ABSTRACT We use offshore seismic reflection data combined with published geologic and geodetic studies to examine the timings and styles of deformation around the South Caspian Basin, an enigmatic aseismic ‘block’ within the Arabia‐Eurasia collision that moves relative to its surroundings. We show that the presently active tectonics of the South Caspian began relatively recently, likely in the range 1.8–2.4 Myr, with components of both shortening and lateral‐slip along its northern and western edges. Along the northern margin (the Absheron Ridge), structures related to the active tectonics are superposed on an older tectonic configuration related to north–south shortening and subduction of the South Caspian, ending in the Miocene at ~6 Ma. In the west (Kura margin), the present‐day oblique right‐lateral transpression began at 1.8–2.4 Myr, replacing an earlier phase of shortening. The tectonic history is hence one of initial Miocene shortening and northward‐directed subduction of SCB basement, followed by a hiatus in deformation along the Absheron ridge, which we infer to result from a switch to oroclinal bending in the ranges surrounding the basin until 1.8–2.4 Myr. At that time the active tectonic configuration was established, and the basin began to be expulsed to the northwest. This change led to the development of major transpressional fault systems along its north, east, and southern margins, the likely resumption of underthrusting of SCB basement northward beneath the Absheron ridge, and the start of underthrusting and incipient subduction beneath the Kura basin along its western margin. The various short‐lived stages in the destruction of the South Caspian have potential for insight into the late stages of basin closure and the transition to mature continental collision.
ABSTRACT This study investigates the footwall crest and fault scarp geomorphology of crustal‐scale fault blocks developed during Late Jurassic rifting on the Greater Tampen Spur, northern North Sea, using 3D seismic data and 84 exploration wells. The Snorre and Visund footwalls represent contrasting geomorphic end members. Snorre is characterised by a broad, plateau‐like erosion surface that truncates tilted Triassic strata and its fault scarp comprises degraded slopes, centrally developed U‐shaped gullies and canyons and hanging‐wall rider blocks developed in an area of overlapping fault segments. The Visund footwall, in contrast, is convex‐up and asymmetric, locally preserves Late Jurassic strata at its crest and is associated with a steep fault scarp containing structurally guided V‐shaped gullies and canyons, larger U‐shaped canyons and a large submarine landslide. These contrasting morphologies reflect different balances between fault‐related uplift, regional subsidence and the timing and duration of drowning. At Snorre, large displacement on the Snorre–Zeta Fault, combined with additional uplift from west‐dipping faults bounding the Marulk Basin, kept the footwall at shallow levels long enough for prolonged wave ravinement. At Visund, local footwall uplift was offset by hanging‐wall subsidence related to the Snorre–Zeta Fault system and by greater regional tectonic subsidence toward the North Viking Graben. This lower structural position led to earlier drowning, favouring submarine diffusive hillslope retreat, repeated mass wasting and preservation of fault‐scarp incisions. Together, these results show that displacement history, structural position within a fault network, drowning history, bedrock lithology, secondary faulting and geomorphic preservation collectively govern footwall and fault‐scarp morphology in marine rift basins. Recognising these controls improves reconstruction of sediment source areas and routing pathways and provides a more robust basis for predicting syn‐rift depositional architecture in adjacent hanging‐wall basins.
ABSTRACT Pockmarks are geomorphological depressions observed in diverse marine environments, typically associated with fluid release from subsurface reservoirs. In addition to seafloor features, buried paleo‐pockmarks offer valuable records of past fluid escape events. However, the processes, fluid types and conditions controlling their formation and recurrence remain poorly constrained. This study examines potential subsurface fluid sources and pathways that contribute to the cyclic formation of a prominent pockmark field located on the southwestern Chatham Rise, offshore New Zealand. Previous geochemical analyses have revealed no evidence for biogenic or thermogenic methane in the shallow subsurface, necessitating alternative interpretations of the fluid source(s). Here, we use quasi‐3D multichannel seismic reflection data to examine the relationship between subsurface fluid flow pathways and spatio‐temporal pockmark distribution. A polygonal fault system (PFS), characterized by low fault network connectivity and preferential fault orientations, indicates the influence of the regional stress field. Additionally, a distinct, discordant reflection with relatively high‐amplitude, interpreted as a diagenetic front, is hosted within the PFS. Vertically extensive faults extend above the PFS and align spatially with basement faults, forming West–East trending half‐grabens that may provide pathways for buoyancy‐driven fluid migration. However, spatial pattern analysis reveals no statistically significant spatial association between pockmarks and faults, indicating primarily diffuse fluid flow in the upper subsurface. Generally, the upslope extent of pockmarks on the Chatham Rise appears to be lithologically controlled and coincides with the limit of the PFS in underlying strata. Recurring pockmark formation is interpreted to reflect a combination of compaction‐related dewatering during polygonal faulting and silica diagenesis, coupled with deeply sourced, possibly CO 2 ‐rich fluids from subducted limestones migrating along basement structures. These findings are combined into a conceptual model of the subsurface fluid flow system, offering new insights into the controls on recurring pockmark formation in non‐hydrocarbon settings.
ABSTRACT The timing of the India – Asia collision along the western margin of the Indian Plate remains highly debated, with estimates ranging from 60 to 40 Ma, associated with different models for the sequence of collision events. To examine the collisional record along the western margin of the Indian Plate, we integrate detrital zircon U–Pb ages and Nd–Sr isotopic signatures with zircon and apatite (U–Th)/He and apatite fission track thermochronology. Late Cretaceous to Palaeocene strata (Pab and Ranikot formations) are characterised by detrital zircon populations dominated by Precambrian to early Palaeozoic grains (1 Ga–500 Ma), together with highly negative εNd (0) values (−17.2 to −12.6) and radiogenic 87 Sr/ 86 Sr ratios, indicating derivation from Indian‐affinity sources. Across the Palaeocene–Eocene transition and into the Early Eocene, the first appearance of younger zircon populations (178–55 Ma) in the Ghazij and Nisai formations coincides with a shift toward less negative εNd (0) values (−12.0 to −8.3) and lower 87 Sr/ 86 Sr ratios, recording the initial addition of Asian‐affinity material to a system still dominated by Indian‐margin sources. The proportion of younger zircon populations is low in the Early Eocene Ghazij Formation of the Sulaiman (∼1%–3% Cretaceous–Paleogene grains), but is higher in coeval Nisai strata of the Katawaz (∼13% 55–96 Ma population), and increases further in Oligocene units (Chitarwatta and Khojak formations), where Jurassic–Oligocene zircon populations (194–32 Ma) become more prominent. This progressive increase is matched by sustained trends toward less negative εNd (0) values and lower 87 Sr/ 86 Sr ratios, indicating an increasing contribution from Asian‐affinity sources through time. These observations are further supported by shifts in paleocurrent directions, changes in sedimentary facies, and Eocene ZHe cooling ages from the Sulaiman hinterland. The first arrival of Asian‐affinity detritus on the Indian Plate provides an important sedimentary constraint on the timing of India–Asia collision in this sector. Our results indicate that collision was likely underway by the Palaeocene–Eocene transition (~56 Ma), followed by continued convergence and progressive integration of sediment‐routing systems during the Eocene and younger intervals. These findings are consistent with an early Paleogene initiation of a single‐stage collision along the western margin of the Indian Plate and are difficult to reconcile with models proposing significantly younger or strongly diachronous collision.
ABSTRACT Although mixed siliciclastic‐carbonate deposits in lacustrine rift basins are recognized as sensitive recorders of tectonic‐climatic‐sedimentary interactions, their high‐resolution spatial and temporal evolution within a parasequence framework has rarely been documented. This study integrates cores, well logs, 3D seismic data and geochemical proxies from the lower Shahejie Formation (Es 1 ) in the Baxian Sag to define four genetic shoal‐bar types and to reconstruct their response to rapid lacustrine transgression during early rift subsidence. Four genetic types of shoal‐bar deposits are identified based on sediment provenance and composition: terrigenous clastic shoal‐bars, endogenic carbonate shoal‐bars, mixed siliciclastic‐carbonate shoal‐bars and reworked carbonate shoal‐bars. Seismic stratigraphic slices and well correlations reveal that these shoal‐bars are preferentially developed in three palaeogeomorphic settings: the terminations of strike‐slip faults, the footwalls of weakly active normal faults and submerged low‐relief uplifts. Vertically, shoal‐bar facies exhibit a symmetric stacking pattern around the maximum flooding surface, transitioning from terrigenous clastic, through mixed siliciclastic‐carbonate, to endogenic carbonate and back to mixed and terrigenous clastic facies. This stacking pattern defines a complete parasequence set (retrogradational to aggradational to progradational) that records high‐frequency lake‐level fluctuations. The initial stage of rift subsidence coincided with rapid lacustrine transgression and significant reduction in terrigenous sediment supply, such that wave reworking of sediment‐starved deltas provided the primary material for shoal‐bar construction, whereas coeval arid conditions promoted endogenic carbonate precipitation. The combination of these processes thus facilitated the mixing of terrigenous clastics and endogenic carbonates. Consequently, the types and spatial distribution of shoal‐bars provide robust proxies for reconstructing the intricate interplay among sediment supply, palaeoclimate and early rift tectonic dynamics.
