Patterns and Failure Modes of Fractures Resulting From Forced Folding of Cohesive Caprocks – Comparison of 2D vs. 3D and Single-vs. Multi-Layered Analog Experiments

Knowledge of the formation mechanisms and geometries of fracture systems in sedimentary rocks is crucial for understanding local and basin-scale fluid migration. Complex fracture networks can be caused by, for instance, forced folding of a competent sediment layer in response to magmatic sill intrusion, remobilisation of fluidized sand or fluid overpressure in underlying porous reservoir formations. The opening modes and geometries of the fractures mainly determine the bulk permeability and sealing capacity of the folded layer. In this study, we carried out laboratory analog experiments to better comprehend patterns and evolution of the fracture network during forced folding as well as differences of the fracture patterns between a 2D and 3D modelling approach and between a homogenous and a multi-layered cover. The experimental layering consisted of a lower reservoir layer and an upper cover, which was either a single high-cohesive layer or an alternation of low- and high-cohesive layers. The two configurations were tested in an apparatus allowing quasi-2D and 3D experiments. Streaming air from the base of the model and air injected through a needle valve was used to produce a regional and a local field of fluid overpressure in the layers. The experimental outcomes reveal that the evolution of the fracture network undergoes an initial phase characterized by the formation of a forced fold associated with dominantly compactive and tensile fractures. The second phase of the evolution is dominated by fracture breakthrough and overpressure release mainly along shear fractures. Structures observed in 2D cross sections can be related to their expressions on the surface of the 3D respective experiments. Furthermore, the experiments showed that the intrusion network is more complex and laterally extended in the case of a multi-layered cover. Our results can be instructive for detecting and predicting fracture patterns around shallow magmatic and sand intrusions as well as above underground fluid storage sites

Die interne Struktur von Subduktionszonen : das Zusammenspiel geophysikalischer Methoden ergibt ein detailliertes Bild

Geophysical research in subduction zones is based on bathymetric, seismic, magnetic, gravimetric measurement as well as numerical and analog modeling. Their combined interpretation leads to an image of the sub-surface and the dynamic processes related with subduction type and to estimate fluid and mass transfer within the subduction complex. The top of the subducting oceanic plate can be imaged seismically, which could have pracical implications for more precise earthquake hazard analysis in the areas investigate

Morphotectonic and morphometric analysis of the Nazca plate and the adjacent offshore Peruvian continental slope - implications for submarine landscape evolution

We use new swath bathymetry data acquired during the RV Sonne cruise GEOPECO and complement them with swath data from adjacent regions to analyse the morphotectonics of the Peruvian convergent margin. The Nazca plate is not covered with sediments and therefore has a rough surface along the entire Peruvian trench. The styles of roughness differ significantly along the margin with linear morphological features trending in various directions, most of them oblique to the trench and roughness magnitudes of a few to several hundred meters. The lower slope is locally very rough and at the verge of failure throughout the entire Peruvian margin, as a result of subduction erosion causing the lower slope to over-steepen. Using curvature attributes to quantitatively examine the morphology in the Yaquina and Mendaña areas revealed that the latter shows a larger local roughness both seaward and landward of the trench. However, the amplitude of morphological roughness is larger in the Yaquina area. We identified a 125 km2 large slump on the Lima middle slope. Morphometric dating suggests an age of 74500 years within 35 to 40% error. Estimated incision rates on the upper slope are between 0.1 and 0.3 mm per year suggesting that landscape evolution on the Peruvian submarine continental slope is similarly slow than that in the Atacama desert

Upward delamination of Cascadia Basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux

The Cascadia convergent margin is a first-order research target to study the impact of rapid sedimentation processes on the mechanics of frontal subduction zone accretion. The near-trench part of the accretionary prism offshore Washington is affected by strongly increased glacial age sedimentation and fan formation that led to an outstanding Quaternary growth rate with landward vergent thrust faulting that is rarely observed elsewhere in accretionary wedges. Multichannel seismic reflection data acquired on the ORWELL project allows us to study the structure and dynamics of the atypical frontal accretion processes. We performed a kinematical and mechanical analysis of the frontal accretion structures, and developed a dynamic Coulomb-wedge model for the landward-verging backthrust formation. Backthrusting results from heterogeneous diffuse strain accumulation in the mechanically heterogeneous Cascadia basin sediment succession entering the subduction zone, and strain partitioning along a midlevel detachment that is activated by gravitational loading caused by rapid glacial age sedimentation. These complex deformation processes cause the passive “upward” delamination of the upper turbidite beds from the basal pelagic carbonate section similar to triangle-zone formation and passive backthrust wedging in foreland thrust belts caused by rapid burial beneath syntectonic sediment deposits. The deformation mechanism at the tectonic front of the Cascadia margin is an immediate response to the strongly increased late Pleistocene sediment flux rather than to atypical physical boundary conditions as generally thought

Ridge subduction at an erosive margin - the collision zone of the Nazca Ridge in southern Peru

The 1.5-km-high, obliquely subducting Nazca Ridge and its collision zone with the Peruvian margin have been imaged by wide-angle and reflection seismic profiles, swath bathymetry, and gravity surveying. These data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust. The lowermost layer contributes 10–12 km to the total crustal thickness of the ridge. The sedimentary cover is 300–400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges. At the collision zone of ridge and margin, the following observations indicate intense tectonic erosion related to the passage of the ridge. The thin sediment layer on the ridge is completely subducted. The lower continental slope is steep, dipping at ∼9°, and the continental wedge has a high taper of 18°. Tentative correlation of model layers with stratigraphy derived from Ocean Drilling Program Leg 112 cores suggests the presence of Eocene shelf deposits near the trench. Continental basement is located <15 km landward of the trench. Normal faults on the upper slope and shelf indicate extension. A comparison with the Peruvian and northern Chilean forearc systems, currently not affected by ridge subduction, suggests that the passage of the Nazca Ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long-term erosive regime

Klima- und Energierelevanz von Gashydraten: Vortrag im Rahmen des MINT-Festivals Jena

Vortrag für Schülerinnen und Schüler der Sekundarstufe II und die interessierte Öffentlichkeit Gashydrate, eine feste Substanz bestehend aus Wasser und Methan, prägen viele submarine Kontinenthänge und auch Permafrostgebiete. Sie sind nur bei gleichzeitig niedrigen Temperaturen und hohen Drücken stabil. Da sie eine hohe Festigkeit aufweisen, stabilisiert ihre Anwesenheit submarine Hänge. Werden sie instabil, etwa durch Temperaturerhöhungen, können solche Hänge instabil werden und es kann zu großen Hangrutschungen kommen. Freiwerdendes Methan wird meist im Meerwasser gelöst, kann unter einigen Bedingungen aber auch in die Atmosphäre gelangen. Die thermodynamischen Prozesse bei der Gashydratbildung und -zersetzung sind sehr kompliziert und gegenwärtig ein wichtiger Gegenstand der aktuellen Forschung vor allem in den marinen Geowissenschaften