In this model, the overturned limb of the fault-propagation fold is preserved . Run Formation to the north), right-side up Middle Ordovician New Market . the fold-fault relationship shows a transition from folding to faulting with. Faulting - when tension and compression associated with plate movement is so great that blocks of rock fracture or break apart. - process can. We also find that the position of drainage divides at the Earth's surface has a complex relationship to the underlying fold axial surface locations.
Although the majority of normal faults are indeed high-angle, low-angle normal faults also occur because fault surface is not necessarily isotropic. A very low-angle normal fault at the base of an extending block is called a detachment fault. A low-angle normal fault that develops on top of, parallel but in an opposite direction to a thrust sheet is a lag fault.
Such an extensional fault forms almost simultaneously with the thrust fault at the base of the thrust sheet, and plays an important role in the tectonic exhumation of deep-seated rocks. A reverse fault is a dip-slip fault in which the hanging-wall has moved upward, over the footwall. Reverse faults are produced by compressional stresses in which the maximum principal stress is horizontal and the minimum stress is vertical.
However, in nature steeply or shallowly dipping reverse faults do occur because of variations in the properties of rocks such as their relative strength on a fault surface. Rasoul Sorkhabi A thrust is a low-angle reverse fault. In orogenic belts, such as the Alps, a thrust fault may transport a thick package of folded rocks over many kilometers; such a thrust sheet is called nappe in French and decke in German. Nappes, some of which have moved for over km, have long been a paradoxical phenomenon in structural geology, and geologists have tried to explain them as a result of gravity gliding of rock on an orogenic slope towards foreland ; hydrothermal fluid lubrication along thrust planes; and incremental movement of the thrust over millions of years.
Probably all these processes happen. Plate collisional tectonics provides the fundamental stress mechanism for the generation of large thrust sheets.
Seismic images from orogenic belts show that thrust faults are often rooted in a basal detachment or decollement. Moreover, in orogenic belts, thrust faults become younger toward the foreland; this sequence is referred to as foreland-propagating or piggy-back faults.
The terms overthrust and underthrust are sometimes used for low-angle, regional thrust faults with the implication that hanging-wall and footwall respectively was the active element in the thrust movement although it is difficult to verify this. Upthrust is a high-angle thrust with a great amount of uplift, often involving basement rupture.
Reverse faults and associated folds may deform the basement rocks thick-skinned deformationor only sedimentary cover detached from the basement thin-skinned deformationor occasionally both the basement and sedimentary cover respectively in the hinterland and foreland of a mountain system.
Rasoul Sorkhabi A strike-slip fault is a nearly vertical dip-slip fault in which fault blocks move horizontally, parallel to the fault strike. In this kind of fault, both the maximum and minimum principal stresses are horizontal while the intermediate stress is vertical. The direction of strike slip may be left-lateral sinistral or right-lateral dextral with respect to an observer. Large strike-slip faults are also called wrench or transcurrent faults.
A mega-shear is a continental-scale zone of deformation produced by strike-slip movement. Regional strike-slip faults are usually composed of several strands. Sometimes two segments of a strike-slip fault partly overlap but are also separated by a step-over, jog or bend; the latter area is usually deformed by transtensional releasing bend or transpressional straining bend structures depending on the directions of strike-slip movements and step-overs.
A Historical and Bibliographic Note Historically, fault terminology is biased toward the regions which have been studied in greater detail than other regions. For example, the terminology of thrust faults and folds was primarily developed in the Alps and in the Rockies, that of extensional faults in the East African-Red Sea rift system and the south-west USA Basin-and-Range province, and that of strike-slip faults in the San Andreas fault system.
Fault terminology can be complex. Geologists have tried to standardize definitions of fault-related terms as structural geology has advanced. Lower-hemisphere equal-area stereographic plots of poles to bedding from detailed geologic mapping along the North Mountain fault zone. Willis [ 22 ] originally distinguished break thrusts that formed by thrusting the connecting limb of an anticline-syncline pair, where the hanging wall anticline was overthrust and, thus, preserved the footwall syncline.
Erosion has removed the hanging wall cutoff [ 23 ]; therefore, the major synclinal structure of the North Mountain thrust sheet, Massanutten synclinorium, is in thrust contact with the large regional overturned synclines such as the Back Creek, Mount Pleasant, Supin Lick, and West Mountain synclines from north to south Figure 1.
Low-angle thrusts in the Appalachians are well documented, particularly in the Southern Appalachians [ 24 ]. However, the North Mountain fault zone in the Central Appalachians appears to be generally of moderate angle. For most of the length of the North Mountain fault zone, the fault is recognized topographically by Little North Mountain. The geometry of the various faults within the zone suggests fault dips of at least 30 to 40 degrees. This is determined by the elevation difference between the faults and the top of Little North Mountain, which gives a minimum angle for individual faults within the zone.
Suppe [ 25 ] recognized fault-propagation folds that form as stratigraphic layers are folded during propagation of a thrust through a sedimentary sequence and act similar to break thrusts. In other words, the fold forms as the fault propagates through the sedimentary sequence with progressive transfer of slip from the fault to the fold.
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Fault-propagation folds are the result of flexural bending of a layered sequence of rock in advance of the actual rupture and development of the fault plane. Though this model has been applied to thrust faults in the fold and thrust belt of the Appalachians, evidence from the North Mountain fault zone suggests that the folding occurred early in Alleghanian deformation and was not initiated by a propagating thrust fault Figure Conceptual model of detachment and folds showing progression of overturned folds and zones of shearing.
Mitra [ 26 ] described faulted detachment folds that form in rock units with high competency contrasts and transition from folding to fault propagation as shortening increases. One characteristic of faulted detachment folds is that the fold-fault relationship shows a transition from folding to faulting with footwall synclines and decapitated anticlinal fold geometries [ 26 ]. The North Mountain fault zone occurs in rocks that have high competency contrasts between the Cambrian and Ordovician carbonates and the shaly units of the upper Middle Ordovician Figure 3.
Note that the change from right-side up to overturned horses in the North Mountain fault zone occurs in the transition from competent to less competent Middle Ordovician units.
Erslev [ 27 ] and Erslev and Mayborn [ 28 ] proposed trishear fault propagation through folds where the deformation zone is bounded by the axial planes of anticlinal hinges and the corresponding synclinal hinges. In this model, shear is distributed from the tip of the fault to a widening zone on the limb of steep and overturned beds.
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This distribution of shear may be the origin of subsidiary faults that led to the development of horses within the North Mountain fault zone.
Conclusions Detailed examination of horses in the North Mountain fault zone document a fold-to-fault progression along the overturned southeast limb of a syncline to the northwest and the adjacent upright anticline to the southeast Figure Horses on the eastern part of the fault zone were derived from the upright limb or a hanging wall anticline.
They structurally overlie overturned beds in the west part of the zone that were derived from the overturned limb of the subthrust syncline. Faulting occurred following the majority of the folding. The North Mountain fault plane is on a frontal ramp and is noncoaxial planar or oblique to an overturned anticline.
This anticline is inferred since it is no longer exposed and the hanging wall cutoff has been removed by erosion.
Evidence of this anticline exists in horses exposed along the present leading edge of the fault zone from central Virginia northward to the eastern panhandle of West Virginia.
The moderate dip of the fault zone and consistent nature of the horses suggest that the North Mountain fault probably has less than 10 miles 16 kilometers of horizontal displacement.
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Detailed geologic mapping is key to understanding complex structures in fold-and-thrust belts. Understanding the relationships of structures, transitions, and competencies of rock units and right-side up to overturned nature of rocks in horses aids in determination of relative fold and fault timing in major thrust zones.
Conceptual model showing fold-to-fault progression and relationship of upright and overturned horses in the fault zone. Acknowledgments The author thanks Arthur P.
Schultz and Robert C. Leslie, British Geological Survey, and Robert Hatcher, University of Tennessee, for their suggestions and critical review of the paper. Also, the author appreciates the in-depth discussions and outcrop reviews on the structural geology of the Central Appalachians and North Mountain fault zone with C. Scott Southworth, David J. Brett Waller, and Kent D. View at Google Scholar B.
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View at Google Scholar S. View at Google Scholar M. View at Google Scholar C. Geological Survey Open-file Reportscale 1: Gathright III, and R. Virginia Division of Mineral Resources Publicationscale 1: Geological Survey Open-file Report View at Google Scholar R. View at Google Scholar E. Geological Survey Annual Report 13,