Fold Belt Research Articles

Mélange Signatures and Low Oxygen Fugacity in Eclogite Xenoliths From the Crust‐Mantle Transition Below a Mesoproterozoic Collision Belt

AbstractMass transfer across the crust‐mantle boundary is a fundamental process governing planetary differentiation, the evolution of geochemical reservoirs and ore formation, controlled by physicochemical conditions at the crust‐mantle interface. In situ trace‐element, clinopyroxene 87Sr/86Sr and garnet Fe3+/ΣFe of kimberlite‐borne eclogite xenoliths from the deep (∼50 km) crust‐mantle transition below the ca. 1.2–1.0 Ga Namaqua‐Natal Fold Belt (southwestern Kaapvaal craton margin) were determined to elucidate their origin and evolution, and to constrain the oxygen fugacity of this pivotal but largely inaccessible environment. Based on a garnet source signature (NMORB‐normalized Er/Lu > 1) in pristine “gabbroic” eclogites with pronounced positive Eu, Sr, and Pb anomalies, the suite is interpreted as originating as plagioclase‐rich cumulates in oceanic crust from melts generated beneath mature oceanic lithosphere, subsequently subducted during the Namaqua‐Natal orogeny. Enriched eclogites have higher measured 87Sr/86Sr in clinopyroxene (up to 0.7054) than gabbroic ones (up to 0.7036), and show increasing bulk‐rock Li, Be and Pb abundances with increasing δ18O in clinopyroxene, and muted Eu‐Sr‐Pb anomalies. These systematics suggest interaction with a siliceous fluid sourced from seawater‐altered oceanic sediment in a subduction mélange setting. Garnet Fe3+/ΣFe in deep crustal eclogites is extremely low (0.01–0.04, ±0.01 1σ), as inherited from the plagioclase‐rich cumulate protolith, and owing to preferred partitioning into clinopyroxene at low temperatures (∼815–1000°C). Average maximum oxygen fugacities (∆logƒO2(FMQ) = −3.1 ± 1.0 to −0.5 ± 0.7 relative to the Fayalite‐Magnetite‐Quartz buffer) are higher than in deeper‐seated on‐craton eclogite xenoliths, but mostly below sulfate stability, limiting the role of S6+ species in oxidizing the mantle wedge.

Kinematic-structural modeling of hybrid fold-thrust belt systems: Insights from the Southern Patagonian Andes

This paper focuses on the analysis of the Southern Patagonian fold-thrust belt at ca. 50° SL, employing a kinematic-structural approach that makes it possible to define its step-by-step structural evolution. Seismic interpretation is combined with outcrop structural data to provide the basis for an integrated evolutionary model including a Jurassic extensional stage, followed by contraction events since the Late Cretaceous. Our interpretation and results reveal a hybrid fold-thrust belt system, with a high decoupling between basement structures and the sedimentary cover. The basement is deformed by tectonic inversion of the Jurassic rift fault system and duplex stacking geometries, while the sedimentary cover is folded by the action of predominantly west-vergent low-angle faults. The Jurassic rifting model identifies listric faults with detachment depths ranging from −6000 to −6500 m.a.s.l. and 11 %–14 % magnitude extension. Shortening calculations yield values of less than 6 %. Growth strata and thickness variations in Cretaceous units suggest that part of the shortening occurred during the Coniacian/Santonian. Overall, this research contributes to the knowledge of fold-thrust belts, highlighting the importance of doing a comprehensive structural modeling including previous deformational stages, and describing steps to provide reliable measurements of deformation.

Structural inheritance through 3D reservoir modeling and ant-tracking attribute implications for structural configuration of Upper Cretaceous Pab Sandstone, Kirthar Fold Belt, Pakistan

This research presents a comprehensive investigation to enhance understanding of subsurface structural complexities within the Upper Cretaceous Pab Sandstone reservoir level in a gas-producing field within the Kirthar Fold Belt of the Southern Indus Basin, Pakistan. The seismic interpretation for accurate structural delineation and characterization of intricate fault systems is often challenging in Fold and Thrust Belt settings. The 3D structural maps of Upper Cretaceous Pab Sandstone at the reservoir level and the application of the ant-tracking attribute for fault extraction improve the structural understanding of the gas-producing field in the Kirthar Fold Belt in the Southern Indus Basin, Pakistan. Attribute-assisted 3D seismic interpretation has revealed several subsurface structure elements, including a large thrusted anticline extended towards the north-south, the pattern of north-south oblique ramp thrusts on the southeastern flank, and a combination of easterly vergent thrusts with a counter-back thrust creating a local pop-up structure in the area. A 3D structural model at the reservoir level was generated using horizon and fault framework volume-based modeling approach, serving as a structural model for Pab sandstone. The structural smoothing, variance and ant-tracking attribute were applied iteratively to get the best results and correlated with manual interpretation. 3D seismic interpretation and application of ant-tracking for fault extraction identifies discrete structural styles and revealed that all thrusting occurred due to compressional tectonics of the Plio-Pleistocene period. The apparent different styles result from the reactivation of earlier extensional fault systems and probably during phased periods of compression. The resulting 3D reservoir model can be used as an input to populate petrophysical properties and results can be extended for future field development plans. Integration of well and 3D seismic data interpretation and the application of automatic fault extraction techniques are highly recommended in structurally complex areas and have an equal implication for worldwide basins with similar conditions for reliable structural interpretation in Fold and Thrust Belts with structural complexity.

Influence of multiple detachments on structural vergence and evolution of the thin-skinned fold-and-thrust belt in the eastern Sichuan Basin: Insights from numerical modeling

The thin-skinned fold-and-thrust belt with multiple detachments in the eastern Sichuan Basin is characterized by a series of fault-detachment folds with a backward preferred structural vergence, which is rare in nature and can't be explained well by the classic critically taper theory. Here, three discrete element simulations were conducted to study the influence of multiple detachments on the structural vergence and evolution of the eastern Sichuan thin-skinned fold and thrust belt. The experimental results reveal that multiple detachment layers affected the bulk mechanical strength and facilitated a dually vergent thrust wedge with a backward preferred structural vergence. The addition of intermediate detachment layers influences structural vergence and fault activity of thrust wedges. With the increase in intermediate weak layers, the preferred structural vergence of these model results progressively moved from forward to nearly symmetric and eventually to backward. These results exhibit first-order structural similarities to the structural vergence and fault activity in the eastern Sichuan fold and thrust belt, which indicates that the influence of the internal distribution of weak layers within the eastern Sichuan Basin should not be ignored. The limited Cambrian evaporite layer and the widespread Silurian shale and Lower-Middle Triassic evaporite layers had significant impacts on the deformation of the fold-and-thrust belt in the eastern Sichuan basin. Consequently, we propose a kinematic model for the coupled geodynamic processes of the eastern Sichuan fold-and-thrust belt. The framework of the eastern Sichuan Basin with a number of backward fault-detachment folds was formed by Mesozoic northwestward propagation, and Cenozoic eastward propagation reactivated the folding and thrusting. The Huayingshan fault was affected by the limited Cambrian evaporite layer.

Cretaceous stratigraphic framework and sedimentary architecture, Pakistan: signatures of rift–drift tectonics and submarine volcanism along the western continental margin of the Indian Plate

Abstract The lithostratigraphy, biostratigraphy and depositional environments of the Cretaceous successions in Pakistan were investigated to reveal the major geological events, including local, regional and global-scale tectonics, phases of volcanism, basin condensation, emergence of structural highs and episodes of non-deposition. The succession is entirely missing at Salt Range but is excellently exposed in the Kirthar–Sulaiman Fold-Thrust belts. Such deviating nature (e.g. Hazara, Kala Chitta, Surghar and Kohat), suggesting episodical tectonic activities, is associated with inter–intra rifting and drifting of the India–Madagascar–Seychelles and Antarctica–Australian plates during the Cretaceous. Correspondingly, several unconformities within the Cretaceous succession at numerous stratigraphic intervals consistently attest to the effect of tectonics throughout the period. The recognized unconformities over Kawagarh, Lumshiwal, Parh and Fort Munro formations testify to episodic uplifts on local and/or regional scales during the Coniacian–Santonian, Coniacian–Maastrichtian, Campanian and Maastrichtian in response to the separation of Madagascar from the India–Seychelles Plate. In contrast, there was continuous sedimentation of carbonate, clastic and/or submarine volcanics in the Kirthar and Indian–Eurasian suture zone. Significant geological events in the restricted regions, e.g. local-scale submarine volcanism (Bibai Formation), ironstone deposition (Dilband Formation) and basin condensation (Khuzdar), were also associated with such rifting–drifting phases along the WNW margin of the Indian Plate.

Petrography, geochemistry and detrital zircon U–Pb dating of the Pliocene‐Pleistocene Dupi Tila Formation from the Lalmai Anticline, Bengal Basin: Regional tectonic implications

The Bengal Basin is a well‐known foreland basin that archives the tectonic evolution of the Himalayan and Indo‐Burman Ranges (IBR). The Dupi Tila Formation is the youngest among the stratigraphic successions of the Bengal Basin, which witnessed the last phase of the Himalayan uplift. However, there is still controversy regarding sediment provenances since the Himalayan and IBR are in the late Neogene exhumation stage. In this study, the petrographical and geochemical compositions of the Dupi Tila sandstones from the Lalmai Anticline of the Bengal Basin are examined to infer the source rocks, tectonic settings, palaeoclimate and weathering intensity in provenance areas. The detrital modes and geochemical discriminations, as well as Chondrite‐normalized rare earth elements patterns with enrichment in light rare earth elements and nearly flat heavy rare earth elements and negative Eu anomalies (average Eu/Eu* = 0.77) indicate dominantly felsic source rocks. The geochemical data also suggest that these source rocks in provenance areas were influenced by weak to moderate chemical weathering (chemical index of alteration: 65.64–75.62) and multiple recycling under warm and humid to semi‐humid climatic conditions. The detrital zircon ages show three dominant peaks at ca. 100, 500 and 1000 Ma with minor 1700 Ma. The amount of young zircon grains (<120 Ma) is noticeably higher (~18%) in this Lalmai outcrop compared to the Dupi Tila samples from the Sylhet Trough and Chittagong‐Tripura Fold Belt. Results from integrated zircon geochronology and geochemistry indicate that the detritus are likely derived from a recycled orogen provenance related mainly to active continental margin setting with subordinate influence from passive setting. Therefore, the regionally extensive Himalayan Orogen appears to be the primary sediment source, with additional mixing from the IBR and recycled sediments originating from the Gangdese batholith of the Trans‐Himalaya.

Geomechanical modeling of fault-propagation folds: A comparative analysis of finite-element and the trishear kinematic model

Fault-propagation folds are common structures within fold and thrust belts. The trishear kinematic model has been widely used to understand the kinematics and geometry of these folds, effectively reproducing various characteristics. However, the resulting geometry of natural prototypes may diverge from the predictions of the trishear model depending on the rheological properties involved in the deformation. In order to address this limitation, finite element viscoplastic numerical models were implemented. The analysis revealed that in models with a 15° fault angle, these simulations develop a mechanically weaker discontinuity, which is defined as the low viscosity zone (LVZ). The LVZ induces faulting and absorbs slip, causing deviations of velocity vectors from parallel alignment with the main reverse ramp. In models with fault angles set at 25° or 35°, the kinematic vectors of the hanging wall aligned parallel to the ramp, and a zone of progressive rotation of the velocity vectors was observed in the forelimb, resembling the theoretical trishear zone. In these scenarios, the resulting folds exhibited greater symmetry. However, in cover layers with a viscosity equal to 1020 Pa s, the forelimb exhibits the highest velocities, which is attributed to material flow toward the footwall.

Evolution of orogenic gold mineralization in the southern and eastern framing structures of the Siberian craton

The purpose of the research is to compare the evolution of gold ore mineralization in the structures of the Baikal-Patom and Verkhoyansk-Kolyma marginal continental orogenic belts that arose on the passive margin of the Siberian continent in the Neoproterozoic-Paleozoic and Paleozoic-Mesozoic, and hosting the largest gold deposits in Russia. The similarities are noted as they reveal themselves in the characteristics of the mineralization stages, and in the typomorphism of pyrite that is the main mineral of ore formations. The study demonstrated the role of polychronic orogenic events of the Paleozoic and late Mesozoic times, which led to the formation of large fold belts in the development of large-scale gold mineralization of the southern and eastern framing of the Siberian craton. These events caused precious metals to mobilize and redistribute from precious metal-rich stratigraphic horizons as corresponding orogenic plutonometamorphic belts were formed. A comparative analysis of the Paleozoic-Mesozoic and Neoproterozoic-Paleozoic metallogenic profiles of the southern and eastern margins of the Siberian craton demonstrates the influence of the formation processes of orogenic gold mineralization, regardless of the composition of the host strata and mineralization age. General metallogenic characteristics of paleogeodynamic formation settings of noble metal mineralization of the orogenic-type, regardless of its age, can serve as the basis for geological-genetic and forecasting predictions as well as for the development of integrated assessment criteria for their prospects within the regions under investigation.

Concentration and distribution patterns of rare earth elements (REEs) in stream sediments of Pakistan

Currently, the metals of Rare Earth Elements (REEs) have become very critical to various modern technologies ranging from cell phones and televisions to LED light bulbs and wind turbines. This work was carried out as the Pakistani contribution to the National-scale Geochemical Survey of South and Central Asia (NGSSCA) project. The objective of this study was to document and study the amounts and distribution of REEs in stream sediment of Pakistan, where such national-scale geochemical mapping was launched for the first time. A total of 4277 regular and 168 duplicate samples were collected, encompassing approximately 360,000 km2 of Pakistani outcrop regions. The findings from the geochemical analysis of stream sediment in Pakistan revealed a spectrum in the total Rare Earth Elements (ΣREE) concentration, spanning from 10.22 mg/kg to 1403.1 mg/kg, averaging at 132.5 mg/kg. LREEs are relatively enriched compared to HREEs with an average of LREEs/HREEs of 3.08. The distribution patterns of REEs in stream sediment are demonstrated. It's has been concluded that high concentrations of REEs are mainly distributed in the Himalayan fold and thrust belt, the Karakoram terrane and the Kohistan island arc in the northern part of Pakistan. The bedrock mineralogy and lithology significantly affect the distribution patterns of REEs in stream sediments. The national geochemical mapping of Pakistan provides a useful method for locating alkaline igneous rocks and identifying potential REE mineralization.

Whole-Rock Geochemistry and Mica Compositions in Lijiagou Pegmatite Spodumene Deposit, Western Sichuan, China

The Lijiagou pegmatite spodumene deposit, located in the middle of the Songpan–Garze Fold Belt and southeast of the Ke’eryin ore field, is a newly discovered super-large deposit. In order to reveal the metallogenic tectonic environment and evolution process of pegmatite, based on the study of the geological characteristics of pegmatite, we carried out a whole-rock geochemical analysis of Ke’eryin two-mica granite and Lijiagou pegmatite and carried out a detailed electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of mica minerals in each zonal pegmatite. The results show that the Ke’eryin two-mica granite is mainly formed in the transition period from syn-collision to post-collision. After the end of the continental collision, the crust is squeezed and thickened in the post-collision extensional transition tectonic environment. Mica from the microcline pegmatite zone (MP) to the albite spodumene pegmatite zone (ASP) in pegmatite show different compositions and structural characteristics, with the evolution trend in the direction from muscovite to Li-bearing mica. The type of mica from MP to AP is mainly muscovite, and Li-bearing mica appears in ASP, which is secondary and metasomatic at the edge of primary muscovite. From MP to ASP, there was a negative correlation between Nb/Ta, K/Rb and the Li, Rb, and Cs contents of mica, while the contents of Li, Rb, Cs, and F in the Li-bearing mica of ASP increased sharply. This evidence illustrates that the favorable tectonic environment contributed to the formation of the Lijiagou pegmatitic spodumene deposit. Lijiagou pegmatite experienced the magmatic–hydrothermal evolution process and has a high degree of differentiation and evolution from MP to ASP, which gradually increased. Combined with the change in mica type, it is considered that ASP formed from the stage of magmatic transition to hydrothermal and was a hydrothermal environment, and Li, Rb, and Cs mainly began to enrich at the stage of magmatic–hydrothermal transition.

First record of Reticulinella? kaeveri in the Laffan and Ilam formations (Zagros fold and thrust belt, SW Iran): new constraints on the chronostratigraphic calibration of the Arabian Plate sequence stratigraphy

Over the past two decades, sequence stratigraphy has proven to be a valuable tool in addressing challenges arising from the inconsistent application of lithostratigraphic nomenclature across diverse geological provinces. It has been particularly successful in achieving regional correlations within the Cretaceous deposits of the Arabian Plate. The key sequence stratigraphic surface in the currently adopted standard sequence stratigraphic scheme is the K150 SB, marking the transition from a passive to an active margin along the eastern edge of the Arabian plate. Despite its importance, the chronostratigraphic age of this surface is poorly constrained. In the Zagros FTB, the uppermost part of the Sarvak Formation, just below the K150 SB, was recently assigned to the upper part of middle Turonian to upper Turonian, based on the presence of Reticulinella? kaeveri. Our research reveals that the range of R.? kaeveri extends above the K150 SB as this species is also present in the Laffan and base of Ilam formations. The Last Occurrence of this taxon provides a new chronostratigraphic marker of the upper Turonian within the Ilam Formation. This finding proves that more Turonian deposit are present in the study area than was previously recognized. We have studied the deposits enclosing the K150 SB in outcrop sections and wells along a transect from basin to platform margin to inner platform setting. Based on planktonic foraminifera and calcareous nannofossils biostratigraphy of basinal sections and sequence stratigraphic correlation to carbonate platform sections, the age of the uppermost part of the Sarvak Formation, immediately below the K150 SB, is firmly constrained within the interval lower Turonian–lower part of middle Turonian. Sequence stratigraphic correlation from basin to platform interior reveals also that the oldest deposits above the K150 SB are not younger than late Turonian. Our new data represents a significant improvement in the chronostratigraphic calibration of sequence stratigraphy in the Cretaceous of the Zagros fold-and-thrust belt.

Automatic thrust/fault and edge location with gravity data across the Shillong plateau and Mikir hill complex in northeastern India using the most positive and most negative curvature interpretation

Abstract Northeast India encompasses numerous thrusts, faults, and lineaments with undulated surface topography and is one of the utmost tectonically active regions in the world. Owing to the results of the collision of the Indian Plate under the Tibetan Plate and Burmese Plate, respectively, this area has affected the highest seismic potential zone-V, triggering many earthquakes. The current study area is located in and around the Shillong plateau, Mikir Hills, Naga Hills, Arakan-Yoma fold belt, Bengal basin, and Mishmi hills of the Himalayan foothills and that fall under the northeast of India. The thrusts and faults information available in this area are very scanty due to limited availability of geoscientific data and revealing seismic survey. Henceforth, it is necessary to get enhanced geoscientific learning for a better understanding of thrusts, faults, and lineaments information, the most positive and most negative curvature attribute analyses have been carried out using ground gravity data in this area. The significant derived results from this study encourage supplementary findings of thrust, fault, and lineament information, which also correlate well with the previously found results of 3D Euler deconvolution and source edge detection. Although, gravity data interpretation has its own limitations, however, the current derived results using the latest curvature analysis approach utilizing gravity data show realistic invigorated solutions for a better understanding of the thrust, fault, and lineament locations in this area.

Control of décollement strength and dip on fault vergence in fold-thrust belts and accretionary prisms

Fault vergence in fold-and-thrust belts and accretionary prisms is characterized by mainly forward verging thrusts and pop-up structures, and only few examples exist where backthrusting dominates. However, backthrusting and triangle zones are known from most if not all fold-thrust belts in the world. The circumstances under which backthrusts instead of forethrusts form are still incompletely understood. Previous studies suggest that the strength and dip of the basal décollement of Coulomb wedges plays a key role. However, a systematic study of these parameters is still missing. In this study we model numerically the formation of brittle-ductile wedges, systematically, varying dip and strength of the basal décollement. To characterize wedges, we use cumulative vorticity as a measure of dominant fault vergence within the entire wedge. We find that backthrust-dominated wedges form in setups of very low basal dip (≤ 0.5°) and a weak décollement. Increasing décollement strength or basal dip results in the formation of pop-up-dominated wedges, before forethrust dominated wedges form. While our models corroborate the idea that thrust vergence and basal dip may control thrust vergence, comparison with natural examples indicates this cannot be the only explanation for the formation of backthrusts. Consequently, additional factors such as syntectonic sedimentation or changes of décollement rheology across strike may be required to explain backthrusting in fold-thrust belts.

Contrasting Styles of Salt‐Tectonic Processes in the Ionian Zone (Greece and Albania): Integrating Surface Geology, Subsurface Data, and Experimental Models

AbstractThe Ionian Zone (IZ) is one of the key elements of the fold and thrust belt (FTB) of the Albanian and Hellenides orogen and contains large outcrops of Triassic evaporites. The IZ consists of various thrust sheets with a general westward vergence, stacking over the Apulian and Pre‐Apulian zones, and repeating a thick carbonate sequence of Upper Triassic to Eocene age. Thrusting becomes younger toward the west with a piggyback sequence, starting during the latest Oligocene Epoch in the Internal Ionian and ending in the Pliocene in the External Ionian. We have studied the IZ in southern Albania and northwestern Greece using field observations and borehole data and by fully interpreting a recently acquired 2D seismic data set. Our objectives are to establish the geometry and nature of the contacts associated with the major Triassic outcrops, to unravel precursor salt diapirs, and to assess their role during the Alpine contraction. Salt structures include gentle salt pillows, isolated salt plugs and diapirs, thrust welds, and salt walls. Combining these observations with experimental modeling results, we show how these structures control the geometry and kinematics of the Alpine thrusts or the location and kinematics of recent strike‐slip faults. Salt minibasins have also been identified, demonstrating salt mobility conditioned Mesozoic sedimentation in the Ionian Basin. The use of salt‐tectonics principles to evaluate the structural style and evolution of the IZ FTB also opens new directions for interpreting the subsurface structure and evolution of the region.

Investigation of the ground effects of Mw7.8 and Mw7.5 Maraş doublet earthquake on the south-east region (Bİtlİs-Zagros fold-thrust belt) of Turkey

Investigation of the ground effects of Mw7.8 and Mw7.5 Maraş doublet earthquake on the south-east region (Bİtlİs-Zagros fold-thrust belt) of Turkey

Late Quaternary deformation, strain partitioning, and fold-thrust belt widening in the Eastern Himalayan Syntaxis, India

The easternmost Himalaya near the 96°E longitude shows a sharp ~90° orogenic bend known as the Eastern Himalayan Syntaxis. A complicated configuration of the region and related space problems, combined with extreme vigorous erosion and surface processes makes it difficult to understand the strain release pattern, slip rate on the participating thrust/s, and the seismic potential of the geological structures. To understand the deformation pattern in the region, luminescence and radiocarbon dating methods were used on deformed landforms in the frontal Siang and Mishmi ranges of the Eastern Himalayan Syntaxis. We obtained fault slip rates of Late Quaternary period. The Holocene rates range over 7.5 ± 0.2 mm/yr in the northern segment, 1 ± 0.1 to 10.1 ± 1.5 mm/yr in the central segment, and 8 ± 0.5 mm/yr in the southern segment of the Mishmi Thrust, whereas the Late Pleistocene rates are of the order of 1 ± 0.1 mm/yr in the central segment and 0.9 ± 0.1 mm/yr in the southern segment of the Mishmi range. The obtained Late Pleistocene rates are lesser than the Holocene rates, but consistent with exhumation rates reported earlier from the northern segment. Holocene fault slip rates of the range 2–9.1 mm/yr across the Manabhum Anticline suggest foreland strain partitioning due to widening of the fold-thrust belt and the locked zone. A widening of the locking width implies an increase in the seismic potential of the region. We suggest that the Manabhum Thrust is possibly a proto-thrust or else a footwall imbrication of the Mishmi Thrust. Contrastingly, a lesser Holocene fault slip rate of 2.3 ± 0.3 mm/yr obtained from the eastern Siang range along the Main Boundary Thrust indicates considerable strain partitioning in the hinterland. Results demonstrate that in tectonically active regions, representative rates of rock uplift inferred from incised bedrock should be integrated over at least one complete seismic and inter-seismic cycle.

Primary source of titanomagnetite ores at the Orokolo Bay placer deposit: Implications for petrogenesis

AbstractCoastal and river sand sequences of Holocene age at Orokolo Bay in Papua New Guinea are host to secondary titanomagnetite ore deposits. The resource developer (Mayur Resources) declares a measured orebody of 139 million tons at grades of 11.35% Fe, 1.94% Ti, and 712 ppm Zr. Orokolo Bay is located near the boundary between the Papuan Thrust and Fold Belt and the Aure Fold Belt. This region is characterized by sedimentary rocks of the Papuan Basin, which are intruded and overlain by a diverse array of magmatic intrusive and volcanic rocks, ranging from mafic alkaline to felsic calc‐alkaline types related to arc–continent collisions in the Cenozoic. The Vailala and Purari rivers located to the east and west of Orokolo Bay, respectively, are the main sediment pathways. Earlier provenance studies have suggested that the deposited sediments mainly come from the erosion of volcanic rocks found within the catchments of these two rivers. However, these studies did not specifically identify the primary source of the Fe‐Ti ores within this region. Here, we report on a potential primary source of the Fe‐Ti ores by examining the occurrence and chemical compositions for detrital titanomagnetites, and associated pyroxene and amphibole minerals. The purpose of this was to discern specific attributes of the primary source for its identification. Lithic sediments comprised of magnesio‐hornblende (Mhbl), tschermakaite (Tsr) and magnesio‐hastingsite (Mhst) amphiboles, clinopyroxene (Cpx), and Fe‐Ti oxide minerals dominate the heavy sands. Feldspar is more abundant than quartz, and Cr‐spinel is rare. Two primary titanomagnetite types were categorized based on their homogeneous grain textures and TiO2 content, Tmt1 (average 28 wt% TiO2) and Tmt2 (average 7 wt% TiO2). Pleonaste exsolutions and zircon inclusions distinguish Tmt1 from Tmt2, while apatite and quenched glass inclusions occur in both. A portion of both primary titanomagnetites exhibits hydrothermal overprinting, this subset was categorized as Tmt3 (average 12.5 wt% TiO2). Fe‐Ti oxides are associated with Cpx and amphiboles as inclusions, and as pseudomorphs in opacitic amphiboles. Tmt1 coexists predominantly with Mhst‐Tsr but also appears less frequently with Mhbl and Cpx. Tmt2 occurs almost exclusively with Mhbl and Cpx. Titanomagnetite chemistry is low in Cr, Mg, and V content suggesting crystallization from evolved melts. Tmt3 is shows enriched Si, Ca, Al, and Mg concentrations that are characteristic of late‐stage fluids derived from calcareous rocks. Amphibole and Cpx chemistry indicate they were derived from a subalkaline source magma of arc affinity. Fe‐Ti thermobarometry reveals Tmt1 (990 to 967°C, −0.51 to −0.18 ΔNNO) formed at higher temperatures and in less oxidized conditions than Tmt2 (834 to 689°C, 0.33 to 3.2 ΔNNO). Amphibole thermobarometry reveals Mhst‐Tsr and Mhbl formed under similar conditions as Tmt1 and Tmt2, respectively. The lack of Cr‐spinel, olivine, and the abundant occurrence of zircon and Mhbl indicate an evolved subalkaline source characterized by felsic rocks related to arc magmatism. The only volcanic source which aligns with these characteristics is Mount Yelia within the Vailala River catchment. Compositional variation between Tmt1 and Tmt2 titanomagnetites and associated silicate minerals is attributed to heterogeneous temperature and ƒO2 conditions within the melt during crystallization. The provenance of Ti‐rich titanomagnetite ores from sources related to arc magmatism indicates the potential of other areas associated with arc activity in Papua New Guinea to host secondary deposits rich in Fe‐Ti ores.

Geochemical exploration and baseline from marine sediments in the fjords, channels and straits of Magellan, Chilean Patagonia

AbstractChilean Patagonia has a complex geological history dominated recently by Quaternary glaciers and rivers that have carried sediments from the mountains to depositional environments within marine fjords and channels. Although much is known about onshore placer deposits, little is known about these deposits and depositional systems in the offshore realm. This study analyzes the chemistry and mineralogy of marine sediments collected in the Strait of Magellan and around the region. This study aims to comprehend the distribution, concentration and provenance of critical/strategic elements and precious metals. In collaboration with the Cabo de Hornos research vessel of the Chilean Navy, 63 sediment samples were collected using a box core sampler, complemented with coastal beach samples. These samples underwent petrographic and geochemical analysis. The predominant opaque minerals identified in these samples are mainly iron oxides (magnetite–hematite‐ilmenite) and sulfides (pyrite, exceptional greigite). A comparison of the average element concentration for all samples for different baseline values revealed the following findings. As, Bi, Li, Re, Sb, Se, Te, and Pd exhibit positive anomalies for the Upper Continental Crust (UCC); Re, Se, and Sr. exhibit positive anomalies for Marine Sediments (MS), and Bi, Cs, Li, Re, Se, Sn, and Te show positive anomalies relative to the Folded Belt (FB). The main contrast anomalies occur in the following locations. Almirantazgo Sound (Sn, Ba, Sb, and Tm); Concepción Channel (Re, Sr., and Te); Smyth Channel (Mn, Re, Sb, Sr., Te, and Se); Almirante Montt Gulf (Mn, Re, and Sb); Nelson Strait (Re, Sr., and Te); Ancho Pass (Ti and Sr); Europa Fjord (Bi and Sn); Del Hambre Pass (Sr); Magdalena Channel and Sound (Sr); Froward Pass (Se), and Whiteside Channel (Mn). Regarding precious metals, silver exhibits contrasting anomalies relative to the UCC in seafloor sediments in the Almirante Montt Gulf (0.161 ppm) and Concepción Channel (0.101 ppm). Palladium contrast anomaly is observed in the Peel Estuary (8 ppb) and Platinum contrast anomalies occur in the Concepción (3 and 5 ppb) and Smyth Channels (4 ppb). Finally, gold, in comparison to the South Patagonian Batholith, reveals contrasting anomalies that occur in the Concepción Channel (8 ppb), Peel Estuary (6.7 and 8 ppb), Nelson Strait (7 ppb), Almirante Montt Gulf (6 ppb), Forward Pass (8 ppb), and Ancho Pass (7 ppb). In summary, the presented results provide valuable insights into the geological and geochemical baseline of the region, offering a better understanding of the relationship with depositional basins and source zones onshore during the late‐Quaternary.

Deformed Latest Pleistocene Fluvial Terraces Reveal Complex Active Faulting within Tectonic Transition Zone, Mendocino Triple Junction, Northern California

Abstract The transition between the San Andreas fault (SAF) system and the southern Cascadia subduction zone (CSZ) at the Mendocino Triple Junction (MTJ) encompasses a broad zone of complex deformation, the Mendocino deformation zone. Here, there are discrepancies between types of geological structures (transform or thrust faults) and recorded geodetic velocity vectors of plate motion. Though SAF-type stress is recorded north of the MTJ, there has been little geological evidence for resultant strain at these latitudes on the coast. We focus on the Van Duzen fault (VDF)—a possible subsidiary fault of the Little Salmon fault system, one of the southernmost active thrust faults within the onshore fold and thrust belt associated with CSZ. The VDF deforms young river terraces of varying age, which we use to develop a relative-age framework to contextualize activity along the VDF. Geomorphic analysis and a paleoseismic excavation across the VDF display deformation attributed to compressional stresses, which postdates young (3–11 ka) terrace deposition, at roughly 0.06–0.38 mm/yr. We hypothesize that the transition between CSZ and SAF tectonic regimes is geologically manifest through the orientation of compressional structures (i.e., VDF), which may illuminate dynamics associated with the migrating triple junction, past and present.

Lithostratigraphy and biostratigraphy of the Upper Cretaceous limestones of Bjelopavlići (Montenegro): contribution to evolution and paleogeography of the Adriatic Carbonate Platform

Abstract The Upper Cretaceous limestones of Bjelopavlići (Montenegro) represent a megasequence of facies reflecting a transition from shallow- to deep-water depositional environments, succeeded by re-establishing shallow-water conditions again. The studied carbonate sequences are exposed on the south-eastern edge of the Adriatic Carbonate Platform (AdCP) in central Montenegro. Based on coupled sedimentological and biostratigraphical studies, the associations of eight spatially and temporally related facies were identified. The lower part of the megasequence comprises shallow-water carbonates originating in the inner-platform area, while the middle part is composed of pelagic and allodapic limestones. Shallow-water platform limestones occupy the upper part of the megasequence. The shallow-water carbonates of Bjelopavlići show a good correlation with the shallow-water Upper Cretaceous sediments in the AdCP. Deep-water limestones of Bjelopavlići are regarded as facies correlative to the spatially distinct deep-water limestones of Brač and as temporary and spatially correlative with slope-to-basin facies derived along the north-eastern margin of the AdCP. The drowning of the platform periphery and the evolution of a deep-water basin where the pelagic and allodapic carbonates of Bjelopavlići were deposited, as well as the subsequent exhumation of the platform are predominantly attributed to tectonics. The uppermost Cretaceous dynamics of the basin were controlled by tectonic mobilization of the foreland in front of regional fold-thrust belts that developed north-eastern from the AdCP.