Simon A. Kattenhorn

Joints at high angles to normal fault strike: an explanation using 3-D numerical models of fault-perturbed stress fields

Structural methods based on homogeneous stress states predict that joints growing in an extending crust form with strike orientations identical to normal faults. However, we document a field example where the strikes of genetically related normal faults and joints are almost mutually perpendicular. Field relationships allowed us to constrain the fracture sequence and tectonic environment for fault and joint growth. We hypothesize that fault slip can perturb the surrounding stress field in a manner that controls the orientations of induced secondary structures. Numerical models were used to examine the stress field around normal faults, taking into consideration the eAects of 3-D fault shape, geometrical arrangement of overlapping faults, and a range of stress states. The calculated perturbed stress fields around model normal faults indicate that it is possible for joints to form at high angles to fault strike. Such joint growth may occur at the lateral tips of an isolated fault, but is most likely in a relay zone between overlapping faults. However, the angle between joints and faults is also influenced by the remote stress state, and is particularly sensitive to the ratio of fault-parallel to fault-perpendicular stress. As this ratio increases, joints can propagate away from faults at increasingly higher angles to fault strike. We conclude that the combined remote stress state and perturbed local stress field associated with overlapping fault geometries resulted in joint growth at high angles to normal fault strike at a field location in Arches National Park, Utah. # 1999 Elsevier Science Ltd. All rights reserved.

Integrating 3-D seismic data, field analogs, and mechanical models in the analysis of segmented normal faults in the Wytch Farm oil field, southern England, United Kingdom

We propose a methodology for the analysis of normal fault geometries in three-dimensional (3-D) seismic data sets to provide insights into the evolution of segmented normal fault systems and to improve recovery efforts in fault-controlled oil fields. Limited seismic resolution can obscure subtle fault characteristics such as segmentation and gaps in fault continuity that are significant for oil migration and thus accurate reservoir characterization. Detailed seismic data analyses that incorporate principles of normal fault mechanics, however, can reveal evidence of fault segmentation. We integrate seismic attribute analyses, outcrop analog observations, and numerical models of fault slip and displacement fields to augment the use of 3-D seismic data for fault interpretation. We applied these techniques to the Wytch Farm oil field in southern England, resulting in the recognition of significant lateral and, to a lesser extent, vertical segmentation of reservoir-scale faults. Slip maxima on fault surfaces indicate two unambiguous segment nucleation depths, controlled by the lithological heterogeneity of the faulted section. Faults initiated preferentially in brittle sandstone and limestone units. Subsequent growth and linkage of segments, predominantly in the lateral direction, resulted in composite fault surfaces that have long lateral dimensions and multiple slip maxima. Reservoir compartmentalization is greatest at the level of prevalent segment linkages, which corresponds at Wytch Farm with the predominant hydrocarbon-producing unit, the Sherwood Sandstone. At relatively shallower depths, fault segments are younger and less evolved, resulting in a greater degree of segmentation with intact relay zones.

Distribution of fault slip in outcrop-scale fault-related folds, appalachian mountains

Abstract Current kinematic models of ramp-related folds predict a direct relationship between ramp angle and fold shape and imply specific sequences of deformation. Analyses of outcrop-scale structures in the Valley and Ridge province of the Appalachians reveal configurations that depart from model predictions. The models fail to account for the presence of footwall synclines, and are inconsistent with measured displacement distributions on some natural faults. Observations support the interpretation that faults can grow by propagation both up- and down-dip from a nucleation point. Fault propagation in either direction may result in the formation of folds primarily on the side of the fault that is displaced in the direction of fault propagation.

Characterization and evolution of fractures in low-volume pahoehoe lava flows, eastern Snake River Plain, Idaho

We characterize fracture evolution in pahoehoe lava flows of the eastern Snake River Plain, Idaho, and highlight significant differences to flood-basalt sheet flows and implications for hydrologic models. There are four distinct fracture types in east ern Snake River Plain flows: (1) column- bounding; (2) column-normal; (3) entablature; and (4) inflation fractures. Types 1–3 are driven by thermal stress, whereas type 4 is induced by lava pressure from within the flow. Thermal stress distribution in a flow is dictated by its aspect ratio (width/ height), which controls the shape of isotherms. Isotherms control column-bounding fracture orientations, resulting in increasingly radial fracture patterns as the aspect ratio approaches unity. Column-normal fractures form in response to thermal stress and fracture-induced stress within basalt columns. Overlap in the timing of column-bounding and column-normal fracture growth has resulted in complex fracture relationships. Column-normal fracture growth is strongly influenced by vesicular layers, which act as mechanical heterogeneities, creating preferential pathways for fracture growth as well as causing jogs or terminations along column-bounding fractures. Eastern Snake River Plain entablatures, which preserve the shape of the central lava core during the final stages of cooling, have distinctly different origins and fracture styles compared to sheet flows. Entablatures formed by penetration of the edges of pressurized lava cores by inflation fractures, causing rapid convective cooling. In addition, inflation fractures significantly perturb isotherm shapes in lava flows, affecting flow-scale fracture patterns and densities. The overall effect of all these processes is a complex pattern of fracturing that attests to a strong impact by each fracture type on the growth behavior of all other fracture types.

Varying styles of magmatic strain accommodation across the East African Rift

Observations of active dike intrusions provide present day snapshots of the magmatic contribution to continental rifting. However, unravelling the contributions of upper crustal dikes over the timescale of continental rift evolution is a significant challenge. To address this issue, we analyzed the morphologies and alignments of >1500 volcanic cones to infer the distribution and trends of upper crustal dikes in various rift basins across the East African Rift (EAR). Cone lineament data reveal along-axis variations in the distribution and geometries of dike intrusions as a result of changing tectonomagmatic conditions. In younger ( 10 Ma) in Ethiopia and the Kenya Rift, rift-parallel dikes accommodate upper crustal extension along the full length of the basin.

Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt

Although polygonal fault systems and related features are common in fine-grained sediments in modern submarine basins and have been studied in basins worldwide using three-dimensional (3-D) seismic data, extensive on-land exposures have remained elusive. We report here on the discovery of a polygonal fault system occurring in nearly continuous surface exposure over ∼900 km 2 in chalk of the Cretaceous Khoman Formation near Farafra Oasis, Egypt. Field exposures reveal polygon boundaries defined by clusters of dozens of normal faults with strongly grooved fault surfaces and coarse calcite veins along faults with evidence for multiple fluid flow events. Geometric patterns and fault intersections reveal that mechanically interacting normal faults with multiple orientations were active contemporaneously in a horizontal strain field that was essentially isotropic and extensional. We interpret the very steep dips (∼80°) to reflect fault initiation in response to elevated pore fluid pressures. In the uppermost part of the Khoman Formation, a terrain of isolated circular structures displaying shallow inward dips overlies the polygonal fault network. The spatial relationship to the underlying faults is consistent with these small circular basins having formed as fluid escape structures as the polygonal fault system evolved. Outcrops in the Khoman Formation provide an unprecedented look into the 3-D geometry of a polygonal fault system, providing context for the analysis of analogous systems in marine basins and other on-land exposures.

Evolution of upper crustal faulting assisted by magmatic volatile release during early-stage continental rift development in the East African Rift

During the development of continental rifts, strain accommodation shifts from border faults to intra-rift faults. This transition represents a critical process in the evolution of rift basins in the East African Rift, resulting in the focusing of strain and, ultimately, continental breakup. An analysis of fault and fluid systems in the younger than 7 Ma Natron and Magadi basins (Kenya-Tanzania border) reveals the transition as a complex interaction between plate flexure, magma emplacement, and magmatic volatile release. Rift basin development was investigated by analyzing fault systems, lava chronology, and geochemistry of spring systems. Results show that extensional strain in the 3 Ma Natron basin is primarily accommodated along the border fault, whereas results from the 7 Ma Magadi basin reveal a transition to intra-rift fault–dominated strain accommodation. The focusing of strain into a system of intra-rift faults in Magadi also occurred without oblique-style rifting, as is observed in Ethiopia, and border fault hanging-wall flexure can account for only a minor portion of faulting along the central rift axis (∼12% or less). Instead, areas of high upper crustal strain coincide with the presence of hydrothermal springs that exhibit carbon isotopes and N2-He-Ar abundances indicating mixing between mantle-derived (magmatic) fluids and air saturated water. By comparing the distribution of fault-related strain and zones of magmatic fluid release in the 3 Ma Natron and 7 Ma Magadi basins, we present a conceptual model for the evolution of early-stage rifting. In the first 3 m.y., border faults accommodate the majority of regional extension (1.24–1.78 mm yr–1 in Natron at a slip rate ranging 1.93–3.56 mm yr–1), with a significant portion of intra-rift faulting (38%–96%) driven by flexure of the border fault hanging wall. Fluids released from magma bodies ascend along the border fault and then outward into nearby faults forming in the flexing hanging wall. By 7 m.y., there is a reduction in the amount of extension accommodated along the border fault (0.40–0.66 mm yr–1 in Magadi at a slip rate ranging from 0.62 to 1.32 mm yr–1), and regional extension is primarily accommodated in the intra-rift fault population (1.34–1.60 mm yr–1), with an accompanying transition of magmatic volatile release into the rift center. The focusing of magma toward the rift center and concomitant release of magmatic fluids into the flexing hanging wall provides a previously unrecognized mechanism that may help to weaken crust and assist the transition to intra-rift dominated strain accommodation. We conclude that the flow of magmatic fluids within fault systems plays an important role in weakening lithosphere and focusing upper crustal strain in early-stage continental rift basins prior to the establishment of magmatic segments.

Blunt-ended dyke segments

Two examples of blunt-ended dykes from the Rooi Rand dyke swarm in South Africa are examined in order to determine the mechanism by which such features form. Although other interpretations of blunt-ended dykes have been proposed, evidence in the Rooi Rand examples suggests that dilation was transferred along a zone of shear at the dyke tip oriented at a high angle to the dyke plane. Microscopic analysis of samples from blunt-ended tip regions reveals cataclasis and mineral straining in the dyke walls in the zone of dyke linkage. The indication is that adjacent, blunt-ended, en Cchelon dyke segments dilate along a shear zone, producing cataclasis of the host rock. Both segments dilate in this manner and are blunt-ended prior to linkage. Horns may develop at the outer corners of the blunt tips so that, subsequent to linkage, the overall geometry resembles that predicted by the conventional model of bridge failure between overlapped en tchelon dykes. However, permanently strained bridges predicted by that model are not necessary for the model described here. In addition, blunt-ended dykes that dilate along a cross-linking shear zone do not need to overlap in order to link together, in contrast to existing model predictions. Dilation adjacent to a blunt-ended dyke may also be accommodated by intrusion of magma into shear zone fractures that vary in orientation with respect to the main dyke. Near the dyke, the near- tip stress field overrides the remote stress field and generates magma-filled shear-related fractures at high angles to the dyke plane. With increasing distance from the dyke, the remote stress field becomes dominant and resultant shear-related fractures are oriented at successively lesser angles to the dyke plane.

Is lithostatic loading important for the slip behavior and evolution of normal faults in the Earth's crust?

Normal faults growing in the Earths crust are subject to the effects of an increasing frictional resistance to slip caused by the increasing lithostatic load with depth. We use three-dimensional (3-D) boundary element method numerical models to evaluate these effects on planar normal faults with variable elliptical tip line shapes in an elastic solid. As a result of increasing friction with depth, normal fault slip maxima for a single slip event are skewed away from the fault center toward the upper fault tip. There is a correspondingly greater propagation tendency at the upper tip. However, the tall faults that would result from such a propagation tendency are generally not observed in nature. We show how mechanical interaction between laterally stepping fault segments significantly competes with the lithostatic loading effect in the evolution of a normal fault system, promoting lateral propagation and possibly segment linkage. Resultant composite faults are wider than they are tall, resembling both 3-D seismic data interpretations and previously documented characteristics of normal fault systems. However, this effect may be greatly complemented by the influence of a heterogeneous stratigraphy, which can control fault nucleation depth and inhibit fault propagation across the mechanical layering. Our models demonstrate that although lithostatic loading may be an important control on fault evolution in relatively homogeneous rocks, the contribution of lithologic influences and mechanical interaction between closely spaced, laterally stepping faults may predominate in determining the slip behavior and propagation tendency of normal faults in the Earths crust.

Global contraction/expansion and polar lithospheric thinning on Titan from patterns of tectonism

We investigate the underlying physical processes that govern the formation and evolution of Titans tectonic features. This is done by mapping mountain chains and hills using Cassini RADAR data obtained during Titan flybys T3 to T69. Our mapping of mountain chains and hills reveals a global pattern: east-west orientations within 30° of the equator and north-south between 60° latitude and the poles. This result makes Titan one of the few solar system bodies where global processes, rather than regional processes, dominate tectonism. After comparison with five global stress models showing theoretical mountain chain orientations, we suggest that either global contraction coupled with spin-up or global expansion coupled with despinning could explain our observations if coupled with a lithosphere thinner in Titans polar regions.