High-amplitude lake-level changes in tectonically active Lake Issyk-Kul (Kyrgyzstan) revealed by high-resolution seismic reflection data

A total of 84 seismic profiles, mainly from the western and eastern deltas of Lake Issyk-Kul, were used to identify lake-level changes. Seven stratigraphic sequences were reconstructed, each containing a series of delta lobes that were formed during former lake-level stillstands or during slow lake-level increase or decrease. The lake level has experienced at least four cycles of stepwise rise and fall of 400 m or more. These fluctuations were mainly caused by past changes in the atmospheric circulation pattern. During periods of low lake levels, the Siberian High was likely to be strong, bringing dry air masses from the Mongolian steppe blocking the midlatitude Westerlies. During periods of high lake levels, the Siberian High must have been weaker or displaced, and the midlatitude Westerlies could bring moister air masses from the Mediterranean and North Atlantic regions

New evidence for important lake-level changes in Lake Baikal during the Last Glaciation

In recent years, a number of estimates have been proposed of fluctuations of the Baikal lake level caused by climate changes. They were mainly based on the interpretation of reflection seismic data from the Selenga delta area (eastern coast of Lake Baikal). These estimates range between 2 m [Colman, 1998] and 600 m [Romashkin et al., 1997]. Better-constrained values of lake-level changes during the last 100 ky were presented by Urabe et al. [2004]. According to their reflection seismic data from the Selenga delta area, the level of Lake Baikal was significantly lower than the present-day level during the two last cold stages (i.e. -45 m during MIS2 and -73 m during MIS4). To precise and verify these values, we carried out an additional high-resolution reflection seismic study in the area of Olkhon Gate (western shore of Lake Baikal). The maximum water depth in this area does not exceed 40 m. The seismic data were collected using two different types of seismic sources: i) a multi-electrode CENTIPEDE sparker with a frequency range of 350-1400 Hz, and ii) the “Sonic-2” seismic system with a frequency range of 2-5 kHz. They allow investigation of the sedimentary record with a resolution of about 1 m (to 300 m depth) and 15-20 cm (to 30 m depth), respectively.Interpretation of these new data allowed distinguishing several seismic units separated by unconformities (erosion surfaces) in the upper part of the seismic profiles. These unconformities could be traced across the entire study area. The uppermost two erosion surfaces are more sharply defined. In the deepest parts of the channel (at 37-40 m water depth) the uppermost unconformity occurs at 5-10 ms below the lake floor, and the second unconformity at 15-20 ms below the lake floor. Both unconformities are interpreted as subaerial erosion surfaces and thus mark a lowstand of the lake level during a prolonged time. For calculation of the thickness of these two units, we used the acoustic logging data from the BDP-98 borehole [BDP Members, 2000]. According these data p-wave velocities vary from 1.6 to 1.8 km/s. The thickness of our upper two seismic units can thus be converted to 4-8 m and 12-16 m, respectively. This implies that the uppermost unconformity occurs at 41-48 m, and the second unconformity at 52-64 m below present-day lake level, which is approximately at the same depth as the two unconformities in the Selenga delta area that were studied by Urabe et al. [2004] and attributed with the MIS2 and MIS4 cold periods, respectively.Our new data thus support the growing amount of evidence of a lowering of the Lake Baikal water level by 40-65 m during glacial/cold periods. The lowstands are probably caused by water redistribution in the Lake Baikal watershed due to climate changes (i.e. glaciation and atmospheric circulation). These data also allow making quantitative assessments of water balance and paleoclimate parameters in the past

Determining the structure of a large tilted block between two major boundary faults in a continental rift (central Lake Baikal): a reflection seismic study

Between the major boundary faults of the central part of Lake Baikal (ie. the Ol’khon fault and the Primorsky fault), a structurally complex tilted area exists that is strongly influenced by the interaction between these two faults. This area, that is about 30 kilometer wide and a 100 kilometers long, consists of three main parts: Pri-Ol’khon, Ol’khon-island and the submerged Maloe More depression. It is believed that the area formed by the gradual propagation of the Primorsky fault in a southeast direction towards the Ol’khon fault.During the summer of 2001 a large amount of high resolution reflection seismic profiles were shot in Maloe More (>600 km), that could be used to get a better insight in the structural development of the area, and in the geometry of its different sub-blocks and basins. In a first stage we have investigated the morphology of the basement underneath the sedimentary cover, and we determined which structures were fault related and which not. Age constraints on the subsequent evolution came from the correlation of the sedimentary units in Maloe More with deposits on Ol’khon-island, and with data from the long BDP-cores in a nearby area (Academician Ridge).The depth of the basement gradually increases from the southwest towards the northeast, and its morphology is characterised by several ridge structures and faults that strike at high-angle to the main faults. Several of these ridges border basins that contain relatively old sediments (Miocene age; Unit A) later overlain by younger units. Therefore the main basement structures of the Maloe More area should be older than the general believed age for the southward propagation of the Primorsky fault (1 Ma according to earlier models). Moreover the occurrence of relatively thick deposits of unit A in the southwestern extremity of Maloe More and in Ol’khon-gate contradicts the idea that these parts of the area are the youngest, being submerged only recently.Instead, older (isolated) sedimentary traps and lacustrine environments must have existed in this area. Faulting in the younger sediments however shows that the presentday activity of the major boundary faults, still has a pronounced effect on the local structure between them. Some of the formed basins are still determined by displacements on the older structures.For this study we have tried to determine the evolution of the Maloe More area, based on its interpreted structure and the relation with overlying sedimentary deposits, and we have tried to link our observations with existing models for the development of the Primorsky and Ol’khon faults

Propagation of the Primorsky Fault in the central part of Lake Baikal and the evolution of Maloe More

The Primorsky Fault is one of the two major western boundary faults in the central part of Lake Baikal. According to the existing fault growth model (e.g. Agar and Klitgord, 1995), this fault has propagated gradually in a southward direction. During this propagation, the Primorsky Fault has cut through the footwall of the Ol’khon Fault, which is the other major boundary fault 35–40km to the south-east. This propagation has controlled the submergence of the Ol’khon Region which forms a large tilted block between both faults.Based on the interpretation of high-resolution reflection seismic profiles of the submerged part of the Ol’khon Region (ie. Maloe More), different depocentres have been identified in the hanging-wall region of the Primorsky Fault. These depocentres correspond to small basins that are separated from each other by distinct basement ridges, with an orientation that strikes almost perpendicularly to the Primorsky Fault. The occurrence of the oldest sedimentary deposits (Unit A, Miocene age) in depocentres in the southern part of Maloe More, indicates that old sedimentary traps and lacustrine environments must have existed in the area. This finding contradicts the existing growth model for the Primorsky Fault, which assumes that only a recent (ca. 1Ma) and gradual propagation of the fault is responsible for the increasing subsidence in Maloe More. In the different sub-basins, younger sediments (Unit B, Upper Pliocene) overlie the deposits of Unit A. Nevertheless, the upper parts of Unit B are also present on the different basement ridges. The thickness of Unit B is on the northeastern ridges in Maloe More considerably greater than on those more to the south-west, indicating that they have been submerged for a longer time. Careful investigation of a RESURS satellite image of the area has revealed a possible segmentation of the Primorsky Fault, with segment boundaries occurring at the location of the different basement ridges in Maloe More.We believe that the growth of the Primorsky Fault can therefore be described in two different stages. A first stage, during the deposition of Unit A, was characterised by the evolution of 5 different (isolated) segments that defined small basins in Maloe More. The observed basement ridges corresponded at that time to intrabasin highs that resulted from the displacement deficit between the different fault segments. Increasing extension lead to the further growth of the segments, causing a final linkage between them. This linkage marks the onset of a second stage, which was achieved during the deposition of Unit B. Linkage between fault segments caused a displacement increase (mainly at the former location of the segment boundaries), resulting in the submergence of the basement ridge. Seen the thicker deposits of Unit B on the northeastern ridges in Maloe More, we believe that the segment linkage was first established between the northernmost fault segments of the Primorsky Fault. Subsequent linkages between other segments more to the south, and the associated post-linkage displacement increases, caused the further submergence of Maloe More towards the southwest in later stages

Scaled physical models of continental rifting: application to the Baikal Rift Zone

Scaled physical models constructed with dry sand layers have proven to be a useful tool for the simulation of the structural patterns that are commonly observed in natural rift systems. With this study we have tried to simulate the evolution of the Baikal Rift Zone as to get better insight in the importance of some of the processes controlling its development. For this purpose, models have been constructed with different baseplate geometries. These models allowed us to observe the possible basement controls on the present-day fault structures in the Baikal Rift Zone.Baseplates having similar shapes as the Siberian Craton caused in the models the development of the stepwise fault deflection that is characteristic for the western border fault system of Lake Baikal. During the initial evolution of the modelled faults, several relay zones were formed between isolated fault segments. Such relay zones are also common in the border fault system of Lake Baikal. In later stages of the modelling, further extension lead to the linkage between fault segments, causing the eventual disappearance of the different relay zones.The development of the models was continuously monitored using digital photographs. Animating the sequence of these photographs allowed to carefully study the kinematic evolution of the experiments. After certain amounts of extension (usually 1 or 2cm) the different basins that had formed in the models were filled with syn-kinematic sand layers. Completed models have subsequently been impregnated and sectioned either vertically or horizontally in 1cm intervals. This technique reveals the internal geometry of the formed fault structures. 3D reconstructions of the models have been produced by digitising certain reference levels on the different crosssections.Such 3D images clearly illustrate the variations in fault displacements in the different parts of the models. Moreover, 3-dimensional representations of the experiments can easily be compared with the available digital terrain models of the Baikal Rift Zone, to test the validity of the modelling results.In this study we have examined in detail the kinematic evolution and the growth of faults in different sandbox experiments, and we have compared our observations with structural interpretations that have already been made for the Baikal Rift Zone

Sandbox simulations of relay ramp evolution

The interaction between two offset overlapping normal faults is characterised by the presence of a relay ramp. In order to investigate the way these structures develop, sandbox experiments were carried out. To simulate the brittle crust, we used dry quartz sand that was extended by means of a rubber sheet located at its base.We imposed the initial configuration of the two interacting normal faults by placing silicone bars at the base of the sand-pack, above the rubber sheet. These, under extension, generated a velocity discontinuity responsible for the development of the normal faults, which later interacted and grew further within the sand package. By varying the initial configuration of the silicone bars, we could easily vary the spacing (distance) between the segments, their overlap, their length and their orientation, and test the influence of these parameters on the development of the ramp between the segments. The modeled faults had aspect ratio’s varying between 2.5 and 5.The relay structures in the experiments were characterised by birth, growth and decay. Birth of a relay ramp marked the onset of interaction and was inferred when a tilt of the sand surface could be observed between the two overlapping faults. Growth was characterised by the propagation of the two interacting faults, increasing the distance of overlap and the tilting of the sand layers. During this growth stage often the deflection of one of the fault traces could be observed. Decay occurred when the two initially isolated faults eventually got connected with each other and the ramp breached.A large part of the relay ramps that were formed in the models were breached — or were getting breached — before the final amount of extension was reached (ß ˜ 20%). For 55% of these ramps it was the hanging-wall fault that propagated towards the footwall fault, for 27% the footwall fault linked up with the hanging-wall fault, and for 18% of these breached ramps, a new fault developed that cross-cut the ramp. The new fault developed only in those cases where the original spacing of the faults was very small compared to their length. An experimental relation between the overlap and spacing of two segments was also determined and compared with earlier theoretical work.Finally, relay ramp evolution in the experiments was also characterised sometimes by several minor-order features which are not commonly observed in natural examples, such as: the further propagation of the fault tips after breaching, an increased displacement gradient just outside the relay ramp instead of inside, etc..

Holocene earthquake-triggered mass-wasting events recorded in the sediments of Lake Puyehue (South-Central Chile)

Despite South-Central Chile’s high seismicity and the occurrence of earth’s largest instrumentally recorded earthquake (AD 1960; Mw: 9.5), paleoseismic data is still scarce for this region. In this study, very high-resolution reflection seismic profiles (3.5 kHz) in Lake Puyehue (41°S) were utilized to trace giant seismic events back into time. The seismic profiles show repeated occurrences of multiple mass-wasting deposits (slumps, debris flows, homogenites) occurring at a same seismic-stratigraphic horizon, indicating that they are coeval and caused by a single mass-wasting event of basin-wide importance. An age-depth model, based on 9 AMS radiocarbon datings and varve-counting on an 11 m-long sediment core, has been used to develop a “seismic chronostratigraphy”. It allows dating of the mass-wasting events by interpolation between dated seismic horizons to the distal parts of the mass-wasting deposits. The mass-wasting events are assumed to be earthquake-triggered because:The recentmost mass-wasting events correlate with the devastating historical earthquakes of AD 1575 and AD 1960.Synchronicity of multiple slope failures (mass-wasting events) requires a strong regional trigger, such as an earthquake. Consequently, local slope oversteepening at delta fronts or local fluid expulsion could not initiate such widespread events.South-Central Chile has been historically subjected to several strong (M > 8) subduction earthquakes and subduction processes have been constantly active since Mesozoic times.Multiple slope failures occur at water depths > 70 m, which rules out shallow instability triggers, such as storm wave action and lake-level fluctuations.This study reveals nine paleoseismic events during the Holocene with a mean recurrence rate of about 1000 yr, but with an overall relatively aperiodic occurrence (ranging between 400-2000 yrs.). The most prominent event took place around 1660 cal. yr. BP, evidenced by at least 29 simultaneous mass-movements and a homogenite deposit. Quantitative comparison of mass-wasting events related to the historical earthquakes of AD 1960 and AD 1575 showed significant differences (respectively 17 and 4 observed mass-wasting deposits) although these earthquakes are assumed to have had a comparable strength. This can be attributed to a lowered sedimentation rate on the potentially unstable slopes in the period 3000 cal. yr. BP – 500 cal. yr. BP, which would have made lacustrine earthquake recording less likely in AD 1575. The absence of mass-wasting deposits associated with other historical earthquakes (e.g.: AD 1737 (Ms: 7.5) and AD 1837 (Ms: 8)) indicates that only mega-earthquakes (Mw >8.5) within a range of about 300 km are recorded in the sedimentary sequence of Lake Puyehue.Reflection seismic profiles also show vertical fluidisation structures with large-scale sediment injections, which disturb the upper sedimentary sequences. The top of these fluidisation structures and diverse deformation levels could be spatially linked to seismically induced mass-wasting deposits and consequently indicate an additional method for lacustrine paleo-earthquake tracing.Several reconaissance seismic surveys on other glacigenic lakes in the Chilean Lake District also show promising paleoseismic records, which will offer the opportunity to correlate lacustrine records to reveal South-Central Chile’s paleoseismic history in detail and the earthquake registration capacities of its glacigenic lakes

Mechanical losses in low loss materials studied by Cryogenic Resonant Acoustic spectroscopy of bulk materials (CRA spectroscopy)

Mechanical losses of crystalline silicon and calcium fluoride have been analyzed in the temperature range from 5 to 300 K by our novel mechanical spectroscopy method, cryogenic resonant acoustic spectroscopy of bulk materials (CRA spectrocopy). The focus lies on the interpretation of the measured data according to phonon-phonon interactions and defect induced losses in consideration of the excited mode shape.Comment: 4 pages, 4 figures, proceedings of the PHONONS 2007, submitted to Journal of Physics: Conference Serie

Paleoproductivity of Puyehue Lake (Southern Chile) during the last millenium: climatic significance

Southern Chile is a key site for the understanding of past climatic variations since it is influenced by the El Nino Southern Oscillation (ENSO). We investigated high resolution climate changes during the last millennium by a multi-proxy analyse of short cores (60 cm long) collected in Puyehue lake (40°S): magnetic susceptibility, grain-size, mineralogy, density, gamma-density, LOI, biogenic silica content and bulk XRF geochemistry. According to age-depth model (210Pb, 137Cs and varve counting - Boës et al., this session), the cores cover the last 600 yr. The sediment is characterized by volcanic minerals and a high diatom content, due to the important lacustrine silica supply characteristic of volcanic environments. Moreover, the active volcanism of the Chilean Lake District is responsible of a high number of tephra deposits. Our main aim is to quantify biogenic particles fluxes throughout the last millennium by Na2CO3 dissolution and by normative calculation based on bulk XRF analyses. The result shows that volcanic eruptions do not influence the biogenic productivity of the lake. From 1400 to 1880 yr. AD, paleoproductivity shows a global trend from low to high biogenic production. Important paleoproductivity changes are observed over the last 120 yr. Results are compared with historical data and discussed in terms of climate changes and/or anthropic influence

The 600 yr eruptive history of Villarrica Volcano (Chile) revealed by annually laminated lake sediments

Lake sediments contain valuable information about past volcanic and seismic events that have affected the lake catchment, and they provide unique records of the recurrence interval and magnitude of such events. This study uses a multilake and multiproxy analytical approach to obtain reliable and high-resolution records of past natural catastrophes from similar to 600-yr-old annually laminated (varved) lake sediment sequences extracted from two lakes, Villarrica and Calafquen, in the volcanically and seismically active Chilean Lake District. Using a combination of micro-X-ray fluorescence (mu XRF) scanning, microfacies analysis, grain-size analysis, color analysis, and magnetic-susceptibility measurements, we detect and characterize four different types of event deposits (lacustrine turbidites, tephra-fall layers, runoff cryptotephras, and lahar deposits) and produce a revised eruption record for Villarrica Volcano, which is unprecedented in its continuity and temporal resolution. Glass geochemistry and mineralogy also reveal deposits of eruptions from the more remote Carran-Los Venados volcanic complex, Quetrupillan Volcano, and the Huanquihue Group in the studied lake sediments. Time-series analysis shows 112 eruptions with a volcanic explosivity index (VEI) >= 2 from Villarrica Volcano in the last similar to 600 yr, of which at least 22 also produced lahars. This significantly expands our knowledge of the eruptive frequency of the volcano in this time window, compared to the previously known eruptive history from historical records. The last VEI >= 2 eruption of Villarrica Volcano occurred in 1991. Based on the last similar to 500 yr, for which we have a complete record from both lakes, we estimate the probability of the occurrence of future eruptions from Villarrica Volcano and statistically demonstrate that the probability of a 22 yr repose period (anno 2013) without VEI >= 2 eruptions is <= 1.7%. This new perspective on the recurrence interval of eruptions and historical lahar activity will help improve volcanic hazard assessments for this rapidly expanding tourist region, and it highlights how lake records can be used to significantly improve historical eruption records in areas that were previously uninhabited