo-Tethys Ocean in the Western Tethyan realm, instead of multi-ocean and multi-continent scenarios.

Meliatic blueschists and their detritus in Cretaceous sediments: new data constraining tectonic evolution of the West Carpathians

The Western Carpathian Meliata Superunit (Meliaticum) includes a heterogeneous group of units—the blueschist-facies Borka Nappe and the very low-grade chaotic complexes—polygenous melange containing material derived from various tectonic settings (Jaklovce Unit) and Jurassic oceanic sediments with olistostrome bodies (Meliata Unit s.s.). The high pressure/low temperature (HP/LT) metamorphism, development of tectonic melanges and synorogenic sediments took place simultaneously during the Middle–Late Jurassic in connection with closing of the Meliata Ocean. We present some new data concerning composition of variegated melanges related to the subduction–accretion processes of the Meliata Ocean and its continental margins. The polygenous melange contains a mixture of fragments of HP/LT up to unmetamorphosed sedimentary and volcanic rocks, including blueschist-facies radiolarites. New electron microprobe chemical age data of monazites from metasediments of the Borka Nappe cluster in two peaks—earliest Cretaceous ages are interpreted in terms of post-exhumation, renewed burial during formation of the Meliatic accretionary wedge. The mid-Cretaceous ages might record the thermal relaxation during the thick-skinned nappe stacking and exhumation of the neighbouring Veporic metamorphic dome. In addition, we describe two distinct types of tourmalines and epidotes occurring in the Borka blueschists that document the prograde HP and retrograde LP metamorphic events. Detritus of the blueschist-facies rocks appears for the first time in the heavy mineral spectra of the Barremian–Aptian platform limestones. These limestones occur as clasts, together with glaucophanite pebbles, in the Albian–Cenomanian flysch formations of the Pieniny Klippen Belt (Klape Unit). We also review occurrences of other blueschist and ophiolitic erosional products in Cretaceous clastic formations and suggest that all have a Meliatic provenance.

Cenozoic deformation history of the Qilian Shan (northeastern Tibetan Plateau) constrained by detrital apatite fission-track thermochronology in the northeastern Qaidam Basin

The Cenozoic deformation history of the Qilian Shan, which is a reactivated fold-thrust belt in the northeastern region of the Tibetan Plateau, is poorly understood but features prominently in the geodynamic mechanism of the evolution of the plateau. The northeastern Qaidam Basin, which is located on the south flank of the Qilian Shan, provides a valuable record of the timing and pattern of the Cenozoic deformation of the Qilian Shan in its synorogenic sediments. This study analyzes the apatite fission-track (AFT) thermochronological signatures of the middle Miocene-Quaternary detrital sediments in the northeastern Qaidam Basin and modern stream sands draining the Qilian Shan as a proxy for the timing of tectonic deformation in the Qilian Shan. Our detrital AFT ages indicate that a Cenozoic initial rapid exhumation event occurred in the Qilian Shan in the late Paleocene-early Eocene (~60–54 Ma), followed by another rapid exhumation event during the middle-late Eocene (~42–38 Ma) and some exhumation during the Oligocene-middle Miocene (~33–14 Ma). Distinct changes in detrital AFT ages at depositional ages of ~12 Ma and ~2.1 Ma suggest that further tectonic deformation affected the Qilian Shan at these times. Our results, along with those of previous studies, suggest that tectonic deformation propagated into the northeastern edge of the Tibetan Plateau almost simultaneously with the India-Asia collision. The spatial evolution of Cenozoic deformation in the Qilian Shan is likely based on the distribution of weak structures in the upper crust.

Stratigraphic correlation and structural position of Lower Cretaceous flysch-type deposits in the eastern Southern Alps (NW Slovenia)

Lower Cretaceous syn-orogenic sediments derived from the obducted ophiolites of the Meliata–Maliac–Vardar (Neotethys) Ocean are typically found in the Dinarides and the Austroalpine units. Correlative flysch-type deposits linking both regions through the Southern Alps had been reported from the Bohinj area (NW Slovenia), but their stratigraphic and structural framework remained poorly known. Our research focused on stratigraphic and structural field studies in a 50 km2 area between Lake Bled and Lake Bohinj in the Julian Alps. The mixed carbonate–siliciclastic sediments, informally named the Studor formation, range in age from the Valanginian (possibly late Berriasian) to the Aptian. They occur on top of two different stratigraphic successions, which we assign to two separate nappes. The first succession consists of deep-water Middle Triassic to Lower Cretaceous deposits of the Bled Basin and belongs to the Pokljuka Nappe, which is the uppermost nappe of the Julian nappe stack. The second succession consists of Upper Triassic to Lower Jurassic platform carbonates and a thin Jurassic–Cretaceous deep-water sequence. This succession was deposited in the marginal area of the Julian Carbonate Platform/Julian High and now belongs to the underlying Krn Nappe. The original (Dinaric) thrust contacts between the Pokljuka and Krn nappes are obliterated by younger deformations. The present-day boundaries between these two nappes are steep NE–SW and younger NW–SE trending faults. The post-nappe deformation sequence characterizing the Alps–Dinarides transition zone has been recognized: (1) Oligocene–Early Miocene NW–SE contraction; (2) Early–Middle Miocene extension; and (3) Late Miocene to recent inversion and transpression.

Origin of the megabreccias in the Katanga Copperbelt (D.R.Congo)

The megabreccias in the Katanga part of the Neoproterozoic Central African Copperbelt contain up to several km-long blocks and fragments of the Mines Subgroup which host most of the stratiform Cu-Co deposits. New observations, particularly on cores from boreholes drilled at Luiswishi indicate three types of fracturing: 1) brittle post-folding in the Mines Subgroup; 2) hydraulic; and 3) ductile in soft incompetent siltstones of the R.A.T. and Dipeta subgroups. These fracturing phases dislocated the Roan succession into blocks and fragments and, in particular, clearly showed that there is an evolution from an in situ hydraulic fracturing, to a heterometric brecciation implying some movement and abrasion of the fragments. The process points to significant compression, and was accompanied by fluid expulsion and precipitation of dolomite after decompression. Fluid inclusion microthermometry in dolomite grains shows that the fluids were of high salinity and high temperature, suggesting dissolution of evaporites most likely contained in the Roan sedimentary pile. These saline fluids allowed the fluidization of the breccias, facilitating the displacement of the nappes, pinching out (extrusion-like) megabreccias along thrust-faults, and resulting in intrusion of breccias between the blocks or into large fractures. Breccias between the blocks are clearly identified as friction breccias. They contain a fine material, as part of the matrix, resulting from abrasion of the fragments during transportation. Abrasion and attrition explain the rounding of the fragments. A late cementation phase from less saline and lower temperature fluids suggests the addition of meteoric water in the system, and the mixing with the ambient fluids. The minimum burial depth of the meteoric water incursion is estimated at 2.8 km. Such under-saturated fluids may have contributed to the dissolution of residual evaporites and of the evaporitic material from the Kiubo rocks at the base of the nappes, and led to further brecciation, possibly explaining the multi-phase features of the breccia. The megabreccias occur at the base of the thrusts sheets and are marked by thrust-fault zones. Results of the study support a process of formation of the megabreccias related to a fold-and-trust event, and invalidate a syn-orogenic sedimentary origin as an olistostrome formed by subaqueous conglomeratic debris flows and clastic syn-orogenic sediments. They also contradict a pure salt tectonic hypothesis that propose the extrusions and enlargements of allochthonous evaporites-gigabreccia before the Lufilian deformation. However, the model is compatible with a “fluid behaviour” of pressured saline fluids trapped in folds and/or thrust sheets, and resulting from evaporites dissolution at variable depth.