High-angle Reverse Faults Research Articles

Andean deformation and rift inversion, eastern edge of Cordillera Oriental (Guateque–Medina area), Colombia

In the Guateque–Medina area, Paleozoic basement and Mesozoic rift basins have been uplifted and exhumed during the Andean orogeny (12 Ma to present). Surface exposures and subsurface data constrain the deformation style and the rift geometry. We have mapped a regional transect and restored a cross section. We have also reconciled existing stratigraphic data, from cordillera, foothills and foreland basin, and have added new data of our own. In Early Cretaceous shales, there is evidence for fault-controlled sedimentation. A brecciated evaporitic layer, which is locally emerald bearing, has acted as a regional detachment. The underlying basement, composed of Paleozoic sediments, crops out as the Quetame Massif. It was uplifted during the Andean orogeny on a series of high-angle reverse faults. The main SE-verging Tesalia fault has resulted from Andean reactivation of an Early Cretaceous normal fault, which bounded a half-rift. A series of NW-verging back-thrusts may have resulted from Andean reactivation of Paleozoic faults. Between the back-thrusts and the Tesalia fault is a basement pop-up. It may be part of a flower structure, because components of right-lateral slip have been identified. These are attributed to eastward subduction of the Nazca plate beneath South America. In general, the style and timing of Andean deformation in the Guateque–Medina area are compatible with the plate tectonic setting of the northern Andes.

The Pennine Anticline, northern England – a continuing enigma?

SUMMARY The westward-verging Pennine Anticline extends along the length of the central Pennines. It is an important Variscan fold of flexuous N–S trend developed on the Variscan Foreland. It defines the eastern margins of the Blackburn–Burnley and South Lancashire coalfields, and is commonly viewed as having been formed by the inversion of an underlying down-east Dinantian syn-depositional fault. Seismic reflection and potential field data provide some evidence on the deep structure of the anticline along much of its length. Modelling of gravity data does not provide unique solutions to basement and deep crustal structure beneath the Pennine Anticline. However, the models are consistent with both a basement/upper crustal density boundary and with variations in the Moho topography. In the southern part of the study area, these gravity data appear to define an important NNW–SSE trending, down-east fault that, when traced north, dies out and diverges from the Pennine Anticline. Where imaged by seismic reflection data, particularly over the Central Lancashire High, the fold is underlain by a high-angle reverse fault at basement level. Unusually in this part of northern England, this fault shows little evidence of a Dinantian extensional precursor, perhaps because its trend lay sub-parallel to the Dinantian extension direction. Although the structure remains enigmatic, it is suggested that the two en echelon anticlines mark folding in the Carboniferous cover rocks, developed above a through-going N–S zone of weakness in the basement comprising structures of varying trends. Under NW–SE directed Variscan compression, these structures

Evidence for Cenozoic extensional basin development and tectonic inversion south of the flat-slab segment, southern Central Andes, Chile (33°–36°S.L.)

The mainly volcanic Cenozoic deposits that make up much of the western part of the Principal Cordillera in Central Chile are generally subdivided into two major units: an older Abanico or Coya-Machalı́ Formation and a younger Farellones Formation. Difficulty in differentiating these units has led to considerable debate. On the basis of the wide distribution, great thickness, and presence of sedimentary intercalations, it has been postulated that these arc volcanics were deposited in an intermontane basin; more recently, it has been proposed that this basin developed under extensional conditions and underwent subsequent tectonic inversion. We present field, geochronologic, geochemical, and thermal maturity data that support the latter interpretation. Collectively, this new information clarifies the stratigraphic, tectonic, and paleogeographic evolution of these deposits. The vast geographic extent of the Abanico Formation and lateral equivalents, which reach from at least 32°30′ to 44°S along the Principal Cordillera, its great thickness, and the presence of repeated thick fluvial and lacustrine intercalations all indicate deposition in a large, strongly subsident, and probably north–south oriented basin, developed between middle to late Eocene and Oligocene. The unconformable contact with underlying Mesozoic units observed at several localities indicates that deposition followed a substantial erosional episode during late Cretaceous and/or early Cenozoic time. Basal deposits of the Abanico Formation near Termas del Flaco increase rapidly in thickness to the west. Still further to the west, a thick Abanico section contains, in its upper part, mammal fossils older than those found in the basal deposits near Termas. This evidence indicates a major space of deposition west of this locality, which had been filled before deposition took place at Termas. The east-vergent, high-angle El Fierro thrust fault on the east side of the westward-growing deposits is interpreted as an inverted normal fault associated with initial basin development and deposition. High-angle thrust faults observed elsewhere on the eastern outcrop margin of the Abanico Formation (i.e. the Chacayes-Yesillo Fault in the Maipo section and the Espinoza Fault in the Cachapoal–Las Leñas section) also have been interpreted as inverted normal faults. The irregular folding style of the Abanico Formation, with its highly variable amplitude, longitude, tightness, and vergency, suggests that deformation is attributable to the inversion of faults associated with basin development. Geochemical characteristics of the Abanico Formation indicate a relatively thin crust during early basin development. Thermal maturity data reflect a deep burial of the deposits during accumulation, and thermal modeling indicates high heat flow conditions during burial. These data support a major extensional episode of the crust and the development of a large depositional space (basin) in this region. On the basis of this evidence, we suggest that deposition of the Abanico Formation is related mostly to crustal extension and its deformation to tectonic inversion. In the western Las Leñas river valley, a growth structure indicates that deformation occurred between 20.8 and 16.1 Ma, while the Abanico Formation was still being deposited. Deformation apparently did not occur coevally throughout the region; however, sedimentation and volcanic deposition in the basin apparently occurred uninterrupted. This argues against a single, obvious unconformity separating the Abanico and Farellones Formations. Instead, it supports the existence of local angular unconformities where fault inversion affected the basin fill. Comparison of the timing of extensional basin development and subsequent contraction (inversion) with the convergence rates between the Nazca and South American plates during the Cenozoic period shows a correspondence with periods of decreasing and increasing convergence rates, respectively. Tectonic and volcanic events on the east versant of the Andes [Journal of South American Earth Sciences 15 (2002)], which are coeval with the basin inversion and crustal thickening episodes presented herein and, therefore, with the previously mentioned period of increasing convergence rate, are assumed to correspond with the same episode of major tectonic accommodation of the crust in this Andean region. It is not yet possible to determine if the collisional event of the Juan Fernández ridge at approximately 15 Ma in the flat-slab segment region had a local or a more regional effect on the late Cenozoic tectonic evolution of the continental margin in the Central Andes.

Formation of diapiric structure in the deformation zone, central Indian Ocean: A model from gravity and seismic reflection data

Analyses of bathymetry, gravity and seismic reflection data of the diffusive plate boundary in the central Indian Ocean reveal a new kind of deformed structure besides the well-reported structures of long-wavelength anticlinal basement rises and high-angle reverse faults. The structure (basement trough) has a length of about 150 km and deepens by up to 1 km from its regional trend (northward dipping). The basement trough includes a rise at its center with a height of about 1.5km. The rise is about 10 km wide with rounded upper surface and bounded by vertical faults. A broad freeair gravity low of about 20 mGal and a local high of 8 mGal in its center are associated with the identified basement trough and rise structure respectively. Seismic results reveal that the horizontal crustal compression prevailing in the diffusive plate boundary might have formed the basement trough possibly in early Pliocene time. Differential loading stresses have been generated from unequal crust/sediment thickness on lower crustal and upper mantle rocks. A thin semi-ductile serpentinite layer existing near the base of the crust that is interpreted to have been formed at mid-ocean ridge and become part of the lithosphere, may have responded to the downward loading stresses generated by the sediments and crustal rocks to inject the serpentinites into the overlying strata to form a classic diapiric structure.

Syntectonic deposits and punctuated limb rotation in an Albian submarine transpressional fold (Mutriku village, Basque-Cantabrian basin, northern Spain)

Research Article| March 01, 2002 Syntectonic deposits and punctuated limb rotation in an Albian submarine transpressional fold (Mutriku village, Basque-Cantabrian basin, northern Spain) L.M. Agirrezabala; L.M. Agirrezabala 1Estratigrafia eta Paleontologia Saila, Euskal Herriko Unibertsitatea, 644 postakutxa, 48080 Bilbao, Spain Search for other works by this author on: GSW Google Scholar H.G. Owen; H.G. Owen 2Natural History Museum, Cromwell Road, London SW7 5BD, UK Search for other works by this author on: GSW Google Scholar J. García-Mondéjar J. García-Mondéjar 3Estratigrafia eta Paleontologia Saila, Euskal Herriko Unibertsitatea, 644 postakutxa, 48080 Bilbao, Spain Search for other works by this author on: GSW Google Scholar Author and Article Information L.M. Agirrezabala 1Estratigrafia eta Paleontologia Saila, Euskal Herriko Unibertsitatea, 644 postakutxa, 48080 Bilbao, Spain H.G. Owen 2Natural History Museum, Cromwell Road, London SW7 5BD, UK J. García-Mondéjar 3Estratigrafia eta Paleontologia Saila, Euskal Herriko Unibertsitatea, 644 postakutxa, 48080 Bilbao, Spain Publisher: Geological Society of America Received: 28 Jun 2001 Revision Received: 22 Aug 2001 Accepted: 26 Oct 2001 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2002) 114 (3): 281–297. https://doi.org/10.1130/0016-7606(2002)114<0281:SDAPLR>2.0.CO;2 Article history Received: 28 Jun 2001 Revision Received: 22 Aug 2001 Accepted: 26 Oct 2001 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation L.M. Agirrezabala, H.G. Owen, J. García-Mondéjar; Syntectonic deposits and punctuated limb rotation in an Albian submarine transpressional fold (Mutriku village, Basque-Cantabrian basin, northern Spain). GSA Bulletin 2002;; 114 (3): 281–297. doi: https://doi.org/10.1130/0016-7606(2002)114<0281:SDAPLR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Deep-water syntectonic deposits and angular unconformities record the denudation and deformation history of the middle Cretaceous Aitzeta structure, interpreted here as a monoclinal syncline associated with the high-angle reverse Mutriku fault (Basque- Cantabrian basin, northern Spain). Sedimentological and structural analyses, combined with a precise chronostratigraphy based on ammonites, permit us to document in detail the history of this syncline during ∼0.53 m.y. (Late Albian, Callihoplites auritus Subzone). Tectonism occurred during two major and two minor short- lived pulses. The sedimentary signatures of these pulses include unconformities and resedimented deposits derived from the uplifted rocks, showing inverted clast stratigraphy. In contrast, intervening phases of tectonic calm were characterized by gentle hemipelagic sedimentation. The major tectonic pulses were characterized by strong rotation and uplift of the syncline limb. The two major pulses coincided with the lower and upper limits of the auritus Subzone, respectively, and resulted in the following limb-rotation and uplift values: 42° and 250 m in the first pulse and 66° and 100–140 m in the second pulse. A maximum duration of 0.12 m.y. for each major pulse has been calculated, from which limb-rotation and uplift rates of 0.35°/k.y. and 2.08 m/k.y. (first pulse) and 0.55°/k.y. and 0.83–1.16 m/ k.y. (second pulse) are estimated. Growth strata and angular unconformity geometries, temporal variation of deformation rates, and bedding-parallel faulting indicate folding by progressive limb rotation and associated flexural slip. Local northwest- southeast compression is deduced in formation of the Aitzeta syncline. The presence of bedding-parallel oblique faulting and minor drag folds suggests a component of right-oblique movement along the Mutriku fault. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Crustal section based on CROP seismic data across the North Tyrrhenian–Northern Apennines–Adriatic Sea

Using deep seismic reflection data from the Italian lithospheric exploration project CROP in the Central Mediterranean region, a 400-km-long section, composed of three different profiles crossing the Northern Tyrrhenian Sea (CROP M-12A profile), the Northern Apennines (CROP-03) and the Adriatic Sea (CROP M-16) is reconstructed and discussed. New data allow us to outline a seismically consistent tectono-stratigraphic setting for the crust and upper mantle of the Northern Apennines thrust–belt system and its Adriatic foreland. Time–space analysis of the deformation of the investigated chain and identification of existing macrostratigraphic crustal intervals and tectonic units allow a reasonably controlled interpretation of the geodynamic evolution and of the main orogenic stages. Careful seismic reprocessing and application of advanced techniques to key zones of the explored area (such as the Tuscan Archipelago) were determinants in obtaining fundamental information for understanding of the complex lithospheric structures and their evolution. Profile interpretation supports that the Northern Apennine chain is dominated by a compressive thrust system. Crustal extension, assumed by some authors as the dominating tectonic process for the whole Tuscan Apennine area, represents a subordinate geodynamic event of the last stage (Tyrrhenian). In the Early Cretaceous–Late Jurassic, the paleogeographic framework consisted of the Europe and Adria plates separated by the Alpine Tethys Ocean. During the Late Cretaceous–Early Eocene, Adria–Europe convergence (eo-Alpine stage) and subduction beneath the Adria plate closed the Alpine Tethys Sea, with the Tethyan slab being clearly seismically imaged. The first Apenninic geodynamic stage occurred in the Late Oligocene–Early Miocene with the opening of the Balearic Basin, which generated a first “lithospheric root” of the Apenninic chain in the Tuscan Archipelago area. This root is represented by Adria-verging thrust faults that progressively flatten eastward. Upper parts of the west-verging eo-Alpine thrust blocks were truncated by the east-verging thrust faults of the Balearic stage. A deeper seismic reflector, attributed to the top of the asthenosphere, forms a mantle high below the Elba Island. From the Late Miocene to Present, the Corsica basin and western hinterland area were affected by extensional tectonics related to the Tyrrhenian opening, whereas compressional tectonics continued in the eastern hinterland and mostly on the eastward migrating foreland, with development of a second “lithospheric root” constituted by high-angle thrust faults. These faults give rise to a huge basement culmination below the main Apennines watershed. Impressive E-directed gravity-sliding of sedimentary blocks over their sloping basement occur, generating the Umbria–Marche shallow seismicity. Crustal shortening of the Apennines system amounts to 170 km, 14 km of which are due to the eo-Alpine stage, 71 km to the Balearic and 85 km to the Tyrrhenian one. In the frame of Africa–Europe convergence, the Tyrrhenian–Apennines tectonodynamics were mainly conditioned by the Mesozoic paleogeography.

Present-day crustal motion along the Longitudinal Valley Fault, eastern Taiwan

The NNE-striking Longitudinal Valley Fault (LVF) in eastern Taiwan is an extremely active high-angle thrust fault. It bounds the Coastal Range and the Longitudinal Valley, which is considered a collision boundary between the Philippine Sea and the Eurasian plates. Repeated GPS data in the Longitudinal Valley area from 1992 to 1999 are utilized to study the spatial variation of crustal motion along the LVF. With respect to Penghu in the Chinese continental margin, velocities for stations on the western side of the LVF (Longitudinal Valley and eastern Central Range) are 18–35mm/yr in directions 283–311°, whereas those on the eastern side of the LVF, the Coastal Range, are 28–68mm/yr in directions 303–324°. A major discontinuity of about 30mm/yr on the rate of crustal motion across the Longitudinal Valley is attributed to the aseismic slip along the LVF as revealed by trilateration data previously. To the south of Fengping, the block motions of the Coastal Range are 31–40mm/yr in 317–330° relative to the Central Range, while the near-fault motions are 13–33mm/yr in 309–336°. Various partitions on the left-lateral strike-slip and convergent components along the LVF are found. In the southern Longitudinal Valley crustal motion is mainly accommodated on the LVF and the Luyeh Fault. In contrast, those in the central and northern Longitudinal Valley are partly taken up on the faults to the east of the LVF or result in the elastic deformation of the Coastal Range. The crustal motion in the northern Longitudinal Valley area is likely to be distributed in the several NE-striking thrusts in a horsetail pattern and obliquely cut the northern Coastal Range, with a small portion of fault-slips along the LVF. Data from dense-deployed GPS networks across the LVF can be employed to give better estimates of near-fault motions and delineate the surface traces of the LVF. Repeated GPS and leveling data from two stations on both ends of the Yuli Bridge that are 575m apart clearly indicate that the surface trace of the LVF passes beneath the bridge with oblique horizontal motion of 23mm/yr in 306° and uplift rate of 24mm/yr.

Geology and structural controls of the Twin Peaks epithermal gold deposit, East Junggar, Xinjiang, China

The Twin Peaks epithermal gold deposit, located in East Junggar, China, is hosted by the late Paleozoic Kulankazigan island-arc complex. The deposit is structurally controlled by the extensional radial fractures associated with development of the Early Carboniferous Twin Peaks volcanic dome. It is stratigraphically restricted to the upper Batamayineishan Formation, which is subdivided into two units: the lower andesite unit that contains the ore bodies, and the upper rhyolite unit. Four hydrothermal alteration zones have been identified in both East and West Zones of the deposit. Each zone is characterized by a highly silicified core, and then there is an outward progression through adularia–sericite, argillic, and propylitic zones. The presence of adularia and sericite, in addition to the sulfide mineral association, suggests that the gold deposit is of the "adularia-sericite" or "low sulfidation" type. The East Zone ore body is offset by post-mineralization, high-angle reverse faults that strike parallel to the zone and small-scale, strike-slip faults that strike perpendicular to the zone.

Neotectonic structure of the Lakonikos gulf

Detailed single-channel seismic reflection survey has been carried out in the Lakonikos Gulf, Southern Péloponnèse, aiming to a better understanding of the neotectonic structure of the Lakonikos Basin. Our survey showed that, contrary to the model of a simple N-S asymmetric graben previously considered, a tectonic horst occurs within the tectonic graben of Lakonikos. A subsidence of more than 1000m is produced by the N-S marginal faults, whereas, the N-S faults creating the central horst structure within the Lakonikos graben are high-angle reverse faults, which have uplifted the sea bottom together with the Pleistocene and Holocene sediments by about 100m. Several E-W vertical transcurrent faults, with strike-slip motion deform the N-S structures. The central tectonic horst structure is very recent as the observed deformation of the Upper Pleistocene and Holocene sediments indicated and a transtensional geodynamic regime is suggested.

Analogue models of collapse calderas and resurgent domes

Collapse calderas and resurgent domes are a common association related to inflation–deflation processes in volcanic systems. The structure of calderas and domes depends upon the permitted, relative movements of crustal volumes at depth (the so-called “space problem”). In order to study the structures of collapse calderas and resurgent domes and to take the space problem into account, several analogue models were made. Dry-quartz sand was used to simulate the rheology of the brittle crust, while newtonian silicone putty, located at the base of the sand-pack, simulated the ductile behaviour of the magma. A piston moved the silicone putty downward or upward, inducing collapse and doming within the sand. Three separate sets of experiments simulated: (1) caldera collapse; (2) resurgence; and (3) superimposition of resurgence on collapse and vice versa. Collapse experiments are characterized by the development of two concentric depressions; the first-formed depression is bordered by outward dipping reverse ring faults; the subsequent, outer concentric depression is bordered by inward dipping normal ring faults. The deformation pattern during resurgence is a function of the overburden thickness (T) and the dome diameter (D). For higher T/D ratios a dome forms, bordered by inward dipping high angle reverse ring faults; outward dipping normal ring faults develop at late stages. For lower T/D ratios, the dome shows, at late stages, a crestal depression accompanied by radial fractures; subsequently, an apical extrusion of silicone occurs. The superimposition of resurgence over collapse (and vice versa) is characterized by the complete reactivation, with opposite kinematics, of all the pre-existing ring faults during inversion. Both in caldera and resurgence, reverse ring faults form in the early stages due to differential uplift; extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults. The experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences.

Characteristics and evolution of mesozoic foreland fold and thrust belt in western Kunlun Shan, China

The authors’ analysis of the chemical components and sedimentary characteristics of the well developed Triassic strata in the southeastern part of western Kunlun Shan led them to conclude that the sediments comprise a set of typical deep-water to semi-deep-water flysch that formed in the passive continental margin of the Qiangtang Block. This suit of strata had undergone strong deformation giving rise to a SW-thrusting duplex, imbricate fans, high-angle thrust fault, recumbent fold, SW-inverted fold, etc.. The deformational intensity weakens gradually southeastward. This is a foreland fold and thrust belt caused by the collision between the Qiangtang Block and the island are on the southern margin of the the Tarim Plate at the end of late Triassic.

Modelling the evolution of asymmetrical basins bounded by high-angle reverse faults with application to foreland basins

The evolution of asymmetrical basins bounded by high-angle reverse faults in a 10- and 20-km-thick strong upper crust with a Byerlee-type strength envelope has been modelled by finite element analysis. The substratum is assumed to be inviscid, and the basin is filled to datum by sediments. The results are compared with those of comparable extensional half grabens, as previously studied. As the layer is shortened by progressive increments, zones of plastic failure extend until throughgoing failure occurs, with the basin progressively narrowing as it deepens. At this stage, further subsidence becomes negligible, placing a limit on the depth, which depends on the layer thickness (and sediment density). During the earlier stages of evolution, the flexure profiles are very similar to those of the otherwise equivalent extensional half graben, but during later stages, substantially deeper and narrower compressional basins can form because of the greater crustal strength under compression. Beyond throughgoing failure, further shortening causes plastic compressional folding, which develops on top of the earlier gentle flexural undulations, eventually producing quite strong anticlines and synclines of progressively decreasing wavelength, which decay away from the fault on both sides. Compressional basins as modelled range from under 25 km to over 150 km in width, and they can be up to nearly 8 km deep, with greater widths and depths for a thicker layer. The most important application of the results is to the origin of foreland basins and foredeeps, providing an alternative and possibly complementary mechanism to the supracrustal loading hypothesis for the formation of such basins. In particular, the asymmetrical thrust basin hypothesis explains how the narrow and deep foreland basins characteristic of European mountain belts can form.

Structural Style and Stratigraphy of the Huwayyah Anticline: an Example of an Al-Ain Tertiary Fold, Northern Oman Mountains

ABSTRACT Detailed field mapping and structural studies in the Jebel Auha-Jebel Huwayyah area northeast of Al-Ain indicate that folding of neoautochthonous sedimentary rocks produced the north-northwest-trending Huwayyah Anticline. The anticline at the surface is composed of the Maastrichtian Qahlah and Simsima formations unconformably overlain by shallow-marine carbonate rocks that are correlated on faunal grounds with the Middle Eocene Dammam Formation. The investigation of the Huwayyah Anticline has identified three microfacies of bioclastic packstone, nummulitic packstone, and nummulitic packstone-grainstone in the local Dammam Formation. Diagenesis in the form of silicification, cementation, recrystallization, dissolution, compaction and neomorphism is widespread. The Huwayyah Anticline is a fault-propagation fold above a thrust ramp. The ramp developed from a pre-existing Late Cretaceous basal thrust within the Semail Ophiolite on the Oman Mountain Front. The anticline was formed as a result of regional compressive deformation due to rejuvenation of the Late Cretaceous thrust in post-Middle Eocene times. Westward-directed high-angle reverse faults of Jebel Auha trend parallel to the fold axis of the anticline. The Auha faults probably originated as west-dipping thrusts on the western flank of the anticline and were subsequently rotated to their present attitude as the flank of the anticline became steeper due to compression from the east.

Thick-skinned and thin-skinned thrusting in the La Ramada fold and thrust belt: crustal evolution of the High Andes of San Juan, Argentina (32°SL)

The Cenozoic structural evolution of the La Ramada fold and thrust belt in the High Andes of Argentina, between 32°00′S and 32°30′S, shows in time and space the interaction of thin-skinned and thick-skinned deformations. Based on geometric and kinematic constraints, three different deformation stages have been recognized. The first began with thin-skinned NNW-trending structures detached in Upper Jurassic gypsum. In the second stage, the basement was involved by tectonic inversion of Triassic normal faults producing a thick-skinned fold and thrust belt. This inversion, combined with the ductile rheology of the Jurassic gypsum, resulted in refolding of older structures. The uplift of the basement by high-angle reverse faults at the Ramada–Espinacito Massif, produced a sticking point in the foreland propagation of the thrust belt. This was responsible for the third stage of deformation characterized by thin-skinned NNW out-of-sequence thrusts developed in the westernmost sector. The total Andean shortening calculated in the area was 21–24 km (near 30%). The structure of the La Ramada fold and thrust belt was integrated into a crustal-scale cross-section at 32°S latitude. Based on gravimetric models and surface geology, a crustal model is proposed for the Andes of Chile and Argentina. A shortening of 180 km was calculated between the Chile trench and the Sierra de Pie de Palo. This implies a mean deformation rate of 2.8 mm year −1 during Cenozoic times. An initial crustal thickness of ca. 33 km was computed. A two-layer model with a middle detachment level is proposed to explain the spatial decoupling between maximum shortening at the surface and maximum crustal thickening. The upper layer was balanced based on surface structural information. The lower layer is interpreted to have been constructed by an imbricate thrust system controlled by the Mercedario rift master-shear and the Precordillera–Chilenia suture.

1918年信州大町地震の断層運動

A fault movement of the 1918 Omachi earthquake is estimated based on the vertical movement along the levelling route in Omachi and its vicinities. The vertical movement during the period of 1891-1920 is analyzed by Miyabe's method. In the 1918 Omachi earthquake, the land block of the northwestern part of Omachi was upheaved and tilted down 6.0×10-5 rad to N75°W. Changes in the heights of the bench marks projected onto the N75°W-575°E direction suggest that the land block was thrust up eastward by a high angle reverse fault dipping to the west. This fault coincides with that of the 1964 Niigata earthquake and is similar to the topographical feature along the Shinano River Seismic Zone. It is presumed that the plate boundary between the Okhotsk Plate and the Eurasian Plate is a collision type, not a simple eastward subduction type as is accepted so far.

Style and origin of mid-Carboniferous deformation in the Illinois Basin, USA — Ancestral Rockies deformation?

The integration of outcrop, borehole, and seismic reflection data from the Illinois Basin and adjacent eastern Ozark Dome in Illinois and Missouri sheds new light on the style and origin of intra-cratonic deformation. Typical structures of this region are high-angle reverse faults in Precambrian basement that propagated upward to monoclines and asymmetrical anticlines in Paleozoic sedimentary cover. These are compressive-block structures directly analogous to (although smaller than) `Laramide-style' structures of the Colorado Plateau and Rocky Mountain foreland. Central Illinois Basin structures were active chiefly during late Chesterian through Atokan (i.e., late Mississippian to middle Pennsylvanian; mid-Carboniferous) time, with continued intermittent movement through the late Pennsylvanian. Both the style and timing of deformation match those of the `Ancestral Rocky Mountains' orogeny of the southern Midcontinent and Rocky Mountain region of the USA. Deformation in the central Illinois Basin has generally been attributed to the nearby late Paleozoic Appalachian–Ouachita orogeny, even though the Illinois Basin's compressive block structural style is foreign to the Appalachian foreland. We suggest that the Ancestral Rockies event may have played a significant role in the development of Pennsylvanian-age compressive-block structures in Illinois and Missouri.

An example of neotectonism in a continental interior — Thebes Gap, Midcontinent, United States

Some of the most intense neotectonic activity known in the continental interior of North America has been recently discovered on a fault zone in the Thebes Gap area, Missouri and Illinois. This faulting almost assuredly was accompanied by large earthquakes. The zone is located approximately 30 km north of the New Madrid seismic zone and consists of complex north-northeast- to northeast-striking, steeply dipping faults that have had a long-lived history of reactivation throughout most of the Phanerozoic. Geophysical studies by others suggest that the faults are rooted in the deeply buried Late Proterozoic and Early Cambrian Reelfoot rift system. Quaternary deposits are cut by at least four episodes of faulting, two of which occurred during the Holocene. The overall style of neotectonic deformation is interpreted as right-lateral strike-slip faulting. At many locations, however, near-surface displacements have stepped from one fault strand to another and produced normal and oblique-slip faults in areas of transtension and high-angle reverse faults, thrust faults, and folds in areas of transpression. There is evidence of reactivation of some near-surface fault segments during the great 1811–1812 New Madrid earthquakes. Quaternary faulting at Thebes Gap demonstrates that there are additional seismic-source zones in the Midcontinent, U.S., other than New Madrid, and that even in the absence of plate-margin orogenesis, intense neotectonic activity does occur over long time periods along crustal weaknesses in continental interiors.

Salt-Related Fault Families and Fault Welds in the Northern Gulf of Mexico

Salt-related faults and fault welds in the northern Gulf of Mexico are classified based on the three-dimensional geometry of the faults or welds, deformed strata, and associated salt. Kinematic or genetic criteria are not used in the classification. Only documented fault styles are considered; those styles produced by experimental or numerical modeling, but not yet observed in the Gulf, are not included. Extensional faults comprising symmetric arrays include peripheral faults, which occur at the landward margin of the original salt basin; crestal faults, which are growth faults rooted in reactive diapirs; and keystone faults, which occur at the crests of anticlines. Asymmetric arrays of normal faults are grouped according to the dominant dip direction. Faults that dip primarily basinward include roller faults, which are listric growth faults that sole into a subhorizontal salt layer; ramp faults, which extend upward from the landward margin of bulb-shaped salt stocks; and shale-detachment faults, which sole into a shale decollement that merges into a salt layer. Counterregional faults are landward-dipping asymmetric arrays that link cylindrical, basinward-leaning salt stocks. Asymmetric arrays with variable dip direction include flap faults, whose footwalls comprise diapirs with uplifted and rotated roof strata, and rollover faults, which occur at the hinges of monoclinal folds. Two families of contractional faults are described: toe thrusts, which are basinward-vergent thrusts that ramp up from a salt or shale decollement, and break thrusts, which are high-angle reverse faults that cut one or both limbs of detachment folds. Fault arrays that strike parallel to the regional dip direction are termed lateral faults. Six types of fault welds are defined: primary welds are those at the autochthonous level; roho welds are subhorizontal, allochthonous welds into which roller faults detach; counterregional welds comprise both subhorizontal and landward-dipping segments beneath growth monoclines; bowl welds are elliptical and upwardly concave; thrust welds are landward-dipping surfaces that separate repeated stratigraphic sections; and wrench welds are steep and strike parallel to the regional dip direction. Groups of geometrically classified fault families and fault welds are kinematically and genetically linked to each other and to associated salt bodies and welds. Linked fault systems can contain extensional, contractional, and strike-slip components. Extensional fault families are formed by basinward translation, subsidence into salt, or folding. Those fault families that accommodate basinward translation are balanced by salt extrusion or contractional fault families. Strike-slip fault families commonly provide hard links, although various fault components also can be soft linked. We illustrate five associations of linked fault systems that are directly related to five types of salt systems: autochthonous salt, stepped counterregional, roho, salt-stock canopy, and salt nappe.

Late Quaternary tectonism along the Pacific coast of the Californias: a contrast in style

Abstract Marine terraces and associated features are examined along 1500 km of coast at the seaward margin of that part of the North American plate that has been captured by the Pacific plate. This coast embraces parts of three morphotectonic provinces, each with its own distinctive tectonic style, namely the southern Coast Ranges, western Transverse Ranges, and northern Peninsular Ranges. Based primarily on the 125 ka shoreline, four magnitudes of late Quaternary deformation are recognized: (1) high uplift rates exceeding 1.0 m per 1000 years; (2) moderate uplift rates between 1.0 and 0.1 m per 1000 years; (3) low uplift rates less than 0.1 m per 1000 years; and (4) variable net subsidence. The highest rates of uplift and subsidence are associated with the intense fold-thrust tectonics of the western Transverse Ranges nearest the Big Bend in the San Andreas fault, and diminish in magnitude to east and west. Moderate uplift rates occur in both the southern Coast Ranges and the northern Peninsular Ranges adjacent to the western Transverse Ranges, commonly associated with block uplift and subsidence between high-angle reverse faults within a broader strike-slip scenario. This suggests that compression and transpression are significant features of these areas and that the continued rotation and relative westward motion of the western Transverse Ranges, within the broader context of shear distributed across the San Andreas fault system, continue to have an impact on regions immediately to the north and south. Uplift diminishes with distance from the Transverse Ranges and is low for Baja California south of Punta Banda. The style of late Quaternary deformation is similar to that defined by earlier Quaternary marine limits, but intensity varies, most notably in the western Transverse Ranges.

Studies of three dimensional lithospheric structure and tectonics using seismic tomographic methods: Applications to resource assessment

New seismic techniques, utilizing signals from the large numbers of micro-earthquakes occurring in tectonically active areas, can now provide detailed 3-D seismic velocity structure images and 3-D maps describing on-going tectonic deformation. The deformation is expressed in terms of spatial variations in the magnitude of the stress released by micro-earthquake fracturing, the spatial variations in total strain energy release by brittle fracture and also in terms of the 3-D determination of the (time dependent) energy release levels from rapid creep deformation along faults. Taken together, the seismic velocity structure and the spatial definition of the tectonic state of the medium can be used to characterize the physical properties of the material and its deformational strain state and to define the large scale faulting dynamics within the medium. This material state and tectonic state information can then be used to infer the presence of mineral deposits based on correlations of imaged intrusive bodies and other structural features (faults, folds, etc.) with the intrinsic material properties inferred from seismic velocity and other material parameter determinations. An example from the Andes Mountains in South America, in an area with deep seated oil and gas deposits, is presented to illustrate the applicability of these seismic methods for resource exploration and the delineation of micro-tectonic features. In this application, a network of ten micro-arrays, each consisting of four sensors with about a half kilometer aperture, was designed to detect and locate earthquakes to very low magnitudes in order to define the tectonic activity and structure in an area of about 2,000 km 2 . All the event locations were used to define active fault zones in three dimensions and were analyzed to infer patterns of stress and energy release, as well as fault mechanisms. The recorded events were also used to obtain detailed tomographic images of the three dimensional P and S wave velocity structure to mid-crustal depths. In addition to monitoring discrete event activity to low magnitude levels, the network was also used to perform Array-based Seismic Emission Tomography which is used to determine the low level seismic energy emitted during rapid creep. The 3-D seismic velocity structure determined from the earthquake P and S wave signal data, showed the presence of major high angle thrust faults to mid-crustal depths and the locations of sedimentary units beneath the network that correlated with known structure from deep wells. The velocity structure was also correlated with both the large and small scale tectonic patterns. Variations of the Poisson's ratio, computed from the P and S wave velocity determinations, showed anomalously high values (above .35) in the area surrounding the major high angle thrust faults. This anomaly is interpreted as a consequence of high densities of fluid filled fractures within and near the faults. This interpretation is supported by the nature of the tectonic activity observed and, in more direct fashion, by the presence of a deep oil production zone centered within the shallowest of the zones identified. Consequently, these seismic methods are able to locate oil/gas deposits directly from Poisson ratio determinations, as well as indirectly through structural and lithological inferences. It is also concluded that the application of these methods, for the precise location of intrusive bodies and major crustal faults, should prove useful in the exploration for other minerals.