Rift Research Articles

Lower Cretaceous volcanic-sedimentary successions of the continental rift basin in the Songliao Basin, northeast China: Implication in high-quality reservoir prediction and hydrocarbon potential

The high-quality reservoirs of volcanic-sedimentary successions in faulted lake basins are important enrichment sites for deep natural gas reservoirs. However, complex volcanic-sedimentary lithology and reservoir characteristics impede the efficient prediction of hydrocarbon in these successions. Therefore, we study the volcanic-sedimentary succession of the Huoshiling Formation (northern Dehui depression, Songliao Basin), focusing on the volcanic-sedimentary stratigraphic cycle (VSC), facies models and the distribution of high-quality reservoirs. Based on the log, core, casting thin-section, seismic and other data, fissure-type and central-type volcanic eruptions are identified. Three VSCs (1–3) are recognised, and six volcanic-sedimentary facies are determined based on lithological characteristics: effusive, explosive, volcanic conduit, volcanic-sedimentary transition, fan-delta and lacustrine. Based on the flow regimes and lithological associations, we develop three models: a fissure-central volcanic effusive model, an intermittent volcanic activity model and a fissure-central explosive model. Three types of pores occur in the volcanic-sedimentary reservoir space: primary pores, dissolution pores and micro-fractures. Lithology and dissolution are critical factors controlling reservoir porosity and connectivity. The distribution of high-quality reservoirs of volcanic-sedimentary successions can be effectively predicted using geostatistical seismic-inversion based on porosity constraints. High-quality reservoir characteristics, source rock and structural characteristics show that crystal debris and breccia-bearing tuff in VSC3 can form structural-lithologic, fault-block and self-generating and -storing hydrocarbon reservoirs. This study's results and methods of high-quality volcanic-sedimentary reservoirs can provide a reference for the efficient exploration and development of oil and gas in similar basins.

Diffuse soil CO2 emissions at rift volcanoes: Structural controls and total budget of the Olkaria Volcanic Complex (Kenya) case study

Continental rift systems are first-order emitters of deep-sourced CO2 but their impact on the global budget is largely unknown. For instance, the estimate of the total amount of deep-sourced CO2 released by the East African Rift (EAR) is largely undocumented due to the small number of direct measurements available and the impracticability of remote sensing methods induced by atmospheric noise. This study explores the degassing budget and the structural control of soil CO2 emissions at an active rift volcano aiming to implement the database of available direct measurements of EAR's volcanoes. We investigated soil CO2 degassing at the Olkaria Volcanic Complex, a large, multicentred, geothermally exploited volcanic complex in the southern portion of the Kenyan Rift. A total of 1158 soil CO2 flux measurements were collected on fumarolic fields or crosscutting unaltered major faults. The contribution of multiple sources of CO2 (biogenic background vs magmatic/hydrothermal) was tested using both graphical statistical analysis and the carbon isotopic signature (13C-CO2). Soil CO2 fluxes reached up to ∼5500 gm-2d-1. The emission of deep-sourced CO2 coupled with the structurally controlled hydrothermal fluid circulation, sums up to ∼280 tCO2 released per day by the entire complex. Finally, we provide an estimate of the total budget for soil CO2 of the rift, summing up the contributions of all volcanoes in the EAR.

The ocean-continent transition in the Western Central Red Sea

The Red Sea is an important example of a continental rift transitioning slowly to an oceanic basin. However, structures that can inform us of how that transition occurred have been poorly reported because deep seismic reflection data capable of imaging basement under the rift sediments are generally lacking publicly. Three lines of multichannel seismic reflection data have recently been published revealing structures on the Nubian side of the central part of the basin. In this study, we reassess these data in the light of recent studies of the central Red Sea. Over continental crust, the data reveal reflection sequences likely due to strata at or near the base of the evaporites, in two cases with varied dips suggesting the presence of syn-rift growth stratigraphy. Almost all of those reflections dip downwards towards the rift axis, not away as would be expected from tilted fault blocks of bookshelf faulting types. That observation, and low relief of basement, confirm inferences made earlier based on gravity anomalies that this part of the Red Sea lacks large-relief fault escarpments and is most likely a syn-rift sag basin. In the transition to oceanic crust, an abnormally broad magnetic anomaly of estimated Chron 5 age is found not to be associated with structures such as sills, so it likely arises from deeper sources. One of the seismic lines traverses a ridge in Bouguer gravity anomalies that runs across the axis. This feature has previously been interpreted as a volcanic ridge similar to those observed at other ultra-slow spreading ridges. The seismic data reveal diffuse basement reflections and confirm that the record immediately above basement lacks reflections typical of sedimentary strata. Both observations are consistent with the presence of oceanic crust. Modelling of gravity anomalies suggests the ridge is likely underlain by igneous intrusive rocks displacing mantle rocks, as expected for a volcanic ridge. The seismic data, combined with recently updated multibeam and high-resolution sparker seismic results, further suggest how the evaporite movements have been modulated by basement topography. These results add to our knowledge of the evaporite movements and continent-ocean transition structures in the central Red Sea.

The evolution of continental rifts revealed from the numerical modeling: A case study of the Baikal rift system, Siberia

We present new results on the Late Cenozoic uplift rates at the flanks of the Baikal rift system (SE Siberia) based on the morphotectonic analysis of basin-adjacent steep-slope mountains and numerical modeling of landscape evolution. As the basic instrument of modeling, we used the CHILD software program by Dr. Greg Tucker (University of Colorado, USA) which allows reconstructing the relief evolution taking into account various orogenic processes. The model problem of the normal-fault movements was solved using the CHILD modification: adding a new tectonic option and developing some adaptations. The objects of modeling were steep normal fault escarpments with clearly defined triangular facets and without young tectonic steps. We assumed that these objects were formed at the fast-rifting stage and may thereby be considered as morphometric indicators of orogenic processes of the Late Cenozoic time. The experiment consisted of two stages. First, we determined a specific set of exogenous parameters which may be conditionally considered general for all similar escarpments of the BRS and for the Late Cenozoic era as a whole. As a reference object there was used one of segments of the Barguzin Ridge with the known uplift rate obtained by AFT. Next, we modeled some other objects from different mountains of the BRS. The results of modeling showed that the approximated Late Cenozoic uplift rate on the considered segments varies over a small range between 0.3 and 0.5 mm/yr. At the final stage, we performed morphotectonic analysis to determine probable uplift rates for steep rift shoulders.

Impact of Mexican Border rift structural inheritance on Laramide rivers of the Tornillo basin, west Texas (USA): Insights from detrital zircon provenance

Abstract Late Cretaceous to Eocene Laramide basement–involved shortening fragmented the Sevier and Mexican foreland basins. This resulted in a major drainage reorganization in response to the emerging topography of Laramide basement–cored uplifts and Mexican inverted Border rift basins. This study presents new depth-profile detrital zircon U-Pb data (3679 ages from 28 samples) from Upper Cretaceous–Eocene fluvial strata of the Tornillo basin in west Texas to determine sedimentary provenance and reconstruct sediment dispersal through the U.S.-Mexico border region. Detrital zircon U-Pb data are dominated by Hauterivian–Coniacian (130–87 Ma; ~20%) and Coniacian–Ypresian (87–52 Ma; ~30%) ages that represent Cordilleran and Laramide arc magmatism, respectively. Subordinate age groups are Paleoproterozoic–Mesoproterozoic (1900–1300 Ma; ~12%), Ectasian–Tonian (1300–900 Ma; ~8%), Tonian–Pennsylvanian (900–300 Ma, ~10%); Permian–Triassic (300–200 Ma; ~8%), and Jurassic–Early Cretaceous (200–130 Ma; ~11%). Detrital zircon maximum depositional ages provide new constraints on the chronostratigraphic framework of the Tornillo Group, the stratigraphic nature of the Cretaceous-Paleogene boundary, and the stratigraphic level of the Paleocene–Eocene thermal maximum. Depth-profile core-rim age pairs yielded Paleoproterozoic–Mesoproterozoic and Jurassic cores with Cretaceous–Paleogene rims, which represent zircons derived from Laramide magmatic rocks that intruded Yavapai-Mazatzal basement and Cordilleran-Nazas magmatic rocks. Zircon grains with Ectasian–Tonian cores and Paleozoic rims likely represent Appalachian-derived and/or Coahuila terrane zircons recycled from the inverted Mesozoic Bisbee basin and Chihuahua trough. These results demonstrate that fluvial strata in the Tornillo basin were sourced from Laramide and Cordilleran magmatic rocks, Yavapai-Mazatzal basement, and recycled Mexican Border rift sedimentary rocks in the southwest United States and northern Sonora, and these sediments were delivered via a large (>103-km-long), axial-trunk river. Additional recycled detritus from Mexican Border rift sedimentary rocks in the Chihuahua fold belt was delivered via transverse tributaries. This drainage reconstruction indicates that the Tornillo river flowed along an inversion-flank drainage corridor adjacent to topography formed by the inverted Mexican Border rift. Therefore, inherited Mexican Border rift architecture represented a first-order control on sediment routing to the Tornillo basin.

Late Permian‐Early Triassic intracontinental tectonic inversion in the Junggar Basin, NW China: New insights from detrital zircon geochronology and seismic reflection data

AbstractThe Junggar Basin is located on the southwestern margin of the Central Asian Orogenic Belt (CAOB). Whether the Late Permian‐Early Triassic tectonic inversion there recorded the final closure of the North Tianshan Ocean or post‐accretionary intracontinental deformation remains controversial. Linking the structural style and provenance analysis of the western and northern margins of the Junggar Basin can provide a better understanding of this tectonic event and its geodynamic mechanisms. Seismic reflection profiles show that Early Permian syn‐rift half‐grabens were followed by the Middle Permian thermal sag, which is characterized by regional onlap and the migration of the depocentre to the centre of the basin. Together with the published isopach and palaeogeography maps in the western margin of the Junggar Basin, the seismic profiles demonstrate that the reactivation of the Ke‐Bai and Wu‐Xia dextral transpressive fault zones between the West Junggar terrane and the Mahu sag controlled the tilting and deformation of pre‐Permian strata and the distribution of Late Permian‐Early Triassic fan deltas. The reported igneous and sedimentological evidence indicates that the southern margin of the Junggar Basin was a rift basin controlled by transtensional strike‐slip faults in the Early Permian, and also was followed by a Middle Permian thermal sag. Quantitative provenance analysis using detrital zircon geochronology and the DZmix program shows that the West Junggar terrane and Tianshan orogenic belts experienced varied uplift, indicative of a transition from the Middle Permian thermal sag peneplanation to the Late Permian‐Early Triassic tectonic inversion involving reactivation of Early Permian normal faults. This intracontinental deformation event in the Junggar Basin was taken up by block counterclockwise rotation during the final amalgamation of the Pangea, which may be the long‐range effect of the final closure of Paleo‐Asia Ocean in the eastern part of the CAOB.

Lomonosov Ridge Composite Tectono-Sedimentary Element, Arctic Ocean

Abstract The Lomonosov Ridge (LR) is a sliver of continental crust, a microcontinent that stretches across the Arctic Ocean and separates the Mesozoic Amerasia Basin from the Cenozoic Eurasia Basin. The LR is thought to have been separated from the northern Barents Sea–Kara Sea passive continental margin by continental rifting at about 56–53 Ma and the subsequent formation of the oceanic crust in the Eurasia Basin. Following the nomenclature of this volume, the sedimentary succession of the LR and nearby area forms a composite tectono-sedimentary element containing an assemblage of the following main tectono-sedimentary elements (TSEs): (1) an undefined group of pre-rift TSEs mostly composed of Mesozoic and poorly known Paleozoic successions; (2) the Cretaceous–Paleocene synrift TSE; and (3) the Eocene–Holocene post-rift TSE. The age of the LR's folded and metamorphosed basement can be inferred based on correlations to basement units of the northern Eurasian passive continental margin, which may include a variety of geological structures formed during Timanian, Caledonian, late Paleozoic and Mesozoic orogenic events. The pre-rift TSE includes the oldest and most poorly known offshore sedimentary succession in the Arctic region. The lithostratigraphic units of these basins were mainly characterized from a handful samples collected at the seafloor and by correlation with better known stratigraphy of the conjugate continental margin. The synrift TSE of Cretaceous–Paleocene age has been defined based on seismic data and samples of sedimentary rocks recovered during the Arctic Coring Expedition (ACEX), as well as from seismic facies correlation between the LR and the Podvodnikov–Makarov, Laptev and East Siberian Sea basins. The post-rift Cenozoic sedimentary record was documented by the ACEX well in the central part of the LR.

Deep electrical structure over the Paleoproterozoic intracratonic Kaladgi rift basin in southwestern India imaged from magnetotelluric studies

The disintegration of the Columbia supercontinent during the late Paleoproterozoic generated major rift basins in the constituent continental fragments. The Kaladgi basin, located between the southern part of the Deccan volcanic province (DVP) and the northern part of the Dharwar craton, is a Columbia rift-related basin in southwestern India that preserves a complex history from initial fault-controlled mechanical subsidence during rifting, thermal subsidence along a collision zone, crustal thinning due to stretching and erosion associated with doming. The Paleoproterozoic basins worldwide show higher uranium concentration and many deposits are also established in the Purana basins of India. In the present study, the lithotectonic architecture of this basin using broadband magnetotelluric (∼320 Hz–3000 s) soundings in the western segment of the Kaladgi rift basin along two profiles. Two-dimensional (2-D) inversion of data using a 2-D nonlinear conjugate gradient algorithm along both profiles provides insights into the deeper structure of the basin. Our results reveal a thin sheet of Deccan volcanic, sedimentary successions belonging to the Badami and Bagalkot groups, and Proterozoic sediments from top to bottom beneath this basin. The crustal structure is highly heterogeneous and associated with deep-seated faults, and its thickness increases from the eastern Dharwar craton (∼30 km) to the western Dharwar craton (∼45 km). The crustal conductors are interpreted as mafic intrusions derived from the underplated basalts. The moderate conductive features may correspond to carbonate fluids trapped within the faults/fractures zone during basin initiation. The conductive features in the lower crust and the Moho are interpreted as fluids derived from underplated intrusions through plume impact. The NNW trending Chitradurga Suture Zone (CSZ) signature and the Bababudan-Nallur Shear (BNS) in the crust and upper mantle depth are imaged along both MT profiles. This study provides insights into the lithology and tectonic architecture of a long-lived rift basin involved in multiple tectonic events from the late Paleoproterozoic to the late Cretaceous.

Tectonic features, genetic mechanisms and basin evolution of the eastern Doseo Basin, Chad

The features of the unconformity, fault and tectonic inversion in the eastern Doseo Basin, Chad, were analyzed, and the genetic mechanisms and basin evolution were discussed using seismic and drilling data. The following results are obtained. First, four stratigraphic unconformities, i.e. basement (Tg), Mangara Group (T10), lower Upper Cretaceous (T5) and Cretaceous (T4), four faulting stages, i.e. Barremian extensional faults, Aptian–Coniacian strike-slip faults, Campanian strike-slip faults, and Eocene strike-slip faults, and two tectonic inversions, i.e. Santonian and end of Cretaceous, were developed in the Doseo Basin. Second, the Doseo Basin was an early failed intracontinental passive rift basin transformed by the strike-slip movement and tectonic inversion. The initial rifting between the African and South American plates induced the nearly N-S stretching of the Doseo Basin, giving rise to the formation of the embryonic Doseo rift basin. The nearly E-W strike-slip movement of Borogop (F1) in the western section of the Central African Shear Zone resulted in the gradual cease of the near north-south rifting and long-term strike-slip transformation, forming a dextral transtension fault system with inherited activity but gradually weakened in intensity (interrupted by two tectonic inversions). This fault system was composed of the main shear (F1), R-type shear (F2-F3) and P-type shear (F4-F5) faults, with the strike-slip associated faults as branches. The strike-slip movements of F1 in Cretaceous and Eocene were controlled by the dextral shear opening of the equatorial south Atlantic and rapid expanding of the Indian Ocean, respectively. The combined function of the strike-slip movement of F1 and the convergence between Africa and Eurasia made the Doseo Basin underwent the Santonian dextral transpressional inversion characterized by intensive folding deformation leading to the echelon NE-SW and NNE-SSW nose-shaped uplifts and unconformity (T5) on high parts of the uplifts. The convergence between Africa and Eurasia caused the intensive tectonic inversion of Doseo Basin at the end of Cretaceous manifesting as intensive uplift, denudation and folding deformation, forming the regional unconformity (T4) and superposing a nearly E-W structural configuration on the Santonian structures. Third, the Doseo Basin experienced four evolutional stages with the features of short rifting and long depression, i.e. Barremian rifting, Aptian rifting–depression transition, Albian–Late Cretaceous depression, and Cenozoic extinction, under the control of the tectonic movements between Africa and its peripheral plates.

Spatio‐Temporal Variations in Sediment Delivery as a Response to Rapid Quaternary Climate Change in the Lake Malawi Rift, East Africa

AbstractThe interplay of rapid climate change and tectonics drives landscape development, sediment routing, and deposition in early‐stage continental rift systems. The Lake Malawi Rift, in the Western Branch of the East African Rift, is an archetype of a juvenile rift and an ideal natural laboratory for evaluating lacustrine source‐to‐sink systems on orbital or shorter timescales. We examine the interplay of these processes over the past 140 kyr using observations from nested seismic reflection data sets tied to scientific drill cores, which calibrate numerical forward models of this closed sedimentary system. Fault slip rates measured from seismic data drive tectonic displacements in the model. Satellite‐derived precipitation maps constrain modern precipitation and are scaled to previous hydrologic balance studies to reconstruct past climates. Our model reproduces known sediment thicknesses across the rift and accounts for 96% of the estimated siliciclastic sediment deposited over the past 140 kyr. The results demonstrate that the onset of arid climate conditions (140–95 kyr BP) causes extreme drainage adjustments downstream and the formation of mega‐catchments that flow axially into a shallow restricted paleo‐lake. Sedimentation rates during this time are twice the present values due to increased sediment focusing via these axial systems into a much smaller, hydrologically closed lake. As the climate became wetter (95–50 kyr BP), the lake rapidly expanded, decreasing both erosion and sedimentation rates across the rift. This closed‐loop approach allows us to evaluate the role of high‐frequency climate change in modulating basin physiography as well as sediment fluxes in juvenile rift systems.

Basin filling and magmatic response to the migration of two-stage rifts: New insights into the late Neoproterozoic tectonics of the northern Tarim Craton

The spatiotemporal evolution of late Neoproterozoic sedimentary basins and their tectono-magmatic context provide essential information regarding the continental rift process of the Tarim Craton during the fragmentation of the Rodinia supercontinent. However, the incomplete exposure of strata and magmatism in the distal continental margin hinders the understanding of different rift phases that trigger continental breakup. The Sawapuqi region is currently located at the northernmost margin of the Tarim Craton and presents an uninterrupted sedimentary sequence and volcanic rocks from the latest Precambrian to the Cambrian period. Herein, we perform an integrated analysis of the Sawapuqi sub-basin and provide critical insights into the Ediacaran stratigraphic and tectono-magmatic evolution that shed light on the final rifting phase. The Sawapuqi sub-basin preserves regressive and transgressive successions, ranging from thick-bedded siltstones and siliceous rocks at the lower unit of the Sugetbrak Formation, through shore gravel-bearing laminated sandstone accumulation (upper unit), and restricted platform stromatolitic carbonates (Qigebrak Formation), to Cambrian mature passive margin sedimentation. An alkaline basaltic layer is interbedded within the coarsening-upward sedimentary packages of the early Ediacaran, recording the final rift-related magmatic pulse in the northwestern Tarim margin. The geochemical signatures of these basalts suggest a relatively shallow-depth derivation from the enriched mantle reservoir in an intraplate setting. The detrital zircon provenance of the coarse-grained siliciclastic to pebbly feldspathic debris sandstone sequence of the upper Sugetbrak Formation indicates the hybrid sediment infills from local metamorphic basements and Neoproterozoic igneous rocks, with progressively increased contribution from the coeval granites in the northern periphery of the Tarim Craton. The stratigraphic evolution from syn-rift clastic rocks to post-rift carbonates and the transition of detrital provenance observed in the Sawapuqi sub-basin suggest the development of a breakup succession at the end of the early Ediacaran period. The margin-localized continental extension and mantle-derived volcanism during the early Ediacaran differed from the distributed extension and bimodal magmatism during the Cryogenian, indicating two-stage extensional modes and subsidences prior to the lithospheric breakup. The rift system of the latter stage, which migrated oceanwards, eventually led to the continental breakup in the northwestern margin of the Tarim Craton. Comparable late Neoproterozoic stratigraphy and magmatic pulses between the Tarim Craton and Yili-Central Tianshan Block (YCTB) support their reconstructions as two conjugate rifted margins. This study indicates that the rift-drift tectonics triggered by this prolonged episodic rifting between the Tarim Craton and the YCTB was closely associated with the final fragmentation of Rodinia.

Hydro-geochemistry and Related Processes Controlling the Composition of Thermal Waters in the Lake Natron Basin, Northern Tanzania

Lake Natron Basin (LNB) forms part of the eastern branch of the East African Rift System (EARS) in Tanzania. The basin is endowed with thermal springs flowing towards the lake from the western, northwestern, southwestern and eastern parts. The western, northwestern and south-western thermal springs emanate from fractures in the basaltic rocks while in the east, they originate from the western flank of the Gelai volcano. There are limited studies on these springs, thus their geochemical differences and distributions of physicochemical parameters are not well understood. A hydrogeochemical study was conducted to interpret available geochemical data from the LNB springs, including cold and thermal water to allow their geochemical characterization. Results have shown that LNB water is dominated by sodium (Na+), chloride (Cl-) and bicarbonate (HCO3-) ions. Three hydro-chemical facies have been identified forming Na-Cl, Na-HCO3 and Ca-HCO3 water types. This study has also revealed that thermal water in the north-western part of the basin is highly mineralised and gradually becomes diluted toward south-west and south due to groundwater incursion. Water-CO2-rock interaction affects the overall chemistry of thermal water leading to HCO3- water, particularly in the east of the basin. While few springs from the western side of the basin indicated maturity of the thermal waters, other springs indicated mixed Cl-HCO3/peripheral water; while the cold waters are HCO3 type. It is recommended to take precautions when planning geothermal projects to avoid corrosion and scaling of production facilities. Similarly, mixing or dilution of thermal water with shallow groundwater can affect the temperature and the composition of Cl-.

Landscape evolution modeling of the normal-fault scarps (Baikal rift system): new experience

The paper considers a new method for determining the quantitative parameters of the relief-forming processes characteristic of the mountainous framing of the basins of the Baikal Rift System (BRS), based on the use of a computer program for numerical simulation of the evolution of the relief CHILD. Particular emphasis was placed on the calculation of approximate values of the rate of tectonic uplift of the near-fault block structures of the Baikal rift. The novelty of the study lies in the adaptation of the CHILD program for solving fault tectonics problems, as well as in the development of a morphotectonic analysis algorithm aimed at searching for specific geomorphological objects that can be considered as uplift indicators. As a result of a series of experiments performed using radioisotope data, three-dimensional models were obtained that simulate the development of near-fault slopes at the Late Cenozoic stage of the development of the BRS, and the probable rates of vertical movements along the faults of the region were determined. It was found that the average Late Cenozoic uplift rates vary within a narrow range of 0,3–0,5 mm/yr, with higher values associated with the northern and northwestern flanks.

The Maracangalha injectite complex: An overlooked hydrocarbon play in the Lower Cretaceous Recôncavo Basin, NE Brazil

The Recôncavo Basin is the third major natural gas producer in Brazil. Lower Cretaceous sandstones of the Maracangalha Formation (Caruaçu and Pitanga Members) hold the largest volumes of non-associated gas in this basin. However, these reservoirs are extremely complex due to a range of soft-sediment deformational process that modify the depositional architecture and impact on the original structure of the porous system. The cause of soft-sediment deformation is still contradictory because both mass movements and post-depositional fluidization has been proposed by previous studies. This uncertainty has drastic implications for reservoir predictability in the subsurface since mass-transport deposits and fluidization-remobilization complexes have very different architectures. To test these hypotheses, this study examines soft-sediment deformation structures (SSDS) in outcrop and core in order to interpret remobilization processes and to infer the main trigger mechanisms involved. Outcrop description was carried out in the coastal zone of the Todos os Santos Bay, NE Brazil, where the Maracangalha Formation is very well exposed. High-resolution core images from four producing fields located northward (Miranga, Jacuípe, Lamarão and Massapê) were also described. SSDS include a great variety of structures: 1) dish, 2) pillar, 3) consolidation lamination, 4) load structures, 5) tilted, convoluted, distorted and disrupted laminations, 6) flames, ball and pillow structures and pseudonodules, 7) large-scale mounded geometries, 8) penecontemporaneous faults and 9) sandstone intrusions/extrusions (dikes, sills, breccias and volcanoes). Intensely deformed sandstones are connected to sandstone intrusions that cut through finer-grained, low-permeability strata. Orientation of normal faults and sandstone intrusions suggests that they are genetically related. The Pitanga and Caruaçu Members are better described as facies, since they differ only by the degree of deformation. Pitanga facies is the product of remobilization of an undeformed (Caruaçu) facies. Unlike previous interpretations that indicated the action of slumps, we concluded that the upward movement of interstitial fluids (liquefaction and fluidization) was the main mechanism of remobilization, deformation and mixing of sediments. The overpressure of pore fluids and syn-rift faults propagation broke up seal-strata, releasing the fluids pressure and driving the upward movement of sediments. Key architectural elements of a sand complex are recognized in outcrop and core, including parent units, host-strata and sandstone intrusions. For the origin of this injectite complex we suggest a seismic trigger related to extensional tectonics of the rift system. The Maracangalha injectite complex comprises a new hydrocarbon play to be considered in further exploration and production (E&P) activities in the Recôncavo Basin.

Early Permian passive-margin magmatism in the Sibumasu Block, northern Thailand: A product of the slab-pull force of the Paleo-Tethys Ocean?

Continental rift-related magmatism provides important clues to continental rifting and breakup, but the petrogenesis and geodynamic processes remain debatable. Here we report a detailed study on zircon and apatite UPb dating, whole-rock geochemistry, and Sr-Nd-Hf isotopes for the hornblende gabbros in northern Thailand. Zircon and apatite UPb age data reveal emplacement of the hornblende gabbros at ca. 290 Ma. The hornblende gabbros are alkaline in composition and characterized by slight negative Nb-Ta-Zr-Hf and marked negative Ti anomalies. They have enriched Sr-Nd-Hf isotopic values [whole-rock (87Sr/86Sr)i = 0.705970–0.706756; εNd(t) = +0.1 to +0.7; zircon εHf(t) = +0.2 to +9.2]. Their geochemical affinities to within-plate basalts suggest a continental rifting setting. These combined features reveal that the hornblende gabbros in northern Thailand originated from a subcontinental lithospheric mantle previously modified by subducted sediments. Our results combined with published data indicate that the continental rift-related magmatism in the Sibumasu Block took place over a long period of ca. 309–282 Ma. We suggest that the slab pull force of the Paleo-Tethys oceanic lithosphere is a likely mechanism for the Late Carboniferous-Early Permian magmatic event in the passive margin of the Sibumasu Block.

Insights into initial continental rifting of marginal seas from seismic evidence for slab relics in the mid-mantle of the Woodlark rift, southwestern Pacific

Abstract The initiation and evolution of marginal seas, especially those developing under a convergent setting, is one of the more enigmatic aspects of plate tectonics. Here, we report the presence of slab relics in the mid-mantle of the Woodlark rift in the southwestern Pacific based on a new map of the topography of the mantle discontinuities from a receiver function analysis and evidence from body-wave tomography. The widespread mantle transition-zone thickening rules out active mantle upwelling, and the revealed slab relics in both the upper and middle mantle may hydrate the upper mantle, which can be expected to further weaken the overlying lithosphere. Such a process can then promote initial continental rifting when this lithosphere is exposed to tensional stress like slab-pull stretching originating from the nearby active subduction.

Microfloristic regions of the Middle–Upper Triassic: New insights into the phytogeographical affinities of southwestern Gondwana

Two phytogeographic units were defined for the Middle–Upper Triassic of Gondwana, the warm humid Onslow and the cool-temperate Ipswich microfloras; nevertheless, many uncertainties remain regarding the geographical scope of such assemblages, as well as to the degree of similarity between phytogeographical regions and the existence of different microfloristic domains within the Laurasian territory. To tackle these questions, a hierarchical cluster analysis complemented with ordination methods and bipartite network analysis was performed over a list of 70 palynomorph species and selected localities of the Anisian–Ladinian, Carnian and Norian time-intervals, with special emphasis on southwestern Gondwana units. Obtained results showed a clear dichotomy between tropical and extratropical associations, within which different microfloristic regions also developed from gradual changes in the frequency distribution of palynotaxa. These findings are significant because they confirm for first time through quantitative methods a phytogeographic separation between ‘Onslow’ and ‘Euro-Mediterranean’ assemblages, considered for a long time to be part of the same microflora. Furthermore, and contrary to expectations, two distinct phytogeographic units were recognized for the Laurasian territory, where the first is related to the hot and dry margins of western Tethys, and the other to the temperate and humid zones of Boreal Pangea. Likewise, a separate and well defined assemblage was recognized for southwestern Gondwanan, during the late Anisian–Carnian of central-western Argentina, named here as the Cuyo microfloristic region. This unit showed a close resemblance to microfloras of northern Laurasia due to the shared presence of several species of miospores that are largely absent from the eastern part of Pangea. We hypothesize that the migration of such species occurred through a trans-equatorial temperate and humid landway, following the rift basins and topographic highs developed along the suture lines of the Variscan orogenic belt and western South America at the beginning of Pangea separation.

Geochemistry and petrology of the Chandarpur-Raipur sandstones: Implications for the evolution of the Chattisgarh Basin, Central India

Depositional processes and stratigraphic evolution of the Chattisgarh basin exhibit a synergistic relation between sedimentation and tectonics. To interpret the sedimentation history and basin evolution, multiproxy approach including field studies, petrography, geochemical, and multivariate statistical analyses of the sandstones of Chandarpur and Raipur groups is employed. The study provides a detailed analysis of the time-connoted petrography and geochemistry of the sandstone units from each formation of the Chandarpur-Raipur sequence of Chattisgarh basin. The sandstones grade from immature wacke through submature arkose to mature quartzarenite from the bottom up the section. These sandstones are broadly characterized by felsic provenance, arid-humid palaeoclimate, and weak-intense palaeoweathering at the source. The assemblage of wacke-arkose-sublitharenite of the basal Lohardih Formation is characteristics of fan delta environment and represents initiation of rifting and basin formation stage. Subarkoses from the younging up sequence of Lohardih-Chaporadih-Kansapathar formations reflect prograding shelf environment that represents craton interior field of the continental block provenance and passive marginal tectonic setting, indicating gradual stabilization of the basin. Quartzarenites from Kansapathar Formation and stratigraphically younger Devsagar Member of Charmuria Formation also represent broadly similar provenance and tectonic setting indicating stable craton interior provenance in a passive marginal setting. Petrographic and geochemical analyses of some of the Kansapathar-Devsagar sandstones suggesting recycled orogen provenance, however, may be attributed to the orogenic activity and/or role of younger bimodal volcanic rock during rejuvenation of multistage basin opening faults. Rifting stage was followed by subsidence stage as indicated by the presence of black limestone capping the sandstone successions. Initiation of basin formation ∼1400 Ma and sedimentation can be correlated with the late-stage fragmentation of the supercontinent Columbia. The basin probably closed ∼1000 Ma during the assembly of the next supercontinent Rodinia. It is estimated that ∼87% of the 2.2-km-thick sedimentary succession in the eastern Chattisgarh basin, were already deposited within this time frame (∼1405 and ∼1007 Ma). The basin is correlatable with other Mesoproterozoic basins of Peninsular India like Vindhyan, Cuddapah basins, as well as with basins like Xiong’er rift basin of North China Craton, Espinhaco basin of eastern Brazil, that are related to the Columbia-Rodinia Supercontinent cycle.

The Impact of Climate‐Driven Lake Level Changes on Mantle Melting in Continental Rifts

AbstractClimate cycles fundamentally control surface processes that affect the distribution of water and sediment, and their associated loads, across the Earth's surface. Here, we use a geodynamic model to examine how water loading can affect mantle melt generation in continental rift settings covered by deep lakes. Our modeling results suggest that lake level fluctuations can modulate the timing and rate of mantle melting. A rapid lake level drop of 800 m has the potential to increase mantle melt volumes by enhancing mantle upwelling beneath the rift, whereas a lake level rise can lead to a reduction of mantle melting. The volume of melt produced driven by lake level fluctuations is also dependent on crustal rheology, extension rate, mantle potential temperature, and lithosphere thickness. Our study identifies the importance of water loading for controlling rift processes, while also demonstrating critical links between changing climate, rift evolution and mantle melting.

Paleo-lake evolution influenced by tectonic processes and climate changes in the Hetao Basin during the late Quaternary

Exploration of paleo-lake evolution is crucial for the understanding of climatic and tectonic roles in earth surface. Previous studies mainly focused on the climatic effect on river-lake transitions and lake evolutions through lacustrine deposits, but with limited consideration of tectonic drivers, especially for the tectonic rift lakes in the Hetao area.Based on the analyses of lacustrine and fluvial piedmont terraces and a 400-m core from the northern Hetao Basin, along with the optically stimulated luminescence and detrital zircon U-Pb dating, we adopted an integrated “tectonics-climate-surface processes” approach to explore the climatic and tectonic effects on the lake evolution of the Hetao rift basin. Results show that lacustrine deposition on the piedmont terrace T1 (60–47 ka), underlying the T3 fluvial (80–60 ka), has an erosion contact with pluvial deposits at 55 ka, implying the paleo-lake formation at ∼ 80–60 ka and shrinkage at ∼ 55–47 ka. Lake formation was forced by basin depression under the tensile stress of tectonic movement, and was recharged by the Yellow River due to obstruction of the outlet at Tuoketuo caused by uplift.Lake shrinkage was due to the combined effect of basin lifting and warm-wet climate conditions that promoted outflow. A substantial decrease in the erosion base level and rapid water drainage induced by river cutting caused erosion of lacustrine deposits in the basin. The limited sedimentary space of the lake basin in early MIS 2 should be considered a factor in lake disappearance in addition to the dry climate. This study highlights that tectonic effects may be more important than climatic effects in the evolution of rift lakes near the young fold mountain belts.