Pablo G. Silva

Fault-generated mountain fronts in southeast Spain: geomorphologic assessment of tectonic and seismic activity

Abstract Tectonic activity of SE Spain is evaluated through the application of a general morphometric analysis over 17 different mountain fronts. The selected fronts are distributed along the two most prominent crustal-scale structures of the Mediterranean sector of Spain: The Eastern Betic Shear Zone (EBSZ) and the Valencia Trough. Geomorphic indexes used in this study are Mountain-front sinuosity index (Smf) and Valley floor/width ratio (Vf). This work is largely based on the Smf/Vf regression analysis of the studied fronts together with previous available data from SW USA. The development of mountain fronts in both different lithologies and tectonic contexts enabled characteristic morphometric signatures for different activity scenarios (tectonic activity classes) to be obtained. This study thus indicates that variations of mountain front geomorphology triggered by different styles of faulting can be detected by morphometric analysis. It is demonstrated that discrete uplift rates of 0.15–0.08 m/ka are sufficient to keep relatively low Smf values down to 1.4 in Active fronts (Class 1). Taking into account the reported uplift rates and preliminary palaeoseismic data for some of the Class 1 studied fronts, recurrence periods for surface rupture events can be theoretically bracketed for SE Spain. Recurrence periods for Class 1 fronts could theoretically range between 10,000 and 12,500 years for maximum slip conditions, and 1750 and 2600 years for minimum slip conditions. Inactive fronts (Class 3) develop under uplift rates down to 0.03 m/ka, and tested recurrence intervals for rupture events range between 25,000 and 50,000 years. Class 2 fronts of intermediate characteristics and activity display uplift rates between 0.07 and 0.03 m/ka.

The impact of Quaternary sea-level and climatic change on coastal alluvial fans in the Cabo de Gata ranges, southeast Spain

Conventionally, a fall in base level is seen as stimulating incision into the distal zones of alluvial fans. In the Cabo de Gata ranges of southeast Spain evidence exists to the contrary. Two sets of Quaternary coastal alluvial fans demonstrate the interaction between climatically-driven variations in the supply of sediment and eustatically-driven changes in base level. The fans are supplied from Miocene volcanic terrain within which no evidence can be found for major tectonic deformation during the period of fan development. The evolution of the east-coast fans has been affected by variations in sediment supply and changes in sea level. The west-coast fans were buffered from the effects of changes in sea level by coastal barriers. Three phases of past sedimentation can be identified on the fans. These can be differentiated on the basis of field observations of soil profiles (particularly colour of the B horizons and accumulation of CaCO3), and laboratory analyses of sequential iron oxide extractions and magnetic mineral properties. The two earlier (major) sedimentation phases were coincident with global glacials (>ca. 135 ka and ca. 85–10 ka, based on the stratigraphy and uranium/thorium dating of the coastal sediments). High sea levels during the intervening interglacial and during the Holocene caused erosion of the distal zones of the east-coast fans which led to channel incision into the fan surfaces. On the west-coast fans no such incision occurred, simply proximal incision by small fanhead trenches. The youngest (relatively minor) phase of fan sedimentation has occurred during the Holocene. These contrasting contexts have produced differing styles of fans, with telescopic fan morphology on the east-coast and stacked morphology on the west-coast fans. The differences are reflected in the fan profiles, with steeper gradients dominating the east-coast fans, and extensive lower gradient distal surfaces on the west-coast fans. Fan morphometry, based on analysis of the residuals from drainage area to fan area and gradient regressions, also differentiates between the fan contexts. The fan building phases appear to be controlled proximally by climatically-driven pulses of sediment supplied to the fans. These occurred during global glacials coincident with low sea levels, and caused fan progradation onto the exposed foreshore. The intervening global interglacials were times of little fan sedimentation, and on the east coast, where high sea levels were able to erode the fan toes, deep through-fan dissection ensued.

Landscape response to strike-slip faulting linked to collisional settings: quaternary tectonics and basin formation in the Eastern Betics, southeastern Spain

Abstract Neotectonic behaviour of the Eastern Betics (southeastern Spain) has been controlled by the presence of a large, left-lateral shear zone. This is an intraplate transcurrent zone, probably decoupled by a mid-crustal detachment horizon. The morphological expression of this crustal structure is a sigmoidal corridor in which the main Quaternary basins are located. Three types of morphostructural domains can be distinguished in the sigmoidal corridor: the central segment, and the northern and southern terminal splays. The central segment is characterized by wrench tectonics, whilst the southern and northern terminal splays are controlled by transtensional and transpressional tectonics, respectively. The Quaternary morphostructural pattern is only the more recent picture of the tectonic processes that have controlled the late orogenic evolution of the Eastern Betics: the indentation of the Aguilas arc, accompanied by thin-skinned thrusting and tectonic escape in its easternmost sector; and extensional collapse, accompanied by thin-skinned stretching, in its westernmost sector.

Fan-surface dynamics and biogenic calcrete development: Interactions during ultimate phases of fan evolution in the semiarid SE Spain (Murcia)

Abstract Pleistocene alluvial fan surfaces of the Campo de Cartagena–Mar Menor Basin (Murcia, SE Spain) are capped by thick mature calcretes. Calcrete profiles consist mainly of six different horizons: prismatic, chalky, nodular, massive, laminar and coated-gravels. Petrographic study of the calcretes has shown the occurrence of features such as alveolar septal structures, calcified filaments, coated grains, spherulites, calcified root cells and calcispheres that indicate the biogenic origin of the calcretes, mainly induced by plant root related microbial activity. The calcretes studied were formed initially in the soil and represented the K horizon. Development of the calcrete profiles took place in six main stages and was driven by multiple phases of soil formation, erosion and reworking. The relationships between these processes caused the formation of different calcrete profiles in proximal and distal fan areas. In the distal areas, which are controlled by limited distal fan aggradation, episodic sediment input, buried previously developed calcretes and generated new space for calcrete growth by plants growing in the overlying unconsolidated materials. This allowed the renewal of calcrete formation and it led to the development of complex composite profiles which are thicker than in proximal areas, where surface stabilisation and/or dissection enabled calcrete reworking and brecciation. These processes of erosion, sedimentation, reworking and renewed calcrete formation initiated by vegetation were repeated through time. They explain the complex macro- and microstructures of these calcretes and indicate that calcrete development, even reaching mature stages, can start before the fan surface is completely abandoned, but it requires episodic sedimentation. Eventually, distal fan aggradation and continuous calcrete development throughout the entire fan surface, led to the ultimate fan surface induration, controlling subsequent landscape evolution. So, fan surface calcretes cannot be envisaged as simple top-surface carbonate accumulations, but as complex feedback systems in which pedogenic, biogenic and sedimentary processes interact in response to the evolving fan-surface dynamics during the terminal phases of fan development in semiarid environments.

Quaternary laminar calcretes with bee nests: evidences of small-scale climatic fluctuations, Eastern Canary Islands, Spain

Abstract Thick laminar calcrete profiles are common on the surface of the eastern Canary Islands of Lanzarote and Fuerteventura. This paper reports the study of one profile from Lanzarote (Macher profile) and one from Fuerteventura (Tefia profile). These profiles are about 2 m thick. The Macher profile is developed on basaltic host rocks and consists of two main horizons: a lower horizon in which white veins of laminated micrite penetrate cracks, and an upper laminar horizon. The Tefia profile is cumulative and consists, from base to top, of massive, laminar, and massive and laminar-brecciated horizons. Highlighted in the study of these profiles are the presence of ooids, the complex structure of the laminar horizons, and the occurrence of fossil bee nests ( Celliforma ). Ooids consist of a nucleus of clay, micrite and etched grains coated with envelopes of micrite and clay (mainly palygorskite). Organic films favoured both the precipitation of carbonate and the adhesion of clays when the ooids formed in the soil. The laminar horizons consist of a centimetre-scale alternation of massive micrite with varied amounts of ooids and detrital grains with laminated micrite. This alternation indicates the small-scale periods of sedimentation, erosion and soil formation in the upper part of a relatively stable surface. These small-scale alternations may reflect climatic vegetation changes in which arid periods are represented by micrite with ooids, while laminated micrite reflects a better-developed vegetation of the wet periods. Celliforma occur as ovoid larval cells with more or less rounded bases and a flat top. The cells are about 3 cm long and 1.5 cm in diameter. Their wall is about 5 mm thick, and is commonly laminated. Celliforma is interpreted as fossil nests of solitary bees. Their presence in the laminar horizon is evidence for the existence of a vegetation cover containing angiosperms.

Paleoseismic indications along ‘aseismic’ fault segments in the guadalentín depression (SE Spain)

Abstract The Guadalentin Depression, located in SE Spain (Murcia Region), is bounded by two of the main NE-SW master faults of the Eastern Betics Cordilleras: The Lorca-Alhama and the Palomares left-lateral strike-slip faults. Available earthquake data indicate that, in the last 600 years, some sectors of the Lorca-Alhama Fault and the entire sector of the Palomares Fault have not been associated with significant historical seismicity. However, they show a wide range of diagnostic features of earthquake surface displacements on late Pleistocene and Holocene alluvial and colluvial surfaces. Aside from the left-lateral offsets recorded along 045–050 ° master fault strands of the Lorca-Alhama Fault, major paleoseismic surface displacements show different kinematics in relation to the broad orientation of the fault strands: (1) vertical normal displacements along 010–020 ° trending faults mainly preserved as degraded fault scarps of 2.5-1.8 m high (Aljibejo site); and (2) vertical reverse displacements, with average offsets of 0.2 – 1 m, along 065–080 ° subsidiary faults. In this last group, the younger one (Carraclaca Baths site) remains as a fault scarp of 0.8 m height affecting a cascade tufa which was active until the Spanish Roman Period (2nd Century B.C. to 6th Century A.D.). In other cases, reverse offsets resulted in smaller displacements (0.26 m) of paleosols, but show a recurrent behaviour (La Escarihuela site). The strongest earthquakes recorded in the study area did not exceed more than Mb 4.5 or MSK Intensity VIII (historical) with no evidence of coseismic rupture. Therefore, the preliminary data presented here seem to indicate that the paleoseismic activity on both faults is capable of producing coseismic surface displacements, probably reaching magnitudes of at least 6.5. These data show that paleoseismic studies based on geomorphological analyses are a useful tool in the assessment of the relative degree of activity of apparently ‘aseismic’ fault traces.

Surface and subsurface palaeoseismic records at the ancient Roman city of Baelo Claudia and the Bolonia Bay area, Cádiz (south Spain)

Abstract The Roman archaeological site of Baelo Claudia (Cádiz, south Spain) is located within the Gibraltar Arch, a region with no significant recent or historical seismicity. However, previous studies have emphasized the occurrence of repeated strong archaeoseismic damage (intensity≥IX MSK) at Baelo Claudia tentatively bracketed in this study around ad 40–60 and ad 260–290. A multidisciplinary study has been carried out including the detailed mapping of surface deformation and building damage, surface geology and geomorphology, collection of structural data, and an extensive ground penetrating radar (GPR) survey. The obtained data are not conclusive when considered separately, but evident links between archaeoseismic damage, structural and GPR data indicate that the destruction of the city was linked to seismic shaking. The analysis of the pattern and orientation of deformation clearly indicates SW–NE directed compression due to ground shaking. This analysis also focuses on localized landslides and liquefaction processes, which appear to be coeval with the earthquakes, but the poor geotechnical parameters of the clayey substratum were determinant to amplify the observed level of destruction. The application of the present Spanish seismic code (NCSE-02) indicates that intensity VIII MSK (0.24–0.26 g) can be reached in this zone for 500 year return periods.

Origin of an assemblage massively dominated by carnivorans from the miocene of Spain.

Carnivoran-dominated fossil sites provide precious insights into the diversity and ecology of species rarely recovered in the fossil record. The lower level assemblage of Batallones-1 fossil site (Late Miocene; Madrid Basin, Spain) has yielded one of the most abundant and diversified carnivoran assemblage ever known from the Cenozoic record of mammals. A comprehensive taphonomic study is carried out here in order to constrain the concentration mode of this remarkable assemblage. Another distinctive feature of Batallones-1 is that the accumulation of carnivoran remains took place in the context of a geomorphological landform (cavity formation through a piping process) practically unknown in the generation of fossil sites. Two characteristics of the assemblage highly restrict the probable causes for the accumulation of the remains: (1) the overwhelming number of carnivorans individuals; and (2) the mortality profiles estimated for the four most abundant taxa do not correspond to the classic mortality types but rather were the consequence of the behavior of the taxa. This evidence together with other taphonomic data supports the hypothesis that carnivoran individuals actively entered the cavity searching for resources (food or water) and were unable to exit. The scarcity of herbivores implies that the shaft was well visible and avoided by these taxa. Fossil bones exhibit a very good preservation state as a consequence of their deposition in the restricted and protective environment of the chamber. Batallones-1 had another assemblage (upper level assemblage) that was dominated by herbivore remains and that potentially corresponded to the final stages of the cavity filling.

Speleoseismology and palaeoseismicity of Benis Cave (Murcia, SE Spain): coseismic effects of the 1999 Mula earthquake (mb 4.8)

Abstract This work describes the coseismic ceiling block collapse within Benis Cave (−213 m; Murcia, SE Spain), associated with the 1999 Mula earthquake (mb=4.8, MSK VII). The collapse occurred at −156 m into the Earthquake Hall, and as a consequence one small gallery became blind. We studied the geology, topography and active tectonic structures relevant to the cave. In addition, we carried out a seismotectonic analysis of the focal mechanism solutions, and also a fault population analysis on slickensides measured in fault planes in the cave. The stress and strain regime is interpreted as being congruent with the palaeoseismic evidence, and agrees with the fault kinematics established for cave galleries developed within fault planes and growth anomalies of coral flowstone. Our analysis suggests that one active segment (NNE–SSW) determined the morphology and topography of the Benis Cave, where strong to moderate palaeoearthquakes (6≤M≤7) took place. As a consequence of this intense seismic activity a small gallery collapsed. A new palaeoseismic structure, or seismothem, has been recognized, namely the effect of palaeoearthquakes affecting the pattern of development of the spatial coral flowstone distribution located at the bottom of the cave.

Geomorphology applied to landscape analysis for planning and management of natural spaces. Case study: Las Batuecas-S. de Francia and Quilamas natural parks, (Salamanca, Spain)

Geomorphology is fundamental to landscape analysis, as it represents the main parameter that determines the land spatial configuration and facilitates reliefs classification. The goal of this article is the elaboration of thematic maps that enable the determination of different landscape units and elaboration of quality and vulnerability synthetic maps for landscape fragility assessment prior to planning human activities. For two natural spaces, the final synthetic maps were created with direct (visual-perceptual features) and indirect (cartographic models and 3D simulations) methods from thematic maps with GIS technique. This enabled the creation of intrinsic and extrinsic landscape quality maps showing sectors needing most preservation, as well as intrinsic and extrinsic landscape fragility maps (environment response capacity or vulnerability towards human actions). The resulting map shows absorption capacity for areas of maximum and/or minimum human intervention. Sectors of high absorption capacity (minimum need for preservation) are found where the incidence of human intervention is minimum: escarpment bottoms, fitted rivers, sinuous high lands with thick vegetation coverage and valley interiors, or those areas with high landscape quality, low fragility and high absorption capacity, whose average values are found across lower hillsides of some valleys, and sectors with low absorption capacity (areas needing most preservation) found mainly in the inner parts of natural spaces: peaks and upper hillsides, synclines flanks and scattered areas. For the integral analysis of landscape, a mapping methodology has been set. It comprises a valid criterion for rational and sustainable planning, management and protection of natural spaces.