Jesús Guerrero

Late Quaternary episodic displacement on a sackung scarp in the central Spanish Pyrenees. Secondary paleoseismic evidence

A sackung scarp has been investigated by trenching in the central Spanish Pyrenees. This feature is located 18 km to the SW of the North Maladeta Fault, which is the most probable source of the Mw 5.3 Vielha earthquake of 1923. Three displacement events have been inferred for the trenched sackung based on colluvial wedge stratigraphy and fault truncation. The increasing amount of deformation in each successive faulting event may be related to the progressive weakening of the slope through time. A minimum vertical slip rate of 0.19 mm/yr has been calculated for the sackung scarp. Several arguments suggest that the episodic displacement of the analysed sackung is controlled by strong seismic shaking: (a) Spatial association of the sackung features with the North Maladeta Fault; (b) Episodic displacement with a millennial recurrence (5.6 kyr) consistent with the expectable earthquake recurrence interval for a low slip rate fault, like the neighbouring North Maladeta Fault. Demonstrating in future investigations that the sackung features in the area constitute archives of large paleoearthquakes would be of great interest for seismic hazard assessments. They might help to improve the catalogue of paleoearthquakes and might provide information on earthquake recurrence intervals and the age of the most recent event (MRE).

Can flexural-slip faults related to evaporite dissolution generate hazardous earthquakes? The case of the Grand Hogback monocline of west-central Colorado

A paleoseismological investigation of flexural-slip faults related to interstratal evaporite dissolution suggests that such gravitational structures might have the potential to generate earthquakes with damaging magnitude. The Carbondale collapse center, in the southern Rocky Mountains of Colorado, is a morpho-structural depression of ~1200 km 2 where Miocene volcanic rocks are downdropped as much as 1200 m due to interstratal dissolution of halite-bearing evaporites. On the western margin of the collapse center, the debuttressing effect related to active evaporite dissolution drives unfolding of the steeply dipping late Laramide Grand Hogback monocline, accompanied by displacement on bedding-parallel faults. These flexural-slip faults rupture unconformable Miocene basalts and Quaternary mantled pediments, generating conspicuous half-graben depressions bounded by antislope fault scarps parallel to the underlying strata of the monocline. Two trenches dug across flexural-slip fault scarps developed in each stratigraphic marker (basalt cap, mantled pediment) revealed unexpected evidence of multiple late Quaternary faulting events (e.g., faulted colluvial wedge, sharp unconformities), with displacement-per-event values of ≥1 m. Three faulting events were inferred from the trench dug in the pediment ( w ) around 6.

Antislope scarps, gravitational spreading, and tectonic faulting in the western Yakutat microplate, south coastal Alaska

Mountain ridges in the western Yakutat microplate are riddled with swarms of antislope scarps and troughs. These landforms were previously interpreted as gravity failures (sackungen), but if partly or wholly tectonic (flexural slip or bending moment faults), they represent part of the strain budget from ongoing plate collision. To determine scarp origin, we mapped landforms, bedrock structure, and trenched scarps at Kushtaka Mountain and south of Martin Lake. The Kushtaka scarps paralleled west-dipping coal and sandstone beds in the Tertiary Kultieth Formation, and were spaced 35–60 m apart with heights of 1–4 m. Structural and paleosol relationships in a 6-m-long, 1.3-m-deep trench indicate the scarp was produced by Holocene creep on a normal fault underlying the scarp. The Kushtaka scarps thus represent toppling-style slip on bedding-plane faults in the eastern limb of a syncline, as the fold “unfolds” due to gravitational spreading. The Martin Lake scarps are more complex and include downslope-facing landslide scarps, upslope-facing flexural toppling scarps, and an oblique-slip tectonic scarp. A 2-m-deep trench across the WNW-trending tectonic scarp exposed the underlying bedrock fault plane with slickensides raking only 17°–20°, indicating mainly left-lateral slip on a sinistral-normal fault. In contrast, swarms of ENE-trending antislope scarps showed normal-oblique (dextral) slip. The two scarp sets may form a conjugate pair that simultaneously accommodates left-lateral tectonic slip and NNE-directed gravitational spreading. Our results, plus those recently published by [Li et al. (2010)][1], show that most antislope scarps in the western Yakutat microplate are formed by normal slip on bedding-plane faults that dip into the mountain, and represent gravitational spreading expressed as toppling-style failure or “unfolding” of strata on fold limbs. Conversely, the sinistral-oblique slip fault at Martin Lake is one in a family of east-trending faults accommodating accretion of the Yakutat microplate into the cuspate syntaxis of the Alaskan plate margin. [1]: #ref-14

Are talus flatiron sequences in Spain climate-controlled landforms?

Summary. This study provides chronological evidence of the influence of climatic variability in the generation of late Quaternary talus flatiron sequences in Spain. The temporal clustering of the OSL and radiocarbon dates obtained from talus flatiron deposits indicates that warm/wet and cold/dry periods controlled the accumulation and incision processes in the slopes, respectively, that led to the development of talus flatirons. These results strongly suggest that talus flatiron sequences constitute valuable paleoclimatic records. Additional and more accurate geochronological data from Spain and other regions of the world would improve the potential of these poorly-known landforms in paleoenvironmental studies.

Salt-dissolution faults versus tectonic faults from the case study of salt collapse in Spanish Valley, SE Utah (USA)

A paleoseismological investigation in Spanish Valley, SE Utah, reveals that faults related to interstratal karstification of salt may show episodic displacement and significantly different parameters than tectonic faults. Spanish Valley is a 25-km-long, 3-km-wide, NW-SE–trending graben formed by the collapse of the crest of a salt anticline. This collapse is related to the karstification of Paleozoic salts, which are several kilometers thick and form the core of the anticline. Differential passive bending of the supra-evaporitic Mesozoic strata produced smaller-scale, NW-SE–trending anticlines and synclines parallel to the axis of the graben on both margins of the collapse valley. Mapping reveals that swarms of synthetic and antithetic normal faults associated with these folds accommodate most of the vertical displacement. A 27-m-long, 4.5-m-deep, trench-like artificial excavation was dug into the hanging wall of the master normal fault of the NE flank with 30–40 m of throw. The excavation exposed a complex structure consisting of a half graben and an asymmetric upper graben separated by a horst. Nine displacement events have been inferred and constrained by consistent AMS (Accelerator mass spectrometry) radiocarbon dates, indicating an anomalously high mean vertical slip rate of 3.07 mm/yr and a very low average recurrence of ∼316 yr. The most recent recorded faulting event took place after 2330 cal. yr B.P. Data derived from detailed maps indicate that the faults have aspect ratios (maximum displacement to fault length) comparable to those reported for tectonic faults. However, they show greater aperiodicity, with coefficient of variation values greater than 1, long-term slip rates between 2 and 25 times greater, and displacement per event values up to 30 times higher than those expected for tectonic faults of the same length.

The application of GPR and ERI in combination with exposure logging and retrodeformation analysis to characterize sinkholes and reconstruct their impact on fluvial sedimentation.

This work illustrates the practicality of investigating sinkholes integrating data gathered by ground penetrating radar (GPR), electrical resistivity imaging (ERI) and trenching or direct logging of the subsidence-affected sediments in combination with retrodeformation analysis. This mutidisciplinary approach has been tested in a large paleosinkhole developed during the deposition of a Quaternary terrace on salt-bearing evaporites. The subsidence structure, exposed in an artificial excavation, is located next to Puilatos, a village that was abandoned in the 1970s due to severe subsidence damage. Detailed logging of the exposure revealed that the subsidence structure corresponds to an asymmetric sagging and collapse paleosinkhole with no clear evidence of recent activity. The sedimentological and structural relationships together with the retrodeformation analysis indicate that synsedimentary subsidence controlled channel location, the development of a palustrine environment and local changes in the channel pattern. GPR profiles were acquired using an array of systems with different antenna frequencies, including some recently developed shielded antennas with improved vertical resolution and penetration depth. Although radargrams imaged the faulted sagging structure and provided valuable data on fault throw, they did not satisfactorily image the complex architecture of the fluvial deposit. ERI showed lower resolution but higher penetration depth when compared to GPR, roughly capturing the subsidence structure and yielding information on the thickness of the high-resistivity alluvium and the nature of the underlying low-resistivity karstic residue developed on top of the halite-bearing evaporitic bedrock. Data comparison allows the assessment of the advantages and limitations of these complementary techniques, highly useful for site-specific sinkhole risk management. This article is protected by copyright. All rights reserved.

Conglomerate Monoliths and Karst in the Ebro Cenozoic Basin, NE Spain

The sedimentary fill of the Ebro Cenozoic Basin, NE Spain, includes thick conglomerate successions in the marginal sectors associated with the surrounding Alpine orogens. These commonly cemented alluvial fan and fan delta conglomerates grade rapidly into less resistant fine-grained facies. Differential excavation of the basin fill, together with erosion processes controlled by vertical fractures in the massive and indurated conglomerates, has resulted in the development of monoliths, locally known as mallos, with vertical walls that may reach more than 300 m in height. The cemented and fractured conglomerates in some sectors of the Catalan margin of the basin, mostly composed of calcium carbonate, display features characteristic of well-developed karst terrains, including sinkholes, karst springs, and multilevel cave systems several kilometers long with spelothems.

Discussion on the article “Paleoseismological analysis of an intraplate extensional structure: the Concud fault (Iberian Chain, Eastern Spain)” by P. Lafuente, L. E. Arlegui, C. L. Liesa, and J. L. Simón (Int J Earth Sci)

This discussion is focused on three aspects of the paper published by Lafuente et al. (Int J Earth Sci, doi:10.1007/s00531-010-0542-1, 2010) on Concud Fault, constitute the fundamental basis to assess the seismic potential of this capable structure: (1) A slip rate estimated for the Concud Fault based on an erroneous displacement value and a questionable correlation, obviating previously published datings, markedly different to those used by the authors. The wrong displacement value introduces an error of more than 25% in the calculated Quaternary slip rate. (2) A new paleoseismological interpretation of the outcrop of Condud Fault at Los Baños, adding two improperly justified paleoearthquakes to the four events previously inferred. (3) The attribution of faults affecting a young terrace to the most recent recorded earthquake on Concud Fault, ruling out implicitly the likely option of a gravitational origin for them, either landsliding or subsidence due to evaporite dissolution.