Kris Vanneste

Late Cenozoic evolution of the western Barents Sea-Svalbard continental margin

Abstract Seven regionally correlatable reflectors, named R7 (oldest) to R1, have been identified in the Upper Cenozoic sedimentary succession along the western continental margin of Svalbard and the Barents Sea. Regional seismic profiles have been used to correlate between submarine fans that comprise major depocentres in this region. Glacial sediment thicknesses reach up to 3 seconds two-way time, corresponding to 3.5–4 km. Despite limited chronostratigraphic control, ages have been assigned to the major sequence boundaries based on ties both to exploration wells and to shallow boreholes, and by paleoenvironmental interpretations and correlations with other regions. Lateral and vertical variations in seismic facies, between stratified and chaotic with slump structures, have major implications for the interpretation of the depositional regime along the margin. The main phases of erosion and deposition at different segments of the margin are discussed in the paper, which also provides a regional seismic stratigraphic framework for two complementary papers in the present volume. Reflector R7 marks the onset of extensive continental shelf glaciations, but whereas the outer Svalbard shelf has been heavily and frequently glaciated since R7 time, this did not occur, or occurred to a much less extent, until R5 time in the southern Barents Sea. The present study provides the background for a quantification of the late Cenozoic glacial erosion of Svalbard and the Barents Sea. The rates of erosion and deposition exhibit large temporal and spatial variations reflecting the importance of glacial processes in the Late Cenozoic development of this nearly 1000 km long margin.

Late Quaternary earthquake-related soft-sediment deformation along the Belgian portion of the Feldbiss Fault, Lower Rhine Graben system

Abstract Trench investigations along the Bree fault scarp in Belgium, in the framework of a first paleoseismological experience in the area of the Lower Rhine Graben system, have not only demonstrated the existence of seismogenic faults almost extending to the present ground surface, but also exposed several types of soft-sediment deformation affecting sandy sediments and soils of inferred late Weichselian to Holocene age. The observed features include asymmetric folding, small-scale normal faulting, possible sand intrusions, and small water escape or load structures. Establishing a seismic origin for the individual features is not always possible based on their sedimentary characteristics only, particularly due to the potential confusion with periglacial phenomena, but their abundance, the close association of different deformational styles, and their vicinity to a known active fault, all seem to be compatible with an earthquake-induced scenario. The features are to some extent comparable to those generated during the recent MS 5.3 Roermond earthquake at the opposite side of the Roer Valley Graben. Stratigraphical and mutual geometrical relationships suggest the occurrence of at least three distinct deformational events since probably 30,000 years, and one event before that time. Since nothing is yet known about their regional distribution, however, estimating the associated paleomagnitudes from these soft-sediment deformations is premature.

The use of geophysical prospecting for imaging active faults in the Roer Graben, Belgium

As part of a paleoseismological investigation along the Bree fault scarp (western border of the Roer Graben), various geophysical methods [electrical profiling, electromagnetic (EM) profiling, refraction seismic tests, electrical tomography, ground‐penetrating radar (GPR), and high‐resolution reflection seismic profiles] were used to locate and image an active fault zone in a depth range between a few decimeters to a few tens of meters. These geophysical investigations, in parallel with geomorphological and geological analyses, helped in the decision to locate trench excavations exposing the fault surfaces. The results could then be checked with the observations in four trenches excavated across the scarp. Geophysical methods pointed out anomalies at all sites of the fault position. The contrast of physical properties (electrical resistivity and permittivity, seismic velocity) observed between the two fault blocks is a result of a differences in the lithology of the juxtaposed soil layers and of a change ...

Active faulting and paleoseismology along the Bree fault, lower Rhine graben, Belgium

Paleoseismic analysis of the 10-km-long Bree fault scarp in the lower Rhine graben yields numerous lines of evidence of earthquake activity in the Holocene and late Pleistocene. This active normal fault, a part of the Feldbiss fault system, dips 70°NE and is expressed at the surface by a prominent NW-SE trending 7 to 20 m high scarp, formed since the deposition of the Maas River main terrace <700 kyr. B.P. Trenches and geophysical prospecting show that the fault, which is known to have ∼100 m of vertical offset since the late Pliocene, breaks late Pleistocene and Holocene deposits. Ground-penetrating radar, seismic refraction, and electric tomography suggest that at shallow depth the amount of displacement is larger than the youngest vertical offset visible in the trenches and corresponds to cumulative fault displacements. The analysis of 36 leveling profiles across the scarp indicates that its height can be classified into three groups, likely corresponding to different events. A morphologic dating gives approximate ages of 2±1.5 kyr B.P., 14±5 kyr B.P., and 41±6 kyr B.P. for the past three surface-faulting earthquakes. Analysis of faulted stratigraphy and earthquake-induced deformation structures exposed in trenches suggests the occurrence of three large earthquakes during the past 45×l03 years and yields 0.07 mm/yr of relative vertical deformation rate. The most recent seismic event occurred between A.D. 610 and 890. The first identification of an active fault with surface ruptures in the lower Rhine graben area emphasizes that large earthquake sources exist within intraplate Europe and that at least some of these events are preserved in the geologic record.

Pseudo-3D imaging of a low-slip-rate, active normal fault using shallow geophysical methods: The Geleen fault in the Belgian Maas River valley

In a detailed site survey for paleoseismic trenching, we applied shallow geophysical prospecting techniques, including ground-penetrating radar (GPR), electric resistivity tomography (ERT), and resistivity mapping to identify, locate, and visualize in 3D the Geleen fault, an active normal fault bordering the Roer Valley graben in northeast Belgium. Because of a low slip rate, the geomorphic expression of this fault is very faint in the relatively young deposits of the Maas River valley. ERT profiles show the fault as a broad, near-vertical anomaly characterized by sharp lateral resistivity contrasts, with an associated vertical offset of sediment layers. We observed offsets of ∼15 m and ∼5 m for the base and top, respectively, of a middle-late Pleistocene fluvial gravel layer. Shallow ERT and GPR profiles indicate that younger sediments are also affected by faulting, but the amount and sense of offset appear to be conflicting: ERT profiles show a near-surface, high-resistivity layer with an apparent rever...

A Model of Composite Seismic Sources for the Lower Rhine Graben, Northwest Europe

Abstract The Lower Rhine Graben (LRG) straddling the border zone of Belgium, the Netherlands, and Germany, is an active tectonic structure in continental northwest Europe. It is characterized by northwest–southeast oriented normal faults, and moderate but rather continuous seismic activity. Many faults have been mapped in the LRG, but so far a model of fault hierarchy or fault segmentation has been lacking. In the frame of a European database of seismogenic sources, we have devised a seismic‐source model for the LRG consisting of so‐called composite seismic sources. Each composite seismic source may encompass one or more segments, but it is unlikely that a segment would extend across more than one source. We distinguish 15 seismic sources based on major stepovers, bifurcations, gaps, and important changes in strike, dip direction, or slip rate. The sources are partitioned into one or more informal fault sections, each with an associated surface trace. For each source, we describe the limits and the composing fault sections, and present the geological arguments for their existence. We have compiled all relevant data concerning the seismic‐source parameters required for the database, putting lower and upper bounds on strike, dip, rake, slip rate, and depth, and an upper bound on earthquake magnitude. This source model should provide a new basis for modeling seismic hazard, as well as for guiding further paleoseismic studies in the LRG. Online Material: Detailed maps of the composite sources in the Lower Rhine Graben, 3D views of the fault model, and a table with parameters of earthquake focal mechanisms and detailed information sheets for each composite seismic source.

Seismic evidence for long-term history of glaciation on central East Greenland shelf south of Scoresby Sund

A seismic reflection transect crossing the central East Greenland continental margin south of the Scoresby Sund fjord system provides information regarding the long-term history of expansion and retreat of the Inland Ice. The shelf and slope sediments can be divided into three first-order units; the upper unit is interpreted to have a glacial origin. Within the up to 1000-m-thick glacial unit, six sequences were identified, representing at least as many phases of extensive ice sheet grounding on the shelf. Varying amounts of progradation and aggradation probably reflect successive phases in the glacial evolution of the region.

Seismic hazard in regions of present day low seismic activity: uncertainties in the paleoseismic investigations along the Bree Fault Scarp (Roer Graben, Belgium)

Earthquake hazard assessment in stable continental regions, such as northern Europe, has traditionally been evaluated on the basis of the instrumentally and historically recorded seismicity, which indicates relatively low hazard levels. Reliability of such estimates is a matter of debate as the long-term potential of large earthquakes usually cannot be determined based on short observational periods generally less than a few hundred years. A significant improvement to this lack of knowledge can be achieved by extending the past observations into the geological time scale. Paleoseismic investigations can provide valuable information to bridge this gap, where the potential for large earthquakes can be quantified both in magnitude and recurrence period, based on the observation of prehistoric earthquakes (paleoearthquakes) in the geological record (particularly in the last 20,000 years). However, using these records in seismic hazard analysis requires systematic treatment of uncertainties. Usually uncertainties are inherent to the interpretation of geological record, which leads, in the end, to the identification of paleoearthquakes. Field observations used in the analysis may satisfy several alternative interpretations. Such interpretations become useless when alternative solutions exist but not documented in detail, and especially when the relative reliability of the favored interpretation with respect to the alternative interpretations is not known. The recently introduced method using logic-tree formalism, which is based on qualitative description of the uncertainties related to the paleoseismic data and especially in its interpretation, is applied in the paleoseismic investigations performed on the Bree Fault Scarp, along the Feldbiss Fault (Roer Graben, Belgium). The cumulative uncertainties associated with the different stages of the study are computed as the combination of the preferred alternative branches in the logic-tree presentation. The final uncertainty and its relative importance in seismic hazard analysis is expressed as the paleoseismic quality factor (PQF), which indicate 0.76. This value can directly be used in seismic hazard analysis.

A 3000-year record of ground-rupturing earthquakes along the central North Anatolian fault near Lake Ladik, Turkey

The North Anatolian fault (NAF) is a similar to 1500 km long, arcuate, dextral strike-slip fault zone in northern Turkey that extends from the Karliova triple junction to the Aegean Sea. East of Bolu, the fault zone exhibits evidence of a sequence of large (M(w) > 7) earthquakes that occurred during the twentieth century that displayed a migrating earthquake sequence from east to west. Prolonged human occupation in this region provides an extensive, but not exhaustive, historical record of large earthquakes prior to the twentieth century that covers much of the last 2000 yr. In this study, we extend our knowledge of rupture events in the region by evaluating the stratigraphy and chronology of sediments exposed in a paleoseismic trench across a splay of the NAF at Destek, similar to 6: 5 km east of Lake Ladik (40.868 degrees N, 36.121 degrees E). The trenched fault strand forms an uphill-facing scarp and associated sediment trap below a small catchment area. The trench exposed a narrow fault zone that has juxtaposed a sequence of weakly defined paleosols interbedded with colluvium against highly fractured bedrock. We mapped magnetic susceptibility variations on the trench walls and found evidence for multiple visually unrecognized colluvial wedges. This technique was also used to constrain a predominantly dip-slip style of displacement on this fault splay. Sediments exposed in the trench were dated using both charcoal and terrestrial gastropod shells to constrain the timing of the earthquake events. While the gastropod shells consistently yielded (14)C ages that were too old (by similar to 900 yr), we obtained highly reliable (14)C ages from the charcoal by dating multiple components of the sample material. Our radiocarbon chronology constrains the timing of seven large earthquakes over the past 3000 yr prior to the 1943 Tosya earthquake, including event ages of (2 sigma error): A. D. 1437-1788, A. D. 1034-1321, A. D. 549-719, A. D. 17-585 (1-3 events), 35 B. C.-A. D. 28, 700-392 B. C., 912-596 B. C. Our results indicate an average interevent time of 385 +/- 166 degrees yr (1 sigma).

Seismic stratigraphy of the Bill Bailey and Lousy Bank area: implications for subsidence history

Abstract The present study discusses some results of a reconnaissance seismic survey carried out in 1988 in the area around the prominent Bill Bailey and Lousy Banks, located SW of the Faeroe Islands. The area is thought to be part of the Rockall-Faeroe microcontinent, which was flooded by Palaeocene plateau basalts. The seafloor topography is largely due to the present-day organization of the basalt surface. Important structural features exhibited by this basement are sub-surface diverging reflectors and a major fault. Sediment accumulation is confined to the basinal area where four unconformities were identified. The lower boundary corresponds to the well-known reflector R4 of the North Atlantic. The other unconformities are proposed to correlate with hiatuses at the beginning and end of the middle Miocene, and at the end of the Pliocene, respectively. The sediment units are briefly described by their upper and lower boundary, seismic facies and thickness variations. About the lithology, however, little is so far known. The sediments are deformed by intraformational faults and a diapir-like structure. Both deformational styles are probably related to temporary overpressurisation in fine-grained sediments, but resulting from different causes. Stratal geometric patterns indicate that initially the subsidence of the basalt surface was rather uniform, but became non-uniform after the Eocene/Oligocene boundary, differentiating the two banks from the surrounding basin.