Along-strike variability of thrust fault vergence

Journal of Structural Geology, 2003

Deformation styles of orogenic belts change along strike across transverse zones (TZs); hence, the kinematics of TZs is indispensable for three-dimensional restoration of thrust belts. We investigate the causes of deviation in strike of fold axes and fault surfaces in two TZs of the southern Appalachians using structural cross-sections and paleomagnetism. In the Rising Fawn TZ of Georgia, hanging-wall lateral-ramp folds yield paleomagnetic rotations and variation in magnitude of thrust translation. Plunge of folds, differential slip, and rotations are associated with lateral-ramp geometry and along-strike variation in the rheology and thickness of units that host lower and upper detachment levels. In Alabama, the internal geometry, strike, and paleomagnetic rotations of the Helena thrust sheet change across the Anniston TZ. These differences resulted from movement of the Helena thrust sheet over different footwall blocks and sub-décollement basement graben structures, and from differential slip along an oblique ramp. Rotations of thrust sheets within TZs occur where a thrust sheet was translated over: (1) an oblique/lateral ramp with contrasting rock strengths at lower and upper detachment levels, (2) a transverse basement fault that separates contrasting basement structural domains, and (3) intersections between frontal ramps and transverse structures. Because of the local causes of rotations in lateral structures, paleomagnetic and structural analyses of TZs are necessary for understanding the kinematics and restoration of both single thrust sheets and large-scale curves in thrust belts. q

The Korean peninsula is widely regarded as being located at the relatively stable eastern margin of the Asian continent. However, more than 10 Quaternary faults have recently been discovered in and reported from the southeastern part of the Korean Peninsula. One of these, the Eupchon Fault, was discovered during the construction of a primary school, and it is located close to a nuclear power plant. To understand the nature and characteristics of the Quaternary Eupchon Fault, we carried out two trench surveys near the discovery site. The fault system includes one main reverse fault (N20 ∞ E/40 ∞ SE) with approximately 4 m displacement, and a series of branch faults, cutting unconsolidated Quaternary sediments. Structures in the fault system include synthetic and antithetic faults, hanging-wall anticlines, drag folds, back thrusts, pop-up structures, flat-ramp geometries and duplexes, which are very similar to those seen in thrust systems in consolidated rocks. In the upper part of the fault system, several tip damage zones are observed, indicating that the fault system propagates upward and terminates in the upper part of the section. Pebbles along the main fault plane show a preferred orientation of long axes, indicating the fault trace. The unconformity surface between the Quaternary deposits and the underlying Tertiary andesites or Cretaceous sedimentary rocks is displaced by this fault with a reverse movement sense. The stratigraphic relationship shows normal slip sense at the lower part of the section, indicating that the fault had a normal slip movement and was reversely reactivated during the Quaternary. The inferred length of the Quaternary thrust fault, based on the relationship between fault length and displacement, is 200– 2000 m. The current maximum horizontal compressive stress direction in this area is generally east-northeast–west-southwest, which would be expected to produce oblique slip on the Eupchon Fault, with reverse and right-lateral strike-slip components.

AAPG Bulletin, 1981

Two-dimensional modeling of loading during the formation of the Idaho-Wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin. The flexural rigidity of the lithosphere is inferred to have been approximately 1023 Nm (1030 dyne cm), on the basis of palinspastic comparison of predicted downwarping, due to the thrust plate loads, to the shape of the sedimentary wedge on the west side of the Cretaceous Western Interior seaway. Erosion of part of the uplifted thrust plates redistributed the load, depositing it farther to the east, thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates. Paleotopography after major Cretaceous thrust events was calculated. The resulting mountainous terrain, gentle alluvial plain, and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the Andes of South America and to paleogeographic reconstructions in the western United States thrust belt. Topography is controlled by the subsurface geometry of the thrust faults, particularly the positions of ramp zones, and by isostatic subsidence.

American Journal of Science, 2005

Most tectonic models for the Sevier Desert basin, west-central Utah, envision it as the result of large-magnitude, normal-sense slip on a regional detachment fault. That interpretation, based principally on seismic reflection data, has helped shape views on the tectonics of the northeastern Great Basin area and, in a larger sense, the historical development of ideas about low-angle normal faulting. In recent years, however, several researchers have suggested, based on rock-mechanical, field, and subsurface evidence, that the hypothesized detachment fault does not exist and that the basin must have another explanation. Even among proponents of the detachment model, opinion has differed regarding the hypothesized extensional fault's total displacement, estimates for which vary widely; the timing of detachment slip; and whether the hypothesized fault represents a "new" Tertiary extensional structure or an extensional reactivation of the Mesozoic Pavant thrust. A comprehensive reinterpretation of available subsurface data for the basin, including several previously unpublished seismic profiles, suggests: (1) that slip on the hypothesized detachment must have ceased by the Miocene in the southern Sevier Desert; (2) that estimates of large-magnitude offset on the hypothesized detachment are essentially unconstrained by structural data and need to be reevaluated; and (3) that models that view the hypothesized detachment as a Tertiary extensional reactivation of a Mesozoic thrust are likely incorrect. The newly available seismic data demonstrate that reflections from the Pavant thrust do indeed closely align with reflections from the Paleozoic/Tertiary contact in many parts of the northern Sevier Desert basin; however, to the south these same thrust fault reflections are directly traceable to a position well above the down dip projection of the presumed detachment within the Paleozoic Cricket Mountains block. Erosional truncation of the thrust faults and the absence in the south of other reflections aligned with the Paleozoic/Tertiary contact preclude extensional backsliding on a Mesozoic thrust fault. These interpretations, if correct, are incompatible with the detachment hypothesis and necessitate alternative explanations for the basin's origins. introduction Over the past three decades, evidence has accumulated, particularly in the North American Basin and Range province, for the role of regional low-angle, normal-sense shear surfaces in extensional processes (see references in Wernicke, 1995). These detachment faults, it has been persuasively argued, are important not merely as local gravity-glide features but as regional-scale structures fundamental to continental geodynamics (Wernicke, 1981; Gans, 1987; Lister and Davis, 1989; Wernicke, 1995, and references therein). Yet, despite extensive field study of detachments, it remains unexplained how rooted, upper-crustal normal faults dipping at angles of only a few degrees to the horizontal can slip, when classical fault mechanics (Anderson, 1942) and the record from seismicity (Jackson, 1987; Collettini and Sibson, 2001) suggest that they should initiate and move at much steeper dips. Explanations proposed to resolve this paradox have included reorientation of the tectonic stress field (Yin, 1989;

Marine and Petroleum Geology, 2009

This paper shows the evolution of time-constrained two-dimensional scaled analogue models of doubly vergent thrust systems in the presence of syntectonic sedimentation. Two sets of experiments were considered: (1) the addition of a syntectonic layer composed of a polymer and overlying sand in the prowedge; and (2) the addition to the previous condition of a progradational sedimentary load. Results from the first set of experiments indicate that the foreland fold-and-thrust belt has a strong relationship with the competence of the syntectonic layers. When the competence is low, the deformation produces tight asymmetric detachment-folds. As the competence increases, the fold-and-thrust belt shows breaktrough folds with longer and better defined foreland-vergence. Results from the second set of experiments indicate that structural vergence is determined by the sense of progradation of the syntectonic layers, and in the case of strong aggradation at the prowedge, extension and reactive diapirism form contemporaneously with the contraction. Three end-members are proposed for mountain front thrust systems formed in the presence of syntectonic polymer and sand sedimentation: (1) outcropping foldand-thrust belt sequence, in the case of no syntectonic sedimentation; (2) long displacement blindthrust sheets, in the case of under-filled basins and (3) short displacement blind-thrust sheets, in the case of over-filled basins. All results indicate that ductile units at the base of syntectonic layers increase the displacement of the underlying frontal thrusts at the prowedge, and reduce the critical taper. Results also indicate that at very high sedimentary rates and hyper-critical taper conditions the prowedge collapses. Conclusions drawn from this research may be applied as an analogue to foreland evolution and to evaluate hydrocarbon generation, migration, and entrapment in thrust belts in areas where seismic imaging is generally poor.

Journal of Structural Geology, 2017

We investigate changes in shortening, displacement and fold geometry to understand the detailed alongstrike structural variation within fold-thrust belts, and infer thrust growth and linkage mechanisms. Field observations from the Vercors in SE France are used to characterise deformation style in the region. Parallel cross sections are constructed, analysed and used to create shortening and thrust displacement profiles from the northern to southern Vercors. Sections show changes in structural style and shortening accommodation from thrust-dominated in the north to fold-dominated in the south. The total shortening distance in the Vercors does not change significantly along strike (3400e4650 m), however displacements along individual thrust zones do vary significantly and displacement profiles show a range in displacement gradients (16e107 m/km). Despite relatively simple shortening patterns in the Vercors, sections show a more complex 3D internal structure of the fold-thrust belt. Thrust displacements and geometries suggest both large-scale thrust zones and small-scale thrusts are soft linked, transferring displacement along strike through transfer zones. Short, soft-linked thrust segments indicate an intermediate stage of thrust growth and linkage, well documented for normal fault systems, which form prior to the formation of thrust branches and hard-linked displacement transfer.

Journal of Structural Geology, 1986

The dominant deformation mechanism within foreland thrust sheets is the displacement of blocks of weakly deformed rock on discrete, mineral-coated minor faults. In highly deformed layers at the bases of three Southern Appalachian foreland sheets, such minor faults are members of families or populations of consistently oriented structures. The deformation style is similar in all three sheets. Rock nearest each thrust was shortened in the direction of sheet transport and thickened by a family of faults oriented at low angles to the thrusts. Rock further above each thrust experienced only the first of these two deformation stages. Statistical analyses of fault populations from these three sheets demonstrate that their deformation is quantitatively as well as qualitatively similar. Deformation due to movement on minor faults is statistically homogeneous at the scale of an outcrop in one of these sheets; strain due to minor faults can be measured in that sheet. A strain profile across this sheet is similar to strain profiles across continuously deformed sheets from internal portions of mountain belts, suggesting that their responses to emplacement are similar.

The mountain Geologist, 2022

This study integrates the results of numerical modeling analyses based on outcrop studies and structural kinematic restorations to evaluate the mechanics of thrust fault initiation and development in mechanically layered sedimentary rocks. A field-based reconstruction of a mesoscopic thrust fault at Ketobe Knob in central Utah provides evidence of thrust ramp nucleation in competent units, and fault propagation upward and downward into weaker units at both fault tips. We investigate the effects of mechanical stratigraphy on stress heterogeneity, rupture direction, fold formation, and fault geometry motivated by the geometry of the Ketobe Knob thrust fault in central Utah; the finite element modeling examines how mechanical stratigraphy, load conditions, and fault configurations influence temporal and spatial variation in stress and strain. Our modeling focuses on the predicted deformation and stress distributions in four model domains: (1) an intact, mechanically stratified rock sequence, (2) a mechanically stratified section with a range of interlayer frictional strengths, and two faulted models, (3) one with a stress loading condition, and (4) one with a displacement loading condition. The models show that early stress increase in competent rock layers are accompanied by low stresses in the weaker rocks. The frictional models reveal that the heterogeneous stress variations increase contact frictional strength. Faulted models with a 20° dipping fault in the most competent unit result in stress increases above and below fault tips, with extremely high stresses predicted in a 'back thrust' location at the lower fault tip. These findings support the hypothesis that thrust faults and associated folds at the Ketobe Knob developed in accordance with a ramp-first kinematic model and development of structures was significantly influenced by the nature of the mechanical stratigraphy. * σ1 is horizontal † σ3 is vertical (downward) Fault µ = 0.85

Low-angle detachment faults and thrust-sheet top basins are common features in foreland basins. However, in stratigraphic analysis their influence on sequence architecture is commonly neglected. Usually, only eustatic sea level and changing flexural subsidence are accounted for, and when deformation is considered, the emphasis is on the generation of local thrust-flank unconformities. This study analyses the effects of detachment angle and repetitive detachment activation on stratigraphic stacking patterns in a large thrust-sheet top basin by applying a three-dimensional numerical model. Model experiments show that displacement over low-angle faults (2–6°) at moderate rates (∼5.0 m kyr−1) results in a vertical uplift component sufficient to counteract the background flexural subsidence rate. Consequently, the basin-wide accommodation space is reduced, fluvio-deltaic systems carried by the thrust-sheet prograde and part of the sediment supply is spilled over towards adjacent basins. The intensity of the forced regression and the interconnectedness of fluvial sheet sandstones increases with the dip angle of the detachment fault or rate of displacement. In addition, the delta plain is susceptible to the formation of incised valleys during eustatic falls because these events are less compensated by regional flexural subsidence, than they would be in the absence of fault displacement.

The Journal of …, 1994

The Axhandle basin in central Utah is a piggyback basin that formed above the Gunnison thrust system. The basin contains-1 km of upper Cretaceous-to-Eocene strata of the North Horn and lower Flagstaff Limestone formations. A significantly more exact chronology, based on magnetic polarity stratigraphy, has been developed for the basinfilling strata. The new data provide a detailed temporal framework for the interpretation of the depositional and tectonic evolution of the basin during chrons 33-21 (-75-49 Ma). Tectonic subsidence curves and stratigraphic relations along the eastern basin margin define two episodes of motion on the Gunnison thrust system. The majority of tectonic shortening on the system occurred between 83-75 Ma and caused widespread non-deposition within the Axhandle basin. A second episode of motion, in detail comprising of four separate periods of backthrusting, occurred between 70-60 Ma. The Canyon Range, Pavant, and Nebo thrust sheets were emplaced 30-50 km to the west of the Axhandle basin between 97 and 74 Ma. The initial episode of motion on the Gunnison thrust system was thus partly coeval with the emplacement of these thrust sheets. Uplift and erosion above the San Rafael Swell, a basement flexure located-100 km east of the Axhandle basin, occurred at the same time-between 70 and-60 Ma-as the second episode of motion on the Gunnison thrust system. These age constraints clearly demonstrate coeval motion on multiple contractional structures. Episodes of thrust wedge reorganization with durations of 5-10 m.y. appear to be separated by quiescent periods of up to 5 m.y. Evolution of the thrust belt in NE Utah and Wyoming was remarkably similar to that in central Utah 300 km to their south. This similarity suggests that the tectonic stresses driving the thrust wedge were relatively uniform along strike.