North-dipping Thrust Faults Research Articles

Formation mechanism and hydrocarbon exploration significance of the box fold in the Qiulitage structural belt, the Kuqa Depression, Tarim Basin, China

Clarifying the deformation characteristics and formation mechanisms of the box fold in the eastern Qiulitage structural belt can provide important references for the reconstruction of the evolution process and petroleum exploration in the Qiulitage structural belt. As a result of the deep geological structure displayed by seismic data and characteristics of faults and fractures within the box fold, the mechanical mechanism and structural evolution of the box fold in the eastern Qiulitage structural belt were investigated, along with the genesis and significance of hydrocarbon exploration of faults and fractures within the box fold. The results show that the surface box fold in the Qiulitage structural belt was formed via the conjugate kinking of the supra-salt structural layer, driven by the intensive southward compression during the Middle and Late Himalayan movements. The box fold has experienced three evolution stages, namely, the tectonically-inactive stage before the deposition of the Kuqa Formation, the fold rudiment stage during the early to middle deposition of the Kuqa Formation (Kuqa period), and the stage of fold finalization and uplift-denudation. The front flank of the box fold develops north-dipping thrust faults and network fracture systems formed during the early to middle Kuqa period and cemented by gypsum due to the precipitation of deep, high-salinity formation water. However, later faulting can cut and dislocate the cement. The upper fold core develops north-dipping tensile faults and near EW tensile fractures, while the lower fold core is associated with small back-thrust structures and near NS shear fractures. The neutral plane is expected to be in the middle-lower part of the fold. The back-flank of the box fold develops south-dipping back-thrust faults and near EW interlayer shear fractures caused via interlayer detachment. The core and back flank of the fold were less affected by the high-salinity formation water, leaving faults and fractures with no considerable cementation. The kink zone and its surroundings have high storage and flow capacities and thus the potential to and develop oil and gas reservoirs. Correctly interpreting kink zones in concealed areas can help expand the scale of original oil and gas reservoirs or discover new petroleum exploration domains. In the Qiulitage structural belt, the connection between deep and shallow fault systems leads to the migration of deep hydrocarbons to shallow layers and subsequent accumulation. The structural-lithologic oil and gas reservoir formed in the Paleogene thin sand layers of the upper part of the Lower Cretaceous and the structural oil and gas reservoir formed in supra-salt sandstone layers of the surface box fold are among The potential exploration domains in shallow layers.

Recent Surface Rupturing Earthquakes along the South Flank of the Greater Caucasus near Tbilisi, Georgia

ABSTRACTFault characterization is a critical step toward improving seismic hazard assessment in the Georgian Greater Caucasus but is largely absent from the region. Here, a paleoseismic trench near the capital city of Tbilisi revealed evidence for recurring surface rupture on a shallowly north-dipping thrust fault. The fault has broken through the overturned forelimb of a fault-propagation anticline that folds a sequence of soils and deposits. Stratigraphic relationships and radiocarbon dating of terrestrial gastropod shells corrected for “old carbon” age anomalies loosely constrain three surface-deforming earthquakes on this fault between ∼40 and ∼3 ka, with variable dip-slip displacements ranging between 0.35 and ∼3 m, and a cumulative displacement of 6.5 ± 0.85 m. Single event slips and recurrence intervals (11, 25, and 3 ka open interval) at this site demonstrate apparent slip rate variations of 3−7× over the last two earthquake cycles on the fault, which we attribute to possible rupture complexity involved in crustal thrust fault earthquakes. This study provides a structural and geochronologic template for future paleoseismic investigations in the Greater Caucasus while highlighting some of the challenges of conducting seismic source characterization in this region.

The 2020 Mw 6.0 Jiashi Earthquake: Coinvolvement of Thin-Skinned Thrusting and Basement Shortening in Shaping the Keping-Tage Fold-and-Thrust Belt in Southwestern Tian Shan

AbstractThe Keping-tage fold-and-thrust belt in southwest Tian Shan is seismically active, yet the most well-recorded earthquakes occurred south of the mountain front. The lack of large earthquakes beneath the fold-and-thrust belt thus hinders our understanding of the orogenic process to the north. The 2020 Mw 6.0 Jiashi earthquake is an important event with surface deformation in the fold-and-thrust belt well illuminated by Interferometric Synthetic Aperture Radar, providing an opportunity to study the present-day kinematics of the thrust front through the analysis of satellite measurements of surface deformations. Here, we employ the surface deformation and relocated aftershocks to investigate the fault-slip distribution associated to this event. Further added by an analysis of Coulomb stress changes, we derive a fault model involving slips on a shallow, low-angle (∼10°) north-dipping thrust fault as well as on a left-lateral tear fault and a high-angle south-dipping reverse fault in mid-crust. Aftershocks at depth reflect the basement-involved shortening activated by a thin-skinned thrust faulting event. In addition, this earthquake uplifted the southernmost mountain front with relatively low topography, indicating the basin-ward propagation of the southwest Tian Shan.

Forecasting seismic activity rates in northwest Himalaya through multivariate autoregressive forecast of seismicity algorithm

In this study, a model based on multivariate autoregressive forecast of seismicity (MARFS) algorithm is adopted to forecast seismic activity rates in northwest Himalaya, using the compiled homogenized moment magnitude (MW) based catalogue. For this purpose, each source zone delineated by Yadav et al. (Pure Appl Geophys 170:283–295, 2012) is divided into a spatial grid interval of 0.5° × 0.5° while the entire catalogue span (1975–2010) is segregated into six time periods/grids to estimate seismic activity rates spatially and temporally. These seismic activity rates which are estimated from spatial density map of hypocenters exhibit high values in Chaman Fault (Zone 1), Hindukush-Pamir region (Zone 3) and the mega thrust systems, i.e., Main Central Thrust, Main Boundary Thrust and Himalayan Frontal Thrust (Zone 4). Then, the seismic activity rates during 2011–2016 could be forecasted by extrapolating (through auto-regression procedure) those observed for previous time periods. The forecast seismic activity rates are estimated within the values of 0 and 7.57 with high values primarily observed in Hindukush-Pamir region of Zone 3 and the gently north-dipping thrust fault systems (Main Central Thrust, Main Boundary Thrust, Himalayan Frontal Thrust) of Zone 4. Finally, the associated area under the curve of receiver operating characteristics graph suggests the superiority of forecasting model with respect to random prediction, whereas results of the data-consistency test, i.e., N test of our model, exhibit consistency in between the observed and simulated likelihoods. Moreover, the hypothetical t test performed in between the spatial grids of forecast seismic activity rates and observed seismic activity rates confirms that the former is consistent with the latter.

Large ancient earthquakes in the western Issyk-Kul basin (Kyrgyzstan, northern Tien Shan)

New paleoseismological and archaeoseismological data from the western Issyk-Kul basin of Kyrgyzstan (northern Tien Shan) provide new insights on active fault in the actively-deforming, basement-involved thrust system of Central Asia. Newly discovered fault scarps follow south-and north-dipping thrust faults which delineate the Kyrgyz and Kungey ranges bordering the Issyk-Kul basin. Motion on these faults generated earthquakes with magnitudes 6.2–7.6 and MSK-64 shaking intensities VIII-XI. Deformation observed in the zone of the Toguz-Bulak fault results from two Holocene earthquakes and another event that occurred about 8000 yr BP. Two more events, at 13,000 and 3000 yr BP, deformed the northeastern periphery of the Kyzyl-Ompul Uplift. Archaeoseismological research in the Northern Sary-Bulun settlement in the western periphery of the Boz-Barmak Uplift, revealed traces of another earthquake of MSK-64 shaking intensity I ≥ VIII dating back to the 12th Century. The obtained data correlates well with results of previous paleoseismological and archeoseismological studies. They show that in the northern Tien Shan there were clusters of strong earthquakes with ages of 14–13, 8, 4–3 ka ago and in the 11–12th centuries AD divided by periods of 4–5 ka. We infer that coseismic slip on these faults may have formed a tectonic dam at the edge of the basin through growth of the Boz-Barmak Uplift, and this dam ultimately deflected the Chu River into its modern channel which bypasses the lake. Lacustrine sediments in the northeastern periclinal segment of the Boz-Barmak Uplift bear signatures of soft-sediment deformation structures (seismites) corresponding to seven M ≥ 5–5.5 (I ≥ VI-VII) earthquakes timed at about 20,000 yr BP.

Active tectonics in the Assam seismic gap between the meizoseismal zone of AD 1934 and 1950 earthquakes along eastern Himalayan front, India

The Assam Seismic Gap has witnessed a long seismic quiescence since the \({ Mw}{\sim }8.4\) great Assam earthquake of AD 1950. Owing to its improper connectivity over the last decades, this segment of the Himalaya has long remained inadequately explored by geoscientists. Recent geodetic measurements in the eastern Himalaya using GPS document a discrepancy between the geologic and geodetic convergence rates. West to east increase in convergence rate added with shorter time span earthquakes like the 1697 Sadiya, 1714 (\({ Mw}{\sim }8\)) Bhutan and 1950 (\({ Mw}{\sim } 8.4\)) Tibet–Assam, makes this discrepancy more composite and crucial in terms of seismic hazard assessment. To understand the scenario of palaeoearthquake surface rupturing and deformation of youngest landforms between the meizoseismal areas of \({ Mw}{\sim }8.1\) 1934 and 1950 earthquakes, the area between the Manas and Dhanshiri Rivers along the Himalayan Frontal Thrust (HFT) was traversed. The general deformation pattern reflects north-dipping thrust faults. However, back facing scarps were also observed in conjugation to the discontinuous scarps along the frontal thrust. Preliminary mapping along with the published literature suggests that, in the eastern Himalayan front the deformation is taking place largely by the thrust sheet translation without producing a prominent fault-related folds, unlike that of the central and western Himalayas.

Crustal structure and tectonics of Bangladesh: New constraints from inversion of receiver functions

An understanding of the sedimentary and crustal structure of the Bengal Basin and of the tectonics deforming it remains elusive due to lack of seismic data from Bangladesh. Taking advantage of recently available data from 11 seismic stations deployed over Bangladesh, we determine the crustal structure beneath each station using 2768 high quality receiver functions (RFs). Inversion of the RFs reveals a highly variable thickness of the overlying sediments beneath the Bengal Basin. The thickness of the sediments increases dramatically across the Hinge Zone of the Early Cretaceous passive margin from 3 to 17km. The thick sediments partly represent progradation of the continental margin due to the influx of clastic sediments from the Himalayas. The Moho shallows across the region. This reflects thinning of the crystalline crust from 38km in the Indian Craton to 34km at the Hinge Zone to <16km in the Bengal Basin. The thickness of the sediments increases dramatically from 3 to 17km south of Madhupur tract which reflects the regions of highest influx of clastic sediments from the Himalayan collision zone. RFs display strong dipping reflectors (strike 67°) for a station over Hinge Zone and seem to be associated with the transition from continental to oceanic crust. The thinning of significant crustal thickness (16–19km) beneath 15–17km of sediment and associated velocities (>4.0km/s) at lower crustal levels supports an influence of the Kerguelen plume igneous activity during rifting. We invert data for a station near the Dauki Fault, which marks the southern boundary of the uplifted Shillong plateau, for dip and anisotropic effects. Our results show the Dauki as a north-dipping thrust fault at Jaflong with a dip angle of 32° and strike (110°) close to its surficial expression. A strong anisotropy (~15%) and the sense of shear (plunge: 24°, trend: 79°) compliment the dipping geometry and deformation seems to be related to the initiation of the Dauki Fault.

A geophysically constrained multi-scale litho-structural analysis of the Trans-Tanami Fault, Granites-Tanami Orogen, Western Australia

The Trans-Tanami Fault in the poorly exposed Paleoproterozoic Granites-Tanami Orogen of Western Australia is an ∼100 km long curvilinear structure with ∼6 km right lateral displacement. Multi-scale integration and analysis of aeromagnetic, gravimetric, reflection seismic and remote sensing data have constrained the relative timing and architectural relationship of this structure. Interpretation of regional scale long-wavelength potential field (gravity and magnetic) anomalies, which are commonly used to define first-order structures, show that the fault is not a terrane boundary. Structural interpretation of short-wavelength potential field data illustrates that the structural domains on either side of the fault represent the products of a non-homogeneous stress regime developed between rigid granitic plutons. Additionally, 2D joint forward modelling of gravity and magnetic data and interpretation of reflection seismic data confirms the vertical displacement across this fault to be negligible indicating a predominant lateral displacement. The lateral displacement along a portion of this structure has exploited a pre-existing plane of a north-dipping thrust fault. Where this early thrust fault terminates, the Trans-Tanami Fault displaces previously unfaulted rock as a wrench fault step-over. These observations differ from previous findings in the area by constraining the absolute displacement of this structure and through the recognition of a wrench fault system that includes lateral step-overs between re-activated early thrust fault planes.

Integrated geophysical investigation of the St. James Fault Complex: A case study

We noninvasively detected the characteristics and location of a regional fault in an area of poor bedrock exposure complicated by karst weathering features in the subsurface. Because this regional fault is associated with sinkhole formation, its location is important for hazard avoidance. The bedrock lithologies on either side of the fault trace are similar; hence, we chose an approach that capitalized on the complementary strengths of very low frequency (VLF) electromagnetic, resistivity, and gravity methods. VLF proved most useful as a first-order reconnaissance tool, allowing us to define a narrow target area for further geophysical exploration. Fault-related epikarst was delineated using resistivity. Ultimately, a high-resolution gravity survey and subsequent inverse modeling using the results of the resistivity survey helped to further constrain the location and approximate orientation of the fault. The combined results indicated that the location of the fault trace needed to be adjusted 53 m south of the current published location and was consistent with a north-dipping thrust fault. Additionally, a gravity low south of the fault trace agreed with the location of conductive material from the resistivity and VLF surveys. We interpreted these anomalies to represent enhanced epikarst in the fault footwall. We clearly found that a staged approach involving a progression of methods beginning with a reconnaissance VLF survey, followed by high-resolution gravity and electrical resistivity surveys, can be used to characterize a fault and fault-related karst in an area of poor bedrock surface exposure.

The Dauki fault at the Shillong plateau-bengal basin boundary in northeastern India: 2D finite element modeling

The Dauki fault is a major fault along the southern boundary of the Shillong plateau that may be a source of destructive seismic hazards for the adjoining areas, including northeastern Bangladesh. In this study, we simulated the present stress distribution and the ongoing convergence deformation within the study area using numerical simulation. This simulation attempts to determine the proper dip angle of the Dauki fault from a variety of differently established models but with the same realistic boundary conditions. The simulated stress field helped to predict the type of tectonic environment and the faulting patterns of the study area. The best-fit model can account for the compressive stress regime in the study area, except in the uppermost part of the Shillong plateau, the Bengal basin, and Assam areas where extensional stress regimes exists. In the model, the elements for which Mohr-Coulomb’s failure occurs correspond with the observed earthquake data that refer to the Dauki fault as a north-dipping thrust fault.

Structural decoupling in a convergent forearc setting (southern Crete, Eastern Mediterranean)

A multidisciplinary approach is used to investigate the structure of the southern Cretan margin, which is located in one of the most seismically active forearc regions in Europe. Bathymetric, seismic-reflection, and fault plane solution data were used to identify the main tectonic features on the margin, correlating their evolution with the main sedimentary sequences recognized on Crete. In contrast to the majority of forearc settings in the Pacific and Indian Oceans, southern Crete is a region of predominantly oblique movement above well-defined detachment zones. North-dipping thrust faults identified on seismic-reflection profiles reveal significant crustal shortening during the Miocene due to the westward propagation of the Hellenic fold-and-thrust system. In addition, east-dipping thrust faults rooted on top of pre-Neogene strata were also identified, but only a few of these thrusts affect Neogene to Holocene strata. Small-scale domes derived from evaporitic (Messinian) intrusions deform Pliocene–Quaternary strata. West- and east-dipping normal faults were also recognized within the Mesozoic and Cenozoic successions, and these are related to regional extension during forearc convergence. In such a setting, the fault-bounded continental slope of Crete effectively separates a region of uplift (Crete) from subsiding troughs to the south. Our work shows that structural segmentation at depth is complex, with multiple crustal levels recording contrasting styles of deformation and distinct moment-tensor solutions. This complexity derives from the oblique style of convergence recorded south of Crete, which reactivates distinct crustal levels depending on their rheology and relative degree of metamorphism inherited during Alpine compression. As a result, a strong correlation between seafloor morphology and transtensional movements is recorded in the upper 10–15 km of the crust, while transpression prevailed after the Serravallian below these depths.

Taking apart the Big Pine fault: Redefining a major structural feature in southern California

New mapping along the Big Pine fault trend in southern California indicates that this structural alignment is actually three separate faults, which exhibit different geometries, slip histories, and senses of offset since Miocene time. The easternmost fault, along the north side of Lockwood Valley, exhibits left‐lateral reverse Quaternary displacement but was a north dipping normal fault in late Oligocene to early Miocene time. The eastern Big Pine fault that bounds the southern edge of the Cuyama Badlands is a south dipping reverse fault that is continuous with the San Guillermo fault. The western segment of the Big Pine fault trend is a north dipping thrust fault continuous with the Pine Mountain fault and delineates the northern boundary of the rotated western Transverse Ranges terrane. This redefinition of the Big Pine fault differs greatly from the previous interpretation and significantly alters regional tectonic models and seismic risk estimates. The outcome of this study also demonstrates that basic geologic mapping is still needed to support the development of geologic models.

Cretaceous-Tertiary shortening, basin development, and volcanism in central Tibet

The geologic map pattern of the Qiangtang terrane in central Tibet defines a >600-km-long and up to 270-km-wide east-plunging structural culmination. It is characterized by early Mesozoic blueschist-bearing melange and upper Paleozoic strata in the core and mainly Triassic–Jurassic strata along the limbs. In the western Qiangtang terrane (∼84°E), the culmination is unconformably overlain by weakly deformed mid-Cretaceous volcanic flows and tuffs. Along the Bangong suture to the south (32°N, 84°E), mid-Cretaceous nonmarine red beds and volcanic rocks lie unconformably on Jurassic suture zone rocks. These relationships demonstrate that west-central Tibet was above sea level during the mid-Cretaceous and experienced significant denudation prior to mid-Cretaceous time. Growth of the Qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early Mesozoic melange and upper Paleozoic strata during the Early Cretaceous Lhasa-Qiangtang collision. The north-south width of the inferred thrust sheet provides a minimum slip estimate of ∼150 km at 84°E, decreasing eastward to ∼70 km at 87°E. Paleogene deformation in the Qiangtang terrane is characterized by widely distributed, mainly north-dipping thrust faults that cut Eocene–Oligocene red beds and volcanic rocks in their footwall. Along the Bangong suture, the north-dipping Shiquanhe-Gaize-Amdo thrust system cuts 64 and 43 m.y. old volcanic tuffs in its footwall and accommodated >40 km of post–mid-Cretaceous shortening. The Tertiary south-dipping Gaize–Siling Co backthrust bounds the southern margin of the Bangong suture and marks the northernmost limit of mid-Cretaceous marine strata in central Tibet. Cretaceous deformation and denudation in central Tibet is attributed to northward underthrusting of the Lhasa terrane beneath the Qiangtang terrane along the Bangong suture. This model implies that (1) Cretaceous strata along the Bangong suture and in the northern Lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the Qiangtang terrane, and (2) the central Tibetan crust was thickened substantially prior to the Indo-Asian collision. Although its magnitude is poorly known, Tertiary shortening in the Qiangtang terrane is more prevalent than in the Lhasa terrane; this difference may be attributed to the presence of underthrust melange in the deeper central Tibetan crust, which would have made it weaker than the Lhasa terrane during the Indo-Asian collision.

Stratigraphy, palynology, and provenance of the Colorado Creek basin, Alaska, USA: Oligocene transpressional tectonics along the central Denali fault system

New sedimentologic, biostratigraphic, and compositional data from a 415-m-thick section of siliciclastic and volcanic strata document Oligocene synthrusting sedimentation south of the McKinley segment of the Denali fault system. Strata of the Colorado Creek basin are presently exposed on the south side of the central Alaska Range in the footwalls of north-dipping thrust faults. New measured sections define a three-part stratigraphy. Lowermost strata consist of a ~30-m-thick unit of marine sandstone and mudstone that contain Late Cretaceous dinoflagellate taxa. The middle unit consists of ~330 m of conglomerate, sandstone, and mudstone interpreted as braided stream and floodplain deposits. This middle unit contains early Oligocene pollen and spore assemblages. The upper unit is 55 m thick and contains lava flows, tuff, and pumice interpreted as the product of subaerial volcanic eruptions. Direct age data are lacking from the upper unit. Compositional data from the middle unit indicate that detritus was derived from sedimentary and igneous source terranes exposed on both the north and south side of the McKinley fault. Matching source lithologies north of the McKinley fault with conglomerate clast types in the Colorado Creek basin implies 30–33 km of maximum post-early Oligocene dextral displacement along the fault. We interpret the Oligocene strata of the Colorado Creek basin as a product of transpressional deformation that produced north-dipping thrust faults associated with strike-slip displacement on the central Denali fault. Our data from the Colorado Creek basin, in combination with previous studies, document a major episode of middle Eocene – late Oligocene synorogenic sedimentation along the Denali fault from British Columbia to southwestern Alaska.

Interpretation of the Seattle Uplift, Washington, as a Passive-Roof Duplex

We interpret seismic lines and a wide variety of other geological and geophysical data to suggest that the Seattle uplift is a passive-roof duplex. A passive-roof duplex is bounded top and bottom by thrust faults with opposite senses of vergence that form a triangle zone at the leading edge of the advancing thrust sheet. In passive-roof duplexes the roof thrust slips only when the floor thrust ruptures. The Seattle fault is a south-dipping reverse fault forming the leading edge of the Seattle uplift, a 40-km-wide fold-and-thrust belt. The recently discovered, north-dipping Tacoma reverse fault is interpreted as a back thrust on the trailing edge of the belt, making the belt doubly vergent. Floor thrusts in the Seattle and Tacoma fault zones, imaged as discontinuous reflections, are interpreted as blind faults that flatten updip into bedding plane thrusts. Shallow monoclines in both the Seattle and Tacoma basins are interpreted to overlie the leading edges of thrust-bounded wedge tips advancing into the basins. Across the Seattle uplift, seismic lines image several shallow, short-wavelength folds exhibiting Quaternary or late Quaternary growth. From reflector truncation, several north-dipping thrust faults (splay thrusts) are inferred to core these shallow folds and to splay upward from a shallow roof thrust. Some of these shallow splay thrusts ruptured to the surface in the late Holocene. Ages from offset soils in trenches across the fault scarps and from abruptly raised shorelines indicate that the splay, roof, and floor thrusts of the Seattle and Tacoma faults ruptured about 1100 years ago. Manuscript received 9 September 2003.

Mid-crustal detachment and ramp faulting in the Markham Valley, Papua New Guinea

Earthquakes and geodetic evidence reveal the presence of a low-angle, mid-crustal detachment fault beneath the Finisterre Range that connects to a steep ramp surfacing near the Ramu-Markham Valley of Papua New Guinea. Waveforms of three large (M w 6.3 to 6.9) thrust earthquakes that occurred in October 1993 beneath the Finisterre Range 10 to 30 km north of the valley reveal 15° north-dipping thrusts at about 20 km depth. Global Positioning System measurements show up to 20 cm of coseismic slip occurred across the valley, requiring that the active fault extend to within a few hundred meters of the Earth9s surface beneath the Markham Valley. Together, these data imply that a gently north-dipping thrust fault in the middle or lower crust beneath the Finisterre Range steepens and shallows southward, forming a ramp fault beneath the north side of the Markham Valley. Waveforms indicate that both the ramp and detachment fault were active during at least one of the earthquakes. While the seismic potential of mid-crustal detachments elsewhere is debated, in Papua New Guinea the detachment fault shows the capability of producing large earthquakes.

Triangle Zones and Underthrusting Along the Southern Margin of the Cordillera de la Costa, North-Central Venezuela

Along the southern margin of the Cordillera de la Costa, the overthrust foothills give way southwards first to a discontinuous zone of multiple thrusts, then to laterally persistent south-dipping and overturned strata and finally to open folds as the shallow deformation dies out within the upper stratigraphic units of the Eastern Venezuelan Basin. Northwest-southeast trending strike-slip faults testify to right-lateral shearing, as well as south-directed shortening, during a transpressive phase that terminated in latest Tertiary time. We interpret the Foothills Belt, the Thrust Fault Zone, the Overturned Belt and the Gentle Dips Belt as surface manifestations of the southward rise of a basal detachment surface up stratigraphic ramps. North-dipping thrust faults splay upwards from this detachment surface which terminates in a triangle zone located beneath the south-dipping and overturned strata, but the site and depth from which it propagates are unknown. Surface mapping suggests that, beneath much of the northern Foothills Belt, the proposed detachment zone follow a decollement surface within the Upper Cretaceous Querecual formation. Beneath the southern part of the Foothills Belt, it appears to climb into the Paleocene Guarico Formation, from which it rises into the Oligocene Roblecito Formation. Deformation terminates southward in a triangle zone. Balancedmore » cross sections illustrate the kinematics of the overthrusting and highlight the implications for hydrocarbon generation and entrapment.« less

Gravity and Magnetic Maps of the Santa Maria Province, California

In this paper the authors display an isostatic residual gravity map, contour interval 5 mGal, and an aeromagnetic map, which is a mosaic of existing data . Two computer-derived maps, also at a scale of 1:250,000 display the locations of the steepest gravity gradients and of the steepest gradients on the pseudogravity transform of the magnetic map. These gradients indicate boundaries between rock masses with differing physical properties. The gradient boundaries, where steep and linear, are presumably cause by faults. The gravity reduction removes the isostatic effects of the continental margin, a 60 mGal gradient across the Santa Maria basin, leaving a maximum basin low of 40-50 mGal caused by the low-density basin rocks and bounded by various steep gradients at the basin margin faults. Offshore gravity lows indicate local sedimentary basins trending north to north-west. The magnetic anomalies are caused primarily by serpentinite in the basement of Franciscan assemblage and form irregular belts trending northwest across the basin and basin margins, terminating against the offshore Hosgri fault. The magnetic patterns west of the Hosgri are more irregular in character and may reflect a different basement. The Hosgri fault appears to extend south beyond Point Arguello (about 2 km offshore)more » according to the gravity map. The right-lateral slip on this fault is thus presumably accommodated on north-dipping thrust faults on the north side of the Santa Barbara Channel and perhaps along the south side of the Santa Maria basin as well. Along the east side of the Hosgri fault, high gravity values indicate local uplift.« less

相模湾の活構造とテクトニクス

Sagami Trough is regarded as the boundary between the Philippine Sea plate and the northeast Japan plate. Previous studies have suggested that a great earthquake will probably occur in the near future in the Sagami Bay area, the northwest portion of Sagami Trough. In this study, active geological structures and topographies in Sagami Bay are described on the basis of detailed analyses of multi-channel and single-channel seismic profiles and topographic maps, and the tectonics of this region are discussed.The Sagami Bay fault and its landward extension, the Kozu-Matsuda fault were generally thought by previous workers to be a northeast-dipping oblique-slip fault, on which most of the relative motion between the two plates are accommodated. The Manazuru Bank situated 5-10 km west of this fault, however, are formed with strongly deformed trough fill sediments. Moreover the rupture surface of the 1923 Kanto earthquake (M. 7.9) deduced from geodetic and seismological data extends across the Kozu-Matsuda-Sagami Bay fault. These suggest that the Manazuru Bank is underlain by a north-dipping thrust fault, on which the plate motions are accommodated. I speculate that the leading edge of the thrust fault is now propagating further west to Izu Peninsula. This could explain the late Quaternary deformation of Izu Peninsula and Ashigara Plain.

Near‐surface expression of the eastern part of the San Cayetano Fault: A potentially active thrust fault in the California Transverse Ranges

Examined in this study are the near‐surface geometry, late Cenozoic geologic history, and seismic potential of the eastern part of the San Cayetano fault, a major north dipping thrust fault of the Transverse Ranges, California. The fault generally dips with a low angle close to the surface, but the dip increases up to 40° at about 5 km below sea level. The fault may become nearly horizontal at depth and root into a midcrustal detachment surface of the Transverse Ranges. The late Cenozoic reverse movement along the San Cayetano fault may have begun during the deposition of the lower part of the Saugus Formation. The fault has also moved after the deposition of the Saugus Formation within the last 0.2 m.y. The San Cayetano fault should be considered, at least, potentially active and may be capable of producing damaging earthquakes. This conclusion is based collectively on (1) the presence of several surficial geological features suggestive of recent movement along the fault, (2) the presence of microearthquakes on the fault plane, and (3) the structural similarity of the fault to seismically active north dipping thrust faults of the region.