North-dipping Thrust Faults Research Articles

Seismicity and active compressional tectonics in Santa Monica Bay, southern California

During 1973–1986 the seismicity in Santa Monica Bay consisted of an average of four events per year of ML ≥ 2.5, in addition to the 1979 Malibu (ML = 5.0) mainshock‐aftershock sequence. The 1979 sequence ruptured a north dipping thrust fault at 10–15 km depth beneath the bay. A diffuse distribution of seismicity extends across the bay from the terminus of the Palos Verdes fault to Point Dume. Two thirds of the single‐event focal mechanisms of 36 events that are located within the bay show mostly thrust or reverse faulting on west to west‐northwest striking planes. A third of the focal mechanisms exhibit strike‐slip faulting on north to north‐northwest striking planes. A stress inversion of the focal mechanism data shows that the maximum principal stress is oriented N13°E in a horizontal plane and the minimum principal stress is oriented N102°E and plunging 47°. Hence the tectonics in Santa Monica Bay are dominated by compression, with thrust, reverse, and strike‐slip faulting. The coexistence of thrust and strike‐slip faults suggests that the bay is not a part of the Peninsular Ranges terrane but forms a transition zone that accommodates the change from strike‐slip in the Peninsular Ranges to the south to reverse faulting in the Transverse Ranges to the north. Mapped anticlines and the previously unrecognized thrust faults beneath the bay form two fold and thrust belts that present additional seismic hazards to Los Angeles and the southern California coastal zone through seismic shaking and possible generation of small tsunamis.

The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: Seismological and tectonic analysis

The October 1, 1987, Whittier Narrows earthquake (ML = 5.9) was located at 34°2.96′N, 118°4.86′W, at a depth of 14.6±0.5 km in the northeastern Los Angeles basin. The focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25°±5° and 65°±5°, respectively. The aftershocks define an approximately circular surface that dips gently to the north, centered at the hypocenter of the mainshock with a diameter of 4–6 km. Hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25° north dipping thrust fault striking west and is confined to depths from 10 to 16 km. Although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting, they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock. A cluster of reverse faulting events on north striking planes occurred within hours after the mainshock, 2 km to the west of the mainshock. The largest aftershock (ML = 5.3) occurred on October 4 and showed mostly right‐lateral faulting on the same north‐northwest striking plane within the hanging wall. Similarly, several left‐lateral focal mechanisms are observed near the eastern edge of the mainshock rupture. The earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays. When relocating the blast using the new models and delays, the absolute hypocentral location bias is less than 0.5 km. The mainshock was followed by nearly 500 locatable aftershocks, which is a small number of aftershocks for this magnitude mainshock. The decay rate of aftershock occurrences with time was fast, while the b value was low (0.67±0.05) for a Los Angeles basin sequence.

Seismic reflection images of the crust of the northern part of the Chugach Terrane, Alaska: Results of a survey for the Trans‐Alaska Crustal Transect (TACT)

Deep crustal seismic reflection data show strong reflections from the middle and lower crust of the convergent continental margin near the eastern end of the Aleutian trench. These data were collected across the Border Ranges fault system, a major suture zone that separates the Peninsular and Chugach tectonostratigraphic terranes. The Chugach terrane consists of metamorphosed, strongly deformed flysch, subduction melange as well as blueschist‐ and greenschist‐facies rocks. Even so, highly reflective rocks lie at great depth within or beneath this terrane. The shallow part of the seismic section (0–5 s) reveals only a few reflections, which undulate and interweave, indicating that some of them originate outside of the plane of the seismic section. Other undulating events could reveal early Cenozoic granitic plutons or antiformal stacks of rock imbricated along thrust faults. In contrast to this shallow reflection pattern, three reflection bands contain most of the events on the middle part of the seismic section (5–12 s or 12–34 km). We interpret divergent reflections within the uppermost band (5–5.5 s) as gently north dipping thrust faults, and subparallel reflections truncated at the boundaries of the upper and middle bands may image a metamorphic foliation or thrust faults. If the truncated reflections represent thrust faults, then the reflection geometry suggests that subparallel roof and floor thrust faults bound imbricated rocks, forming duplex structures. The upper and middle bands are separated by an area on the seismic section that reveals few reflections. This area correlates approximately in depth with a high velocity (7.5 km/s) layer evident in refraction models. The poorly reflective, high velocity unit might be volcanic rocks like those exposed in the structurally lowest part of the Chugach terrane. If so, then the bottom of the poorly reflective rocks indicates the base of the Chugach terrane. Rocks that cause the upper and middle reflection bands and the intervening, poorly reflective volcanic rocks may form a midcrustal shear zone that is about 10 km thick. The deepest band of subparallel reflections (30–35 km) correlates closely in depth with the top of the Wadati‐Benioff zone associated with the underthrusting Pacific plate. The reflections probably stem from the decollement that separates lithospheric plates within the subduction zone. This reflection band increases abruptly in thickness, possibly indicating local underplating of subducted sediment. No events from the oceanic Moho have been recognized. Neither the Border Ranges fault system, the suture between the Chugach and Peninsular terranes, nor deep crustal layers of the Peninsular terrane are evident in these seismic reflection data.

Thick- and thin-skinned tectonics in the Pyrenees

A comparison is made between the Gavarnie thrust and the Mérens Fault in the Axial zone of the Pyrenees. The former has a gentle dip and quite a large displacement (at least 12 km) but does not cut through either Hercynian or Alpine isograds. The latter has a smaller displacement (~ 5 km) but dips steeply and cuts through both Hercynian and Alpine isograds at a high angle. On this basis and on the basis of shear zone geometries immediately north of it, it is proposed that the Mérens Fault nucleated as a steeply (65°–80°) dipping structure, while the Gavarnie thrust nucleated with a shallow attitude. The Mérens Fault is not a backward-rotated thrust fault, nor is it the root zone for any major nappe structure. Similar steep ductile structures occur within the Gavarnie nappe and may reflect considerable internal strain in basement lithologies. The relationship between steep and shallow structures is not yet clear; the shear zones may pre-date the thrusting in which case they may be thick-skinned structures affecting the whole lithosphere, or they may be contemporary with thrusting reflecting only local thickening above a décollement. Rheological models can be used to test proposed geometrical and kinematic models for the lithosphere-scale evolution of the Pyrenees. Suggested models are dominated by a cool, rigid, high-level mantle wedge beneath the North Pyrenean zone which probably controlled the location of north-dipping thrust faults. Thick-skinned shortening is possible in thick crust in the Axial zone but is very unlikely in the North Pyrenean zone where steeply rooted structures would have to cut through the strongest part of the lithosphere.

Structure and deformation history of the Bitlis suture near Lake Hazar, southeastern Turkey

Detailed mapping of the Bitlis suture zone near Lake Hazar shows that it is composed of three tectonostratigraphic units which crop out as distinct north-dipping thrust slices. From south to north, these are the Puturge Metamorphic Complex, the Maden Melange, and the Elazig Igneous Complex. The Puturge Metamorphic Complex consists of pre-Tertiary, continental-margin sediments metamorphosed to the greenschist facies during the Campanian-Maastrichtian and deformed by four generations of structures. These are, in sequence (1) isoclinal folds and a transposition foliation, (2) open folds, crenulation cleavage, and north-dipping thrust faults, (3) kink bands, and (4) small-displacement faults. The Maden Melange represents middle Eocene, back-arc-basin sediments and volcanics metamorphosed to the greenschist facies and deformed by three generations of structures: (1) a north-dipping cleavage, (2) kink bands, and (3) small-displacement faults. The Elazig Igneous Complex comprises an imbricated Maastrichtian-early Eocene island arc and young marginal-basin terrain. Thrust faults between units are north-dipping, listric, and form a thin-skinned system. These features suggest the following deformational and tectonic history. The Puturge Metamorphic Complex was generated by isoclinal folding and metamorphism of sediments composing the Arabian continental margin during Campanian-Maastrichtian ophiolite obduction to the south. The Elazig arc developed on the deformed margin because of subsequent southward subduction of oceanic lithosphere. The arc migrated to the north, opening a back-arc basin. By the early Eocene, the back-arc basin was filling with volcaniclastics of the Maden Melange, and the Elazig arc collided with a continent to the north. During middle to late Eocene, convergence caused thrust stacking of the Elazig, Maden, and Puturge Complexes; second-generation deformation in the Puturge Complex; and first-generation deformation in the Maden Melange. Continued convergence with the Arabian plate in the late Miocene caused kink banding in the Puturge and Maden Complexes. Since the late Miocene, this convergence has been accommodated by shortening and thickening along numerous internal faults.

Structural Style--Brooks Range Mountain Front, Alaska: ABSTRACT

The Brooks Range mountain front between the Sagavanirktok River and Kurupa Lake is characterized by thrust sheets of Lisburne rocks, which dominantly have stratigraphic tops to the north and either dip northward or are overturned to the south. The early (Jurassic-Neocomian) thrust belt strikes obliquely (N70°W) into the mountain front and can be traced on seismic sections into the foothills. Individual thrust structures die out westward into overturned folds and ultimately into plunging noses as the thrust belt plunges to the west and successively higher structural levels are exposed. The total thrust displacement remains essentially constant because of transfer of motion to higher thrusts. Most of the west occurs along narrow zones of instant plunge with essentially zero occurring along trend. The present east-west mountain front is oblique to the original thrust vergence (N20°E) of Jurassic-Neocomian age and is due to later (Albian and younger) Brooks Range core uplift, folding, and comparatively minor thrusting. The Lisburne folds at the mountain front respond almost plastically to the late core uplift and gravitationally slide downward on rotated north-dipping thrust faults to form cascading folds with easiest relief upward and northward. Core uplift has rotated original thrust sheets 90° in some instances so that geologic map patterns are plunge-projection cross sections of thrust plates. The pattern at Eskimo Creek (Table Mountain quadrangle, eastern Brooks Range) illustrates how a view can reveal the true structure of the thrust belt. Examples of mountain-front structures are given at Killik River, Kurupa Lake, Kaikshak Hill, Akmagolik Creek, Atigun River, and Ivishak River. End_of_Article - Last_Page 660------------

Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska

Major portions of the eastern Alaska Range, south of the Denali fault, in the McCarthy, Nabesna, Mount Hayes, and eastern Healy quadrangles, consist predominantly of the Maclaren and Wrangellia tectono-stratigraphic terranes. The Maclaren terrane consists of the Maclaren Glacier metamorphic belt and the regionally deformed and metamorphosed East Susitna batholith. The Maclaren Glacier metamorphic belt is composed of argillite, metagraywacke, and sparse andesite flows that are progressively regionally metamorphosed from lower greenschist facies to middle amphibolite facies near the East Susitna batholith. The East Susitna batholith is composed of gabbro, quartz diorite, granodiorite, and sparse quartz monzonite. Isotopic ages are as old as a K-Ar hornblende age of 87.5 m.y., possibly reset, and a U-Pb zircon age of 70 m.y. The batholith is intensely deformed and regionally metamorphosed under conditions of the middle amphibolite facies. The Wrangellia terrane is divided into two subterranes: (1) the Slana River subterrane, composed of late Paleozoic andesite to dacite flows, tuff, limestone, and argillite, unconformably overlying massive basalt flows of the Triassic Nikolai Greenstone, Late Triassic limestone, and younger Mesozoic flysch; and (2) the Tangle subterrane, a deeper-water equivalent of the Slana River subterrane, composed of late Paleozoic and Early Triassic aquagene tuff, chert, minor andesite tuff and flows, limestone, unconformably overlying pillow basalt and massive basalt flows of the Triassic Nikolai Greenstone, and Late Triassic limestone. Both subterranes are intruded by locally extensive gabbro and diabase dikes and by cumulate mafic and ultramafic sills. Less extensive terranes (two) are the Clearwater terrane, a sequence of intensely deformed chlorite schist, muscovite schist, marble, and greenstone of Late Triassic age; and an unnamed terrane of ultramafic and associated rocks of presumable Paleozoic or Mesozoic age. Each terrane or subterrane generally has (1) a distinctive time-stratigraphic sequence reflecting a unique geologic history; (2) a missing provenance for bedded sedimentary or volcanic rocks; and (3) bounding thrust or strike-slip faults, interpreted as accretionary sutures. The Maclaren and Wrangellia terranes are juxtaposed along the Broxson Gulch thrust, which consists of an imbricate series of north-dipping thrust faults. Paralleling the Broxson Gulch thrust, a few kilometres to the south, is the north-dipping Eureka Creek thrust, along which are juxtaposed the Slana River and Tangle subterranes. The Maclaren terrane is correlated with the Kluane Schist and the Ruby Range batholith in the southern Yukon Territory, which represent the northward extension of the Taku and Tracy Arm terranes. If correct, this correlation defines a minimum displacement of the Maclaren terrane along the Denali fault of ∼400 km. The Maclaren terrane is interpreted to have formed in a synorogenic Andean-type arc setting on the west margin of Mesozoic North America in the middle to late Mesozoic and early Cenozoic. The Wrangellia terrane is interpreted to have initially formed in an island-arc setting during the late Paleozoic. Subsequently in the Late Triassic, the Wrangellia terrane underwent rifting near the paleoequator, with formation of the Nikolai Greenstone and associated mafic and ultra-mafic igneous rocks. In the middle and late Mesozoic, Wrangellia migrated toward, and was accreted during, the middle Cretaceous to the Maclaren terrane along the Broxson Gulch thrust. Subsequent dispersion of both the Maclaren and Wrangellia terranes along the Denali fault and the Broxson Gulch thrust commenced during the early Tertiary and continues through the present.

Tectonic History and Progressive Development of Fold-Thrust Belt in Eastern Gulf of Alaska: ABSTRACT

The geology of the Gulf of Alaska, east of Kayak Island, records the temporal variation of three fundamentally different tectonic settings that developed owing to the interaction between plates along the western margin of North America. A late Meosozic to early Tertiary convergent margin setting is indicated by nearly contemporaneous plutonic belts, forearc-basin sequences, and accretionary terranes. In contrast, the middle Tertiary continental margin in the eastern Gulf of Alaska was relatively stable and is characterized by sedimentation in a subsiding basin with local extensional tectonism. The present tectonic setting was probably initiated during the End_Page 928------------------------------ Miocene and reflects a combinatyion of dextral strike-slip motion and oblique convergence. The proposed tectonic model suggests that most of the eastern Gulf of Alaska is underlain by early Tertiarty and older oceanic crust and that large-scale lateral displacement of the Yakutat block may not be necessary. Late Tertiary convergence in the central Gulf of Alaska, or Yakataga district, has controlled the structural evolution of a complexly deformed, fold-thrust belt consisting of numerous subparallel folds and north-dipping thrust faults. Moving from north to south, deformation in this belt becomes progressively younger and less intense; most of the structures in the offshore part of the Yakataga district are late Pliocene to Pleistocene in age. Industry leased a number of these relatively young structures in April 1976, and subsequently drilled ten dry holes. The drilling results suggest that geopressures, resulting in part from tectonic stresses, low geothermal gradients, and poor sand development appear to be characteristic of these structures and contributed to the lack of success.End_of_Article - Last_Page 929------------

Quaternary flake tectonics of the California Transverse Ranges

P -residual studies suggest that the Pacific-American plate boundary in the upper mantle lies in the central Mojave Desert, northeast of its crustal position at the San Andreas fault. Focal mechanisms of some relatively deep microearthquakes suggest a flattening with depth of north-dipping thrust faults, resulting in a mid-crustal detachment in which the central Transverse Ranges move south relative to the mantle. This makes sense geologically if the central Transverse Ranges and western Mojave Desert are considered as tectonic flakes of brittle crystalline rocks underlain by ductile Pelona Schist or Catalina Schist, which makes up the zone of decollement. Such ductile schists would occur today at depths comparable to those at which they were metamorphosed. The detachment breaks through to the surface at the Point Dume–Malibu Coast fault zone and the Red Mountain–San Cayetano–Santa Susana–Sierra Madre–Cucamonga fault zone, the latter coinciding with the southern edge of the Palmdale bulge. The Transverse Ranges flake now moves west-northwest relative to the western Mojave flake, south-southwest relative to the Pacific plate, and northwest relative to the Great Valley–Sierra Nevada. The mantle boundary may rejoin the San Andreas fault north of a southwest-trending zone of high seismicity in the Tehachapi Mountains which may be produced by convergence. The age of the present tectonic system may be the same as the age of the break-through thrust faults; these are probably no older than 2 m.y. and may be even younger.

Holocene Pacific–North American plate interaction in southern Alaska: Implications for the Yakataga seismic gap

The St. Elias, Alaska, earthquake (magnitude 7.1 M s ) on February 28, 1979, occurred along the complex Pacific–North American plate boundary between Yakutat Bay and Prince William Sound, rupturing only a fraction of the seismic gap identified in that region. To aid in evaluating the potential for, and likely site of, a future earthquake occurring in the remainder of the gap, we have formulated a kinematic model of neotectonic deformation in southern Alaska from available geologic and seismic data. In this model the part of the North American plate bordering on the Gulf of Alaska is divided into three subblocks, which are partially coupled to the Pacific plate. On the basis of the model, the gap-filling rupture or ruptures would most likely be along the north-dipping thrust faults of the Pamplona zone between Icy Bay and the eastern end of the Aleutian Trench. If the accumulated strain of 3.8 m postulated for this region were released suddenly in one event involving the remainder of the gap, the result would be an earthquake as large as magnitude 8.