Outcrop studies are essential for characterizing fractured carbonate systems, as they provide the most comprehensive understanding of how vertical and lateral facies variability and associated diagenetic events affect fracture network geometry and, thus, fluid flow in the subsurface. Outcrop analog studies on carbonate platforms and basins have revealed relationships between fracture size and depositional geometries to define fracture stratigraphy frameworks; however, studies on carbonate slope to toe-of-slope settings are sparse. This study constrains the fracture stratigraphy along depositional strike and dip in the grainstone and rudstone-dominated Pinery Member of the Permian-aged Bell Canyon Formation, Texas, USA. Fractures exhibit northwest/southeast and east-northeast/west-southwest strike orientations. In the proximal toe-of-slope setting, very thick-bedded upslope-shingling carbonate debrites exhibit compensational stacking with downslope-dipping carbonate turbidite complexes. The proximal debrite complex contains opening-mode (Mode I) throughgoing fracture networks that form a single mechanical unit. The distal toe-of-slope is organized into a weakly confined interbedded grainstone turbidite complex with a higher abundance of intrabed and bed-bound fractures. Fractures at this location form a positive linear correlation with bed thickness and form multiple vertically stacked mechanical units. Overall, fracture heights range from 0.01 m to 12 m with a median of 0.64 m, and fracture-set distributions are best characterized by truncated power-law or exponential distributions. Results of this study have implications for predicting facies- and tectonics-controlled fracture network geometries in the subsurface for oil and gas, enhanced geothermal, and groundwater reservoir characterization.
The formation of the Tibetan Plateau and the subsequent postcollisional convergence have left profound imprints on the lithosphere, yet how these processes have modified the underlying upper mantle remains poorly constrained due to the lack of direct magmatic records. Continental ocean-island-type basalts, although volumetrically minor, provide an exceptional window into such compositional overprinting by deep geodynamic processes. Here, we probe heterogeneity in the postcollisional asthenosphere by integrating mineralogical, elemental, and Mg-Zn-Sr-Nd isotopic constraints from continental basalts in the Tongguan region, southeastern Tibetan Plateau. These basalts erupted during the Quaternary (ca. 1 Ma; zircon U-Pb ages) and exhibit typical ocean-island basalt (OIB)–type trace element signatures with depleted Sr-Nd isotopic compositions (εNd ≤ +8.6). Mineral and whole-rock thermometry, hygrometry, and oxybarometry collectively indicate a relatively hydrous (H2O ≤ 3.5 wt%) yet low-fO2 (FMQ+0.1) primary magma that formed under normal mantle thermal conditions (<1368 °C). Olivine chemistry and whole-rock elemental systematics consistently require derivation from a dominantly Stage 2 pyroxenitic asthenospheric mantle with minor peridotite involvement. The basalts display systematically light δ26Mg (−0.60‰ to −0.49‰) and heavy δ66Zn (+0.34‰ to +0.42‰) values yet lack elemental signatures of carbonate metasomatism. This marked isotope–element decoupling compellingly traces recycled marine carbonates that underwent deep subduction-induced decarbonation, leaving a “ghost carbonate” imprint in the mantle source. Together with geophysical evidence, these features directly link Tongguan basaltic magmatism to dehydration of stagnant oceanic slabs in the mantle transition zone (MTZ). The Mg-Zn isotopic variations further delineate a clear regional imprint of recycled sedimentary carbonates in the asthenospheric mantle south of ~26°N as a consequence of deep slab stagnation, in stark contrast to the relatively noncarbonated asthenosphere to the north, where slab tearing dominates. Taken together, our study demonstrates that continental OIB-type magmatism in the southeastern Tibetan Plateau records pronounced postcollisional asthenospheric heterogeneity dictated by spatially variable subduction dynamics. More broadly, this case highlights the pivotal role of stagnant slabs in the MTZ in governing deep carbon recycling and generating asthenosphere-derived continental OIB-type magmatism.
The presence of ocean-island basalt (OIB)-type magmas in convergent margins implies complex mantle geodynamic processes. This study reports an Early Permian (ca. 282 Ma) association of OIB-type and Nb-enriched arc-type rocks from the Northern Qiangtang Terrane, Tibet. Both suites share predominantly depleted Nd-Hf isotopic compositions, suggesting derivation from an asthenospheric mantle containing enriched components. The Nb-enriched arc-type rocks additionally record slab interaction. Spatial distribution with OIB-type rocks located proximal and arc-type rocks distal to the trench supports a sub-slab origin for the enriched component. After a systematic evaluation of various potential geodynamic models, we interpret this origin as reflecting slab tearing in the vicinity of the trench. Spatiotemporal, geochemical, and thermal comparisons suggest that this mantle source may be linked to the 290–285 Ma Qiangtang-Panjal large igneous province, which was located >2000 km to the south. This linkage implies long-distance transport of plume material across the Paleo-Tethys Ocean and subsequent plume-subduction zone interaction beneath the Northern Qiangtang margin. We propose that slab tearing facilitated the upwelling and melting of this previously enriched material, providing the setting for long-distance plume material transport and plume-subduction interaction in ancient convergent systems. The transport of plume material was likely driven by large-scale northward mantle flow. This large-scale mantle-flow framework may also provide a qualitative working hypothesis for linking plume-related thermal weakening and basal traction to the initial rifting and northward drift of the Cimmerian microcontinents.
The greater Red River system of Texas and Oklahoma, USA, is historically underutilized due to high concentrations of dissolved solids (i.e., Na+, Cl−, SO42−). Catchment geology consists largely of N-striking Permian units that transition downstream from younger, highly soluble marine carbonates and evaporites to older, less soluble terrestrial siliciclastic units. Because Permian marine evaporite-bearing units have distinct Sr and S isotopic compositions, these constituents may effectively trace stratigraphic origins and downstream mixing behavior of the dissolved solid load. This hypothesis was tested for the Wichita River Watershed, an important sub-watershed of the upper Red River Basin. Major ions and stable isotopes (C, H, O, S, Sr) prove to be effective solute tracers in this system because of compositional contrasts between evaporite, carbonate, and silicate lithologic endmembers and the large 87Sr/86Sr range between Permian seawater (<0.707) and continental siliciclastic rocks (>0.710). Western headwaters are close to isotopic equilibrium with outcropping Blaine Formation gypsum, which prospectively correlates with San Andres Formation evaporites in the Permian basin based on comparable 87Sr/86Sr and δ34SSO4 values. Progressive downstream interaction with more radiogenic siliciclastic watershed units increases the dissolved solute 87Sr/86Sr signal to values approaching modern seawater (0.709) above its confluence with the Red River. Sulfur isotopes confirm a persistent Permian evaporite influence over sedimentary pyrite oxidation. These signatures contrast markedly from stream segments whose headwaters do not intersect evaporites. Study results demonstrate the efficacy of Permian evaporite geochemical tracers to characterize spatial variations in the dissolved solid load that directly impact water quality in watersheds intersecting Permian evaporite basins.
Siliciclastic-dominated environments are considered inhospitable for reefs. Yet through glacial cycles, these localities may undergo periods of sediment starvation, which opens the environment for biogenic buildups. The Levant shelf is such an environment, indirectly receiving sediment from the Nile delta. Here, using high-resolution seismic data and targeted coring, a previously unknown, mostly buried, reef system at the edge of the Levant shelf is presented. This reef system, about a kilometer in width, spans most of the Levant shelf with a range of morphologies from mounded to flat top. It represents a lull in the sediment supply to the Levant shelf in the aftermath of the Last Glacial Maximum (LGM). The reef system developed atop an abrasion surface related to the LGM and persisted for several thousand years, during which it transitioned from the euphotic zone to the mesophotic zone. Termination of the reef phase ca. 8 ka coincided with renewal and intensification of sediment supply during the African Humid Period and Sapropel S1. The input of nutrients and sediment during these phases is considered here the kill mechanism. This suite of biogenic buildups represents a prime example of the mode exchange between siliciclastic and carbonate systems.
