Seismic expression of shear zones: Insights from 2-D point-spread-function based convolution modelling

Shear zones are common strain localization structures in the middle and lower crust and play a major role during orogeny, transcurrent movements and rifting alike. Our understanding of crustal deformation depends on our ability to recognize and map shear zones in the subsurface, yet the exact signatures of shear zones in seismic reflection data are not well constrained. To advance our understanding, we simulate how three outcrop examples of shear zones (Holsnøy - Norway, Cap de Creus - Spain, Borborema - Brazil) would look in different types of seismic reflection data using 2-D point-spread-function (PSF)-based convolution modelling, where PSF is the elementary response of diffraction points in seismic imaging. We explore how geological properties (e.g. shear zone size and dip) and imaging effects (e.g. frequency, resolution, illumination) control the seismic signatures of shear zones. Our models show three consistent seismic characteristics of shear zones: (1) multiple, inclined reflections, (2) converging reflections, and (3) cross-cutting reflections that can help interpreters recognize these structures with confidence.publishedVersio

From Caledonian collapse to North Sea Rift: The extended history of a metamorphic core complex

Extensional systems evolve through different stages due to changes in the rheological state of the lithosphere. It is crucial to distinguish ductile structures formed before and during rifting, as both cases have important but contrasting bearings on the structural evolution. To address this issue, we present the illustrative ductile‐to‐brittle structural history of a metamorphic core complex (MCC) onshore and offshore western Norway. Combining geological field mapping with newly acquired 3‐D seismic reflection data, we correlate two distinct onshore basement units (BU1 and BU2) to corresponding offshore basement seismic facies (SF1 and SF2). Our interpretation reveals two 40 km wide domes (one onshore and one offshore), which both show characteristic kilometer‐scale, westward plunging upright folds. The gneiss domes fill antiformal culminations in the footwall of a >100 km long, shallowly west dipping, extensional detachment. Overlying Caledonian nappes and Devonian supradetachment basins occupy saddles of the hyperbolic detachment surface. Devonian collapse of the Caledonian orogen formed dome and detachment geometries. During North Sea rifting, brittle reactivation of the MCC resulted in complex fault patterns deviating from N‐S strike dominant at the eastern margin of the rift. Around 61°N, only minor N‐S faults (<100 m throw) cut through the core of the MCC. Major rift faults (≤5 km throw), on the other hand, reactivated the detachment and follow the steep flanks of the MCC. This highlights that inherited ductile structures can locally alter the orientation of brittle faults formed during rifting.publishedVersio

Rift kinematics preserved in deep-time erosional landscape below the northern North Sea

Our understanding of continental rifting is, in large parts, derived from the stratigraphic record. This record is, however, incomplete as it does not often capture the geomorphic and erosional signal of rifting. New 3D seismic reflection data reveal a Late Permian-Early Triassic landscape incised into the pre-rift basement of the northern North Sea. This landscape, which covers at least 542 km2, preserves a drainage system bound by two major tectonic faults. A quantitative geomorphic analysis of the drainage system reveals 68 catchments, with channel steepness and knickpoint analysis of catchment-hosted palaeo-rivers showing that the landscape preserved a >2 Myr long period of transient tectonics. We interpret that this landscape records a punctuated uplift of the footwall of a major rift-related normal fault (Vette Fault) at the onset of rifting. The landscape was preserved by a combination of relatively rapid subsidence in the hangingwall of a younger fault (Øygarden Fault) and burial by post-incision sediments. As such, we show how and why erosional landscapes are preserved in the stratigraphic record, and how they can help us understand the tectono-stratigraphic evolution of ancient continental rifts.publishedVersio

Silica diagenesis and physical properties of Cenozoic rocks in the North Viking Graben, northern North Sea

Silica diagenesis has the potential to drastically change the physical and fluid flow properties of the host strata and may therefore play a key role in the development of sedimentary basins. To investigate the role of silica diagenesis in the North Viking Graben, northern North Sea, mineralogical-, well- and 3-D seismic data are combined in this study. Optical microscopy, scanning electron microscopy and X-ray diffraction are used to identify the opal-A/CT transformation in the Cenozoic succession of the North Viking Graben, northern North Sea. The effect of the opal-A/CT transformation on the host rock properties is investigated by combining quantitative mineralogical data obtained by X-ray diffraction with wireline data of sixteen exploration wells using multiple linear regression analysis. The analysis shows that opal-A content explains host rock porosity to a large extent and opal-CT and pyrite content explain host rock porosity to a lesser degree. The overall decline in opal-A content with depth is interpreted to reflect increasing biogenic silica production between the Eocene and Miocene. Focussed reductions in opal-A content that coincide with increased opal-CT contents are probably the result of opal-A/CT transformation. Because the observed opal-A/CT transformation does not coincide with major lithology variations, it is assumed that the transformation is primarily a function of time and temperature. This assumption allows the spatial and temporal evolution of the opal-A/CT transformation to be modelled. Modelling results indicate that opal-A/CT transformation probably started in the Balder Formation in the Middle-to-Late Eocene, migrated upwards through the lower Hordaland Group, and fossilised as a result of Middle-Miocene sea-level fall and erosion. Based on the basin modelling results and a detailed fault analysis, silica diagenesis could have led to the nucleation and growth of the polygonal faults in the North Viking Graben. This study highlights that silica diagenesis is a complex process that can significantly impact compaction and deformation of siliceous sedimentary successions.Open Acces

SHIMURA CURVES OVER FINITE FIELDS AND THEIR RATIONAL POINTS(Algebraic Number Theory and Related Topics)

Layer-bound, low-displacement normal faults, arranged into a broadly polygonal pattern, are common in many sedimentary basins. Despite having constrained their gross geometry, we have a relatively poor understanding of the processes controlling the nucleation and growth (i.e., the kinematics) of polygonal fault systems. In this study we use high-resolution 3-D seismic reflection and borehole data from the northern North Sea to undertake a detailed kinematic analysis of faults forming part of a seismically well-imaged polygonal fault system hosted within the up to 1,000 m thick, Early Palaeocene-to-Middle Miocene mudstones of the Hordaland Group. Growth strata and displacement-depth profiles indicate faulting commenced during the Eocene to early Oligocene, with reactivation possibly occurring in the late Oligocene to middle Miocene. Mapping the position of displacement maxima on 137 polygonal faults suggests that the majority (64%) nucleated in the lower 500 m of the Hordaland Group. The uniform distribution of polygonal fault strikes in the area indicates that nucleation and growth were not driven by gravity or far-field tectonic extension as has previously been suggested. Instead, fault growth was likely facilitated by low coefficients of residual friction on existing slip surfaces, and probably involved significant layer-parallel contraction (strains of 0.01–0.19) of the host strata. To summarize, our kinematic analysis provides new insights into the spatial and temporal evolution of polygonal fault systems

Evolution of Rift Systems and Their Fault Networks in Response to Surface Processes

Continental rifting is responsible for the generation of major sedimentary basins, both during rift inception and during the formation of rifted continental margins. Geophysical and field studies revealed that rifts feature complex networks of normal faults but the factors controlling fault network properties and their evolution are still matter of debate. Here, we employ high‐resolution 2D geodynamic models (ASPECT) including two‐way coupling to a surface processes (SP) code (FastScape) to conduct 12 models of major rift types that are exposed to various degrees of erosion and sedimentation. We further present a novel quantitative fault analysis toolbox (Fatbox), which allows us to isolate fault growth patterns, the number of faults, and their length and displacement throughout rift history. Our analysis reveals that rift fault networks may evolve through five major phases: (a) distributed deformation and coalescence, (b) fault system growth, (c) fault system decline and basinward localization, (d) rift migration, and (e) breakup. These phases can be correlated to distinct rifted margin domains. Models of asymmetric rifting suggest rift migration is facilitated through both ductile and brittle deformation within a weak exhumation channel that rotates subhorizontally and remains active at low angles. In sedimentation‐starved settings, this channel satisfies the conditions for serpentinization. We find that SP are not only able to enhance strain localization and to increase fault longevity but that they also reduce the total length of the fault system, prolong rift phases and delay continental breakup.Plain Language Summary: Continental rifting is responsible for breaking apart continents and forming new oceans. Rifts generally evolve according to three types: wide rift, symmetric rift, and asymmetric rifts, which also shape the final geometry of the continental rifted margin. Geophysical data shows that the evolution of rifts depends on a multitude of factors including the complex interactions between fault networks that accommodate extension and the processes of erosion and sediment deposition. Here we run 2D computer simulations to investigate fault network evolution during active rifting that include changes to the surface through erosion and sedimentation. By using a new python tool box, we extract the fault network from the simulation and determine individual fault properties like the number of faults, displacement, age, and length through time. We find that regardless of the rift type, rifts evolve according to five phases that can be assessed through the evolution of the fault network properties. Additionally, we find that greater erosion and sedimentation can prolong rift phases and delay the breakup of continents.Key Points: We apply a new fault analysis toolbox to coupled numerical models of tectonics and surface processes. Fault network evolution of the major symmetric, asymmetric, narrow, and wide rift types can be described in five distinct phases. Surface processes reduce fault network complexity and delay breakup by enhancing strain localization and increasing fault longevity.Helmholtz Young InvestigatorsNational Science FoundationDeutsche Forschungsgemeinschaft (DFG)https://doi.org/10.5281/zenodo.575314

Seismic expression of shear zones: Insights from 2-D point-spread-function based convolution modelling

Shear zones are common strain localization structures in the middle and lower crust and play a major role during orogeny, transcurrent movements and rifting alike. Our understanding of crustal deformation depends on our ability to recognize and map shear zones in the subsurface, yet the exact signatures of shear zones in seismic reflection data are not well constrained. To advance our understanding, we simulate how three outcrop examples of shear zones (Holsnøy - Norway, Cap de Creus - Spain, Borborema - Brazil) would look in different types of seismic reflection data using 2-D point-spread-function (PSF)-based convolution modelling, where PSF is the elementary response of diffraction points in seismic imaging. We explore how geological properties (e.g. shear zone size and dip) and imaging effects (e.g. frequency, resolution, illumination) control the seismic signatures of shear zones. Our models show three consistent seismic characteristics of shear zones: (1) multiple, inclined reflections, (2) converging reflections, and (3) cross-cutting reflections that can help interpreters recognize these structures with confidence

Seismic expression of shear zones: insights from 2-D convolution seismic modelling

During extension, compression or strike-slip motion, shear zones accommodate large amounts of strain in the crust. Our understanding of these processes critically depends on our ability to recognize shear zones in the subsurface. The exact signature of shear zones in seismic reflection data is however not well understood. To advance our understanding, we simulate how three outcrop examples of shear zones (Holsnøy, Cap de Creus, Borborema) would look in different types of seismic reflection data using 2-D convolution seismic modelling. We explore how geophysical effects (e.g. frequency, illumination) and geological properties (e.g. shear zone dip and aspect ratio) affect the seismic signature of shear zones. Our models show three consistent seismic characteristics of shear zones: (1) multiple, inclined reflections, (2) converging reflections and (3) cross-cutting reflections that can help interpreters recognizing these structures with confidence