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1.
We present GPS observations in Morocco and adjacent areas of Spain from 15 continuous (CGPS) and 31 survey-mode (SGPS) sites extending from the stable part of the Nubian plate to central Spain. We determine a robust velocity field for the W Mediterranean that we use to constrain models for the Iberia-Nubia plate boundary. South of the High Atlas Mountain system, GPS motions are consistent with Nubia plate motions from prior geodetic studies. We constrain shortening in the Atlas system to <1.5 mm/yr, 95% confidence level. North of the Atlas Mountains, the GPS velocities indicate Nubia motion with respect to Eurasia, but also a component of motion normal to the direction of Nubia-Eurasia motion, consisting of southward translation of the Rif Mountains in N Morocco at rates exceeding 5 mm/yr. This southward motion appears to be directly related to Miocene opening of the Alboran Sea. The Betic Mountain system north of the Alboran Sea is characterized by WNW motion with respect to Eurasia at ～1–2 mm/yr, paralleling Nubia-Eurasia relative motion. In addition, sites located in the Betics north of the southerly moving Rif Mountains also indicate a component of southerly motion with respect to Eurasia. We interpret this as indicating that deformation associated with Nubia-Eurasia plate motion extends into the southern Betics, but also that the Betic system may be affected by the same processes that are causing southward motion of the Rif Mountains south of the Alboran Sea. Kinematic modeling indicates that plate boundary geometries that include a boundary through the Straits of Gibraltar are most compatible with the component of motion in the direction of relative plate motion, but that two additional blocks (Alboran-Rif block, Betic Mountain block), independent of both Nubia and Eurasia are needed to account for the motions of the Rif and Betic Mountains normal to the direction of relative plate motion. We speculate that the southward motions of the Alboran-Rif and Betic blocks may be related to mantle flow, possibly induced by southward rollback of the subducted Nubian plate beneath the Alboran Sea and Rif Mountains.  相似文献   

2.
The presence of continuous upper crustal blocks between the Iberian Betics and Moroccan Rif in the western and middle Alboran Sea, detected with tomography, can add new information about the lithosphere structure and geodynamic evolution in this region. A large volume of seismic data (P and S wave arrival times) has been collected for the period between 1 December 1988 and 31 December 2008 by 57 stations located in northern Morocco (National Institute of Geophysics, CNRST, Rabat), southern Portugal (Instituto de Meteorologia, Lisbon) and Spain (Instituto Geografico National, Madrid) and used to investigate the lithosphere in the western Alboran Sea region. We use a linearized inversion procedure comprising two steps: (1) finding the minimal 1-D model and simultaneous relocation of hypocenters and (2) determination of local velocity structure using linearized inversion. The model parameterization in this method assumes a continuous velocity field. The resolution tests indicate that the calculated images give near true structure imaged at 5 km depth for the Tanger peninsula, the Alhoceima region and southern Spain. At 15, 30 and 45 km depth we observe a near true structure imaged in northern Morocco, and southern Spain. At 60 and 100 km, southern Spain and the SW region of the Alboran Sea give a near true structure. The resulting tomographic image shows the presence of two upper crustal bodies (velocity 6.5 km/s) at 5–10 km depth between the Betics, Rif, western and central Alboran Sea. Low velocities at the base of these two bodies favor the presence of melt. This new evidence proves that the Tethysian ocean upper crust was not totally collapsed or broken down during the late Oligocene–early Miocene. These two blocks of upper crust were initially one block. The geodynamic process in the eastern of the Mediterranean is driven by slab rollback. The delamination process of the lithospheric mantle terminates with the proposed slab rollback in the western part of the Mediterranean. This can be explained by the removal of the major part of the lithosphere beneath the area, except in the SW part of the Alboran Sea where a small part of the lithospheric mantle is still attached and is extends and dips to SE beneath the Rif, slowly peeled back to the west. A second detached lithospheric mantle is located and extends to eastern part of the Rif and dips to the SE. The removal of lithosphere mantle from the base of the crust was replaced and heated by extrusion of asthenospheric material coming from depth to replace the part of crust detached. A combination of isostatic surface/topographic uplift and erosion induced a rapid exhumation and cooling of deep crustal rocks.  相似文献   

3.
Geological, geodetic and seismological data have been analyzed in order to frame the Lipari–Vulcano complex (Aeolian archipelago, southern Italy) into the geodynamic context of the southeastern Tyrrhenian Sea. It is located at the northern end of a major NNW–SSE trending right-lateral strike-slip fault system named “Aeolian–Tindari–Letojanni” which has been interpreted as a lithospheric discontinuity extending from the Aeolian Islands to the Ionian coast of Sicily and separating two different tectonic domains: a contractional one to the west and an extensional one to the north-east. Structural field data consist of structural measurements performed on well-exposed fault planes and fractures. The mesostructures are mostly represented by NW–SE striking normal faults with a dextral-oblique component of motion. Minor structures are represented by N–S oriented joints and tension gashes widespread over the whole analyzed area and particularly along fumarolized sectors. The analyzed seismological dataset (from 1994 to 2013) is based on earthquakes with magnitude ranging between 1.0 and 4.8. The hypocenter distribution depicts two major alignments corresponding to the NNW–SSE trending Aeolian–Tindari–Letojanni fault system and to the WNW–ESE oriented Sisifo–Alicudi fault system. GPS data analysis displays ∼3.0 mm/yr of active shortening between the two islands, with a maximum shortening rate of about 1.0 × 10−13 s−1, between La Fossa Caldera and south of Vulcanello. This region is bounded to the north by an area where the maximum values of shear strain rates, of about 0.7 × 10−13 s−1 are observed. This major change occurs in the area south of Vulcanello that is also characterized by a transition in the way of the vertical axis rotation. Moreover, both the islands show a clear subsidence process, as suggested by negative vertical velocities of all GPS stations which exhibit a decrease from about −15 to −7 mm/yr from north to south. New data suggest that the current kinematics of the Lipari–Vulcano complex can be framed in the tectonic context of the eastward migrating Sisifo–Alicudi fault system. This is dominated by transpressive tectonics in which contractional and minor extensional structures can coexist with strike-slip motion.  相似文献   

4.
The intermontane Ronda Basin, currently located in the Western Betics External Zones, started as an embayment of the Betic foreland basin during the Tortonian. We have characterized a post-Serravallian, basin-related deformation event that overprinted the former fold-and-thrust belt. Updated structural and kinematic maps allow us to identify NW–SE basinward-dipping normal faults at the southwestern and northeastern boundaries of the basin and NE–SW shortening structures (large-scale folds and reverse faults) affecting both the outcropping basement and partially the basin infill. In order to test the possible tectonic activity of these structures during the last 5 Ma, exhaustive geomorphologic analyses in the Ronda Basin area have been done. This included the qualitative study of relief and drainage network, together with the characterization of quantitative indices (SLk, Smf, Vf and HI). These results obtained from this analysis are coherent with structural data and suggest that the identified post-Serravallian structures were active up to at least 5 Ma. We also conclude that the Ronda Basin was generated by along strike segmentation of the relief in the Western Betics induced by NE–SW (arc-parallel) stretching accompanied with NW–SE shortening. In the NW basin boundary, the strain was partitioned into ENE–WSW dextral strike-slip faults and NE–SW shortening structures, which gave rise to a Messinian transpressive structural high that disconnected the former Ronda Basin from its parental foreland basin.  相似文献   

5.
—The plate boundary between Iberia and Africa has been studied using data on seismicity and focal mechanisms. The region has been divided into three areas: A; the Gulf of Cadiz; B, the Betics, Alboran Sea and northern Morocco; and C, Algeria. Seismicity shows a complex behavior, large shallow earthquakes (h < 30 km) occur in areas A and C and moderate shocks in area B; intermediate-depth activity (30 < h < 150 km) is located in area B; the depth earthquakes (h 650 km) are located to the south of Granada. Moment rate, slip velocity and b values have been estimated for shallow shocks, and show similar characteristics for the Gulf of Cadiz and Algeria, and quite different ones for the central region. Focal mechanisms of 80 selected shallow earthquakes (8 mb 4) show thrust faulting in the Gulf of Cadiz and Algeria with horizontal NNW-SSE compression, and normal faulting in the Alboran Sea with E-W extension. Focal mechanisms of 26 intermediate-depth earthquakes in the Alboran Sea display vertical motions, with a predominant plane trending E-W. Solutions for very deep shocks correspond to vertical dip-slip along N-S trends. Frohlich diagrams and seismic moment tensors show different behavior in the Gulf of Cadiz, Betic-Alboran Sea and northern Morocco, and northern Algeria for shallow events. The stress pattern of intermediate-depth and very deep earthquakes has different directions: vertical extension in the NW-SE direction for intermediate depth earthquakes, and tension and pressure axes dipping about 45 ° for very deep earthquakes. Regional stress pattern may result from the collision between the African plate and Iberia, with extension and subduction of lithospheric material in the Alboran Sea at intermediate depth. The very deep seismicity may be correlated with older subduction processes.  相似文献   

6.
The central part of Europe north of the alpine orogenic belt is generally seen as a relatively stable area of the western tip of the Eurasian plate. Indeed, up to now, no geodetically significant motions have been detected although an active rift system running roughly in SSE–NNW direction along the Rhine valley could have some effect on the stability of this region. Presently, the increasing accuracy of geodetic point motions should allow the study of small motions at levels down to nearly 0.1 mm/yr. We start our investigation with a closer look at the ‘true’ accuracy and significance of GPS derived point velocities of permanent stations. We compare and discuss the different levels of formal errors obtained by the three analysis centers considered in this study (EPN, JPL and SOPAC) and present additional ways of assessing the accuracy using the redundancy offered by different independent analyses and multiple systems operating at one site. On the average, all results indicate that a one-sigma level of ±0.3 mm/yr can be seen as a conservative estimate for the horizontal accuracy of point motions in central Europe. On the basis of this assumption we find that at present, the actual velocity field as determined by different analysis groups and centers does not show any significant east–west extensional deformation. We do however see a prominent north–south compressional velocity gradient of about 1 mm/yr/1000 km (1 nanostrain/yr) which could be associated with the Alpine thrust in conjunction with a south-directed horizontal component of the Fennoscandian Glacial Isostatic Adjustment.  相似文献   

7.
In this study, we present new GPS observations in Azerbaijan to provide an improved basis for determining the distribution of crustal deformation throughout the country and surrounding areas. The deformation field in the region has been analyzed with a dense GPS network configuration and a reliable quantification of the ongoing deformation was achieved. Results show that while contraction is dominant over the whole region, it is mostly concentrated on the middle and eastern parts of Caucasus Thrust Fault reaching up to 6.4 ± 0.2 mm/yr and Lesser Caucasus Fault does no accommodate more than 1–2 mm/yr of contraction. New network also clearly substantiates that the West Caspian Fault, which is a continuation of Caucasus Thrust Fault in the south, accommodates right-lateral slip rates of 7.1 ± 0.3 mm/yr in addition to 5.5 ± 0.3 mm/yr contraction rates.  相似文献   

8.
Mount Etna is located in a particular region of convergence of African and Eurasian plates where intense post-collisional tectonics caused considerable uplift. However we present arguments supporting the hypothesis that volcanism and associated seismic activity would result from a local mantle uprise leading to a “horst”, probably linked to a deep-rooted hot spot. It ensued deformation and fracturing of the overlying crust with emission of aphyric tholeiitic basalts directly from their mantle source, and subsequent development of a “deep reservoir” (or complex of intrusions) at the top of a mantle diapir near 30 km depth. This is advocated by the appearance of porphyritic alkaline lavas whose mineral equilibria and differentiation processes are consistent with an 8–10 kbar pressure, and by the development of central volcanoes. The horst itself appears to have begun in the SW sector of the present volcanic area. Its uplift was greater westward, as seen from the trend of the terraces along the Simeto river, and became later obvious toward the SE. These differential movements produced fractures and faults which are to day evident in the southern area of Mt Etna. The growth of the horst then proceeded in a NE direction, following the regional tectonic lines and with a greater intensity along the side facing SE, crossed by the regional NNW–SSE line (Aeolian–Maltese escarpment).The seismicity and ground deformation registered over the last twenty years support the proposed model. Earthquakes are unfrequent in the lower southern and western areas of the volcano, whereas they are numerous and stronger to the north-east, in the summit area above 1600 m a.s.l., and in the eastern sector along the NW–SE faults and fractures. Finally, a digital elevation model recently published reveals the existence of two tectonic domains. The first one is associated with the horst and contains prevalently NE–SW oriented faults, whereas the second is mainly linked to regional tectonics with NNW–SSE and NW–SE faults and fractures.  相似文献   

9.
Seismic analysis and geochemical interpretations provide evidence that two separate hydrothermal cells circulate within the greater Lassen hydrothermal system. One cell originates south to SW of Lassen Peak and within the Brokeoff Volcano depression where it forms a reservoir of hot fluid (235–270 °C) that boils to feed steam to the high-temperature fumarolic areas, and has a plume of degassed reservoir liquid that flows southward to emerge at Growler and Morgan Hot Springs. The second cell originates SSE to SE of Lassen Peak and flows southeastward along inferred faults of the Walker Lane belt (WLB) where it forms a reservoir of hot fluid (220–240 °C) that boils beneath Devils Kitchen and Boiling Springs Lake, and has an outflow plume of degassed liquid that boils again beneath Terminal Geyser. Three distinct seismogenic zones (identified as the West, Middle, and East seismic clusters) occur at shallow depths (< 6 km) in Lassen Volcanic National Park, SW to SSE of Lassen Peak and adjacent to areas of high-temperature (≤ 161 °C) fumarolic activity (Sulphur Works, Pilot Pinnacle, Little Hot Springs Valley, and Bumpass Hell) and an area of cold, weak gas emissions (Cold Boiling Lake). The three zones are located within the inferred Rockland caldera in response to interactions between deeply circulating meteoric water and hot brittle rock that overlies residual magma associated with the Lassen Volcanic Center. Earthquake focal mechanisms and stress inversions indicate primarily N–S oriented normal faulting and E–W extension, with some oblique faulting and right lateral shear in the East cluster. The different focal mechanisms as well as spatial and temporal earthquake patterns for the East cluster indicate a greater influence by regional tectonics and inferred faults within the WLB. A fourth, deeper (5–10 km) seismogenic zone (the Devils Kitchen seismic cluster) occurs SE of the East cluster and trends NNW from Sifford Mountain toward the Devils Kitchen thermal area where fumarolic temperatures are ≤ 123 °C. Lassen fumaroles discharge geothermal gases that indicate mixing between a N2-rich, arc-type component and gases derived from air-saturated meteoric recharge water. Most gases have relatively weak isotopic indicators of upper mantle or volcanic components, except for gas from Sulphur Works where δ13C–CO2, δ34S–H2S, and δ15N–N2 values indicate a contribution from the mantle and a subducted sediment source in an arc volcanic setting.  相似文献   

10.
In the peripheral basins of the Alboran Sea, five stratigraphic units (latest Messinian-Pliocene) separated by discontinuities and representing transgressive–regressive cycles have been recognized. The first unit (LM) is latest Messinian in age and precisely characterizes the Lago-Mare event at the end of the Messinian Salinity Crisis, i.e. just before the opening of the Strait of Gibraltar at the beginning of the Pliocene. The three following units (Pl-1, Pl-2 and Pl-3) are Zanclean in age, whereas the last one (Pl-4) is Piacenzian. These four Pliocene units consist of alluvial, deltaic, and littoral deposits in the marginal areas, changing to open marine deposits with planktonic components in the basinal areas, although their extension varies in each basin. Regionally, these units do not necessarily stack in a single stratigraphic succession because of tectonics that controlled their hosting basins. Thus, the LM and Pl-1 units occur only in the Malaga and Estepona-Marbella basins, revealing that the onset of the sedimentation after the Messinian evaporitic stage and the Pliocene transgression was not a single and synchronous event in the western Alboran Sea. Moreover, the Pl-3 and Pl-4 units do not appear in all basins, so that the subsequent continentalization process of these Alboran peripheral areas during the Pliocene was also diachronous.The sedimentary evolution of the peripheral basins was controlled mainly by tectonics. During the latest Messinian-early Pliocene, the sedimentation took place in a context marked by a NNW–SSE compression and ENE–WSW perpendicular tension. The onset of the sedimentation (LM and Pl-1 units) could be linked to preexisting E–W faults that mark part of the borders of the Malaga basin and the Estepona-Marbella sector. During the deposition of the Pl-2 unit, the movements of E–W, NW–SE, and NE–SW normal faults determined a continuous subsidence in several basins, resulting in the accumulation of thick clastic marine sequences (i.e. Malaga, Vélez-Málaga, and Nerja basins in Spain and Tirinesse basin in Morocco). Tectonic activity during the early Zanclean leads to a new paleogeographic configuration of the Alboran peripheral areas. The main features are: (i) continentalization of the Nerja sector in the Betics, Tetouan, and Oued Laou-Tirinesse sectors in the Rif; (ii) on the contrary, a period of intense subsidence started in the coastal sectors between Torremolinos and Manilva, allowing the development of the Pl-3 unit directly on the substratum; and (iii) the Malaga, Vélez-Málaga, and Malalyine basins maintained the marine regime, so their sedimentary infilling recorded the Pl-2–Pl-3 unconformity. Nevertheless, these last basins emerged shortly afterwards, before the end of the early Zanclean (FO of Globorotalia puncticulata), probably in relation to the beginning of the sea-level fall which characterizes the upper part of the TB 3.4 cycle by Haq et al. (1987).During the late Zanclean, sedimentation occurred only in the Betic basins, where NNE–SSW faults – conjugated with NW–SE faults – induced a major subsidence, permitting better development of the Pl-3 unit. On the contrary, NW–SE faults in the sector between Malaga and Nerja, and NE–SW faults in the Tirinesse basin, became practically inactive. Before the end of the Zanclean, the subsidence ceased also in the westernmost Betic basins, thus causing emersion, firstly in the sector between Torremolinos and Manilva and, slightly later, in the San Roque-Algeciras sector. Thus, the whole geodynamic activity conditioned the time–space evolution of the northern edge of the Alboran Sea, which was emerging throughout the Zanclean, successively from the E to the W. A similar E to W continentalization trend can be suggested for the Rifian Pliocene sectors when taking into account the Oued Laou-Tirinesse basins that emerged before the Malalyine one.Moreover, the unit boundaries do not coincide with those of the familiar Exxon coastal aggradational curve, but rather with the local or regional tectonic activity. Consequently, the correlation of the unit boundaries with those of the Pliocene deposits of the eastern Betic basins remains difficult. According to the biostratigraphical data, the Pl-1, Pl-2, and Pl-3 units correspond to the Pliocene-I by Montenat (1990), while the Pl-4 unit may be equivalent to the Pliocene-II.  相似文献   

11.
We present fundamental-mode Rayleigh-wave azimuthally anisotropic phase velocity maps obtained for the Great Basin region at periods between 16 s and 102 s. These maps offer the first depth constraints on the origin of the semi-circular shear-wave splitting pattern observed in central Nevada, around a weak azimuthal anisotropy zone. A variety of explanations have been proposed to explain this signal, including an upwelling, toroidal mantle flow around a slab, lithospheric drip, and a megadetachment, but no consensus has been reached. Our phase velocity study helps constrain the three-dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western United States. The dispersion measurements were made using data from the USArray Transportable Array. At periods of 16 s and 18 s, which mostly sample the crust, we find a region of low anisotropy in central Nevada coinciding with locally reduced phase velocities, and surrounded by a semi-circular pattern of fast seismic directions. Away from central Nevada the fast directions are ～ N–S in the eastern Great Basin, NW–SE in the Walker Lane region, and they transition from E–W to N–S in the northwestern Great Basin. Our short-period phase velocity maps, combined with recent crustal receiver function results, are consistent with the presence of a semi-circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust. At longer periods (28–102 s), which sample the uppermost mantle, isotropic phase velocities are significantly reduced across the study region, and fast directions are more uniform with an ～ E–W fast axis. The transition in phase velocities and anisotropy can be attributed to the lithosphere–asthenosphere boundary at depths of ～ 60 km. We interpret the fast seismic directions observed at longer periods in terms of present-day asthenospheric flow-driven deformation, possibly related to a combination of Juan de Fuca slab rollback and eastward-driven mantle flow from the Pacific asthenosphere. Our results also provide context to regional SKS splitting observations. We find that our short-period phase velocity anisotropy can only explain ～ 30% of the SKS splitting times, despite similar patterns in fast directions. This implies that the origin of the regional shear-wave splitting signal is complex and must also have a significant sublithospheric component.  相似文献   

12.
The Kachchh province of Western India is a major seismic domain in an intraplate set-up. This seismic zone is located in a rift basin, which was developed during the early Jurassic break-up of the Gondwanaland. The crustal strain determined from the GPS velocity data of post-seismic time period following the 2001 Bhuj earthquake indicates a maximum strain rate of ∼266 × 10−9 per year along N013°. Focal mechanism solutions of the main event of 26 January 2001 and the aftershocks show that the maximum principal stress axis is close to this high strain direction. Maximum shear strain rate determined from the GPS data of the area has similar orientation. The unusually high strain rate is comparable in magnitude to the continental rift systems. The partitioning of the regional NE–SW horizontal stress (SHmax) by the pre-existing EW-striking boundary fault developed the strike–slip components parallel to the regional faults, the normal components perpendicular to the faults, NE-striking conjugate Riedel shear fractures and tension fractures. The partitioned normal component of the stress is considered to be the major cause for compression across the regional EW faults and development of the second-order conjugate shear fractures striking NE–SW and NW–SE. The NE-striking transverse faults parallel to the anti-Riedel shear planes have become critical under these conditions. These anti-Riedel planes are interpreted to be critical for the seismicity of the Kachchh region. The high strain rate in this area of low to moderate surface heat flow is responsible for deeper position of the brittle–ductile transition and development of deep seated seismic events in this intraplate region.  相似文献   

13.
Located on the margin of the west Alboran basin, the Gibraltar Arc (Betic-Rif mountain belt) displays post-Pliocene vertical movements evidenced by uplifted marine sedimentary basins and marine terraces. Quantification of vertical movements is an important clue to understand the origin of present-day relief generation in the Betic-Rif mountain chain together with the causes of the Messinian Salinity Crisis. In this paper, we present the results of a pluridisciplinary study combining an analysis of low temperature thermochronology and Pliocene basins evolution to constrain the exhumation history and surface uplift of internals units of the Rif belt (Northern Morocco). The mean (U-Th)/He apatite ages obtained from 11 samples are comprised between 14.1 and 17.8 Ma and display a wide dispersion, which could be explained by a great variability of apatite chemistries in the analyzed samples. No correlations between altitude and age have been found along altitudinal profile suggesting a rapid exhumation during this period. Thermal modeling using our (U-Th)/He apatite ages and geochronological data previously obtained in the same area (40Ar/39Ar and K/Ar data on biotite, zircon and apatite fission track) allow us to propose a cooling history. The rocks suffered a rapid cooling at 60–100 °C/Ma between 22.5 and 19 Ma, then cooled to temperatures around 40 °C between 19 and 18 Ma. They were re-heated at around 110 °C between 18 and 15 Ma then rapidly cooled and exhumed to reach the surface temperature at around 13 Ma. The re-heating could be related to a renewal in thrusting and burying of the inner zones. Between 15 and 13 Ma the cooling resumed at a rate of 50 °C/Ma indicating an exhumation rate of 0.8 mm/y considering an average 40 °C/km geothermal gradient. This exhumation may be linked to the extension in the Alboran Sea. Otherwise biostratigraphic and sedimentological analysis of Pliocene basins of the internal Rif provided informations on the more recent events and vertical movements. Pliocene deposits of the Rifian coast represent the passive infilling of palaeo-rias between 5.33 and 3.8 Ma. The whole coastal area was uplifted at slow average rates (0.01–0.03 mm/y) in relation with a northeastward tilting of 0.2–0.3° since the Lower-Pliocene. A late Pliocene to present extensional tectonics associated to uplift has been identified all along the coastal ranges of the Internal Zone of the Rif chain. This extension was coeval with the major late Pliocene to Pleistocene extensional episode of the Alboran Sea and appears to be still active nowadays. No significant late Messinian uplift was evidenced, thus calling into question the geodynamic models relating the closure of the marine gateways and the MSC to slab roll back.  相似文献   

14.
Space-based tectonic studies on the western part of the North Anatolian Fault Zone (NAFZ) have been conducted over two decades. After the August 17, 1999, Izmit earthquake (Mw = 7.4), this region attracted greater scientific interest, and the collected data became more valuable. The Geodesy Department of the Kandilli Observatory and Earthquake Research Institute (KOERI) at Bogazici University established three micro-geodetic networks to the east of Akyazi, east of Iznik, and west of Lake Sapanca in the eastern part of the Marmara region; GPS data have been continually collected at these locations since 1994. The NAFZ branches out in the western part of the Marmara region and extends up to the Aegean Sea. Segments of the fault passing through the Marmara Sea are considered active, and this has increased concern regarding imminent earthquakes. Conventional geodetic measurements made between 1990 and 1994 are not sufficient for monitoring small movements. However, GPS has played a very important role in detecting such deformations in the area after 1994. The Iznik network, with 10 points, is bilaterally located on the Iznik-Mekece fault. Six years of GPS data for 2004–2010 collected for the monitoring of crustal deformation showed that the Iznik-Mekece fault segment moves westward at about 22 ± 1 mm/yr with respect to the Eurasia fixed reference frame. The GPS observations show that there is no strain accumulation in the area.  相似文献   

15.
We analyzed receiver-function data recorded by a temporary broadband array deployed as part of the BOLIVAR project and the permanent seismic network of Venezuela to study the mantle transition zone structure beneath the Caribbean-South American plate boundary and Venezuela. Significant topography on both the 410-km and the 660-km discontinuities was clearly imaged in the CCP (common-conversion-point) stacked images. Beneath the southeastern Caribbean, the 410-km is featured by a narrow (～ 200 km EW) ～ 25-km uplift extending in the NS direction around 63° west, while the 660-km is depressed by ～ 20 km in a narrow region slightly west to the uplift, a scenario that is more consistent with westward descent of the oceanic South American plate rather than a break-off of NNW dipping proto-Caribbean oceanic lithosphere along the El Pilar Fault. We also found a thick transition zone beneath the Falcon region in northwestern Venezuela, possibly associated with the subducted Nazca plate. A flat 410-km was observed beneath the Guayana shield, suggesting that the shield has a stable and moderately deep keel, which has little effect on the underlying transition zone structure.  相似文献   

16.
The Al Hoceima Mw 6.4 earthquake of 24 February 2004 that occurred in the eastern Rif region of Morocco already hit by a large event in May 1994 (Mw 5.9) has been followed by numerous aftershocks in the months following the event. The aftershock sequence has been monitored by a temporary network of 17 autonomous seismic stations during 15 days (28 March–10 April) in addition to 5 permanent stations of the Moroccan seismic network (CNRST, SPG, Rabat). This network allowed locating accurately about 650 aftershocks that are aligned in two directions, about N10-20E and N110-120E, in rough agreement with the two nodal planes of the focal mechanism (Harvard). The aftershock alignments are long enough, about 20 km or more, to correspond both to the main rupture plane. To further constrain the source of the earthquake main shock and aftershocks (mb > 3.5) have been relocated thanks to regional seismic data from Morocco and Spain. While the main shock is located at the intersection of the aftershock clouds, most of the aftershocks are aligned along the N10-20E direction. This direction together with normal sinistral slip implied by the focal mechanism is similar with the direction and mechanisms of active faults in the region, particularly the N10E Trougout oblique normal fault. Indeed, the Al Hoceima region is dominated by an approximate ENE-SSW direction of extension, with oblique normal faults. Three major 10–30 km-long faults, oriented NNE-SSW to NW-SE are particularly clear in the morphology, the Ajdir and Trougout faults, west and east of the Al Hoceima basin, respectively, and the NS Rouadi fault 20 km to the west. These faults show clear evidence of recent vertical displacements during the late Quaternary such as uplifted alluvial terraces along Oued Rihs, offset fan surfaces by the Rouadi fault and also uplifted and tilted abandoned marine terraces on both sides of the Al Hoceima bay.However, the N20E direction is in contrast with seismic sources identified from geodetic inversions, which favour but not exclusively the N110-120E rupture directions, suggesting that the 1994 and 2004 events occurred on conjugate faults. In any event, the recent seismicity is thus concentrated on sinistral N10-20E or N110-120E dextral strike-slip faults, which surface expressions remain hidden below the 3–5 km-thick Rif nappes, as shown by the tomographic images build from the aftershock sequence and the concentration of the seismicity below 3 km. These observations may suggest that strain decoupling between the thrusted cover and the underlying bedrock and highlights the difficulty to determine the source properties of moderate events with blind faults even in the case of good quality recorded data.  相似文献   

17.
Recent improvements in the seismological networks on the Ibero-Maghrebian region have permitted estimation of hypocentral location and focal mechanisms for earthquakes which occurred at South Spain, Alboran Sea and northern Morocco of deep and intermediate depth, with magnitudes between 3.5 and 4.5. Intermediate depth shocks, range from 60 to 100 km, with greater concentration located between Granada and Málaga. Fault-plane solutions of 5 intermediate shocks have been determined; they present a vertical plane in NE-SW or E-W direction. Seismic moments of about 1015 Nm and dimensions of about 1 km have been determined from digital records of Spanish stations.P-wave forms are complex. This may be explained by the crustal structure near the station, discontinuities in the upper mantle and inhomogeneities near the source. Deep activity at about 650 km has only 3 shocks since 1954 (1954, 1973, 1990). Shocks are located at a very small region. Fault-plane solutions show a consistent direction of the pressure axis dipping 45° in E direction. For the 1990 shock seismic moment is 1016 Nm and dimensions 2.6 km. TheP-waves are of simpler form with a single pulse. The intermediate and deep activities are not connected and no activity has been detected between 100 and 650 km. The intermediate shocks may be explained in terms of a recent subduction from Africa under Iberia in SE direction. The very deep activity must be related to a sunk detached block of lithospheric material still sufficiently cold and rigid to generate earthquakes.  相似文献   

18.
We analyzed records of eight seismic stations of the autonomous broadband seismograph network of a joint project between Utrecht University (the Netherlands), California Institute of Technology, and Centro de Investigación Científica y de Estudios Superiores de Ensenada (CICESE). These stations recorded the Mw 5.6 earthquake that occurred on 12 November 2003 at Salsipuedes basin in the middle of the Gulf of California 2 km west of the island Angel de la Guarda. This event was located at 29.16º N and 113.37º W, 30 km northeast of Bahia de los Angeles. A foreshock and hundreds of aftershocks were recorded in the 48 hours after its origin time. With the location of 29 earthquakes we identified the active segment, perpendicular to the main transform fault NW–SE of Canal de Ballenas, representing the transtensional boundary between the Pacific and North American plates. The direction of the active fault described is consistent with the normal fault mechanism reported by the National Earthquake Information Center (strike=39º, dip=34º, slip=–44º).From the duration magnitude of 456 aftershocks, we calculated a b-value of 1.14±0.28; furthermore, we calculated a seismic moment of (3.5 ±3.3) X1017Nm, a source radius of 3.7 ± 2.63 km, and a static stress drop of 3.94 ± 1.15 MPa (39.4 ± 11.5 bar.).  相似文献   

19.
The aim of this paper concerns Miocene igneous activity in the Alboran Sea and Peri-Alboran area (northern Morocco, western Algeria and Betic Cordilleras in Spain), considering its age and its location with regard to major tectonics structures.We have compiled previous K-Ar isotopic ages of lavas and plutonic boulders and intrusives with an error of ±1σ and completed this set by a new K-Ar isotopic age for andesitic tuffites from Alboran Island. Geochemistry of most of these samples has been considered after previous analyses completed with new data for Spain magmatism. These two sets of data allow us to place the magmatic activity within the regional stratigraphy and tectonics and their chronological framework of the three major tectonic phases of the Maghrebian orogen, at 17 Ma (Burdigalian), 15 Ma (Langhian) and 9 Ma (Tortonian). Petro-geochemical characteristics are compared through time and geographical locations. A major goal of this coupled approach is to help the elaboration of possible geodynamical processes.As an application, we present the case study of the Dellys, Djinet and Thenia region (east of Algiers) where the successive magmatic events between 19.4 ± 1 and 11.6 ± 0.5 Ma are closely related to the local tectonics and sedimentation.The Peri-Alboran igneous activity is placed in a multidisciplinary framework. Timing of activity is defined according to the ages of the neighbouring sedimentary units and the K-Ar ages of igneous rocks.In Spain, the Cabo de Gata-Carboneras magmatic province displays late Oligocene and early Miocene leucogranitic dikes, dated from 24.8 ± 1.3 to 18.1 ± 1.2 Ma; three following andesitic to rhyolitic events took place around 15.1 ± 0.8 to 14.0 ± 0.7 Ma, 11.8 ± 0.6 to 9.4 ± 0.4 Ma, 8.8 ± 0.4 to 7.9 ± 0.4 Ma; this last event displays also granitic rocks. Lamproitic magmas dated between 8.4 ± 0.4 and 6.76 ± 0.04 Ma were emplaced after the Tortonian phase.In Morocco, after the complex building of the Ras Tarf volcanic edifice, major calc-alkaline to shoshonitic volcanoes were built between 9.0 ± 0.5 and 4.8 ± 0.5 Ma, in particular the large Gourougou volcanic complex. Near Oujda, volcanic activity of alkaline affinity leads to multiple emissions of basalts throughout Pliocene times until the beginning of Pleistocene, between 6.2 ± 0.3 and 1.5 ± 0.1 Ma.In the Alboran domain, an age of 19.7 ± 0.8 Ma is reported (this study) for the andesitic tuffites that form the emergent part of the Alboran Island. This age is comparable to that of the Algerian tuffites and cherts “silexites” and the Malaga ones in Spain. Younger activity, completely separated from the previous one, forms the low-K basaltic andesitic dikes from Alboran Island, dated between 9.1 ± 0.5 and 7.5 ± 0.3 Ma. Along the Alboran Ridge both low-K and high-K andesites to dacites were emitted in the estimated range of 10.7–8.7 Ma. Low-K and high-K andesites to dacites sampled at ODP sites 977 and 978 into the East Alboran Basin, are dated between 12.1 ± 0.2 and 9.3 ± 0.1 Ma.We propose to relate with the Trans-Alboran lineament only the post-Tortonian igneous activity younger than 9 Ma.  相似文献   

20.
Increasing evidence from fission track studies in Sweden indicate that large parts of the Fennoscandian Shield have been affected by a large-scale thermotectonic event in the Palaeozoic. In this study the results of 17 apatite fission track analyses from central Sweden are presented collected along three NW–SE transects trending from the Bothnian Sea to the Caledonides. On the Bothnian coast samples have been collected directly from the Sub-Cambrian Peneplain. The sedimentary cover protecting this surface until recently is responsible for the thermal increase detected through apatite fission track (FT) thermochronology.The apatite FT ages range between 516 ± 46 Ma (±1σ) on the Bothnian coast around sea level to 191 ± 11 Ma in the Caledonides (～500–1500 m.a.s.l.). The mean track lengths vary from 11.3 ± 2.2 μm (±1σ) in the east to 14.2 ± 2.8 μm in the west, indicating a longer stay in the PAZ in the east, versus a continuous cooling pattern in the west. This pattern in combination with other geological constraints indicates that the crystalline basement rocks near the Caledonian deformation front in the west experienced higher temperatures after the formation of the Sub-Cambrian Peneplain followed by denudation, compared with the basement rocks in the east near the Bothnian coast.The apatite FT data near the Caledonian deformation front indicates prevailing temperatures of more than 110 ± 10 °C prior to the Mid Palaeozoic, causing a resetting of the apatite fission track clock. The temperatures were progressively lower away from the deformation front. Apatite fission track analysis of samples collected from the Sub-Cambrian Peneplain along the Bothnian coast indicate maximum temperatures of 90 ± 15 °C during Late Silurian–Early Devonian time. This heating event is argued to be the result of burial beneath a developing foreland basin in front of the Caledonian orogeny. Assuming a geothermal gradient of 20 °C/km, this temperature increase can be converted to a total burial of the samples. The resulting geometry of this basin can be described as an asymmetrical basin at least 3.5 km deep in the vicinity of the Caledonian deformation front decreasing to about 2.5 km on the Bothnian coast, continuing further onto Finland. The width of this basin was in thus in the order of 600 km. Whether this was formed completely synorogenic or partly synorogenic, broadening after cessation of the orogeny, could not be revealed.The Late Palaeozoic and Mesozoic thermal evolution of this area is related to the extensional tectonics in the North Atlantic Domain.  相似文献   

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