The Eurasian‐African NW‐SE oblique plate convergence produces shortening and orthogonal extension in the Alboran Sea Basin (westernmost Mediterranean), located between the Betic and Rif Cordilleras. ...A NNE‐SSW broadband of deformation and seismicity affects the Alboran central part. After the 1993–1994 and 2004 seismic series, an earthquake sequence struck mainly its southern sector in 2016–2017 (main event Mw = 6.3, 25 January 2016). The near‐surface deformation is investigated using seismic profiles, multibeam bathymetry, gravity and seismicity data. Epicenters can be grouped into two main alignments. The northern WSW‐ENE alignment has reverse earthquake focal mechanisms, and in its epicentral region recent mass transport deposits occur. The southern alignment consists of a NNE‐SSW vertical sinistral deformation zone, with early epicenters of higher‐magnitude earthquakes located along a narrow band 5 to 10‐km offset westward of the Al Idrisi Fault. Here near‐surface deformation includes active NW‐SE vertical and normal faults, unmapped until now. Later, epicenters spread eastward, reaching the Al Idrisi Fault, characterized by discontinuous active NNE‐SSW vertical fractures. Seismicity and tectonic structures suggest a westward propagation of deformation and the growth at depth of incipient faults, comprising a NNE‐SSW sinistral fault zone in depth that is connected upward with NW‐SE vertical and normal faults. This recent fault zone is segmented and responsible for the seismicity in 1993–1994 in the coastal area, in 2004 onshore, and in 2016–2017 offshore. Insights for seismic hazard assessment point to the growth of recent faults that could produce potentially higher magnitude earthquakes than the already formed faults.
Key Points
The 2016–2017 seismic sequence is related to the wide NNE‐SSW sinistral fault zone located in the central part of the Alboran Sea
Epicentral sea bottom deformations include mass transport deposits and recent faults
Seismicity and sea bottom deformations are located west of the main Al Idrisi Fault, supporting the westward widening of the fault zone
Deception Island shows the most recent exposed active volcanism in the northern boundary of the Bransfield Trough. The succession of the volcanic sequence in the island is broadly divided into pre- ...and post-caldera collapse units although a well-constrained chronological identification of the well-defined successive volcanic episodes is still needed. A new paleomagnetic investigation was carried out on 157 samples grouped in 20 sites from the volcanic deposits of Deception Island (South Shetlands archipelago, Antarctic Peninsula region) distributed in: (1) volcanic breccia (3 sites) and lavas (2 sites) prior to the caldera collapse; (2) lavas emplaced after the caldera collapse (10 sites); and (3) dikes cutting pre- and the lowermost post-caldera collapse units (5 sites). The information revealed by paleomagnetism provides new data about the evolution of the multi-episodic volcanic edifice of this Quaternary volcano, suggesting that the present-day position of the volcanic materials is close to their original emplacement position. The new data have been combined with previous paleomagnetic results in order to tentatively propose an age when comparing the paleomagnetic data with a global geomagnetic model. Despite the uncertainties in the use of averaged paleomagnetic data per volcanic units, the new data in combination with tephra occurrences noted elsewhere in the region suggest that the pre-caldera units (F1 and F2) erupted before 12,000 year BC, the caldera collapse took place at about 8300 year BC, and post-caldera units S1 and S2 are younger than 2000 year BC.
The coexistence of shortening and extensional tectonic regimes is a common feature in orogenic belts. The westernmost end of the Western Mediterranean is an area undergoing shortening related to the ...5 mm/yr NNW‒SSE convergence of the Nubia and Eurasia Plates. In this region, the Central Betic Cordillera shows a regional ENE‒WSW extension. Here, we present GNSS‐derived geodetic data along a 170 km‐long transect orthogonal to the main active normal faults of the Central Betic Cordillera. Our data indicate that the total extension rate along the Central Betic Cordillera is 2.0 ± 0.3 mm/yr. Extension is accommodated in the eastern (0.8 ± 0.3 mm/yr in the Guadix‐Baza Basin) and western (1.3 ± 0.3 mm/yr in the Granada Basin) parts of the Central Betic Cordillera, while no extension is recorded in the central part of the study area. Moreover, our data permit us to quantify, for the first time, short‐term fault slip rates of the Granada Fault System, which is one of the main seismogenic sources of the Iberian Peninsula. We deduce a fault slip rate of ∼1.3 ± 0.3 mm/yr for the whole Granada Basin, with 0.9 ± 0.3 mm/yr being accommodated in the Granada Fault System and 0.4 ± 0.3 mm/yr being accommodated in the southwestern sector of the Granada Basin, where no active faults have been previously described at the surface. The heterogeneous extension in the Central Betic Cordillera could be accommodated by shallow high‐angle normal faults that merge with a detachment at depth. Part of the active extension could be derived from gravitational instability because of underlying over‐thickened crust.
Plain Language Summary
We present here high‐precision GPS data obtained in S Spain. In this area tectonic deformation of the Earth crust is related to the convergence between Nubia (Africa) and Eurasia Plates. Under this general convergence setting, our study area is undergoing extensional deformation. GPS data permit us to quantify and characterize this extension. The total extension in our study area is 2.0 ± 0.3 mm/yr. But this extension is heterogeneous, as it concentrates in two areas to the east and to the west, separated by a zone with no extension. Moreover, our data permit to quantify, for the first time, short‐term fault slip rates of the Granada Fault System (0.9 ± 0.3 mm/yr). This parameter is essential to characterize the seismic hazard of this structure, which is one of the main seismogenic sources of the Iberian Peninsula. We also discuss the subsurface geometry of the faults accommodating this heterogeneous extension and the crustal mechanism responsible for that.
Key Points
We quantify active extension within a collisional orogen (Betic Cordillera, SE Spain)
Extension in the Central Betic Cordillera is heterogeneous
We quantify, for the first time, short‐term slip rate of the Granada Fault System, one of the main seismogenic sources of Spain
Since the Miocene, the thinned continental crust below the Alboran Sea and its overlying sedimentary cover have been undergoing deformation caused by both convergence of Eurasia and Africa and by ...deep processes related to the Tethyan slab retreat. Part of this deformation is recorded at the Xauen and Tofiño banks in the southern Alboran Sea. Using swath bathymetry and multichannel seismic reflection data, we identified different stages and styles of deformation. The South Alboran Basin is made up of Early Miocene to Pliocene sedimentary layers that correlate with the West Alboran Basin depocenter and are dominated by E‐W trending folds and thrusts. The Xauen and Tofiño Banks first recorded the phase of extension and strike‐slip movement during the slab retreat, followed by the phase of compressional inversion since the Tortonian and are now structured by tight folds, thrusts, and mud bodies. This study proposes that the Banks were located on the southern‐inherited Subduction Tear Edge Propagator (STEP) fault related to the westward migration of the Alboran domain during the Miocene. The STEP fault zone, acting as a boundary between the African block and the Alboran block, was located along the onshore Jebha‐Nekor fault and the offshore Alboran Ridge and the Yusuf fault zone. Thick‐skinned and thin‐skinned shortening occurred when slab retreat stopped, and inversion began. The present‐day style of the deformation seems to be linked to a decollement level made of undercompacted shale on top of the Ghomaride complex.
Key Points
The reactivation of the Subduction Tear Edge Propagator fault zone on the southern edge of the Alboran Domain forms the South Alboran Ridge
The style of the present‐day structure of the South Alboran region implies an interaction between thick‐skinned and thin‐skinned tectonics
Undercompacted shales and inherited crustal structures are involved in the structural evolution of the South Alboran Basin
In this study we contrast the results of different geophysical methods in order to describe the karst system surrounding of the Gruta de las Maravillas cave (Aracena, Spain). A comprehensive study of ...the geophysical responses of the known cavity was carried out, after which several sections were studied to detect the karst architecture and cave continuity. To ensure precision, the inner 3D-topography of the cave was determined by classical geodetic techniques and a digital terrain model was performed with differential GPS. The microgravity method was used to obtain the residual gravity map of the entire study zone, whose minima could be related to caves. Then, the negative gravity anomalies were analyzed to plan several lines for implementing further geophysical methods: magnetic profiles (MP), electrical resistivity tomography (ERT), induced polarization (IP), 2D seismic prospection (refraction tomography and reflection sections) and ground penetrating radar (GPR). The resulting models for each line explored were integrated with detailed geological maps to establish the unknown continuity of the caves. Finally, we discuss the suitability of each geophysical technique for cave detection in marble with sulfur host rock and propose the best procedures to constrain their geometries.
•Combined geophysical methods are integrated in cave prospecting.•Microgravity survey with differential GPS is the best reconnaissance method.•ERT, seismic, GPR and magnetic researches provide precise results.•The response in a known cave is extrapolated to research unknown areas.•Aracena cave, located in marbles, is formed by sulfur related acid waters.
The suitability of the combined microgravity and electrical tomography to detect and characterize caves deeply buried in limestones is tested. We have selected the buried Algaidilla cave, in the ...Estepa range (western Betic Cordillera), which is partially submerged below the water table and which was intersected between 26 and 66m in depth by two boreholes. At that location, microgravity, electrical resistivity tomography (ERT) and induced polarization (IP) data was collected along four profiles. Algaidilla cave is associated with a −0.5mGal residual gravity anomaly minimum. Microgravity models reveal an approximately horizontal N–S elongated shape with a maximum length of 150m and a width of 40m. Resistivity variations provide information on the cave sectors remaining above, as well as below the water table. Forward modeling and depth of investigation (DOI) support the suitability of ERT to constrain the cave geometry. The cave is identified as having an intermediate to low-resistivity feature, which approximately matches the 250Ω·m contour line along its ceiling. In addition, induced polarization models show high chargeability anomalies probably associated with decalcification clays. The location and approximately geometry of Algaidilla cave estimated from geophysical modeling suggests that nearby overburden may develop cave-ins and collapse sinkholes. Microgravity is proved as a powerful tool to detect caves at this depth, but this method alone fails to estimate the geometry. ERT results delineate the cavity both above and below the water table. Although the deepest sectors of the models should be interpreted with caution since they are less constrained by the data, below the water table the cavity shows great resistivity contrast with regard to the background carbonate. In addition, this study points out the usefulness of the IP method for detecting decalcification clays, often present at the base of karstic caves.
•We use the combination of nondestructive geophysics methods to detect deeply caves.•We use microgravity, electrical resistivity tomography and induced polarization.•Forward modeling supports the suitability of the response of ERT and IP profiles.•The DOI index discriminates the unreliable parts of the electrical profiles.•The high resolution of the measurement allows obtaining a trusted model of the cave.
Eight independent magnetotelluric (MT) campaigns, carried out in the western Mediterranean area and Antarctica, have been analyzed with the aim of extracting information from Schumann resonances ...(SR). The advantages and drawbacks of MT data with respect to single‐purpose fixed stations for SR research are evaluated. Two different methods of signal processing have been employed: spectral analysis through the fast Fourier transform (FFT) and Rescaled Range analysis (R/S), calculating the Hurst exponents. The first permitted a study of the source contributions, the effects caused by local changes in the observation area, interseasonal behavior, and the relations between electric and magnetic horizontal fields. The average central frequencies obtained for the first three resonances are 7.8, 14, and 20.5 Hz respectively, but there are fluctuations in them. These variations seem to respond to the characteristics of the principal active storms that generate the resonances. These frequency shifts are stronger for the second and third resonances. Owing to the broadband registering of MT, the second method of signal processing could be applied to the low noise signals from Antarctica with high resolution, revealing the persistent nature of SR. A numerical simulation indicates a way for inferring lightning rates from R/S analysis.
Elephant Island, located in the vicinity of the present-day active boundary of the South Shetland Block and the Antarctic and Scotia plates, is a region of particular interest for understanding the ...past and present geodynamic evolution of the southern Scotia Arc. Lineament from different data sources, field-measured fractures and geomorphological evidences have been analysed in this context. The lineaments extracted from aerial photographs (1,624), from a DEM (348) and from RADARSAT-2 satellite data (1,365) indicate four dominant lineament sets with NE–SW, NW–SE, N–S and W–E strikes. All data sources identified similar lineament families, but differences in the frequency distributions and subsequently on the dominant orientations were observed. The measurements direct of fractures were obtained from 23 sites in the field at which 278 planes were measured. Fracture planes indicate main modes trending in the NNE–SSW and NNW–SSE directions and a secondary mode in the E–W. The major trends of the fracture measurements and the lineaments display a good correlation in the E–W direction. However, there is an angular variation in the azimuth values of the NNE–SSE and NNW–SSE fractures with respect to the N–S, NE–SW and NW–SE orientations of the lineaments of approximately 20°. This trend deviation may be due to the fact that mapped lineaments are composed of small fracture sets that may be related to shear fractures that cannot be distinguished at the aerial photograph or radar satellite data scales. Submerged sea-floor morphological feature orientations match the studied morphostructures on the island and the main tectonic structures in this part of the Scotia Arc. A linkage of the main lineament families to the tectonic stages from the Oligocene to the present has been proposed, taking into account the information of the orientation and sense of movement of the fractures and stresses in the Elephant Island region.
The detection of caves developed in gypsum and marls through geophysical methods is tested in the gypsum karst of Sorbas (SE Spain). We applied microgravity and electrical resistivity tomography ...(ERT) in the Covadura cave system, which features a large variety of shapes developed in a multilayer structure. The response of caves in different conditions of host rocks and cave dimensions is analysed by means of synthetic models. Gravity studies require very accurate conditions in data acquisition and processing to highlight the associated minima, given the low density contrast between caves and gypsum or marl host rock. Different microgravity station spacing was tested to find the best configuration for the detection of such caves. The suitability of ERT under different conditions of soil humidity is discussed in conjunction with the low resistivity contrast between gypsum bedrock and cave conduits. After measuring 2D ERT parallel profiles coincident in space, yet in wet versus dry soil conditions, the results were compared and it was determined that the ERT profiles measured under wet soil conditions are better than in dry ones for cave detection. A 3D model was drawn up for the two different humidity conditions to obtain the fitted morphology of the cave. The best results for cave detection are obtained when combining gravity and ERT in wet conditions, as they provide for a higher contrast in physical properties and an optimal setting for instrumental measurements.
•ERT and microgravity prospection are applied to a cave in gypsum and marls.•A combination of geophysical methods is recommended to distinguish caves in gypsum.•Resistivity measurements under different humidity soil conditions are compared.•Different microgravity station spacings are tested to optimize the gravity survey.
•Acequias de careo constitute the oldest managed aquifer recharge system of Europe.•In Middle Ages, Muslims derived meltwater and recharged it through uncoated channels.•The managed aquifer recharge ...technique helped ensure water availability downstream.•This case study illustrates a paradigm that should be used in future water crises.•This MAR technique may prove key for adaptation to climate change in Alpine regions.
In Sierra Nevada (southern Spain), the highest mountain range in southern Europe, the application of an ancestral Integrated Water Resources Management system (IWRM), based on the conjunctive use of groundwater and surface water, provides water resources for irrigation and supply in the driest months of the year in this semiarid mountain region. Meltwater is derived from the headwaters of the mountain streams and rivers through a set of uncoated channels excavated in the ground (locally known as acequias de careo) to infiltrate at the upper part of the valleys. Water infiltrated along the acequias de careo slowly flows down the hillsides, through the weathered zone of the hard rock aquifer and the glacial and periglacial sediments. The recharge accomplished through this Managed Aquifer Recharge technique (MAR) activates numerous springs located halfway down the hillside and increases the base flow of the rivers. In this study, focused on a careo channel located on the southern slope of Sierra Nevada called Acequia de El Espino, different archaeological, sedimentological, geophysical and hydrogeological techniques are applied to determine the age and the efficiency of this ancestral example of a MAR and IWRM system. Results suggest that the acequias de careo may be the oldest MAR system in Europe, and that this MAR technique could be applied in other high mountain alpine watersheds to mitigate the effects of climate change.
PDF
