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Abstract: Growing macroscale fold structures are one of the most dynamic and complex systems in geology, where during fold growth pore fluid pressure, fluid temperature, and fluid composition (PTX) conditions do not remain static but instead vary continually at all points within the deforming fold such that no two points within the fold will have the same deformation or fluid history. These PTX conditions in turn directly affect fluid storage and mobility as well as rock strength, which has direct feedback on the mechanical development of the fold. In this review, we first outline the research methodologies that have yielded significant insights, and review four examples of well-constrained fold-related fluid systems. We then discuss the fluid-related, fold-related, and mesoscopic deformation processes that are relevant to fold-related fluid systems. Finally, we close by presenting several conceptual models for fold-related fluid system structure, and highlighting key areas for future work. [Copyright &y& Elsevier]

Abstract: This study uses fluid inclusions in quartz and calcite veins to characterize the fracture-controlled paleohydrology of a map-scale, evaporite-cored detachment fold in the Sierra Madre Oriental of northeastern Mexico. Field observations indicate that the veins are tectonic in origin, and that they formed in a general sequence that corresponds to four broad stages of progressive folding. We collected samples from each vein stage in various structural and stratigraphic positions across the fold. After determining the mineral paragenesis and origin of inclusions in each sample, we used standard microthermometric techniques to measure the homogenization temperatures (Th ), salinities and eutectic temperatures of the available two- and three-phase aqueous inclusions. Neither Th , salinity nor eutectic temperature varied systematically with inclusion origin or mineral type. The data are also not significantly correlated with structural position or vein type, but are strongly partitioned by stratigraphy, suggesting that the area was a vertically stratified hydrologic system consisting of three regional paleohydrostratigraphic units. An upper unit comprised the Indidura Formation through Difunta Group, and is characterized by Th near 150°C and salinity <5wt% NaCl equivalent. A middle unit comprised the Taraises and Cupido Formations, and is characterized by Th near 150°C and salinities near 12 or 22wt% NaCl equivalent. The lower unit comprised the Zuloaga and La Casita Formations, and is characterized by an average Th near 225°C and salinities in three groups near 12, 22 or 36wt% NaCl equivalent. The preservation of multiple, distinct fluid types in many veins suggests that fracture development during folding and uplift created a second paleohydrologic system that overprinted the vertically stratified system. Fracture-controlled fluid migration in this second system occurred in periodic, repeated, and spatially variable pulses. [Copyright &y& Elsevier]

The fluid system within a critically tapering orogenic wedge is governed by complex interactions between topographic drive, thermal gradients, prograde dehydration reactions, internal structure, and regional tectonic compaction. Despite this complexity, topography is widely known to be the primary driving potential responsible for basin-scale fluid migration within the upper 7-10 km of an orogenic wedge. In recent years, investigators have revisited the problem of basin-scale fluid flow with an emphasis on depth-decaying permeability, which is a geologic phenomenon that is seldom accounted for in regional flow models. These recent investigations have shown that depth-dependent permeability at the basin scale strongly influences the relationship between local- and regional-scale flow paths. Here we investigate topography driven fluid flow within an orogenic wedge using a numerical modeling experiment designed to assess first-order fluid system behavior when permeability decreases systematically with depth. Critical taper theory is invoked to define two-dimensional basin geometry, and three subaerially exposed orogenic wedge models are presented with critical taper angles of 2°, 4°, and 10°. To assess the combined influence of topographic slope and depth-dependent permeability, a constant rate infiltration is applied at the wedge surface and a transient simulation is performed within each model for 20 m.y. Our results suggest that (1) depth-dependent permeability severely limits the penetration depth of infiltrating water within broadly tapering orogenic wedge systems, (2) fluid system evolution within a narrowly tapering orogenic wedge (i.e., ≤2°) is governed by local-scale topography superimposed on the regional gradient, (3) the influence of subbasin topography on local-scale fluid circulation is suppressed as the regional topographic gradient increases, and (4) the spatial distribution of groundwater residence time is fundamentally different when topographic slope exceeds 3°. [ABSTRACT FROM AUTHOR]

Oxygen, carbon, and strontium isotope variations in vein-filling calcite and quartz Cements and their host rocks are used to elucidate the origin, spatial and temporal evolution, and migration pathways of fluids in the detachment Nuncios fold complex, northeastern Mexico. The folded Mesozoic sedimentary sequence contains two regional paleohydrostratigraphic units separated by a unit of low permeability. Two main generations of cements are present in both paleohydrostratigraphic units. Distinct differences exist between δ18O, δ13C, 87Sr/86Sr, and fluid-inclusion temperatures of early vein-filling cements in the lower and the upper units. These differences, together with a strong correspondence between early cement and host-rock δ18O and δ13C values, suggest that early diagenetic fluids were compartmentalized between the two units. Late vein-filling cements have isotopic compositions and fluid-inclusion temperatures that converge to similar values, indicating a change to open fluid flow between the lower and upper units. We hypothesize that the fluid history of the Nuncios fold complex evolved in two main stages: (1) burial diagenesis and early folding, during which fluids were confined within individual units, and (2) late-stage folding, during which increased deformation associated with fold tightening caused the expulsion of fluid from the lower unit into the upper unit. [ABSTRACT FROM AUTHOR]

Structural geologists now recognize detachment folds as fundamental structural features in many contractional settings. Several two-dimensional geometric and kinematic models exist to describe the development of such detachment folds; however, most are not available in a computer-based format that permits the forward-modeling and graphical representation of the detachment fold geometry. We developed DETACH, a Microsoft Excel™ spreadsheet, to construct simple cross sections of detachment folds using published geometric and kinematic models. DETACH allows users to assess the range of possible fold geometries and detachment depths that can be constructed using a prescribed set of fold kinematics, and to quickly evaluate and select a best-fit kinematic model for folds of known geometry. We illustrate DETACH''s capabilities by modeling a two-dimensional cross section of a natural detachment fold and by constructing pseudo-three-dimensional models of detachment fold terminations with the assistance of Geosec2D™ and Gocad™ structural modeling software. [Copyright &y& Elsevier]