Journal of Structural Geology最新文献

The Sertengshan Range-front Fault (SRF) and Langshan Range-front Fault (LRF) constitute a normal fault system on the northern boundary of the Linhe Depression in its northwest, and an investigation of the LRF–SRF is significant for understanding the seismic activities of normal faults. We set four trenches at three study sites to reveal paleo-earthquakes in the western segment of the SRF (W-SRF) in this system, and established 5 surface rupture events along W-SRF since the Late Pleistocene. By integrating paleoseismic data from 27 trenches on multiple fault segments, we reconstructed the paleoseismic sequence for the LRF–SRF region since 15 ka, and identified 10 paleoseismic events with corresponding rupture segments and magnitudes. The slip rates at the three study sites decreased gradually from the west to east on the W-SRF, by comparing with previous studies, further evidencing that the tectonic activities of the two faults have gradually synchronized. Besides, based on the timing of the latest paleoseismic event (1.88−2.03 ka BP), earthquakes of > M 8 surface ruptures of >200 km can occur in the northwestern Ordos Block and its seismic behavior revealed the seismogenic possibility of normal faults triggering an M > 8 earthquake.

Megaflaps comprise steeply dipping to overturned panels of the oldest suprasalt strata flanking steep diapirs, and represent the roofs of early inflated salt. These large-scale structures result from salt-sediment interaction at minibasin scales and entail multiple kilometres of folding and vertical relief. They are divided into two end-member types (halokinetic and contractional) and form by some combination of limb rotation and kink-band migration. They can be difficult to image and interpret adjacent to flaring diapirs and beneath allochthonous salt due to steep bedding dips and suboptimal illumination.

Using physical models, we investigate halokinetic megaflaps driven by differential loading. Models with vertically-stacked vs. laterally-shifting loading above a prekinematic layer have been run to determine the main processes and mechanisms controlling the growth and kinematic evolution of megaflaps. Parameters such as the thickness of the prekinematic cover, the width of the proto-salt wall, the synkinematic sedimentation rate, and variations in the mechanical properties of the prekinematic cover have been tested to evaluate their role in megaflap generation. The experimental results demonstrate that in absence of tectonic forces, halokinetic megaflaps are generated by a combination of 1) an early increase of pressure-head gradient between two adjacent minibasins with different rates of sedimentation and subsidence, and 2) the disappearance of this gradient that occurs when welding occurs beneath the more quickly subsiding minibasin. The geometry, kinematic evolution, and degree of small-scale deformation of the megaflaps in our analogue models are consistent with both exposed (e.g., Paradox Basin) and seismically imaged halokinetic megaflaps (e.g., deepwater northern Gulf of Mexico).

Tunisia, located in the Northern African margin, was subjected to Late Permian-Early Cretaceous N–S extension subsequent to Pangea breakup and leading to the Tethyan opening and widening. During the Mid- and Late Cretaceous, the NE–SW crustal extension that established the Pantelleria-Malta-Sirt grabens system underlining the eastern margin stretched also the Tunisian Atlassic domain, creating and/or reactivating many NW-SE extensional structures as much as the grabens in central Tunisia and leading to volcanism and halokinesis along several weak zones. Neogene compressional tectonics inverted the earlier extensional structures and impacted most of the halokinetic features established within the post-Triassic cover. The M'Rhila-Labaied-Trozza fault relay zone, located in the middle of central Tunisia, shows both extensional and compressional structures with Triassic extrusive features and seems to be a key feature for understanding the geodynamic development of the area. In this work, we combined field and geophysical data together with analogue models to decipher the structure and the kinematic evolution of the study area. The results showed that the studied structures are associated with reactive Mesozoic salt ridges established along an extensional fault relay zone that utilized NW-SE, E-W and NE-SW inherited fractures. These initially created extensional corridors allowing Triassic evaporite extrusion and accumulation, then influenced the deformation of the area in transpressional and local pure compressional regimes depending on the fractures and stress orientations during tectonic inversion. Analogue guided interpretation of the successive deformational stages of the study area from salt ridge growth to its tectonic inversion.

The Mid-Polish Anticlinorium is a regional structure that formed during Late Cretaceous inversion of the Permian-Mesozoic Polish Basin. Within the anticlinorium, local salt pillows built of the Upper Permian (Zechstein) evaporites are often located above reverse faults that accommodated basement inversion. Seismic data from the Szubin area in central Poland were used to guide a combined analogue and numerical modelling study to test whether locally thicker evaporites deposited within a half-graben, could indeed give rise to a salt pillow formed above the half-graben's hanging wall during its inversion. The results of the two approaches are internally consistent and prove that such a genetic relationship is fully viable, with the most important variables being the size of the half-graben, the viscosity of the salt, and the presence of any erosion of the pillow structure. Thus, the existence of salt pillows along the major inversion zones of the Mid-Polish Anticlinorium could possibly be used as indicators for the location of syn-depositional half-grabens during deposition of the Zechstein evaporites. This in turn might suggest that even in the basin center, shallower areas might have existed above the footwalls of such half-graben during Zechstein deposition, characterized by smaller thicknesses and somewhat different facies arrangement. Similar concepts likely apply to other intracontinental salt basins that experienced rifting and then inversion.

High-angle misorientations can significantly influence material properties. In this study, optical microscopy, Scanning Electron Microscope-Electron Backscatter Diffraction (SEM-EBSD), and Atomic Force Microscopy (AFM) have been used to investigate high-angle misorientations in quartz-bearing crustal rocks. Thin sections of high-grade quartzofeldspathic rocks were subjected to chemical mechanical polishing (CMP) with colloidal silica. In quartz, high-angle misorientations like random high angle grain boundaries (RHAGBs) and Dauphiné twin boundaries (DTBs) could be discriminated using EBSD techniques but not optical microscopy. In nanoscale AFM images, indented channels are observed along RHAGBs but not DTBs; these result from material removal during CMP, indicating lower compactness of RHAGBs compared to DTBs. Along any RHAGB, EBSD reveals different misorientations across segments between consecutive RHAGB-DTB intersections. Grains adjacent to these RHAGB segments have angles between their c-axes varying from 61-66° with parallel {$10\bar{1}2\}$ planes, and 81–84° with parallel {$11\bar{2}2$} planes, respectively. These symmetries represent the Japan and Sardinian twin laws of quartz, indicating that the RHAGB segments become low-energy twin boundaries, thereby reducing the overall surface energy of the aggregate. Finally, these results suggest that apart from surface topography quantification and high-resolution nanoscale imaging, AFM in conjunction with SEM-EBSD can be used for precisely locating sites for TEM study.

The characterization of salt tectonics and its gravity-driven deformation processes are the key to a better understanding of the structural evolution of salt-bearing rifted margins. Unlike most salt basins that have experienced long-lasting deformation, the Messinian evaporites in the Levant Basin have been moderately deformed, offering the opportunity to study the early stage of salt tectonic deformation. Despite the availability of seismic reflection, borehole and bathymetrical data, some uncertainties still exist about the mechanisms responsible for the deformation and structural features observed in the deep-water Levant Basin. Our study includes physical experiments based on published seismic and structural interpretations conducted in the Levant Basin. Our physical experiments take into consideration the main driving parameters that controlled the development of the deep-water Levant Basin, testing the interplay and impact of gravity gliding and spreading from the Levant Margin, gravity spreading from the Nile Deep Sea Fan, and the influence of the passive buttress of the Eratosthenes Seamount. Deformation was imposed by depositing successive sand lobes and/or by tilting the experimental table. The physical models included a thick viscous silicone layer, analogue of the Messinian evaporitic sequence, overlain by a granular overburden, simulating the brittle clastic post-Messinian succession. Results show that the prominent gravity-driven force affecting the deformation pattern of the deep-water Levant Basin is the gravity spreading from the Nile Deep Sea Fan, whereas gravity spreading and gliding from the Levant Margin affect only the proximal to the margin areas. Additionally, the buttressing effect of the Eratosthenes Seamount and the location of the salt basin pinch-out played an important role in the final deformation pattern of this region of the Eastern Mediterranean.

In-situ shattered rocks are often associated with seismogenic fault zones, but their mechanism of formation is still matter of debate, partly because of the limited number of field studies. Here we describe the characteristics of in-situ shattered rocks distribution along the NW-SE-striking seismogenic Monte Marine Fault (MMF) in the Italian Central Apennines. In the studied area, the MMF cuts through Mesozoic carbonates, is exhumed from <3 km depth and consists of two >5 km-long major hard-linked segments with normal kinematics. The linkage between the two fault segments occurs along a ∼2 km-long step-over zone with E-W trending faults and oblique-slip kinematics. To the northwest, fault-related shear deformation is localized in a ∼5 m-thick cataclastic fault core and off-fault deformation is dominated by in-situ shattered rocks up to ∼40 m-thick. Instead, in the step-over zone to the southeast, the in-situ shattered rocks are up to ∼500 m thick, particularly where MMF crosscuts older low-angle thrust faults.

We integrated detailed field structural surveys with microstructural and grain size distribution analyses of the fault rocks to assess the mechanism of (1) formation of in-situ shattered rocks and, (2) progressive localization of shear deformation along the MMF. The obtained results, after the viability of several formation mechanisms (mechanical models) have been reviewed, support the hypothesis that the formation of in-situ shattered rocks was associated with the propagation of (multiple) seismic ruptures (mainshocks and aftershock sequences) within a mechanically heterogeneous fault zone. Heterogeneity is due to the occurrence of preexisting damage related to previous earthquakes, but also inherited from the older low-angle thrust faults. Therefore, we suggest that the origin of these shattered rocks is more compatible with seismic related processes than only with quasi-static fault growth models. On the other hand, the cataclastic fault core derived from the progressive accommodation of shear deformation within the in-situ shattered rock volumes during several seismic cycles. We conclude that the large volumes of in-situ shattered rocks are the result of seismic-related dissipative processes in a geometrically and mechanically heterogeneous fault zone. In this scenario, large volumes of in-situ shattered rocks are compliant low velocity zones which can influence the propagation of earthquake ruptures.

Basement inherited structures represent a significant factor affecting thrust propagation dynamics during the growth of fold-and-thrust belts. In this study inspirited from seismic data analysis of Gaoquan anticline in the Northern Tianshan foreland basin, we devised an experimental approach to investigate the structural and kinematical evolution of deformation from preexisting basement restraining bend to subsequent contractional deformation. Tested parameters included reactivation of the basement restraining bend and erosion. Results indicated that when preexisting basement restraining bend was reactivated and folded an overlying décollement, subsequent thrust nucleated preferentially at the top of the folded décollement. Erosion helped localize deformation, thereby reducing the width of the deformation zone and promoting “out-of-sequence” thrusting during compression. Finally, as we employed silicone polymer to simulate overpressured mudstone layer in the major décollement, our experiments also provide insights into a better understanding of the relationship between shallow salt-detached thrusting and deep inherited basement structures, such as in the Jura Mountains structures.

2D contractional scaled analogue experiments with composite materials including silica-sand and mica-flakes for overburden and silicone for salt analogue are used to investigate effects of mechanical stratigraphy on the structural evolution and kinematics of salt-detached fold-thrust-belts. Specific parameters tested are mechanical stratigraphy of the overburden and thickness variation of the basal silicone layer. The silicone-detached models in general are characterized by low-taper thrust wedge geometries and non-systematic vergence of folds and thrusts. Strain localization in the undeformed layer occurs as an in-sequence foreland breaking sequence. Strain is nucleating as detachment folds including thrust-bounded and concentric folds. Increased shortening develops break-thrusts in fold limbs. In-sequence frontal thrust interacts with out-of-sequence reactivation of older thrusts in the internal thrust wedge. Syn-kinematic silicone mobilization causes diapirism, allochthonous sheets and source-fed thrust. The specific distribution of discordant and allochthonous silicone structures vary with the mechanical stratigraphy. The impact of the mica-interlayer in the overburden sequence is strain-dependent. It strengthens the undeformed sand-pack compared to initial thrusting while active thrusts with mica-flakes in shear zones are weaker and active for longer than in homogeneous sand-pack. The longevity of thrusts correlates with the transfer of silicone to external domains and hanging-walls of thrusts. The silicone thickness controls the strain nucleation modes whether thrusting-dominated or folding-dominated predating main-thrusting stages. It also governs silicone supply and flow regimes with thick silicone source layers being readily remobilized to source-fed thrust and inflate silicone massifs in the foreland.

Insights from the modelling results are that the formation of large-transport source-fed thrusts such as Quele Thrust (China) and Chazuta Thrust (Peru) observed in salt-bearing FTB's is possibly attributed to salt detachment thickness and anisotropic overburden resulting from mechanically layered stratigraphy.

Convergence between African and European plates generates compressional strain, primarily concentrated along the northern African margin. This is testified on the Algerian margin by numerous earthquakes (e.g. Bougrine et al., 2019) and by the presence of active folds and thrusts. Multi-channel seismic reflection profiles from the MARADJA I survey reveal north-verging thrusts rooted below the Messinian units, and the geometries of the Messinian salt structures. This study examines the characteristics of salt tectonics offshore Algiers and Dellys, focusing on the effect of the positive structural inversion of the former passive margin on geometries, timing, and mechanisms of salt deformation. The interpretation of seismic reflection and multi-beam bathymetric data of the MARADJA I survey, along with its comparison with analogue models, allowed us to reconstruct the salt tectonics processes on the margin and to identify the predominant role of a plateau uplift on salt deformation. Early and ubiquitous salt deformation by downbuilding was followed by a major phase of plateau uplift (end of Messinian Crisis), leading to westward gravity gliding and a slowdown of the salt deformation above the plateau. Km-tall salt structures were developed and thick minibasins deposited. Salt tectonics is nowadays active only where the relationship between salt and overburden thickness is favorable.