Breakaway Fault Research Articles

Tertiary sedimentary rocks associated with the harcuvar core complex in Arizona (U.S.A.): insights into paleogeographic evolution during displacement along a major detachment fault system

Geologic mapping and analysis of Oligocene-Miocene sedimentary rocks in the upper plate of the Buckskin-Rawhide detachment fault system (west-central Arizona) reveals a complex paleogeographic history during fault displacement, involving shifting sediment-source areas and multiple drainage reversals. Within the study area, four upper-plate fault blocks are capped by homoclinal sedimentary sections that display fanning dip relationships indicating concurrent tilting and sedimentation. During deposition of these strata, upper-plate sedimentation was intermittently confined to separate half-graben, while at other times rapid rates of aggradation relative to fault-displacement resulted in burial of ridges separating sub-basins. Four sedimentary assemblages recognized within the study area can be correlated between fault blocks on the basis of lithologic similarity, stratal position, provenance, paleocurrent data, and sparse geochronologic constraints. The basal assemblage consists of lacustrine rocks and interfingering fluvial strata composed of detritus derived from the granitic terrane surrounding the northern part of the study area. These sediments were deposited during the earliest stages of tilting of upper-plate fault blocks, and inferred displacement along the Buckskin—Rawhide detachment fault system. The lower assemblage, which directly overlies the basal assemblage, consists of fine-grained lacustrine deposits, sandy conglomerate and breccia. During lower-assemblage deposition, mass-flow-dominated alluvial fans were derived from source areas consisting mainly of Mesozoic and Paleozoic rocks exposed to the south of the study area. These source areas may have been associated with the breakaway fault of the detachment system, or with fault blocks stranded above structurally elevated parts of the footwall. Tilting of fault blocks continued during deposition of the lower assemblage and strongly affected dispersal patterns and lithofacies distributions; lacustrine sediments were deposited mainly within large, deep sub-basins and, with few exceptions, lower-assemblage streams followed half-graben axes. The middle assemblage consists of conglomerate and sandstone deposited by an extensive south-directed stream system that probably flowed off undistended parts of the hanging wall, across extended parts, and locally onto the footwall. Tilting of upper-plate fault blocks continued during middle-assemblage deposition, but rapid burial of escarpments by sediment permitted relatively unobstructed southward drainage. The upper assemblage consists of sandy conglomerate deposited by a northeast-directed system of broad, shallow streams. These deposits display a variety of clast types, including Tertiary mylonitic, sedimentary, and volcanic rocks that were eroded from the upwarped footwall of the core complex and overlying klippen. Upper-assemblage streams maintained relatively consistent northeasterly flow, even during the early stages of deposition when some fault blocks were still rotating.

Prevolcanic extensional Seaman breakaway fault and its geologic implications for eastern Nevada and western Utah

Stratigraphic and structural relations indicate that a large-displacement east-dipping normal fault, the Seaman fault, is concealed under southern White River Valley, Nevada. The Seaman fault forms the breakaway of an eastward-rooting pre-middle Oligocene (pre-30.6 Ma) extensional detachment system and may connect northward to the breakaway of the Oligocene extensional system exposed in the Snake, Schell Creek, and Egan Ranges. We propose that this regional breakaway zone bounds a single eastward-rooting Oligocene extensional belt, the Snake-Stampede system, that affected much of eastern Nevada and western Utah. The southward migration with time of Tertiary volcanism in the Great Basin caused varying temporal relations between volcanism and deformation in the Snake-Stampede system; approximately synchronous extension is prevolcanic to the south but syn- to postvolcanic to the north. This pattern conflicts with widely accepted temporal and genetic links between volcanism and crustal extension. Basin and Range crustal extension probably neither triggered nor was triggered by magmatism, but instead volcanism and extension operated somewhat independently.

Surface expression of an accommodation zone within the Gulf of Suez rift, Egypt

The Gebel Gharamul region in the western Gulf of Suez offers an excellent outcrop example of the structural geometries associated with an accommodation zone termination. The surface expression of the accommodation zone is dominated by a basement promontory, which protrudes from the rift shoulder and underlies the junction of oblique ramps from two adjacent half-grabens, both of which face the Gulf of Suez. The southern half-graben represents the breakaway fault of the southwest-dipping tilt-block domain, and the northern half-graben represents the collapse of the upper plate above the northeast-dipping tilt-block domain. Basement and Nubian rocks in the hanging walls above the opposing low-angle detachments on either side of the accommodation zone are broken by gulf-parallel synthetic normal faults and orthogonal transfer faults, which act as block terminations and incrementally accommodate the large-scale rotation and displacement between adjacent blocks. The prerift and synrift stratigraphic succession is draped over this fault template, resulting in a complex and variable distribution of structures, the product of an inhomogeneous stratigraphic succession within an accommodation zone.

Cross-faults and differential stretching of hanging walls in regions of low-angle normal faulting: examples from western Turkey

Summary Large oblique-throw hinge faults orientated at high angles to the strike of major breakaway faults in W Turkey are examined. They exhibit variable displacements which commonly increase towards the breakaway fault with which they seem to be associated and bound cross-blocks that in places have rotated around vertical axes by differing amounts and senses, and are interpreted in terms of the possibility that hanging walls of major low-angle breakaway faults (extensional allochthons), may exhibit differential internal extension accommodated by transverse faults that may or may not rotate around vertical axes. Such transverse faults, here called accommodation faults , are different from transfer faults in that they need not offset the main breakaway fault and may accomplish significant map-view distortions of extensional allochthons. Irrotational accommodation faults may give rise to horst and graben complexes or may create the image of domino-style, uniformly tilted normal and/or thrust faults at high angles to the strike of major breakaways, thus giving rise to the misleading image of extension in two directions in an extensional zone or shortening at high angles to regional extension. Although strike-slip offsets along irrotational accommodation faults need not bear any relation to the magnitude of regional extension, their strikes must at least be sub-parallel to the orientation of regional extension. Real shortening at high angles to regional extension may give rise to rotational accommodation faults. Neither the strike-slip offset along such faults nor their azimuth need bear any simple relationship to the magnitude and orientation of regional extension. Rotational accommodation faults bring about an equivalent rotation of all structures that form along them, such as pull-apart basins, and the hanging wall blocks they delimit, together with their pre-existing structures. All three kinds of cross-faults, i.e. transfer faults, irrotational and rotational accommodation faults may coexist in any one extensional region and may grade into one another both in space and in time creating a very complicated structural picture and causing irregular subsidence patterns. It is stressed that cross-faults have an extremely important bearing on our understanding of the geometry and evolution of extensional zones, here called taphrogens , and that only a three-dimensional consideration of the structure of taphrogens is likely to lead to a satisfactory understanding of their nature.

The case for tectonic denudation by the Heart Mountain fault—A response

Two basic concepts pertaining to the history of the Heart Mountain fault of northwestern Wyoming have recently been challenged; one, that there was tectonic denudation, and two, that volcanic rock of the Wapiti Formation was deposited on the exposed fault surface. Tectonic denudation is believed to have occurred as a consequence of the upper plate having broken into numerous blocks that separated as movement progressed along a nearly horizontal fault surface, thus leaving the fault surface exposed between blocks. Volcanic rocks of the Wapiti Formation were then deposited both on the exposed fault surface and against and over the upper-plate blocks. Two formations of Eocene volcanic rocks are involved. The older volcanic unit, the Cathedral Cliffs Formation, and the Paleozoic carbonate rocks are part of the upper plate of the Heart Mountain fault and moved with it, whereas the younger Wapiti Formation was deposited on the fault surface after movement had ceased. In an alternate interpretation recently advanced by T. A. Hauge, subdivisions of the Absaroka Volcanic Supergroup, of which the Cathedral Cliffs and Wapiti Formations are units, are not recognized. The upper plate of the Heart Mountain detachment is interpreted as having been a single, continuous allochthon composed largely of volcanic rocks with small amounts of Paleozoic rocks. During Heart Mountain faulting, extension of the once-continuous slab of Paleozoic sedimentary rock is alleged to have been accompanied by the formation of ten or more grabens, now filled predominantly by Absaroka volcanic rocks. This interpretation further proposes that the volcanic rocks were emplaced while the separating blocks of Paleozoic strata were still moving and that the basal part of the volcanic rock between these blocks is in fault contact rather than depositional contact with the strata beneath the Heart Mountain fault. Many lines of geologic field evidence indicate that the Wapiti Formation is younger than the Heart Mountain fault and was deposited on the technically denuded fault surface. (1) Wapiti rocks bury the break-away fault. (2) Fault breccia at the base of the upper-plate carbonate blocks is composed entirely of carbonate fault breccia and has no volcanic component. (3) Small blocks of upper-plate rocks have been displaced by gravity from the upper part of the allochthon to the detachment fault surface. (4) Eocene stream-channel deposits locally cut into the surface of tectonic denudation and also have been displaced on the Heart Mountain fault. (5) The volume of Wapiti Formation filling the spaces between allochthonous blocks in proportion to the volume of those blocks is much too great for the Wapiti to have been allochthonous. (6) Clastic dikes of carbonate fault breccia penetrate Wapiti volcanic rocks. (7) Some of these clastic dikes of fault breccia contain Precambrian xenoliths and wood phenoclasts requiring surface exposures of the fault breccia before injection as dikes. (8) Wapiti volcanic rocks having chilled borders are in tightly bonded contact with upper-plate Paleozoic rocks. (9) Faults present in the upper-plate blocks do not penetrate the overlying Wapiti Formation. (10) Volcanic fault breccia is absent where volcanic rocks overlie carbonate fault breccia. (11) A mound of carbonate fault breccia is not mixed with overlying Wapiti Formation. The continuous allochthon interpretation is based on several erroneous assumptions that cannot be supported by field observations. (1) Faults to transport and emplace the Wapiti Formation onto and along the Heart Mountain fault do not exist. (2) The contact between volcanic rocks and the allochthon west of Corral Creek at the west end of Cathedral Cliffs, cited by Hauge as a fault in an extending allochthon, is a depositional contact. (3) The volcanic rock adjoining allochthonous Paleozoic rocks north of Pilot Creek cannot be part of an extending allochthon because (a) it is Cathedral Cliffs Formation, which is pre–Heart Mountain fault, and (b) its direction of movement is horizontal rather than down dip, as required in an extending allochthon. (4) Most of the igneous dikes were intruded after the Heart Mountain fault movement ceased, and so they could not accommodate significant extension of the upper plate. (5) Striae reported as indicating fault emplacement of volcanic rock (Wapiti Formation) on the Heart Mountain fault actually lire flow features, formed as the Wapiti Formation was deposited on the exposed fault surface. Tectonic denudation is the only model that is consistent with evidence observable in the field. Although the process by which tectonic denudation was accomplished remains enigmatic, tectonic denudation remains a constraining fact in any model for the origin of the Heart Mountain fault.

Ravia Nappe, Bryan County, Oklahoma: A Gravity Slide Block off the Tishomingo Uplift: ABSTRACT

The Ravia nappe in Bryan County, Oklahoma, is located along the southwestern flank of the Tishomingo uplift, between the Cumberland and East Durant oil fields. This mass of Cambrian-Ordovician through Mississippian sediments tectonically overlies younger Springer shales (Pennsylvanian) of the Ardmore basin. Previously, this feature has been interpreted to have been thrust southward along the Cumberland fault, a fault parallel to the Ravia thrust. Reinterpretation of this area, with additional well data, indicates the Ravia nappe is a gravity slide block off the uplifted Tishomingo mountains. The Ravia nappe subcrops below the Cretaceous unconformity as a 16 mi2 (41 km2) triangular-shaped mass with a maximum thickness of 4,800 ft (1,450 m). The fault surface along its base has a bowl-like shape. This surface is neither the Cumberland fault nor the Ravia fault, but appears to be a separate fault surface. The nappe consists of overturned Caney through Simpson rocks (Mississippian-Ordovician) along its southwestern edge (toe), whereas right-side-up Arbuckle carbonates (Cambrian-Ordovician) occur on the northeastern side (heel) of the nappe. Arbuckle carbonates of the nappe overlie an overturned-to-the-south syncline of Arbuckle through Springer rocks. This overturned syncline, on the footwall side of the Ravia fault, is present north of the Cumberland oi field and trends southeastward into Sec. 9, T6S, R8E. At this location the structural style of the Tishomingo footwall rocks changes from an overturned syncline south of the Ravia thrust to another thrust with a footwall fold. The Ravia thrust possibly terminates near this change in structural style with the other northeast-dipping thrust continuing southeastward to the East Durant oil field. The Ravia nappe is interpreted to have been originally the southwest overturned limb of the Tishomingo uplift. Prior to the major thrusting on the Ravia thrust, but after compressional folding and uplift of the Tishomingo mountains, a breakaway fault formed across the most intensely folded beds. This breakaway fault undercut the overturned southwestern limb of the Tishomingo uplift in a concave-upward fault surface. Gravitational forces caused the Ravia nappe Mississippian Caney rocks to Cambrian-Ordovician Arbuckle rocks to slide rotationally southwestward 2.5 mi (4 km). Topographic relief prior to the slide may have been as much as 9,000 ft (2,700 m). The slide occurred sometime during late Morrowan to early Desmoinesian. Analogs of this type of deformation are present in the Owl Creek Mountains, central Wyoming; Front Range, northern Colorado; Qal'eh Raisi, southwestern Iran; and Belton anticline and Sulphur syncline, Oklahoma. These analogous structures and the Ravia nappe, show three common elements: (1) they are competent, erosion-resistant units which slide off the flanks of folded anticlines across softer shaly units; (2) the heel portion of the slide surface is a bedding plane fault, whereas the toe of the slide surface cuts across bedding planes; and (3) the driving mechanism is the gravitational force associated with tectonic uplift. End_of_Article - Last_Page 1325------------

The Heart Mountain break-away fault, northwestern Wyoming

The Heart Mountain break-away fault was the last of the four phases of the Heart Mountain detachment fault to be discovered and is the only fault of this type yet recognized and described. The descriptive term, “break-away fault,” was introduced in 1960 for the steep surface along which the rocks of the Heart Mountain decollement fault mass separated from the stationary block on the west. The break-away fault, located near the northeast entrance to Yellowstone National Park, has a known linear extent of 37 km. The northern one-third trends north, and the remainder trends southeast and south. Rocks on its western side are Paleozoic carbonate rocks and Eocene volcanic rocks; the volcanic rocks are pre–Heart Mountain fault in age. The rocks on its eastern side are younger and consist of Eocene volcanic rocks which were deposited against and over the fault. Thus the break-away fault is unusual in that one side is a fault surface and the other side is a depositional surface, and in that respect is similar to parts of the bedding-plane phase of the Heart Mountain detachment fault. Slickensides and brecciation in the Madison Limestone adjoining the break-away indicate horizontally directed compressive shear. Movement was probably right lateral, and then laterally southeastward away from the fault at an angle of about 45°. To the north, in Wolverine Pass and in the valley of Soda Butte Creek, the break-away fault extends down to a bed in the basal part of the Bighorn Dolomite where it joins or terminates at the bedding plane phase of the Heart Mountain detachment fault; movement of upper-plate rocks away from the break-away fault increases from about 0.25 km at the northernmost trace of the fault to more than 1.5 km on the south side of Soda Butte Creek valley. Several kilometres southeastward, in Cache Creek valley, the Heart Mountain break-away fault terminates downward in volcanic rocks of the undivided Lamar River and Cathedral Cliffs Formations and joins a low-angle fault in those rocks. This low-angle fault probably steps downward to the east where it reaches the Heart Mountain detachment fault surface. Three kilometres farther to the southeast, the break-away fault is presumably represented by a low-angle fault in these volcanic rocks that probably also steps down to the northeast beneath the volcanic cover to join the Heart Mountain detachment fault. Erosion in this area probably has removed rocks that contained the high-angle part of the fault. The south end of the break-away fault is overlapped and concealed by the Wapiti Formation. It presumably continues southward, but it probably does not join a normal fault near Black Mountain, although Voight (1974) has suggested that it does.