Late Cretaceous Time Research Articles

Organic geochemical investigation of cretaceous source rocks with varying thermal maturity levels in the East Andes (Boyacá, Colombia)

This study encompasses a geochemical investigation of Cretaceous source rocks from the Eastern Cordillera (Colombia). This basin represents an interesting area for geochemical studies due to its abundance of organic matter-rich levels, some of which exhibit mild to extremely high thermal maturities, enabling discussion on long-term carbon sinks. This research work focuses on the study of organic-rich shaly sediments of Barremian to Campanian age from seven (7) outcrop locations in two areas of the Boyacá Department. A set of seventy (70) rock samples have been analysed using several organic geochemical techniques and have shown to cover thermal maturity ranges from the early oil window to the metagenesis stage (measured reflectance values from 0.6% to 2.1%). The OAE2 event was identified based on a 2.5‰–3‰ carbon isotopic excursion in the distal Guaguaquí Group from the section along the Guaquaquí River. Maturity was not found to correlate with depositional burial depths in the so-called distal study area, suggesting that other tectonic events may have played an important role in the genetic history of hydrocarbons particularly in this part of the Boyacá Department. During the mid-to late Cretaceous times, the two study areas consisted of a marine margin with a terrestrial input from the east, which is reflected in a higher terrestrial character of sediments in the eastern foothills (V/Ni < 0.7) compared to the others. Finally, fluorene- and diamondoid-based components were used for enhancing the characterization of the mid-to late Cretaceous organic matter in terms of source in the study sub-region (three-fluorenes and dimethyl-/trimethyl-adamantanes ratios above or below 0.25 and 0.85).

A new armored dinosaur with double cheek horns from the early Late Cretaceous of southeastern China

Ankylosaurines are the iconic armoured dinosaurs that characterize terrestrial vertebrate faunas in the Late Cretaceous of Asia and Laramidia (western North America). The earliest members of this clade are known from the early Late Cretaceous (Cenomanian–Santonian) times of Asia, but little consensus has emerged as to how they are related to the anatomically derived and chronologically younger forms. In southeastern China, the Cretaceous red sand beds crop out across basins from Zhejiang to Guandong provinces. However, the horizons corresponding to the early Late Cretaceous stages remain poorly sampled. Here, we report the first definitive vertebrate skeleton ¾ let alone that of an armoured dinosaur ¾ from the Coniacian/Turonian Ganzhou Formation, Datai yinliangis gen. et sp. nov. Despite the immature ontogenetic status of the type materials, D. yingliangis can be diagnosed with autapomorphic traits in the cranial caputegulae (such as double horns on the quadratojugal) and extensive gular osteoderms. Morphologically, it is intermediate between the chronologically older ankylosaurids from Asia (e.g., Crichtonpelta and Jinyunpelta) and derived post-Cenomanian ankylosaurines (e.g., Pinacosaurus). Phylogenetic analyses broadly corroborate this assessment. The new taxon either falls in the grade of Asian ankylosaurines proximal to the lineages of derived forms or forms a sister lineage to Pinacosaurus. Based on these insights, Datai makes a significant addition to the early Late Cretaceous vertebrate fauna from southeastern China and highlights the future potential in this region for improved understanding of the origin and early evolution of ankylosaurines.

Structural Restorations of the Complete Conjugate US‐Mexico Eastern Gulf of Mexico Margin

AbstractWe present the first sequential structural restoration with flexural backstripping of the Gulf of Mexico US‐Mexico conjugate margin salt basin. We construct four large‐scale (100s of km) balanced, sequential structural restorations to investigate spatio‐temporal patterns of subsidence, geometry of the original salt basin, feedbacks between post‐salt structural and stratigraphic evolution, paleo‐bathymetry, and crustal configurations. The restorations are based on interpretations of 2D and 3D seismic data, and include sequential sedimentary decompaction, flexural isostatic backstripping, and thermal isostatic corrections. The spatially variable crustal thinning factor is directly measured from seismic data, and lithologic parameters are determined by well penetrations. We present a model for the original salt basin and discuss evidence for and implications of a deep water salt basin setting for the GoM. Our analysis suggests a salt basin that contained ∼1–2 km thick salt in a basin 175–390 km across with ∼1 km of bathymetry after salt deposition. The base of salt is mostly smooth with &lt;1 km of local relief in the form of normal faults that disrupt a pre‐salt sedimentary section. We find that supra‐salt extension and shortening are not balanced, with measurable extension exceeding shortening by 18–30 km on each cross‐section. Our subsidence analysis reveals anomalous subsidence totaling 1–2 km during Late Jurassic and Early Cretaceous times that may reflect dynamic topography or depth‐dependent thinning. We offer an interpretation of crustal breakup invoking pre‐salt clastic sedimentation, salt deposition in a deep water syn‐thinning basin, and post‐salt lower‐crustal exhumation.

Kilometer-scale recumbent folding, tectonic attenuation, and rotational shear in the western Anaconda Range, southwestern Montana, USA

Abstract The Eocene Anaconda metamorphic core complex is the most recently documented metamorphic core complex in the North American Cordillera. While much work has focused on constraining the nature and timing of core complex extension, earlier deformation preserved in its footwall is not as well understood. The Anaconda metamorphic core complex footwall contains an anomalously thin, lower- to uppermost-amphibolite-facies section of Mesoproterozoic Belt Supergroup and Paleozoic metasedimentary strata. While the tectonic nature of this thinning is generally accepted, the mechanisms behind it remain enigmatic. Previous workers have hypothesized that footwall strata were attenuated along the upper limb of the Late Cretaceous Fishtrap recumbent anticline, a kilometer-scale, NW-vergent, recumbent fold exposed throughout the west-central metamorphic core complex footwall. New geologic mapping in the west-central Anaconda Range better constrains the nature and timing of tectonic attenuation in this structurally complex area. Two generations of folds were recognized: (1) F1 recumbent isoclines associated with the Fishtrap recumbent anticline and (2) F2 W-vergent asymmetric folds associated with map-scale N-plunging folds. F1 folds, axial planar S1 transposition fabrics, and bedding-parallel faults and shear zones boudinage, transpose, and omit strata of the Belt Supergroup. We suggest that the Fishtrap recumbent anticline tectonically attenuated the Belt Supergroup through Paleozoic section of the west-central Anaconda metamorphic core complex footwall, and we propose that it is a kilometer-scale, regionally significant structure. We further propose that the fold may have developed in response to rotational shear and sinistral transpression along the Lewis and Clark Line, which was further driven by accretion of outboard terranes along the western margin of North America during Late Cretaceous time.

Joint application of fluid inclusion and clumped isotope (∆47) thermometry unravels the complexity of thermal and fluid‐flow history reconstruction in sedimentary basins—Upper Triassic Chaunoy Formation reservoirs (Paris Basin)

AbstractWe investigated calcites and dolomites precipitated during burial diagenesis of the Upper Triassic (Norian) continental siliciclastics from sub‐surface reservoirs of the northern Paris Basin (Chaunoy Formation) that experienced a thermal maximum &gt;100°C during Late Cretaceous times. Relative carbonate precipitation timing was established via petrographic analyses. The diagenetic carbonates were further investigated by fluid inclusion and clumped isotope (Δ47) thermometry. The two thermometric datasets were interpreted by evaluating the possible occurrence of inclusion thermal reequilibration and Δ47 solid‐state reordering, based on the known basin thermal history and the three existing Δ47 reordering models. By considering the fluid inclusion and Δ47 datasets obtained and the various Δ47 reordering models, different carbonate precipitation scenarios, in terms of timing and parent fluid composition (δ18Ofluid), were inferred. These results underline that in samples having experienced thermal maximum &gt;100°C, accuracy and interpretation of fluid inclusion and Δ47 thermometry data (especially on calcite) may be biased by thermal reequilibration and solid‐state reordering. The results converge towards the need of jointly applying fluid inclusion and Δ47 thermometry on the same carbonate phases to evaluate all the possible precipitation scenarios. The most likely carbonate precipitation scenarios, based on Δ47 thermometry data, point at the precipitation of two calcite phases during Early to Late Jurassic times and of one dolomite phase during the Late Cretaceous. The parent fluids possibly were original formation waters of the Chaunoy Fm. that mixed with brines migrating from the East, where time equivalent evaporitic deposits occur. The proposed precipitation model for calcites and dolomites, involving different pulses of brine migration, and the dominance of calcite phases were not recorded by previous studies on the Upper Triassic units. These latter results may be of interest to evaluate the reservoir potential of the Chaunoy Fm. in this underexplored portion of the Paris Basin.

Taxonomic revision of the canaliculate radiolitid (rudist, bivalve) genus Lattenbergites Lupu, 1987 from the Upper Cretaceous of the central and eastern Mediterranean Tethys: palaeobiogeographic distribution

Lattenbergites Lupu was previously considered to be a new genus of canaliculate radiolitid from the middle–upper Coniacian of Lattenberg, Germany. The re-examination of the holotype and paratypes of the type species L. hermi Lupu of this genus reveals diagnostic characters of Joufia milovanovici (Slišković), including the presence in the left valve of a single ring of radial canals in the calcitic outer shell layer, the canals having sub-rectangular cross-sectional shape flanking the myocardinal apparatus in the originally aragonitic inner shell layer, and a pair of tubular structures in its postero-ventral part, opposing the ‘pseudopillar’-like structures of the right valve. Therefore, we propose that Lattenbergites Lupu, is a junior synonym of Joufia milovanovici. J. milovanovici from the Malatya, Haymana-Polatlı Basins and eastern Pontides of Türkiye, as well as the Roşia Basin of Romania is also described and compared to Lattenbergites. The presence of J. milovanovici in the Northern Calcareous Alps during the middle–late Coniacian, Romania during the late Santonian to early–middle Campanian, and Türkiye during the late Maastrichtian, allows us to reconsider the palaeobiogeographic distribution of this species within the Mediterranean Tethys during Late Cretaceous time, as well as for the genus Joufia.

Timing of the Transition From Sevier‐ to Laramide‐Style Tectonism in Southwestern Montana Based on the Provenance of the Frontier Formation, North American Cordillera

AbstractThe Upper Cretaceous Frontier Formation in southwestern Montana is coeval with the transition from Sevier‐ to Laramide‐style tectonism in the Idaho‐Montana sector of the North American Cordillera. To better constrain the timing of initial exhumation above the Laramide‐style Blacktail‐Snowcrest arch, we use biostratigraphic data, sandstone petrography, and detrital zircon (DZ) geochronology to determine the provenance and depositional age of the Frontier Formation. Near Lima Peaks, erosion of Lower Cretaceous strata from the frontal Sevier‐style Tendoy thrust sheet provided sediment to the foreland basin. In the Western Centennial Mountains, sediment sources included those same sources as well as Neoproterozoic and Cambrian strata and Mesoproterozoic plutons in the Belt basin. In contrast, near the Gravelly Range, sediment eroded from Pennsylvanian‐Upper Cretaceous strata atop the Blacktail‐Snowcrest basement‐cored uplift, documenting unroofing to Pennsylvanian‐Permian stratigraphic levels by 87–85 Ma. Palynology corroborates recycling into the Frontier Formation, including from the underlying Blackleaf Formation. The ubiquitous presence of 100–85 Ma DZ ages coupled with different interpreted source regions suggests that an ash‐fall source contributed young zircon grains to the Frontier Formation. The timing of exhumation above the Blacktail‐Snowcrest arch provided by these new data presented herein suggest that Laramide‐style tectonism in Idaho‐Montana may be unrelated to shallow‐angle subduction within a narrow corridor as envisioned by current models. Instead, upper plate controls, such as the locations of inherited faults and basement highs, the distribution and thickness of pre‐orogenic sedimentary cover, and the availability of detachment surfaces, may be responsible for Laramide‐style tectonism during early Late Cretaceous time.

Superimposed structure in the southern periphery of Abu Gharadig Basin, Egypt: Implication to petroleum system

South West Abu-Sennan (SWS) area near the southern periphery of the Abu-Gharadig petroliferous basin in the Egyptian Northern Western Desert is an important hydrocarbon field characterized by being a multi-reservoir with a complex structural architecture. Therefore, in this study, we illustrated the regional structural role that facilitated having Jurassic mature source rocks (Masajid and Khatatba formations) through surgical seismic interpretation and observed the controlling petroleum system of the study area using a set of 20 seismic lines and 4 wells data. The study resulted in having the SWS oil field controlled by a strike-slip movement that occurred during the Upper Cretaceous time accompanying the African Plate movement against the Eurasian Plate superimposing an ENE–WSW strike-slip fault zone in the region and a strong pattern of NW–SE-oriented faults due to numerous phases of extension. The associated structures include horsts, normal fault propagation folds, and strike-slip-related anticlines. The strike-slip tectonics played a major role in forming entrapment for the Jurassic expelled hydrocarbons. The seismic horizons flattening exercise performed on AR/G member and Khoman Formation demonstrated that the main reason for not having hydrocarbon accumulations in the Jurassic levels in the study area is that the trap configuration was ready to accumulate hydrocarbon but after the hydrocarbon generation and migration time. The study concluded that the hydrocarbons migrated along the faults which cut deeply to the source rocks and entrapped in the possible cretaceous reservoirs during the late cretaceous time. The conducted workflow in this study can be utilized to better explore and develop cretaceous reservoirs in the study and neighbouring areas.

Lithospheric evolution of eastern Arabia based on surface wave and receiver function analyses

The geodynamic evolution of Eastern Arabia comprises three major events that must have acted on the entire lithosphere below the present-day passive continental margin. Starting with the Pan-African plate assembly during the Late Proterozoic, NNE-oriented structural trends were established which were reactivated during Infracambrian and Ordovician extension. Permian Pangean breakup and rifting created two rift axes to form the present-day geometry of the eastern and northern passive margins of Eastern Arabia. SW-directed obduction of the Semail Ophiolite during the Late Cretaceous was the latest major tectonic event that shapes the present-day geology of northern Oman. Recent anisotropic ambient noise tomography, surface wave analysis of earthquake data and receiver function analysis reveal the present-day lithospheric thickness of ∼100 km, and the previously unknown, highly variable internal structure of the East Arabian continental crust. We integrate all newly available seismological observations into a consistent geodynamic model that describes the evolution of lithospheric thickness and crustal heterogeneity, identifies crustal segmentation and relates inherited structures to lateral variations within the dynamics of ophiolite obduction.A sharp, NNE-striking contrast of ∼10% in shear wave velocity (VS) in the lower crust separates felsic Arabian continental crust in the NW from intermediate lithologies in the NE of Oman. This crustal segmentation is interpreted as being established during the Pan-African orogeny. High VS in the lower crust of northeastern Oman is likely due to Permian mafic intrusions related to the Pangean breakup. Mild crustal thickening below todays high topography is attributed to Late Cretaceous time, where northward continental subduction below the Neo-Tethys led to underthrusting and thickening of the continental crust and ultimately to obduction of the Semail Ophiolite.After post-obduction relaxation, we argue that lithospheric thickness, margin perpendicular fast directions of anisotropy and contemporaneous mid- to late Eocene basanite intrusions are indicative of an overall relaxed state of stress of the lithosphere at that time. In consequence, we propose marginal erosion of the base lithosphere on the passive Arabian plate margin during the Late Eocene as a geodynamic process that facilitated margin-wide emergence of the Oman Mountains. Altogether, the present configuration of the lithosphere in northeastern Arabia preserves imprints from Late Proterozoic Pan-African assemblage, Permian Pangean break-up, Late Cretaceous subduction and ophiolite obduction, and Cenozoic post-obduction evolution.

Integration of recent drill results with multichannel seismic reflection data: New inferences on the subsurface configuration and sedimentary history of the northwestern Bay of Bengal

The study pertains to the use of two industry wells and one IODP drill site in conjunction with high quality multichannel seismic reflection data in the northwestern Bay of Bengal. The industry wells provide subsurface information from Early Cretaceous period while the IODP drill site U1444 gives information about deposition younger to Miocene. Their locations over the seismic profiles facilitated the assignment of ages to the prominent seismic horizons thereby yielding a revised seismic stratigraphy of the northwestern Bay of Bengal.The ages of the oldest seismic horizons are in agreement with the recently published ages of the underlying oceanic crust. The sediments up to Late Cretaceous time are thick west of 85°E Ridge, while to its east, they are thin. The sediments up to Paleocene time thin towards offshore, implying that they are land derived (from East Coast of India). Fan sedimentation commenced in the Eocene time and was excessive since Miocene. The transparent sequence between the Late Pleistocene and Pliocene unconformities is thin over the 85°E Ridge. The western extents of the Quaternary subfans above the Late Pleistocene unconformity are demarcated in the study area.Further, the study suggests that the emplacement of the 85°E Ridge took place in the Middle Cretaceous period. Faulting/folding at the continental rise occurred around Miocene time and maybe linked to intraplate deformation that occurred in the Central Indian Ocean Basin. Thus, the inferences drawn from this study help to provide a reliable sedimentary history of the northwestern Bay of Bengal forming an updated base for future investigations.

The Kanarra fold-thrust structure—the leading edge of the Sevier fold-thrust belt, southwestern Utah:

The multiple origins proposed for the Kanarra anticline in southwestern Utah as a drag-fold along the Hurricane fault, a Laramide monocline, a Sevier fault-propagation fold, or a combination of these processes, serve to muddy its tectonic significance. This in part reflects the structural complexity of the exposed eastern half of the fold. The fold evolved from open and up-right to overturned and tight, is cross-cut by multiple faults, and was subsequently dismembered by the Hurricane fault. The western half of the fold is obscured because of burial, along with the hanging wall of the Hurricane fault, beneath Neogene and younger sediments and volcanics. We present the results of detailed bedrock geologic mapping, and geologic cross sections restored to Late Cretaceous time (prior to Basin and Range extension), to demonstrate the Kanarra anticline is a compound anticline-syncline pair inextricably linked with concomitant thrust faulting that formed during the Sevier orogeny. We propose the name Kanarra fold-thrust structure to unambiguously identify the close spatial and temporal association of folding and thrusting in formation of this prominent geologic feature. We identify a previously unrecognized thrust, the Red Rock Trail thrust, as a forelimb shear thrust that was in a favorable orientation and position to have been soft-linked, and locally hard-linked, with the thrust ramp of the basal detachment to form a break thrust. The east verging Red Rock Trail thrust is recognized by a distinctive cataclasite in the Lower Jurassic Navajo Sandstone. The hanging wall of the Red Rock Trail thrust is displaced eastward over the Middle Jurassic Carmel Formation and Upper Cretaceous formations and can be traced for at least 27 km and possibly farther. We contend the Kanarra fold-thrust structure unambiguously defines the leading edge of the Sevier fold-thrust beltin southwestern Utah. Stratigraphic relationships in the southern and northern part of the Kanarra fold-thrust structure constrain its development between the early and late Campanian (about 84 to 71 Ma) but possibly younger. In southwestern Utah, initial movement along the Iron Springs thrust at about 100 Ma (Quick and others, 2020) and subsequent eastward advancement of the Sevier deformation front to the Red Rock Trail thrust at about 84 to 71 Ma coincided with well-documented magmatic flare ups in the Cordilleran arc in the hinterland of the Sevier fold-thrust belt. This temporal relationship between magmatic flare ups and thrusting is consistent with a close correspondence between arc-related processes and episodic foreland deformation.

Late extension of a passive margin coeval with subduction of the adjacent slab: The Western Alps and Maghrebides files

The evolution of the Alpine Tethys margins during the beginning of the African-Eurasian convergence was little studied compared to their evolution during the post-Pangea rifting and oceanic expansion,i.e., from the Early Jurassic to the early Late Cretaceous. The present work firstly aims to make up for this shortcoming in the case of the distal European margin of the Alpine Tethys, namely the Briançonnais domain of the Western Alps. We show that this margin was affected by strong post-rifting extension mainly in Late Cretaceous-Paleocene times and propose to make it the type of the (rare) “Late Extension Passive Margins”. Remarkably, this extension shortly preceded Lutetian times, when Briançonnais margin encroached the SE-dipping subduction zone under the Adria microplate. Secondly, we assess the post-rifting evolution of the north-Tethyan paleomargin in the Maghrebides transects,i.e., south-west of the Briançonnais transect along the same European-Iberian paleomargin. For this purpose, we consider the Triassic-Eocene series of the “Dorsale Calcaire” in the Alkapeca Blocks located along southeastern Iberia until the Eocene then transported onto the North African margin. Examination of the literature shows that the Tethyan margin of the Alboran block was strongly affected by normal faulting as early as Late Jurassic-Early Cretaceous times whereas post-rifting extension of the Kabylian blocks mainly occurred in the Late Cretaceous-Paleocene like in the Briançonnais. We propose that post-rifting extension of the Alboran block southern margin resulted from the sinistral movement of Africa relative to Iberia while the later extension of the Kabylian blocks can be related to the further convergence kinematics. Subduction of the Ligurian-Maghrebian slab under the North African margin would have occurred at that time in the southward continuation of the Alpine subduction. The overriding Adria and North African margins did not experience significant compression at that time. During the Eocene, a subduction polarity reversal occurred, which was associated with the relocation of the subduction zone along the Alkapeca block. This was the beginning of the Apenninic subduction, which triggered the back-arc opening of the Mediterranean basins.

Protolith and metamorphic age of the Sieggraben Eclogites: Implications for the Permian to Cretaceous Wilson cycle in the Austroalpine unit

The Austroalpine eclogite-bearing Sieggraben Complex, exposed at the eastern margin of the Eastern Alps, is considered to represent a subducted and then exhumed fragment of the continental rift, which led to the formation of the Meliata oceanic basin. Combined zircon U-Pb dating, whole rock geochemistry and Hf isotope analysis of the Sieggraben Complex revealed that the eclogite sample shows a protolith age of 242.3 ± 2.6 Ma (Middle Triassic) and displays tholeiitic basalt/N-MORB geochemical characteristics. Associated ultramafic rocks as part of oceanic or subcontinental mantle lithosphere suggest a depleted mantle source and a deep subduction environment. Two zircon grains of eclogites with low Th/U ratios yield ages of 113 ± 2 Ma and 86 ± 4 Ma and represent the approximate age of eclogite metamorphism during Cretaceous. A trondhjemite dike cutting through eclogite gives a crystallization age of 82.19 ± 0.4 Ma and formed by partial melting during decompression. The host metasedimentary rocks are interpreted as old continental crust close to the margin of the Meliata basin were affected by Permian migmatitic metamorphism. The results of this study combined with previous results from the Sieggraben Complex are used to present an updated model for the tectonic evolution of the distal Austroalpine unit associated with the oceanic Meliata relics The Austroalpine Sieggraben Complex represents a location on the distal continental margin during Permian rifting. This piece of continental crust of an inferred paleotectonic position adjacent to the Meliata oceanic lithosphere subducted during Early Cretaceous times to mantle depth. The subducted continental crust was then exhumed incorporating even ultramafic mantle rocks. During exhumation and decompression of mafic rocks, partial melting took place forming the trondhjemite dike in Late Cretaceous times. • A Middle Triassic ptotolith age was found for eclogites of the Sieggraben Complex. • A Cretaceous-aged trondhjemite dike report formed by decompressional melting after eclogite metamorphism. • Paragneiss hosts a Panafrican detrital zircon memory and was affected by Permian migmatization. • A new tectonic model for the distal Austroalpine unit relates with the Meliata Ocean in a Wilson cycle.

Mafic to felsic dyke swarms in coastal Guangdong, China: geochemical and geochronological constraints on the latest Mesozoic geodynamics on the southern margin of the South China Block

AbstractThe coastal region of Guangdong Province, China, is characterized by well-developed dyke swarms. The dykes with widths of decimetres to metres and lengths of tens of metres occur along straight and planar fractures cutting granites or volcanic rocks with Jurassic to Early Cretaceous ages. They show steep attitudes with strikes varying from NNW to NNE and from NE to SE, consistent with a stress regime transition from E–W to N–S extension. Major-element analysis on representative dyke samples reveals a composition range from basaltic to andesitic with a few dacitic outliers. Trace elements of most samples show notable Nb–Ta negative anomalies and Pb positive anomalies on the primitive mantle normalized spidergrams, characteristic of arc-related rocks. Among these, several dacite samples show notable fractionated medium rare earth elements and heavy rare earth elements and high Sr/Y (47–74) and La/Yb (15–21) ratios indicative of adakitic affinity. A few samples lack anomalies in Nb and Ta and have primitive trace-element ratios (e.g. Nb/La and Sr/Nd) or initial ϵNd values resembling ocean island basalt-like rocks. Rare earth element distribution patterns of all samples are right inclined and generally absent of Eu anomalies, which together with other trace-element indexes indicate an origin from heterogeneous mantle sources with depths below the stability field of plagioclase. Radiogenic isotopes, especially those of Nd and Sr (or Nd–Pb), define an array extended from the depleted to enriched mantle (EMII) provenance. LA-ICP-MS dating of zircon obtains ages between 110 and 70 Ma indicating emplacement of these dykes in latest Early to Late Cretaceous time, a time just before initiation of the South China Sea basin. In the context of regional geodynamics, it is proposed that these dykes were formed in a tectonic setting transiting from a Pacific-related back-arc to a passive continental margin pertaining to the development of the South China Sea basin.

Challenging assumptions of sediment routing in retroarc foreland basins: Detrital zircon evidence for axial versus transverse drainages in the Late Cretaceous of southern Utah, USA

AbstractDepositional models for retroarc foreland basin systems commonly imply a direct link between thrust belt dynamics and sediment supply, despite documented complexity between orogen‐transverse and orogen‐parallel (axial) sediment routing systems. Previous detrital zircon provenance studies from the Straight Cliffs Formation in the Kaiparowits Plateau of southern Utah indicate primary axial source areas to the south (Mogollon Highlands), and southwest (Cordilleran magmatic arc), with relatively minor transverse input from the Sevier fold‐thrust belt to the west. Complementary data from 32 new samples are presented to investigate whether these trends persist in correlated strata from sections most proximal to the active fold‐thrust belt, as well as strata proximal to the Mogollon Highlands. This study aims to improve statistical robustness through higher‐n datasets (&gt;300 analyses/sample) and addresses the possibility of recycling of zircon grains from strata in the exhumed Sevier fold‐thrust belt. Based on this provenance dataset, we infer that orogen‐transverse deposition was mainly limited to the most proximal sections (within ca. 75 km of the fold‐thrust belt), but included episodic distribution across the foredeep that was associated with distinct intervals of widespread amalgamated sand and gravel sheet deposits. In contrast, more distal parts of the foredeep record long‐distance fluvial transport (&gt;400 km) of zircons from the active Cordilleran magmatic arc as well as Yavapai‐Mazatzal basement rocks in central Arizona (the Mogollon Highlands). Palaeogeographic reconstructions suggest the apex of a large basin‐axial fluvial system may have been located at the structural syntaxis between the Sevier fold‐thrust belt and western margin of the Mogollon Highlands. This interpretation is supported by detrital zircon data presented here, thus extending the possibility that the Palaeogene ‘California River’ may have existed by at least Late Cretaceous time. Ultimately, these data point to extrabasinal controls on sediment supply, which was largely decoupled from flexural accommodation controls in this archetype retroarc foreland basin.

Provenance of the Late Cretaceous Pab Formation, Sulaiman fold‐thrust belt, Pakistan: Insight from the detrital zircon U–Pb geochronology and sandstone petrography

This study provides an insight to the provenance of the Late Cretaceous Pab Formation exposed in the Sulaiman fold‐thrust belt, Pakistan. The methodology adopted consisted of sandstone petrography and U–Pb detrital zircon geochronology. The sandstone petrography indicates craton interior and recycled orogenic provenances that points to an Indian provenance. The observed basaltic lithics were possibly derived from Rajmahal traps instead of Deccan traps, supported by younger detrital age. The detrital zircon ages exhibit a resemblance to the central and southern Indian provenances, while some detritus also matches the age spectrum of Tethyan Himalayas (~400–800 Ma). Combining sandstone petrography and U–Pb geochronology results, it is suggested that in the Late Cretaceous time, the sediments were mainly fed by the southern and central sources of the Indian Craton. This provenace is possible, as during the Cretaceous time, thermal doming of Indian Plate in the southern part occurred when it was passing over a mantle hotspot. This movement over the hotspot uplifted the southeastern region and increased sediment supply from southern and central Indian provenances and deposited along the western margin as the Pab Formation.

A 3-D crustal shear wave velocity model and Moho map below the Semail Ophiolite, eastern Arabia

SUMMARYThe Semail Ophiolite in eastern Arabia is the largest and best-exposed slice of oceanic lithosphere on land. Detailed knowledge of the tectonic evolution of the shallow crust, in particular during and after ophiolite obduction in Late Cretaceous times is contrasted by few constraints on physical and compositional properties of the middle and lower continental crust below the obducted units. The role of inherited, pre-obduction crustal architecture remains therefore unaccounted for in our understanding of crustal evolution and the present-day geology. Based on seismological data acquired during a 27-month campaign in northern Oman, Ambient Seismic Noise Tomography and Receiver Function analysis provide for the first time a 3-D radially anisotropic shear wave velocity (VS) model and a consistent Moho map below the iconic Semail Ophiolite. The model highlights deep crustal boundaries that segment the eastern Arabian basement in two distinct units. The previously undescribed Western Jabal Akhdar Zone separates Arabian crust with typical continental properties and a thickness of ∼40–45 km in the northwest from a compositionally different terrane in the southeast that is interpreted as a terrane accreted during the Pan-African orogeny in Neoproterozoic times. East of the Ibra Zone, another deep crustal boundary, crustal thickness decreases to 30–35 km and very high lower crustal VS suggest large-scale mafic intrusions into, and possible underplating of the Arabian continental crust that occurred most likely during Permian breakup of Pangea. Mafic reworking is sharply bounded by the (upper crustal) Semail Gap Fault Zone, northwest of which no such high velocities are found in the crust. Topography of the Oman Mountains is supported by a mild crustal root and Moho depth below the highest topography, the Jabal Akhdar Dome, is ∼42 km. Radial anisotropy is robustly resolved in the upper crust and aids in discriminating dipping allochthonous units from autochthonous sedimentary rocks that are indistinguishable by isotropic VS alone. Lateral thickness variations of the ophiolite highlight the Haylayn Ophiolite Massif on the northern flank of Jabal Akhdar Dome and the Hawasina Window as the deepest reaching unit. Ophiolite thickness is ∼10 km in the southern and northern massifs, and ≤5 km elsewhere.

Jurassic true polar wander recorded by the Lhasa terrane on its northward journey from Gondwana to Eurasia

Paleomagnetic data constrain paleogeographic motion of rocks relative to the Earth's spin axis, which is a sum of plate motion relative to the mantle and true polar wander. Discerning between these effects is challenging for studies aiming to reconstruct paleo-plate motions from deformed orogenic terranes. Here, we study the paleolatitudinal drift history of the Lhasa terrane of southern Tibet that migrated from the northern Gondwana to the southern Eurasian margin between late Triassic and early Cretaceous time. Previous work identified a 180 Ma near-equatorial Lhasa latitude and assumed near-constant paleolatitudinal drift. Large-scale true polar wander at this time, however, which has been argued for in previous work, requires highly variable Lhasa plate motion rates relative to Gondwana. Here, we test whether the alternative interpretation of constant plate motion rates provides a better prediction of paleomagnetic data. To this end, we present a new paleomagnetic pole from ∼155 Ma volcanics (here dated by U/Pb zircon) of the Lhasa terrane. Our pole comprises site mean directions from 46 lavas, passes a fold test, and is supported by an extensive rock magnetic and microscopic analysis that reveals no evidence of remagnetization. Our results give an average direction of D±ΔDx=337.1°±2.6°, I±ΔIx=−13.8°±4.9°, and a corresponding paleopole position at λp=45.3°N, φp=295.3°E with K=69.8 and A95=2.5°, predicting a similar near-equatorial, paleolatitude as at 180 Ma. This paleolatitudinal standstill is consistent with predicted paleolatitudes from global APWPs that account for Jurassic TPW and assume a constant Lhasa-Gondwana (India) oceanic spreading rate of ∼8 cm/a during the 215-130 Ma opening of the Neotethys and closure of the Mesotethys oceans. Contemporaneous arc magmatism on the Lhasa terrane was previously interpreted to indicate that the Lhasa terrane was located above a subduction zone during its northward journey to Tibet: our results show that if our reconstruction is accurate, then trench migration rates also must have been ∼8 cm/a. Our results are consistent with the range of rates of global long-lived trench migration that have been reconstructed in other studies, but they are much higher than measured in most modern systems. Our results more completely document the Lhasa terrane's separation from Gondwana and provide a straightforward methodology of using paleomagnetic data to test plate kinematic scenarios, overcoming the problem of the unknown effect of true polar wander on paleomagnetic data from deformed terranes.

Kinematic evolution of the central Andean retroarc thrust belt in northwestern Argentina and implications for coupling between shortening and crustal thickening

Abstract The Andes are the culmination of shortening and crustal thickening that commenced during Late Cretaceous time. First-order questions regarding the tectonic evolution of the central Andes include the magnitude and timing of shortening, and controls on the along-strike variability in observed styles of shortening and deformation. Along-strike differences in the time of surface uplift have spawned two contrasting hypotheses: (1) uplift is related to dynamic and isostatic processes accompanying lithospheric removal and is decoupled in space and time from crustal thickening and shortening, and (2) uplift is directly coupled with shortening and crustal thickening. Although considerable work has been done in Bolivia to address these hypotheses, work in northern Argentina has not yet produced a transorogenic balanced structural cross section from which the total amount and kinematic history of shortening can be evaluated. To help understand the evolution of the thrust belt in northernmost Argentina, we present a regional, retrodeformable cross section at 23°–24°S across the Puna and Eastern Cordillera. New apatite fission-track thermochronological data integrated with other geochronological, sedimentological, and structural data constrain incremental retrodeformation of the cross section between ca. 45 and 6.5 Ma. Regional shortening was facilitated by at least 12 major thrust systems, linked to a regional mid-crustal décollement. Deformation generally propagated eastward through time and involved two major episodes of eastward advance of the orogenic front, separated by periods of internal out-of-sequence shortening and kinematic stagnation of the orogenic front. A new minimum estimate of ∼271 km of total shortening from the Cordillera de Domeyko to the eastern orogenic front explains crustal thickening at northern Puna latitudes. Together with previously published paleoaltimetry data, our new structural and thermochronologic data indicate that regional uplift in the northern Argentine Puna and Eastern Cordillera was synchronous with, and thus directly linked to, crustal shortening and thickening.

Ecomorphospace occupation of large herbivorous dinosaurs from Late Jurassic through to Late Cretaceous time in North America.

Following the Late Jurassic, megaherbivore communities in North America undergo a dramatic turnover in faunal composition: sauropods decline to the point of becoming relatively minor components of ecosystems, stegosaurs become extinct, and hadrosaurids, ceratopsids and ankylosaurs rise in diversity and abundance. Although a variety of causes have been proposed to account for the dramatic decrease in sauropod diversity following the Late Jurassic and could have also been applicable to the disappearance of stegosaurs, the potential for competitive replacement of sauropods by hadrosauroids as an explanation has been previously dismissed due to morphological differences without further investigation. Using twelve ecomorphological correlates of the skull, this study provides a preliminary investigation into ecomorphospace occupation of major megaherbivore clades from the Late Jurassic through to the Late Cretaceous of North America and assess if morphological differences were enough to have potentially facilitated dietary niche partitioning between sauropods and iguanodontians and stegosaurs and ankylosaurs. Overlap in reconstructed ecomorphospace was observed between sauropods (particularly non-diplodocid sauropods) and iguanodontians, as would be expected if morphological differences were not enough to facilitate niche partitioning, contrary to original claims used to dismiss the competitive replacement hypothesis. Overlap was also observed between stegosaurs and ankylosaurs, particularly between Late Cretaceous ankylosaurs. Whether this overlap is reflective competitive replacement or opportunistic occupation of recently vacated niches will require further assessment as sampling of some clades prior to the Late Cretaceous is too poor to make a reliable assessment and several underlying assumptions necessary for competition to occur (e.g., resource limitation) still need investigation. Teasing out the cause(s) of the ‘sauropod decline’ and extinction of stegosaurs in North America following the Late Jurassic will require future research not only into the competitive exclusion hypothesis, but other hypotheses as well with better sampling from Early Cretaceous and Late Jurassic intervals.