Late Cretaceous Early Palaeogene Research Articles

Late Cretaceous–early Palaeogene inversion-related tectonic structures at the northeastern margin of the Bohemian Massif (southwestern Poland and northern Czechia)

Abstract. A brief, regional-scale review of the Late Cretaceous–early Palaeogene inversion-related tectonic structures affecting the Sudety Mountains and their foreland at the NE margin of the Bohemian Massif is presented and complemented with results of new seismic studies. The Sudetes expose Variscan-deformed basement, partly overlain by post-orogenic Permo-Mesozoic cover, containing a wide spectrum of tectonic structures, both brittle and ductile, in the past in this area referred to as young Saxonian or Laramide. We have used newly reprocessed legacy seismics to study these structures in the two main post-Variscan structural units of the area: the North Sudetic and Intra-Sudetic synclinoria. The results were discussed together with regionally distributed examples of tectonic structures from quarries and underground mines as well as those known from the literature. The Late Cretaceous–early Palaeogene tectonic structures in consecutively reviewed Sudetic tectonic units, from the north to the south, typically include gentle to moderately tight buckle folds, locally of detachment type or fault-related and high-angle reverse and normal faults, as well as low-angle thrusts – often rooted in the crystalline basement. The structures termed grabens in the local literature are at the same time frequently interpreted as bounded by reverse faults (hence we use here the term “reverse grabens”) and typically reveal a strongly synclinal pattern of their sedimentary fill. The top of the crystalline basement, as imaged by seismic data in the North Sudetic Synclinorium below the faulted and folded cover, is synformally down-warped with a wavelength of up to 30 km, whereas on the elevated areas, where the basement top is exposed at the surface, it is tectonically up-warped (i.e. antiformally buckled). The compressional structures typically show an orientation that fits the regionally known Late Cretaceous–early Palaeogene tectonic shortening direction of NE–SW to NNE–SSW. The same applies to the regional jointing pattern, typically comprising an orthogonal system of steep joints of ca. NW–SE and NE–SW strikes. All the reviewed structures are regarded as being due to the Late Cretaceous–early Palaeogene tectonic shortening episode, although some of the discussed faults with a strike-slip component of motion may have been modified, or even produced, by late Cenozoic tectonism.

Existence of a continental-scale river system in eastern Tibet during the late Cretaceous\u2013early Palaeogene

The establishment of continental-scale drainage systems on Earth is largely controlled by topography related to plate boundary deformation and buoyant mantle. Drainage patterns of the great rivers in Asia are thought to be highly dynamic during the Cenozoic collision of India and Eurasia, but the drainage pattern and landscape evolution prior to the development of high topography in eastern Tibet remain largely unknown. Here we report the results of petro-stratigraphy, heavy-mineral analysis, and detrital zircon U-Pb dating from late Cretaceous–early Palaeogene sedimentary basin strata along the present-day eastern margin of the Tibetan Plateau. Similarities in the provenance signatures among basins indicate that a continental-scale fluvial system once drained southward into the Neo-Tethyan Ocean. These results challenge existing models of drainage networks that flowed toward the East Asian marginal seas and require revisions to inference of palaeo-topography during the Late Cretaceous. The presence of a continent-scale river may have provided a stable long-term base level which, in turn, facilitated the development of an extensive low-relief landscape that is preserved atop interfluves above the deeply incised canyons of eastern Tibet.

Biostratigraphy and lithostratigraphy of Late Cretaceous–Early Palaeogene deposits in Tang-e-Abolhayat, Zagros basin (SW Iran)

In this study, to consider microbiostratigraphy of Cretaceous and Palaeocene planktic foraminifera, a stratigraphic section of the Gurpi and Pabdeh formations named Tang-e-Abolhayat was selected. On the basis of the identified planktic foraminiferal assemblages, seven biozones and two sub-biozones have been recorded (four biozones from the uppermost part of the Cretaceous, (Gurpi Formation), and three biozones from the lower part of the Lower Palaeocene (Pabdeh Formation). The biostratigraphic correlations based on planktic foraminiferal zonations showed a comparison between the biostratigraphic zones in this study and other equivalents of the commonly used planktic zonal scheme around the Cretaceous–Palaeogene in and outside Iran. Biostratigraphic analysis confirms that the studied section is incomplete across the Cretaceous–Tertiary boundary and a hiatus is suggested by the absence of the Plummerita hantkeninoides, Guembelitria cretacea, Parvularugoglobigerina eugubina Zones and Parasubbotina pseudobulloides Subzone (CFl to Pla zones). These absences could reflect the restriction of the above-mentioned zones to shallower settings, slow sedimentation rates or an interval of erosion or no deposition. There is no obvious evidence of erosion. On the other hand, the Pabdeh Formation overlies Gurpi Formation paraconformably.

The low-temperature thermo-tectonic evolution of the western Tian Shan, Uzbekistan

The Kyzylkum-Nurata region represents a key area in understanding the tectonic evolution of the western Tian Shan. In this study, we present new thermochronological data (apatite fission track and apatite (U-Th-Sm)/He) and associated thermal history models for 45 igneous samples from the Kyzylkum-Nurata Segment of South Tian Shan on the territory of Uzbekistan and Tajikistan. Our data show that the Kyzylkum-Nurata Segment experienced a multi-phase Mesozoic thermal history that differs from previously studied segments of the Tian Shan. A Triassic (~220–200 Ma) cooling signal is widespread throughout the Tian Shan and is interpreted as being associated with exhumation following the closure of the Palaeo-Asian Ocean. Following this period of fast cooling, the Kyzylkum-Nurata Segment experienced a period of slow cooling and erosion in the Early Jurassic (~190–160 Ma). However, in contrast to other parts of the Tian Shan, our study area preserves evidence for rapid cooling during the Late Jurassic–Early Cretaceous (~160–120). Given that this rapid cooling signal was only recorded for samples in association with major structures (e.g., relic suture-shear zones), we interpret this event as a period of fault reactivation related with tectonic processes at the Eurasian margin of the Tethys Ocean. Subsequently, the Late Cretaceous–early Palaeogene (~100–50 Ma) is characterised by slow cooling and erosion. Since the late Palaeogene, the basement of the Tian Shan experienced cooling related to the India-Eurasia collision. The thermal signal of this collision has been extensively recorded in the high-altitude Kyrgyz Tian Shan. Within the low-relief Kyzylkum-Nurata Segment, this Cenozoic overprint is not recorded, allowing for a detailed assessment of the Mesozoic thermal and landscape evolution of the western Tian Shan. Our study demonstrates that the Cretaceous Uzbek Tian Shan was characterised by a series of parallel, linear mountain belts that formed along suture zones during fault reactivation.

Static Dental Disparity and Morphological Turnover in Sharks across the End-Cretaceous Mass Extinction

The Cretaceous-Palaeogene (K-Pg) mass extinction profoundly altered vertebrate ecosystems and prompted the radiation of many extant clades [1, 2]. Sharks (Selachimorpha) were one of the few larger-bodied marine predators that survived the K-Pg event and are represented by an almost-continuous dental fossil record. However, the precise dynamics of their transition through this interval remain uncertain [3]. Here, we apply 2D geometric morphometrics to reconstruct globaland regional dental morphospace variation amongLamniformes (Mackerel sharks) and Carcharhiniformes (Ground sharks). These clades are prevalent predators in today's oceans, and weregeographically widespread during the late Cretaceous-early Palaeogene. Our results reveal a decoupling of morphological disparity and taxonomic richness. Indeed, shark disparity was nearly static across the K-Pg extinction, in contrast to abrupt declines among other higher-trophic-level marine predators [4, 5]. Nevertheless, specific patterns indicate that an asymmetric extinction occurred among lamniforms possessing low-crowned/triangular teeth and that a subsequent proliferation of carcharhiniforms with similar tooth morphologies took place during the early Paleocene. This compositional shift in post-Mesozoic shark lineages hints at a profound and persistent K-Pg signature evident in the heterogeneity of modern shark communities. Moreover, such wholesale lineage turnover coincided with the loss of many cephalopod [6] and pelagic amniote [5] groups, as well as the explosive radiation of middle trophic-level teleost fishes [1]. We hypothesize that a combination of prey availability and post-extinction trophic cascades favored extant shark antecedents and laid the foundation for theirextensive diversification later in the Cenozoic [7-10].

Comparative anatomy of the gill skeleton of fossil Aulopiformes (Teleostei: Eurypterygii)

Gill skeletons provide a rich source of character information for inferring the relationships among extant fishes. However, the difficulties in accessing branchial structures in fossils have limited the use of gill-arch anatomy in phylogenetic studies of extinct fishes. Here we apply micro-computed tomography (µCT) to visualize and describe gill-arch anatomy in three-dimensionally preserved Late Cretaceous–early Palaeogene remains of seven genera attributed to the eurypterygian clade Aulopiformes (lizardfishes), a group for which detailed cladistic character sets describing patterns of variation in the branchial skeleton are available. We evaluate the placement of these fossil taxa based on characters of the gill skeleton in isolation. Our results support an alepisauroid placement for †Apateodus corneti, †Cimolichthys lewesiensis and †Halec eupterygius, and a stem synodontid affinity for †Argillichthys toombsi and †Labrophagus esocinus. These placements are broadly consistent with past hypotheses based either on formal cladistic argumentation or qualitative morphological comparison drawing on other skeletal systems. We find insufficient evidence in the branchial skeleton to place †Aulopopsis depressifrons more specifically than Aulopiformes incertae sedis. Gill-arch anatomy substantially revises past interpretations of †Sardinioides illustrans by providing clear evidence for placement within Aulopiformes generally, and as either a stem aulopid or stem paralopid more specifically. This species must therefore be removed from †Sardinioides, the type of which is a myctophiform and shows conspicuous anatomical differences from †‘S.’ illustrans. Our work provides proof-of-concept for the recovery of detailed information on gill skeleton anatomy in fossils, indicating the potential for the extraction of considerable new morphological data – and phylogenetic information – from suitably preserved fossil specimens.

Taxonomical identification of a new petrified multiseeded capsular fruit from the Deccan Intertrappean beds of Bhutera, India

The fossil flora of the Deccan Intertrappean beds of central India provides a unique opportunity for detailed investigation, taxonomic affinities and biogeographic relationships of plants during the late Cretaceous–early Palaeogene Period. The fossils found in cherts were deposited during quiescent intervals of lava flows.
 The present paper deals with a fossil fruit collected from the Deccan Intertrappean beds of Bhutera (lat. 22º06.58' N, long. 79º08.42' E) of Chhindwara District, Madhya Pradesh, India. The fruit was exposed in transverse oblique plane. The serial peel sections were taken for detailed study. The present fruit is small, dicotyledonous, obovate, unilocular, multiseeded, dehiscent capsular fruit. On comparison with modern dicotyledonous families, the fruit shows a close resemblance with Portulacaceae.

Mineral Chemistry of Clays Associated with the Late Cretaceous-Early Palaeogene Succession of the Um-Sohryngkew River Section of Meghalaya, India: Palaeoenvironmental Inferences and K/Pg Transition

Abstract A unique, continuous, shallow marine succession within the Langpar Formation in the Um-Sohryngkew river section of Meghalaya contains late Maastrichtian through early Danian planktonic foraminiferal biozones (CF4-P1a) and the K/Pg boundary (between CF1 and P0). To resolve compositional overlap [in the three-fold clay mineral based sub-divisions of the section (include 1-2 mm thick yellowish brown clay layer in biozone CF3)] and to understand paleoenvironmental conditions prevalent at the time of K/Pg transition, micro-structural and compositional studies were carried out. The paper discusses accurate compositional limits of solid solubility in smectite and illite. Clay morphological attributes distinctly vary with the changes in the biozonation. Major oxide data plots for the litho-units (samples JP1-16) over binary diagrams, show clustering of plots within the illite compositional field. Plots based on structural formulae and layer charges of illite and kaolinite rich clays (from CF4 to Pla biozones), show closeness with the clay data fields of Agost, Caravaca, Petriccio and El-Kef K/T boundary sections as their charge occupancies at tetrahedral (Zt), octahedral (Zo) and interlayer (Zi) sites are similar. Thermodynamic data plots over ternary [AR23+Si3O10 (OH)2 - R23+SiO4O10(OH)2 - A3AlSi4O10(OH)2] diagram, clustered within the illite compositional fields. Majority of illites shows high K values. They represent occasionally higher Altet. and lower Aloct. layer charges. Calculated palaeotemperature values for illite (from the CF4 to Pla zones) vary from 68 - 232°C. Sudden rise in the temperature (>140°C) of illite formation noticed in the upper part of the biozone CF3 (sample JP-12) is comparable to the K/T boundary layer of Caravaca section. Wide variation in the humid tropical to arid-semiarid climatic and thermal (diagenetic to low grade metamorphic) conditions noticed across the succession is possiby linked with the contemporaneous Abor / Deccan volcanic activities (at the time of their deposition) as also reflected in their clay layer [Si (2.95-3.68)] and interlayer [K+Na (0.4-4.61)] charges.

Sequence stratigraphy and depositional environments of Late Cretaceous–Early Palaeogene succession, North Eastern Desert, Egypt

The foraminiferal contents and geochemistry of 199 samples collected from three surface sections in the southern Galala Sub-basin, North Eastern Desert of Egypt, have been studied in detail. From south to north these sections are situated at Gebel Tarboul, Wadi Tarfa, and Bir Dakhl. The results allow reconstructing of the depositional environments and high resolution sequence stratigraphy of the Upper Cretaceous–Palaeogene succession. The quantitative and qualitative distribution patterns of benthic foraminifera of the Upper Cretaceous–Lower Palaeogene succession suggests a depositional environment from outer neritic to bathyal, at 200 to ~700 m water-depth. Based on sequence stratigraphic analyses, ten complete third order depositional sequences have been recognized. These depositional sequences from base to top are as follows: CaSGB-1 sequence, CaSGB-2 sequence, MaSGB-1 sequence, MaSGB-2 sequence, DaSGB sequence, Da/SeSGB sequence, SeSGB sequence, ThSGB sequence, Sp/YpSGB, and YpSGB sequence. These sequences are controlled by both eustatic sea-level changes and tectonic movements that prevailed during deposition.

Mesozoic reactivation of the Beishan, southern Central Asian Orogenic Belt: Insights from low-temperature thermochronology

The Beishan Orogenic Collage (BOC) is located in the southeast of the Central Asian Orogenic Belt (CAOB) and formed during final consumption of the Palaeoasian Ocean in the late Palaeozoic. This study applies low temperature thermochronology to constrain the Meso-Cenozoic history of the BOC.Apatite fission track and U–Th–Sm/He data obtained for granitoid samples along a north–south transect through the BOC indicate three distinct phases of exhumation during (1) the late Triassic–early Jurassic (~225–180Ma), (2) early Cretaceous (~130–95Ma) and (3) late Cretaceous–early Palaeogene (~75–60Ma). Samples from the northern BOC reveal a more profound early Cretaceous signal and a weaker late Triassic–early Jurassic signal than those in the southern BOC. A possible explanation for this discrepancy is the presence of crustal-scale fault zones in the northern BOC that are interpreted to have undergone repeated reactivation throughout the Mesozoic, exposing deeper exhumed sections of the BOC. These faults may thus have acted as a major control on exhumation in the region. This pattern is consistent with results from elsewhere in the CAOB, such as in the Tianshan and the Altai, where regional widespread exhumation occurred since the early Cretaceous while major fault zones record localised exhumation during the late Cretaceous and Cenozoic. Late Cretaceous–early Palaeogene cooling ages were found only in the south of the BOC, suggesting that exhumation at that time was more localised and did not reach the northern margins of the study area.The Meso-Cenozoic cooling events described here are thought to be related to the progressive closure of the (Palaeo-)Tethys ocean to the south. Associated collision and accretion of micro-continental blocks and island–arcs at the southern Eurasian margin are interpreted to have induced more widespread reactivation and exhumation in Central Asia than previously anticipated, extending to the northern margin of the Tarim Craton. This observation hence refines the existing tectonic history models for Central Asia.

Physils and organic matter-base palaeoenvironmental records of the K/Pg boundary transition from the late Cretaceous-early Palaeogene succession of the Um-Sohryngkew River section of Meghalaya, India

A continuous, shallow marine succession of the Um-Sohryngkew River section is distinct as it contains late Maastrichtian-early Danian planktonic foraminiferal zones and the K/Pg boundary, although K/Pg transition events remain inconclusive. Physils divide entire succession in to lower, middle and upper sub-divisions and represent anomalous values of redox-sensitive elements (δCe, La/Yb and Gd/Yb) in biozone CF3. Organic matter when analyzed show TOC and C spikes in biozone CF3. Illite thermometry also revealed sudden increase in the palaeo-temperature (>140°C) for yellowish brown 1–2mm thick organic rich clay layer in biozone CF3. Interestingly, conspicuous increase in the short chain n-alkanes and fatty acids is observed in the biozone CF3, although, long chain n-alkanes (C27–C33) derived from terrestrial plants show low abundance throughout the succession.High amount of combustion derived fluoranthene, pyrene, chrysene, benzo(a) anthracene PAH compounds found in the biozone CF3 are analogous to those reported from the K/T boundary sections of Stevns Klint, Gubbio, Woodside Creek and Arroyo el Mimbral. The pyrolitic signatures of these organic macro-molecules reflect global fire, caused distress to biota (during the deposition of biozone CF3 layer) which is coincidental with the well documented Ce anomaly layer, but, preceded by planktonic foraminiferal change in biozone P0 and PGE anomaly bearing layer in the biozone CF2. These organic macro-molecules reflect global fire, induced by the heat supplied by the late Cretaceous Abor and/or Deccan extrusions perhaps linked with the K/T transition events as later initiated prior to the K/Pg boundary, however, the main episode of Deccan volcanic activity occurred ∼300ky earlier or at the K/Pg boundary itself. The deposition of 1–2mm thick, yellowish brown, smooth (with conchoidal fracture) pyrite nodules and micro-spherules bearing organic rich clay layer marked with the decrease in the carbonate content (2.43%) that lies at the contact between the silty mudstone and grey calcareous shale located in the biozone CF3 of this succession coincides with the first appearance of Pseudoguembelina hariaensis representing age of 66.83–65.45Ma is also related to the India’s collision with the Eurasia and Burma andextrusion of Abor volcanic.These events also endorse succeeding events such as anomalous concentration of platinum group of elements and concentration of spherules during biozone CF2, which are other end Cretaceous events before the advent of the K/Pg boundary.

Molecular and fossil evidence disentangle the biogeographical history ofPodocarpus, a key genus in plant geography

AbstractAimThe genusPodocarpus(Podocarpaceae) provides an opportunity to contrast biogeographical hypotheses within and among continents, and to analyse divergence between disjunct tropical and temperate forests of South America. We developed a calibrated phylogeny ofPodocarpusto reconstruct the ancestral areas and potential expansion routes within Podocarpaceae.LocationPodocarpusconsists of two extant subgenera:Foliolatusfrom Asia and Oceania, andPodocarpuslocated in Gondwanan continents and north to the Caribbean. The paper focuses mainly on the area occupied by the latter subgenus.MethodsWe combined previously published and novelDNAsequences with fossil records. New species sequenced are members ofPodocarpussubgenusPodocarpusfrom South and Central America. We assembledDNAsequences of the chloroplast (matK andrbcL) and nuclear (ITS1 andITS2) to analyse phylogenetic relationships withinPodocarpussubgenusPodocarpusby Bayesian methods, which were calibrated using macrofossils that could be confidently identified as modern genera. Ancestral areas were inferred using the dispersal–extinction–cladogenesis model.ResultsThe phylogenetic reconstruction inferred a minimum age for the origin ofPodocarpuss.l. in the late Cretaceous–early Palaeogene (63 Ma) and strongly supported monophyly of the genusPodocarpusand of subgeneraPodocarpusandFoliolatus. SubgenusPodocarpusconsists of two monophyletic, latitudinally structured clades. One clade consists of temperate American species while the other includes species from tropical‐subtropical Africa and South America.Main conclusionsThe history of the subgenera withinPodocarpusis older than previously reported: they can be traced back to late Cretaceous–early Palaeocene biogeographical connections between Australasia and South America through Antarctica. Latitudinally disjunct lineages within South America most probably diverged from widespread ancestors as a result of a persistent arid barrier that was established prior to the late Palaeogene. The calibrated age for the Tropical–Subtropical clade suggests an Atlantic–subtropical biogeographical corridor between South America and Africa long after the breakup of Gondwana and the stabilization of the circum‐Antarctic current.

Polycyclic aromatic hydrocarbon compound excursions and K/Pg transition in the late Cretaceous–early Palaeogene succession of the Um Sohryngkew river section, Meghalaya

Polycyclic aromatic hydrocarbon compound excursions and K/Pg transition in the late Cretaceous–early Palaeogene succession of the Um Sohryngkew river section, Meghalaya

Polycyclic aromatic hydrocarbon compound excursions and K/Pg transition in the late Cretaceous–early Palaeogene succession of the Um Sohryngkew river section, Meghalaya

Polycyclic aromatic hydrocarbon compound excursions and K/Pg transition in the late Cretaceous–early Palaeogene succession of the Um Sohryngkew river section, Meghalaya

Chemico-mineralogical attributes of clays from the late Cretaceous–early Palaeogene succession of the Um Sohryngkew river section of Meghalaya, India: Palaeoenvironmental inferences and the K/Pg boundary

A continuous shallow marine 10 m thick succession within the Langpar Formation in the Um Sohryngkew river section of Meghalaya, containing late Maastrichtian through early Danian planktonic foraminiferal zones – CF4, CF3, CF2, CF1, P0, Pα and P1a and the K/Pg boundary (between CF1 and P0) that makes unique of its kind. The section has been re-studied and sampled for clay mineralogy to understand the palaeoenvironmental conditions prevalent in the region and to assess the K/Pg transition. The relative abundances of the clay mineral phases permitted a threefold sub-division of the studied section with a illite, illite/ smectite dominated lower part, illite, kaolinite and abundance of montmorillonite dominated middle part and kaolinite–montmorillonite dominated upper part. Enriched HREEs in the lower part of the succession suggest variations in the pH of alteration solutions. Most of the samples show positive cerium (δCe) and europium (δEu) anomalies, the former reflecting oxidizing conditions at the time of clay formation. Illite dominated clays present a positive Eu anomaly, formed at relatively higher temperatures than the clays with less illite and without Eu anomalies, whereas clays occurring in the lower and upper parts exhibit a prominent negative Eu anomaly. Shifts in the redox condition found in this section are more or less similar to the foraminiferal changes and Au, Pt, Pd anomalies. Clay mineralogical attributes and REE patterns, comparable to those of the known K/Pg boundaries, appeared within the CF3 and CF2 zones in the Um Sohryngkew river section. The sample at the boundary between CF3 and CF2 is marked by a negative <delta>Ce anomaly, high La/Yb and TOC values, suggesting that sea level rise during the upper part of CF3 was caused by tectonism rather than warming. The similar characteristics of clay minerals and REE patterns, attributed to the initiation of tectonic events during the CF3 zone, indicate environmental changes that affected the shelf area and the provenance of these sediments.

An ‘ameridelphian’ marsupial from the early Eocene of Australia supports a complex model of Southern Hemisphere marsupial biogeography

Recent molecular data strongly support the monophyly of all extant Australian and New Guinean marsupials (Eomarsupialia) to the exclusion of extant South American marsupials. This, together with available geological and fossil evidence, has been used to argue that the presence of marsupials in Australia is simply the result of a single dispersal event from South America during the latest Cretaceous or Palaeocene, without subsequent dispersals between the two continents. Here, I describe an isolated ankle bone (calcaneus) of a metatherian from the early Eocene Tingamarra Local Fauna in northeastern Australia. Strikingly, this specimen, QM F30060, lacks the 'continuous lower ankle joint pattern' (CLAJP), presence of which is a highly distinctive apomorphy of the marsupial clade Australidelphia, which includes Eomarsupialia, the living South American microbiotherian Dromiciops and the Tingamarran fossil marsupial Djarthia. Comparisons with a range of marsupials and stem-metatherians strongly suggest that the absence of the CLAJP in QM F30060 is plesiomorphic, and that this specimen represents the first unequivocal non-australidelphian ('ameridelphian') metatherian known from Australia. This interpretation is confirmed by phylogenetic analyses that place QM F30060 within (crown-group) Marsupialia, but outside Australidelphia. Based on these results, the distribution of marsupials within Gondwana cannot be explained by simply a single dispersal event from South America and Australia. Either there were multiple dispersals by marsupials (and possibly also stem-metatherians) between South America and Australia, in one or both directions, or, alternatively, there was a broadly similar metatherian fauna stretching across southern South America, Antarctica and Australia during the Late Cretaceous-early Palaeogene.

Structural control on Meso-Cenozoic tectonic reactivation and denudation in the Siberian Altai: Insights from multi-method thermochronometry

The Siberian Altai forms part of the Central Asian Orogenic Belt and currently is an active intracontinental mountain belt. This Siberian Altai orogen was shaped during multiple phases of intense mountain building and associated denudation throughout the Mesozoic and Cenozoic. We performed a multi-method thermochronometric study on the Altai basement in Siberia in order to distinguish these denudation episodes and to place them in an absolute time-frame. Our data shows that inherited suture zones, such as the Charysh–Terekta–Ulagan–Sayan suture, played a dominant role during this episodic deformation in the Altai. Titanite fission track results reveal Late Triassic–Early Jurassic cooling, which is best expressed in intrusions close to the suture zone. Apatite fission track (AFT) results reveal a second phase of steady basement cooling during the Late Jurassic and Cretaceous. In the suture, AFT and additional apatite (U–Th–Sm)/He (AHe) data reveal a consistent Late Cretaceous–Early Palaeogene phase of fast cooling. We interpret the Mesozoic cooling signals as far-field effects of the Mongol-Okhotsk orogeny. The intense Late Cretaceous–Palaeogene basement cooling signal in the suture zone can be explained as a result of fault-reactivation during the collapse of this orogen and the subsequent re-organization of crustal blocks. During most of the Palaeogene and Early Neogene, basement cooling stagnated, indicating tectonic quiescence. Neogene AHe ages for the Kurai Range, directly adjacent to the suture may reflect renewed rapid cooling as a result of denudation of the present-day Altai mountains. We interpret this most recent cooling as an indirect consequence of stress-propagation from the India–Eurasia collision into the CAOB interior, which thus reached as far as southern Siberia. This study further demonstrates the importance of thermochrological studies on intrusions along major fault-zones in order to date and distinguish fault-reactivation episodes.

Palaeoposition of the Seychelles microcontinent in relation to the Deccan Traps and the Plume Generation Zone in Late Cretaceous-Early Palaeogene time

Abstract The Early Palaeogene magmatic rocks of North and Silhouette Islands in the Seychelles contain clues to the Cenozoic geodynamic puzzle of the Indian Ocean, but have so far lacked precise geochronological data and palaeomagnetic constraints. New 40 Ar/ 39 Ar and U–Pb dates demonstrate that these rocks were emplaced during magnetochron C28n; however, 40 Ar/ 39 Ar and palaeomagnetic data from Silhouette indicate that this complex experienced a protracted period of cooling. The Seychelles palaeomagnetic pole (57.55°S and 114.22°E; A9512.3°, N =14) corresponds to poles of similar ages from the Deccan Traps after being corrected for a clockwise rotation of 29.4°±12.9°. This implies that Seychelles acted as an independent microplate between the Indian and African plates during and possibly after C27r time, confirming recent results based on kinematic studies. Our reconstruction confirms that the eruption of the Deccan Traps, which affected both India and the Seychelles and triggered continental break-up, can be linked to the present active Reunion hotspot, which is being sourced as a deep plume from the Plume Generation Zone. Supplementary material: Experimental data are available at http://www.geolsoc.org.uk/SUP18482 .

The post-Pan-African thermal and extensional history of crystalline basement rocks in eastern Tanzania

Apatite fission track thermochronology from basement rocks in eastern Tanzania reveals a protracted history of denudation, uplift and extensional tectonism for the region after Carboniferous time. The data show that post-Pan-African development of eastern Tanzania was characterised by long periods of slow cooling punctuated by at least three relatively rapid cooling events, during the Early Cretaceous, Late Cretaceous-Early Palaeogene and Late Eocene-Early Oligocene. The relatively rapid cooling resulted from episodes of increased denudation rates related to the formation and reactivation of high-angle block faults that moved in response to intraplate stress. The episodes of denudation are manifested as packages of clastic sedimentary rocks in East African basins. Abrupt offsets in the trends of the apatite fission track data at major faults is consistent with development of a nascent rift valley within the broad region of previously block faulted and mildly extended crust, during Neogene time. The nascent rift valley was focused within a zone of weakness, associated with reactivation of favourably orientated regional-scale Proterozoic structures. This is particularly true of the subsurface boundary between the thick lithosphere of the Tanzanian Craton and the relatively thinner lithosphere of the Proterozoic Mozambique Belt.

Tectonic setting and pre-orogenic sedimentation in the Indus-Tsangpo (Yarlung) suture zone of Ladakh Himalaya, India

There are three tectonically distinguishable zones along the Indus Tsangpo Suture of the Ladakh Himalaya: The Lamayuru Formation of Triassic-Palaeogene age representing accreted oceanic basin sediments on the passive margin of the Indian subcontinent; Ophiolitic Melange Zone of Jurassic_Late Cretaceous age; and the Nindam Formation of Late Cretaceous-Early Palaeogene age consisting of trench and trench slope sediments. The plate tectonic setting of the Lamayuru Formation and the Nindam Formation could be correlated with the modern analogue for the passive setting (trailing-edge) and the subduction (leading-age) related sedimentary deposits respectively. Pre- and syn-tectonism related to subduction of the Indian plate with the Eurasian plate, produced favourable sites for deposition of sediments. The rocks exposed help elucidate the relationship between the ongoing convergence and sediments being shed into the basins; they record the existence of rock masses removed by erosion from tectonic highlands.