Pedro Lucha

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

1:5000 Landslide map of the upper Gállego Valley (central Spanish Pyrenees)

A 1:5000 scale geomorphological landslide inventory map of the glaciated headwaters of the Gállego River in the Spanish Pyrenees is presented. This map, covering an area of 57.6 km2 and mainly produced by direct mapping in the field, provides a more comprehensive and accurate picture of the distribution of slope movements than previous maps. Around 20% of the area is affected by large flow-dominated slope movements developed on Paleozoic slates. Our map shows that some of these post-glacial landslides have blocked the main drainage at several sites creating landslide-dammed lakes. The damage caused by landslides in the mapped area, with an estimated cost of greater 10 million euros, is mostly related to the activity of pre-existing slope movements, some of which have been reactivated or accelerated by adverse human alterations (e.g., excavation at the toe, overloading, higher water infiltration). This study illustrates that the incorporation of detailed cartographic landslide inventories in the planning and development process of alpine areas may contribute to reduce significant landslide-related damage in a cost-effective manner.

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.

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.