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1.
Summary. The present day seismicity of the Zagros seems to occur on high angle reverse faults distributed across the whole width of the belt. It does not indicate activity on a single inclined thrust surface and there do not seem to have been any well located events at intermediate depths. Modelling of the long period teleseismic body waves of seven large earthquakes presented here shows their focal depths to be in the range 8–15 km. This is thought to indicate faulting in the uppermost basement beneath the sedimentary cover, though the absence of published seismic refraction results renders the sediment thickness uncertain. Faulting of this type and depth may occur on inherited normal faults which have subsequently been reactivated as thrusts. Such reactivation allows considerable shortening to take place in the early stages of continental collision without the subduction or excessive thickening of continental crust.  相似文献   

2.
Microseismicity and faulting geometry in the Gulf of Corinth (Greece)   总被引：7，自引：0，他引：7  
During the summer of 1993, a network of seismological stations was installed over a period of 7 weeks around the eastern Gulf of Corinth where a sequence of strong earthquakes occurred during 1981. Seismicity lies between the Alepohori fault dipping north and the Kaparelli fault dipping south and is related to both of these antithetic faults. Focal mechanisms show normal faulting with the active fault plane dipping at about 45° for both faults. The aftershocks of the 1981 earthquake sequence recorded by King et al . (1985 ) were processed again and show similar results. In contrast, the observations collected near the western end of the Gulf of Corinth during an experiment conducted in 1991 ( Rigo et al . 1996 ), and during the aftershock studies of the 1992 Galaxidi and the 1995 Aigion earthquakes ( Hatzfeld et al . 1996 ; Bernard et al . 1997 ) show seismicity dipping at a very low angle (about 15°) northwards and normal faulting mechanisms with the active fault plane dipping northwards at about 30°. We suggest that the 8–12 km deep seismicity in the west is probably related to the seismic–aseismic transition and not to a possible almost horizontal active fault dipping north as previously proposed. The difference in the seismicity and focal mechanisms between east and west of the Gulf could be related to the difference in the recent extension rate between the western Gulf of Corinth and the eastern Gulf of Corinth, which rotated the faults dipping originally at 45° (as in the east of the Gulf) to 30° (as in the west of the Gulf).  相似文献   

3.
Assessing seismic hazard in continental interiors is difficult because these regions are characterized by low strain rates and may be struck by infrequent destructive earthquakes. In this paper, we provide an example showing that interpretations of seismic cross sections combined with other kinds of studies such as analysis of microseismicity allow the whole seismogenic source area to be imaged in this type of region. The Middle Durance Fault (MDF) is an 80-km-long fault system located southeastern France that has a moderate but regular seismicity and some palaeoseismic evidence for larger events. It behaves as an oblique ramp with a left-lateral-reverse fault slip and has a low strain rate. MDF is one of the rare slow active fault system monitored by a dedicated dense velocimetric short period network. This study showed a fault system segmented in map and cross section views which consists of staircase basement faults topped by listric faults ramping off Triassic evaporitic beds. Seismic sections allowed the construction of a 3-D structural model used for accurate location of microseismicity. Southern part of MDF is mainly active in the sedimentary cover. In its northern part and in Alpine foreland, seismicity deeper than 8 km was also recorded meaning active faults within the crust cannot be excluded. Seismogenic potential of MDF was roughly assessed. Resulting source sizes and estimated slip rates imply that the magnitude upper limit ranges from 6.0 to 6.5 with a return period of a few thousand years. The present study shows that the coupling between 3-D fault geometry imaging and accurate location of microseismicity provides a robust approach to analyse active fault sources and consequently a more refined seismic hazard assessment.  相似文献   

4.
Three main shocks M-1, M-2 and M-3 (17 October 2005 at 05:45 UTC, M w 5.4; 17 October at 09:46 UTC, M w 5.8 and 20 October at 21:40 UTC, M w 5.9) and their associated aftershocks within the Gulf of S i ğac i k, 50 km southwest of Izmir, Turkey were studied in detail. A temporary seismic network deployed during the activity allowed the hypocentre of M-3 and subsequent aftershocks to be determined with high accuracy. A relative relocation technique was used to improve the epicentres of M-1 and M-2. All three main shocks have strike-slip mechanisms which agree with the linear trends of the aftershock locations. Two distinct zones were illuminated by the aftershock locations. The zones contain clear echelon patterns with slightly different orientations from the trend of the aftershock distribution. M-2 and M-3 ruptured along of the eastern rupture zone which aligns N45°E. However the strike direction of M-1 is not clearly identified. The alignment of the two rupture zones intersect at their southern terminus at an angle of 90°. The fault zones form conjugate pair system and static triggering is considered as a probable mechanism for the sequential west to east occurrence of M-1, M-2 and M-3. This earthquake sequence provides seismological evidence for conjugate strike-slip faulting co-existing within a region dominated by north–south extension and well-developed east–west trending normal faults.  相似文献   

5.
Summary. An array of 28 portable seismic stations was operated in the region of El Asnam following the magnitude 7.3 ( M s) thrust earthquake of 1980 October 10. Locations of 494 events are presented in this paper and provide an indication of the overall form of the aftershock distribution. Tests to establish location accuracy (particularly depth) reduce this set to 277 events which, it is argued, are well constrained. P -waves alone are used in this study as a consequence of a debate about the reliability of reading S -phases. From the reduced set of 277 events, 81 events provide well-constrained focal mechanisms.
The locations are presented in the form of maps and cross-sections, and discussed in relation to information already derived from field mapping of surface breaks and teleseismic studies of the waveforms of the main event. The zone of surface faulting (including secondary normal faulting) extended for 35 km but the aftershock distribution extends for twice this distance. Along the part of the fault which experienced substantial displacement in the main shock, the fault plane itself appears to be devoid of aftershocks, although many lie in the footwall beneath the fault. At junctions between segments of thrust faulting, strike-slip motion occurs. This is apparent in the aftershock focal mechanisms, and in the surface ruptures in one place.
The large number of aftershocks in the north-east area appears to be due to the reactivation of a fan-like system of smaller reverse faults associated with surface folding. Activity at the south-west end is considerably less than that in the north-east, and is not obviously associated with recognizable geological or morphological features.  相似文献   

6.
We use teleseismic waveform analysis and locally recorded aftershock data to investigate the source processes of the 2004 Baladeh earthquake, which is the only substantial earthquake to have occurred in the central Alborz mountains of Iran in the modern instrumental era. The earthquake involved slip at 10–30 km depth, with a south-dipping aftershock zone also restricted to the range 10–30 km, which is unusually deep for Iran. These observations are consistent with co-seismic slip on a south-dipping thrust that projects to the surface at the sharp topographic front on the north side of the Alborz. This line is often called the Khazar Fault, and is assumed to be a south-dipping thrust which bounds the north side of the Alborz range and the south side of the South Caspian Basin, though its actual structure and significance are not well understood. The lack of shallower aftershocks may be due to the thick pile of saturated, overpressured sediments in the South Caspian basin that are being overthrust by the Alborz. A well-determined earthquake slip vector, in a direction different from the overall shortening direction across the range determined by GPS, confirms a spatial separation ('partitioning') of left-lateral strike-slip and thrust faulting in the Alborz. These strike-slip and thrust fault systems do not intersect within the seismogenic layer on the north side, though they may do so on the south. The earthquake affected the capital, Tehran, and reveals a seismic threat posed by earthquakes north of the Alborz, located on south-dipping thrusts, as well as by earthquakes on the south side of the range, closer to the city.  相似文献   

7.
Expanding spread profile at the northern Jan Mayen Ridge   总被引：1，自引：0，他引：1  
An expanding spread seismic profile at the central northern Jan Mayen Ridge, ESP-5, has yielded a crustal seismic velocity distribution which is similar to observations from the thinned continental crust at the Norwegian continental margin. The profile reveals a post-early Eocene sedimentary sequence, about 1. 5 km thick, overlying 1 km of volcanic extrusives and interbedded sediments. Below, there are about 3 km of pre-opening sediments above the seismic basement. The results indicate that the main ridge block is underlain by a thinned crust, possibly only 13.5 km thick. The results are compatible with a continental nature for the main ridge complex.  相似文献   

8.
《Basin Research》2018,30(Z1):382-400
High‐resolution acoustic and seismic data acquired 100 km offshore Cape São Vicente, image with unprecedented detail one of the largest active reverse faults of the SW Iberian Margin, the Horseshoe Fault (HF). The HF region is an area seismogenically active, source of the largest magnitude instrumental and historical earthquake (Mw > 6) occurred in the SW Iberian Margin. The HF corresponds to a N40 trending, 110 km long, and NW‐verging active thrust that affects the whole sedimentary sequence and reaches up to the seafloor, generating a relief of more than 1 km. The along‐strike structural variability as well as fault trend suggests that the HF is composed by three main sub‐segments: North (N25), Central (N50) and South (N45). Swath‐bathymetry, TOBI sidescan sonar backscatter and parametric echosounder TOPAS profiles reveal the surface morphology of the HF block, characterized by several, steep (20°) small scarps located on the hangingwall, and a succession of mass transport deposits (i.e. turbidites) on its footwall, located in the Horseshoe Abyssal Plain. A succession of pre‐stack depth‐migrated multichannel seismic reflection profiles across the HF and neighbouring areas allowed us to constrain their seismo‐stratigraphy, structural geometry, tectono‐sedimentary evolution from Upper Jurassic to present‐day, and to calculate their fault parameters. Finally, on the basis of segment length, surface fault area and seismogenic depth we evaluated the seismic potential of the HF, which in the worst‐case scenario may generate an earthquake of magnitude Mw 7.8 ± 0.1. Thus, considering the tectonic behaviour and near‐shore location, the HF should be recognized in seismic and tsunami hazard assessment models of Western Europe and North Africa.  相似文献   

9.
In a tectonically active setting large earthquakes are always threats; however, they may also be useful in elucidating the subsurface geology. Instrumentally recorded seismicity is, therefore, widely utilized to extend our knowledge into the deeper crust, especially where basement is involved. It is because the earthquakes are triggered by underground stress changes that usually corresponding to the framework of geological structures. Hidden faults, therefore, can be recognized and their extension as well as orientation can be estimated. Both above are of relevance for assessment on seismic hazard of a region, since the active faults are supposed to be re-activated and cause large earthquakes. In this study, we analysed the 1999 October 22 earthquake sequence that occurred in southwestern Taiwan. Two major seismicity clusters were identified with spatial distribution between depths of 10 and 16 km. One cluster is nearly vertical and striking 032°, corresponding to the strike-slip Meishan fault (MSF) that generated the 1906 surface rupture. Another cluster strikes 190° and dips 64° to the west, which is interpreted as west-vergent reverse fault, in contrast to previous expectation of east vergence. Our analysis of the focal solutions of all the larger earthquakes in the 1999 sequence with the 3-D distribution of all the earthquakes over the period 1990–2004 allows us reinterpret the structural framework and suggest previously unreognized seismogenic sources in this area. We accordingly suggest: (1) multiple detachment faults are present in southwestern Taiwan coastal plain and (2) additional seismogenic sources consist of tear faults and backthrust faults in addition to sources associated with west-vergent fold-and-thrust belt.  相似文献   

10.
Deciphering the evolution of mountain belts requires information on the temporal history of both topographic growth and erosion. The exhumation rate of a mountain range undergoing shortening is related to the erodability of the uplifting range as well as the efficiency of erosion, which partly depends on the available precipitation. Young, rapidly deposited sediments have low thermal conductivity and are readily eroded, in contrast to underlying resistant basement rocks that have a higher thermal conductivity. Apatite fission‐track thermochronology can quantify cooling; thermal models constrain the relationship between this cooling and exhumation. By utilizing geological relations for a datum, we can examine the evolution of rock uplift, surface uplift and exhumation. In the northern Sierras Pampeanas of Argentina, a young sedimentary basin that overlay resistant crystalline basement prior to rapid exhumation provides an ideal setting to examine the effect of contrasting thermal and erosional regimes. There, tectonically active reverse‐fault‐bounded blocks partly preserve a basement peneplain at elevations in excess of 4500 m. Prior to exhumation, the two study areas were covered by 1000 and 1600 m of recently deposited sediments; this sequence begins with shallow marine deposits immediately overlying the regional erosion surface. Apatite fission‐track data were obtained from vertical transects in the Calchaquíes and Aconquija ranges. At Cumbres Calchaquíes, erosion leading to the development of the peneplain commenced in the Cretaceous, probably as a result of rift‐shoulder uplift. In contrast, Sierra Aconquija cooled rapidly between 5.5 and 4.5 Myr. At the onset of this rapid exhumation, the sediment was quickly removed, causing fast cooling, but relatively slow rates of surface uplift. Syntectonic conglomerates were produced when faulting exposed resistant bedrock; this change in rock erodability led to enhanced surface uplift rates, but decreased exhumation rates. The creation of an orographic barrier after the range had attained sufficient elevation further decreased exhumation rates and increased surface uplift rates. Differences in the magnitude of exhumation at the two transects are related to both differences in the thickness of the sedimentary basin prior to exhumation and differences in the effective precipitation due to an orographic barrier in the foreland and hence differences in the magnitude of headward erosion.  相似文献   

11.
汶川特大地震导致了青川县乔庄镇(主城区)遭遇了较为惨重的特大灾难.县城的恢复重建工作受到地震断裂、山体变形、强烈的余震活动等严重制约.尤其是地震断裂问题,更是对能否原址重建起决定性作用.在参考以往研究成果的基础上,结合实地调查,对该镇的地震断裂问题进行了评估,确定了青川-平武断裂(龙门山后山断裂主断裂之一)分三条分支断裂(北支断裂、中间断裂、南支断裂)穿过乔庄镇主城区的大致方位和走势,并且现场发现该三条分支断裂均发生了不同程度的地震破裂(如地表破裂,山体震裂等),已逐渐发展成为中强震的孕震构造.  相似文献   

12.
Reactivation of pre‐existing intra‐basement structures can influence the evolution of rift basins, yet the detailed kinematic relationship between these structures and overlying rift‐related faults remains poorly understood. Understanding the kinematic as well as geometric relationship between intra‐basement structures and rift‐related fault networks is important, with the extension direction in many rifted provinces typically thought to lie normal to fault strike. We here investigate this problem using a borehole‐constrained, 3D seismic reflection dataset from the Taranaki Basin, offshore New Zealand. Excellent imaging of intra‐basement structures and a relatively weakly deformed, stratigraphically simple sedimentary cover allow us to: (a) identify a range of interaction styles between intra‐basement structures and overlying, Plio‐Pleistocene rift‐related normal faults; and (b) examine the cover fault kinematics associated with each interaction style. Some of the normal faults parallel and are physically connected to intra‐basement reflections, which are interpreted as mylonitic reverse faults formed during Mesozoic subduction and basement terrane accretion. These geometric relationships indicate pre‐existing intra‐basement structures locally controlled the position and attitude of Plio‐Pleistocene rift‐related normal faults. However, through detailed 3D kinematic analysis of selected normal faults, we show that: (a) normal faults only nucleated above intra‐basement structures that experienced late Miocene compressional reactivation, (b) despite playing an important role during subsequent rifting, intra‐basement structures have not been significantly extensionally reactivated, and (c) preferential nucleation and propagation of normal faults within late Miocene reverse faults and folds appears to be the key genetic relationship between contractionally reactivated intra‐basement structures and rift‐related normal faults. Our analysis shows that km‐scale, intra‐basement structures can control the nucleation and development of newly formed, rift‐related normal faults, most likely due to a local perturbation of the regional stress field. Because of this, simply inverting fault strike for causal extension direction may be incorrect, especially in provinces where pre‐existing, intra‐basement structures occur. We also show that a detailed kinematic analysis is key to deciphering the temporal as well as simply the spatial or geometric relationship between structures developed at multiple structural levels.  相似文献   

13.
In this work, we explore by means of analogue models how different basin-bounding fault geometries and thickness of a viscous layer within the otherwise brittle pre-rift sequence influence the deformation and sedimentary patterns of basins related to extension. The experimental device consists of a rigid wooden basement in the footwall to simulate a listric fault. The hangingwall consists of a sequence of pre-rift deposits, including the shallow interlayered viscous layer, and a syn-rift sequence deposited at constant intervals during extension. Two different geometries exist of listric normal faults, dip at 30 and 60° at surface. This imposes different geometries in the hangingwall anticlines and their associated sedimentary basins. A strong contrast exists between models with and without a viscous layer. With a viscous décollement, areas near the main basement fault show a wide normal drag and the hangingwall basin is gently synclinal, with dips in the fault side progressively shallowing upwards. A secondary roll-over structure appears in some of the models. Other structures are: (1) reverse faults dipping steeply towards the main fault, (2) antithetic faults in the footwall, appearing only in models with the 30° dipping fault and silicone-level thicknesses of 1 and 1.5 cm and (3) listric normal faults linked to the termination of the detachment level opposite to the main fault, with significant thickness changes in the syn-tectonic units. The experiments demonstrate the importance of detachment level in conditioning the geometry of extensional sedimentary basins and the possibility of syncline basin geometries associated with a main basement fault. Comparison with several basins with half-graben geometries containing a mid-level décollement supports the experimental results and constrains their interpretation.  相似文献   

14.
The Billefjorden Fault Zone represents a major lineament on Spitsbergen with a history of tectonic activity going back into the Devonian and possibly earlier. Recent structural, sedimcntological and stratigraphical investigations indicate that most of the stratigraphic thickness variations within the Mesozoic strata along the Billefjorden Fault Zone south of Isfjordcn are due to Tertiary compressional tectonics related to the transpressive Eocene West-Spitsbergen Orogeny. No convincing evidence of distinct Mesozoic extensional events, as suggested by previous workers, has been recognized. Tertiary compressional tectonics are characterized by a combined thin-skinned/thick-skinned structural style. Decollement zones arc recognized in the Triassic Sassendalen Group (tower Décollement Zone) and in the Jurassic/Cretaceous Janusfjellet Subgroup (Upper Décollement Zone). East-vergent folding and reverse faulting associated with these decollement' zones have resulted in the development of compressional structures, of which the major arc the Skolten and Tronfjellct Anticlines and the Advcntelva Duplex. Movements on one or more high angle east-dipping reverse faults in the pre-Mesozoic basement have resulted in the development of the Juvdalskampcn Monocline, and are responsible for out-of-sequence thrusting and thinning of the Mesozoic sequence across the Billefjorden Fault Zone. Preliminary shortening calculations indicate an eastward displacement of minimum 3-4 km, possibly as much as 10 km for the Lower Cretaceous and younger rocks across the Billefjorden Fault Zone.  相似文献   

15.
Summary. In 1985, 180 km of regional vibroseis profiles were acquired in the Carolinas and Georgia, southeastern United States, as part of the Appalachian Ultra-Deep Core Hole (ADCOH) Site Study. The data quality is excellent, with large-amplitude reflections from faults and crystalline rocks, lower Palaeozoic shelf strata and from within autochthonous Grenville basement. The profiles image the subsurface more clearly than other available data and allow the possibility of alternative interpretations of important elements of the tectonic framework of the southern Appalachians.
The major points in the interpretation are: 1) The Blue Ridge master decollement is at a depth of 2-3 km beneath the Blue Ridge. This thrust increases in dip just NW of the Brevard fault zone. 2) The Brevard fault zone appears to splay from the master decollement at 6 km (2.2 s) near Westminster, S.C., and defines the base of the crystalline Inner Piedmont allochthon. 3) Below the Blue Ridge thrust sheet are images of duplex and imbricate structures ("duplex tuning wedges") connected by other thrust faults that duplicate shelf strata to a thickness of 4–5 km. 4) Subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower Palaeozoic strata as suggested by others. 5) Eocambrian-Cambrian(?) rift basins in the Grenville basement are also imaged.
The ADCOH data were originally recorded with 14–56 Hz bandwidth and 8 s length, but an extended Vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust. Below 8 s, reflections from within the Grenville basement become weak, but are observable as late as 13 s; however, these Moho (?) reflections are generally short segments.  相似文献   

16.
Seismic reflection profiles from the Murray Ridge in the Gulf of Oman, northwest Indian Ocean, show a significant component of extension across the predominantly strike-slip Indian–Arabian plate boundary. The Murray Ridge lies along the northern section of the plate boundary, where its trend becomes more easterly and thus allows a component of extension. The Dalrymple Trough is a 25 km wide, steep-sided half-graben, bounded by large faults with components of both strike-slip and normal motion. The throw at the seabed of the main fault on the southeastern side of the half-graben reaches 1800 m. The northwest side of the trough is delineated by a series of smaller antithetic normal faults. Wide-angle seismic, gravity and magnetic models show that the Murray Ridge and Dalrymple Trough are underlain by a crystalline crust up to 17 km thick, which may be continental in origin. Any crustal thinning due to extension is limited, and no new crust has been formed.
We favour a plate model in which the Indian–Arabian plate boundary was initially located further west than the Owen Fracture Zone, possibly along the Oman continental margin, and suggest that during the Oligocene–Early Miocene Indian Ocean plate reorganization, the plate boundary moved to the site of the present Owen Fracture Zone and that motion further west ceased. At this time, deformation began along the Murray Ridge, with both the uplift of basement highs, and subsidence in the troughs tilting the lowest sedimentary unit. Qalhat Seamount was formed at this time. Subsequent sediments were deposited unconformably on the tilted lower unit and then faulted to produce the present basement topography. The normal faulting was accompanied by hanging-wall subsidence, footwall uplift, and erosion. Flat-lying recent sediments show that the major vertical movements have ceased, although continuing earthquakes show that some faulting is still active along the plate boundary.  相似文献   

17.
Following the 1996 February 18 M L = 5.2 earthquake in the Agly massif in the eastern French Pyrenees, we installed a temporary network of seismometers around the epicentre. In this paper, we analyse 336 well-located aftershocks recorded from February 19 to February 23 by 18 temporary stations and two permanent stations located less than 35  km from the epicentre. Most aftershocks have been located with an accuracy better than 1.5  km in both horizontal and vertical positions. Their spatial distribution suggests the reactivation of a known fault system. We determined 39 fault-plane solutions using P -wave first motions. Despite their diversity, the focal mechanisms yield an E–W subhorizontal T-axis. We also determined fault-plane solutions and principal stress axes using the method developed by Rivera & Cisternas (1990 ) for the 15 best-recorded events. We obtain a pure-shear-rupture tectonic regime under N–S subhorizontal compression and E–W subhorizontal extension. These principal stress axes, which explain the focal mechanisms for at least 75 per cent of the 39 aftershocks, are different from the axes deduced from the main shock. The post-earthquake stress field caused by the main-shock rupture, modelled as sinistral strike slip on three vertical fault segments, is computed for various orientations and magnitudes of the regional stress field, assumed to be horizontal. The aftershock distribution is best explained for a compressive stress field oriented N30°E. Most aftershocks concentrate where the Coulomb failure stress change increases by more than 0.2  MPa. The diversity of aftershock focal mechanisms, poorly explained by this model, may reflect the great diversity in the orientations of pre-existing fractures in the Agly massif.  相似文献   

18.
Formed during an early compressional period in the opening of North Atlantic Ocean, a Tertiary fold-thrust belt extends along the mid-to- southern part of the western coast of Spitsbergen. Complex thrust structures involve the basement (Caledonian and older) and many shallow dipping thrust faults dissect the overlying cover rocks (Devonian and younger) in Oscar II Land in the northern part of the belt. Some of these faults occur within the basement rocks with slivers or fault blocks of the cover rocks from south-western Brøggerhalvøya to innermost St. Jonsfjorden in north-eastern Oscar II Land. Six of the slivers contain Carboniferous rocks and one is a fault-bounded block with Devonian rocks. These steeply west-dipping faults form a complex fault system- EOFC (Engelskbukta-Osbornbreen Fault Complex) - within the basement area. The lithological units of the basement are separated by faults within the EOFC, which is structurally continuous with the Brøggerhalvøya fold-thrust zone to the north and is thought to continue to the fold-thrust zone on the south-eastern coast of St. Jonsfjorden. Some previous authors considered that the two lithologically contrasting Vendian diamictites and intervening Moefjellet Formation are stratigraphically continuous and defined two separate tilloid successions in the present area. This interpretation has been extended over the whole of western Spitsbergen. However, the present study indicates that these two tilloid formations and the Moefjellet Formation are separated by the faults, probably thrusts, within the EOFC and are not in a continuous stratigraphic relation. Therefore, the two-stage history of Vendian glaciation seems questionable.  相似文献   

19.
The style of extension and strain distribution during the early stages of intra-continental rifting is important for understanding rift-margin development and can provide constraints for lithospheric deformation mechanisms. The Corinth rift in central Greece is one of the few rifts to have experienced a short extensional history without subsequent overprinting. We synthesise existing seismic reflection data throughout the active offshore Gulf of Corinth Basin to investigate fault activity history and the spatio-temporal evolution of the basin, producing for the first time basement depth and syn-rift sediment isopachs throughout the offshore rift. A major basin-wide unconformity surface with an age estimated from sea-level cycles at ca . 0.4 Ma separates distinct seismic stratigraphic units. Assuming that sedimentation rates are on average consistent, the present rift formed at 1–2 Ma, with no clear evidence for along-strike propagation of the rift axis. The rift has undergone major changes in relative fault activity and basin geometry during its short history. The basement depth is greatest in the central rift (maximum ∼3 km) and decreases to the east and west. In detail however, two separated depocentres 20–50 km long were created controlled by N- and S-dipping faults before 0.4 Ma, while since ca . 0.4 Ma a single depocentre (80 km long) has been controlled by several connected N-dipping faults, with maximum subsidence focused between the two older depocentres. Thus isolated but nearby faults can persist for timescales ca . 1 Ma and form major basins before becoming linked. There is a general evolution towards a dominance of N-dipping faults; however, in the western Gulf strain is distributed across several active N- and S-dipping faults throughout rift history, producing a more complex basin geometry.  相似文献   

20.
The nature of the transition between the Zagros intra-continental collision and the Makran oceanic subduction is a matter of debate: either a major fault cutting the whole lithosphere or a more progressive transition associated with a shallow gently dipping fault restricted to the crust. Microearthquake seismicity located around the transition between the transition zone is restricted to the west of the Jaz-Murian depression and the Jiroft fault. No shallow micro-earthquakes seem to be related to the NNW–SSE trending Zendan–Minab–Palami active fault system. Most of the shallow seismicity is related either to the Zagros mountain belt, located in the west, or to the NS trending Sabzevaran–Jiroft fault system, located in the north. The depth of microearthquakes increases northeastwards to an unusually deep value (for the Zagros) of 40 km. Two dominant types of focal mechanisms are observed in this region: low-angle thrust faulting, mostly restricted to the lower crust, and strike-slip at shallow depths, both consistent with NS shortening. The 3-D inversion of P traveltimes suggests a high-velocity body dipping northeastwards to a depth of 25 km. This high-velocity body, probably related to the lower crust, is associated with the deepest earthquakes showing reverse faulting. We propose that the transition between the Zagros collision and the Makran subduction is not a sharp lithospheric-scale transform fault associated with the Zendan–Minab–Palami fault system. Instead it is a progressive transition located in the lower crust. The oblique collision results in partial partitioning between strike-slip and shortening components within the shallow brittle crust because of the weakness of the pre-existing Zendan–Minab–Palami faults.  相似文献   

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