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Evidence of earthquake-induced liquefaction features in the Acequión river valley, central western Argentina, is analysed. Well-preserved soft-sediment deformation structures are present in Late Pleistocene deposits; they include two large slumps and several sand dikes, convolutions, pseudonodules, faults, dish structures and diapirs in the basal part of a shallow-lacustrine succession in the El Acequión River area. The water-saturated state of these sediments favoured deformation.

Soft-sediment deformation structures induced by seismic liquefaction and/or fluidization receive much attention in sedimentological, structural and palaeoseismic studies. The direct record of larger earthquakes is restricted to instrumental and historical data; the recognition of prehistoric earthquakes requires criteria to recognize seismites in the geological record. The areal distribution of seismites can sometimes be related to active faults since distances to the epicenter (for a given magnitude) tend to be related to the liquefaction effects of seismic shocks. The use of soft-sediment deformation structures for palaeoseismic studies has limitations, however. Hardly anything is known, for instance, about the effects that modern seismic events have on the sediments in most environments. Moreover, criteria for the recognition of seismites are still under discussion. The following characteristics seem, particularly in combination, the most reliable: (1) Soft-sediment deformation structures should occur in laterally continuous, preferably recurring horizons, separated by undeformed beds; (2) These deformation structures should be comparable with structures known to have been triggered by modern seismic activity; (3) The sedimentary basin should have experienced tectonic activity at the time when the deformations were formed; and (4) The intensity or abundance of the soft-sediment deformation structures in a presumed seismite should change laterally, depending on the distance to the epicenter. It turns out that all of these four criteria have important exceptions. (1) Soft-sediment deformation structures occurring over large lateral distances in a specific layer can be triggered also by other processes. Moreover, in environments with a low sedimentation rate, the time between successive earthquakes is often too short to allow accumulation of beds that remain undisturbed. Furthermore, total liquefaction of a sandy bed may result in the absence of deformation features. (2) No truly diagnostic soft-sediment deformation structures exist to prove seismic activity. Moreover, the final configuration of a soft-sediment deformation structure is independent of the type of trigger. (3) Seismites occur frequently in areas where seismic activity is low today. (4) The lateral changes in the intensity of soft-sediment deformation structures in seismites as a factor presumed to depend on the distances to the epicenter, pose a complicated problem. The 2012 Emilia earthquakes, for instance, affected sandy fluvial channels but not the fine-grained floodplains. It must thus be deduced that specific soft-sediment deformation structures cannot be used without additional evidence to identify seismites. In particular, the magnitude of seismic shocks and the recurrence time of main events (the most important features that allow recognition of seismites) seem to be sedimentological in nature: facies changes in space and time seem the parameters that most strongly control the occurrence, morphology, lateral extent and the vertical repetition of seismites.

High-velocity friction experiments have shown an almost complete loss of strength associated with silica gel formation on slip surfaces. The identification of frictional silica gel products in palaeo-seismic faults is, however, problematic, because there are multiple natural sources of silica gel and recrystallization of gel to quartz complicates preservation. The importance of gel formation on natural faults is therefore unknown. Here, we report a structurally distinct and semi-continuous, 0.5–10 mm thick layer of microcrystalline quartz along a major carbonate-hosted fault, the Olive fault, in the Naukluft Nappe Complex, Namibia. The quartz layer is distinguished by flow banding-like textures and unusual cathodoluminescence characteristics. The layer consists of ∼2–20 μm hexagonal quartz crystals, which include distinct, crystalline, pore-bearing micro-to nano-spheres, separated by pore geometries indicative of volumetric contraction, and with grain boundaries enriched in aluminium. We interpret these features to indicate that the quartz crystals formed from recrystallization and dehydration of a silica gel. Because it is found in a carbonate-hosted fault and crosscuts lithological layering, the silica source is not from comminution of local wall rocks. Rather, the gel likely formed from reshear of a quartz-coated fault surface, or incremental shear slip associated with precipitation of silica driven by co-seismic pressure drops. This example of fault-related silica gel may have formed by a different mechanism than the gels produced in high-velocity friction experiments, but once formed, may have comparable rheological effects. [ABSTRACT FROM AUTHOR]

Abstract: Lake Mucubají is a moraine-dammed lake, crossed by the Boconó Fault, in the Mérida Andes, Venezuela. Four long piston-cores (up to 8 m long) and 24 short gravity cores (0.6 to 0.8 m long) were collected to study the sedimentary fill of the lake. Lithostratigraphy, magnetic susceptibility, organic and inorganic contents, and 14C ages, permit lateral correlations between the cores, and with an emerged part of the former larger lacustrine basin fill. The cored succession spans the last 16,000 years. Several abrupt changes in sediment texture and composition are ascribed to depth and surface modifications. Together with soft-sediment disturbances, they are related to the seismo-tectonic activity of the Boconό Fault: shock-induced phenomena (slumps, seiche effects, liquefaction) and co-seismic scarps. Four major earthquakes indicate a mean minimum recurrence interval of 1200 years, which is consistent with trench data obtained from a neighbouring active trace of the Boconó Fault system. Despite the seismo-tectonic imprint, a general post-LGM climatic trend can be traced and correlated with northern-hemisphere global evolution, in particular for the last 13,000 years. [Copyright &y& Elsevier]