Mark P. Fischer

Predicting the orientation of joints from fold shape: Results of pseudo–three-dimensional modeling and curvature analysis

We treat layers of sedimentary rock as elastic plates and predict the orientations of joints by assuming that they open parallel to the maximum instantaneous stretch of a layer. Because the direction of maximum instantaneous stretch is parallel to the maximum curvature of a surface, we hypothesize that joints will trend parallel to the minimum curvature of an elastically deformed layer. After constructing pseudo–three-dimensional trishear models of Laramide-style uplifts that grow self-similarly, we calculated the direction of minimum-curvature axes during the evolution of the fold. Our analysis of minimum-curvature axes in evolving folds suggests several important characteristics for fold-related joint sets: (1) joints that are neither parallel nor perpendicular to the fold axis may be induced by local fold-related strains; (2) at any time during folding, joint orientations may vary according to the structural position on a fold; (3) at any location on a fold, joint orientation may depend on when a joint forms during the evolution of the fold; and (4) joint patterns in trishear folds may vary with stratigraphic position. Natural folds that evolve along simple geometric pathways may develop fold-related joint sets, the orientation, dominance, abutting relationships, spacing, and continuity of which will vary systematically throughout the structure. This variation in joint-system architecture may reflect the history of fold growth.

Stratigraphic controls on eformation patterns in fault-related folds: a detachment fold example from the Sierra Madre Oriental, northeast Mexico

Abstract Deformation patterns are described by the style, intensity and distribution of structures throughout a volume of rock. In fault-related folds, presently observed deformation patterns are controlled by structural factors like kinematic history, environmental factors like burial history and stratigraphic factors like layer thickness and competence. To demonstrate the role of stratigraphy in controlling fold-related deformation patterns, we document the stratigraphic variability of mesostructures at one structural position on a map-scale detachment fold. Unique assemblages of faults, fractures and cleavages in certain stratigraphic intervals define a mechanical stratigraphy comprised of several units, each exhibiting a characteristic deformation response to the folding process. The varying response of each mechanical unit suggests that folding mechanisms such as orthogonal flexure, flexural slip or flexural flow, were sequentially or coevally operative in each of the units. Using the simple, theoretical strain distributions associated with each folding mechanism and the fact that each mechanical unit accommodates fold-related strain by developing a characteristic set of mesostructures, we are able to predict general deformation patterns elsewhere in the fold. The significance of this approach is that it recognizes that even in a single structure, fold kinematics and deformation response may be dramatically different in various parts of a stratigraphic section. Because of this, the quality and character of reservoir rocks in anticlinal traps may change markedly over short stratigraphic distances. Predictions of fold-related deformation based solely on fold kinematics are unlikely to resolve these variations.

Photogrammetric techniques for analyzing displacement, strain, and structural geometry in physical models: Application to the growth of monoclinal basement uplifts

ABSTRACTPhysical models have been widely used to study geological structures for more than one hundred years. The greatest benefi t of physical models is that with proper scal-ing of model dimensions and materials, researchers can directly observe structural or tectonic processes that take millions of years to occur naturally. Despite this ben-efi t, however, use of physical models is not widespread because their construction and analysis is commonly labor-intensive work that yields largely qualitative information on structural geometry and limited quantitative information on displacement and strain. This paper describes how close-range photogram-metry can be used to obtain quantitative information on the geometry, displacement, and strain patterns in an evolving physical model. The technique is an inexpensive, high-resolution, noninvasive, and effi cient method that uses standard commercial software and a digital camera to determine the x , y , z posi-tions of high-contrast markers placed on the model surface. The model geometry at any given time is defi ned by the positions of all the markers, whereas strain and displace-ment are obtained by comparing, or track-ing, the positions of the markers at different times during an experiment. We present as an example application of the technique, an analysis of scaled physical models of mono-clines that form above basement-involved reverse faults with differing displacement distributions. Using close-range photogram-metry, we are able to link fault displacement and lateral propagation history to unique, evolving, three-dimensional (3-D) geometries and deformation patterns that are unlikely to be revealed by other analysis techniques.Keywords: physical model, monocline, base-ment uplift, photogrammetry, strain.INTRODUCTION

Evolution of fluid compartmentalization in a detachment fold complex

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 δ 18 O, δ 13 C, 87 Sr/ 86 Sr, 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 δ 18 O and δ 13 C 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.

An experimental evaluation of the curvature-strain relation in fault-related folds

Substantial effort has gone into predicting the characteristics of subsurface fracture systems in fault-related folds. Curvature analysis is a common method used to predict the location and characteristics of fracture networks in folded rock layers. In curvature analysis, it is assumed that layers of rock deform like elastic plates so that layer-parallel strains are directly related to the curvature of the folded surface. This article tests the underlying assumption of all curvature analyses: that curvature is a direct proxy for strain in folded rock layers. We test the assumption by analyzing the curvature and strain in a series of scaled physical models of contractional, basement-involved, fault-related folds. Our objective is to constrain the conditions that lead to a strong positive correlation between curvature and extensional strain. Of particular interest is whether curvature and strain correlate over a wide range of fault throws and dips. The analysis of our folds demonstrates that both the distribution and magnitude of a fold-axis normal extension in the surface of the overlying layer appear to vary as a function of fault dip. Our results indicate that the correlation between strain and curvature generally becomes worse with decreasing fault dip. Fault throw also exhibits an effect on the curvature-strain relationship. However, this effect is dependent on the dip of the fault as well and only exhibits an effect on the curvature-strain relationship for moderately dipping faults. A direct correlation between curvature and strain at all stages of fold development is observed only for steeply dipping basement faults. These results suggest that curvature may not be a consistently reliable strain proxy in basement-involved fault-related folds and that the accuracy of curvature-related strain predictions will be strongly dependent on the dip and throw of the underlying basement fault.

Topographically driven fluid flow within orogenic wedges: Effects of taper angle and depth-dependent permeability

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°.

DETACH: an Excel spreadsheet to simulate 2-D cross sections of detachment folds

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(TM) 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 DETACHs 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(TM) and Gocad(TM) structural modeling software.

Fracture-controlled palaeohydrology of a secondary salt weld, La Popa Basin, NE Mexico

Abstract Isotopic and fluid inclusion analyses of veins and host rocks constrain the compositions, temperatures and sources of palaeofluids along the La Popa salt weld. Most veins formed after the salt was evacuated from the precursor salt wall; veins are generally more abundant on the downthrown side of the weld and near a significant bend in the trace of the weld. The spatial distribution of fluid types and temperatures suggests the weld served as a vertical fluid conduit and a horizontal baffle. Stable isotopes indicate there was significant fluid–rock interaction and little vertical fluid communication between rock units in areas away from the weld. Fluid temperatures along the weld ranged from 84 to 207 °C, salinities ranged from 4 to 25 wt% NaCl equiv. and methane was abundant in the weld zone and on the downthrown side of the weld. Strontium isotopes suggest that some of the vein-forming fluids were derived from the evaporites that once occupied the weld. Our results suggest the sealing potential of similar welds may be related to the presence of abrupt changes in weld geometry such as cusps or bends, the amount of shortening across the weld and the amount of vertical displacement across the weld.

Advective Heat Transport and the Salt Chimney Effect: A Numerical Analysis

We conducted numerical simulations of coupled fluid and heat transport in an offshore, buried salt diapir environment to determine the effects of advective heat transport and its relation to the so-called “salt chimney effect.” Model sets were designed to investigate (1) salt geometry, (2) depth-dependent permeability, (3) geologic heterogeneity, and (4) the relative influence of each of these factors. Results show that decreasing the dip of the diapir induces advective heat transfer up the side of the diapir, elevating temperatures in the basin. Depth-dependent permeability causes upwelling of warm waters in the basin, which we show to be more sensitive to basal heat flux than brine concentration. In these model scenarios, heat is advected up the side of the diapir in a narrower zone of upward-flowing warm water, while cool waters away from the diapir flank circulate deeper into the basin. The resulting fluid circulation pattern causes increased discharge at the diapir margin and fluid flow downward, above the crest of the diapir. Geologic heterogeneity decreases the overall effects of advective heat transfer. The presence of low permeability sealing horizons reduces the vertical extent of convection cells, and fluid flow is dominantly up the diapir flank. The combined effects of depth-dependent permeability coupled with geologic heterogeneity simulate several geologic phenomena that are reported in the literature. In this model scenario, conductive heat transfer dominates in the basal units, whereas advection of heat begins to affect the middle layers of the model and dominates the upper units. Convection cells split by sealing layers develop within the upper units. From our highly simplified models, we can predict that advective heat transport (i.e., thermal convection) likely dominates in the early phases of diapirism when sediments have not undergone significant compaction and retain high porosity and permeability. As the salt structures mature into more complex geometries, advection will diminish due to the increase in dip of the salt-sediment interface and the increased hydraulic heterogeneity due to complex stratigraphic architecture.