Role Of Pre-existing Structures Research Articles

Analogue model of the Bala Rud Fault, Zagros: an oblique basement ramp in a fold-and-thrust belt

The role of pre-existing structures during the building of the Zagros fold-and-thrust belt is a matter of debate. The orientation of the pre-existing basement faults varies in different parts of the Zagros belt. Major directions of the basement faulting in the Arabian plate and Zagros are N–S, NE–SW, NW–SE and E–W. Reactivation of these inherited pre-existing faults occurred at the Cretaceous. In this study, analogue modelling was constructed to test the effect of various orientations of the pre-existing basement faults on the deformation style in the northern boundary of Dezful Embayment. The model consisted of sand layers that partly overlay a viscous layer of silicon above wooden plates. Model results explain the development of deflection in the deformation front and the incompatibility location of faults in the basement and surface. The presence of the basal decollement horizon between the basement and overlying sedimentary sequences separated faulting geometry in the basement from sedimentary cover. Model results suggest that the flexure of the Zagros Mountain Front Faults (ZMFF) in the north part of Dezful Embayment is resulted by reactivation of pre-existing Bala Rud fault (BRF) which is an oblique ramp. The close similarity between the major structures produced during the analogue experiments and those observed in nature, suggests that BRF as a pre-existing basement fault has a major role in the deformation of the structures in the study area.

Role of multiple inherited basement structures on orogen geometry and evolution: Insights from analogue modelling

The role of inherited weaknesses in metamorphic basements with respect to the geometry and tectonic evolution of subsequent orogens formation forms a cornerstone of structural reconstructions, especially in non-inverted orogens with old Variscan basements. In this study, we analyse the geometry, kinematics, and tectonic evolution of the complex western termination of the Alpine Pyrenean–Cantabrian orogen using an integrated approach based on analogue modelling, constrained by structural mapping and geochronology, to provide new insights into the role of pre-existing structures and the shift in the orientation of the stress field. Our results demonstrate that pre-existing structures are necessary to explain the complex western termination, which does not follow the same tectonic architecture as the rest of the orogen. The reason for this is the different orientation of the pre-existing Variscan structures in this area, which do not follow the general E–W trend. These western structures (including NNE–SSW and NW–SE strike-slip fault systems and a greater number of E–W thrusts) accommodate the shortening, but have a less favourable orientation. The reactivation of the NNE–SSW strike-slip faults and the southern E–W thrusts in the recent stages indicates a late intraplate tectonic regime, mainly following the shift of the Shmax (from N–S to NNW–SSE) which occurred when the convergence between the Iberian, Eurasian, and African plates changed.

Late Carboniferous dextral transpressional reactivation of the crustal-scale Walls Boundary Fault, Shetland: the role of pre-existing structures and lithological heterogeneities

The Walls Boundary Fault in Shetland, Scotland, formed during the Ordovician–Devonian Caledonian orogeny and underwent dextral reactivation in the Late Carboniferous. In a well-exposed section at Ollaberry, westerly verging, gently plunging regional folds in the Neoproterozoic Queyfirth Group on the western side of the Walls Boundary Fault are overprinted by faults and steeply plunging Z-shaped brittle–ductile folds that indicate contemporaneous right-lateral and top-to-the-west reverse displacement. East of the Walls Boundary Fault, the Early Silurian Graven granodiorite complex exhibits fault-parallel fractures with Riedel, P and conjugate shears indicating north–south-striking dextral deformation and an additional contemporaneous component of east–west shortening. In the Queyfirth Group, the structures are arranged in geometrically and kinematically distinct fault-bounded domains that are interpreted to result from two superimposed tectonic events, the youngest of which displays evidence for bulk dextral transpressional strain partitioning into end-member wrench and contractional strain domains. During dextral transpressional deformation, strain was focused into pelite horizons and favourably aligned pre-existing structures, leaving relicts of older deformation in more competent lithologies. This study highlights the importance of pre-existing structures and lithological heterogeneity during reactivation and suggests the development of a regional transpressional tectonic environment during the Late Carboniferous on the Shetland Platform.

Structural Control of Ore Deposits: The Role of Pre-Existing Structures on the Formation of Mineralised Vein Systems

The major role played by pre-existing structures in the formation of vein-style mineral deposits is demonstrated with several examples. The control of a pre-existing decollement level on the formation of a crustal extension-related (collapse) gold deposit is first illustrated in the Quadrilátero Ferrífero from Brazil. Shear zone and decollement structures were also examined and shown to control veins formation by three distinct processes: (i) re-aperture and re-using of wrench shear zones in the case of Shila gold mines (south Peru); (ii) remobilisation of metal in volcanic-hosted massive sulphide (VHMS) deposit by subsequent tectonic events and formation of a secondary stockwork controlled by structures created during this event (Iberian Pyrite Belt, Spain); (iii) formation of economic stockwork by contrasting deformation behaviours between ductile black schist versus brittle more competent dolomite (Cu-Ifri deposit, Morocco). Two examples involve changing of rheological competence within zones affected by deformation and/or alteration in order to receive the mineralisation (case studies of Achmmach, Morocco, and Mina Soriana, Spain). The last case underscores the significance of the magmatic–hydrothermal transition in the formation of mesothermal gold deposits (Bruès mine, Spain). All these examples clearly demonstrate the crucial role played by previously formed structures and/or texture in the development and formation of ore deposits.

Crustal structure of Baffin Bay from constrained three-dimensional gravity inversion and deformable plate tectonic models

Mesozoic to Cenozoic continental rifting, breakup and spreading between North America and Greenland led to the opening, from south to north, of the Labrador Sea and eventually Baffin Bay between Baffin Island, northeast Canada and northwest Greenland. Baffin Bay lies at the northern limit of this extinct rift, transform and spreading system and remains largely underexplored. With the sparsity of existing crustal-scale geophysical investigations of Baffin Bay, regional potential field methods and quantitative deformation assessments based on plate reconstructions provide two means of examining Baffin Bay at the regional scale and drawing conclusions about its crustal structure, its rifting history and the role of pre-existing structures in its evolution. Despite the identification of extinct spreading axes and fracture zones based on gravity data, insights into the nature and structure of the underlying crust have only been gleaned from limited deep seismic experiments, mostly concentrated in the north and east where the continental shelf is shallower and wider. Baffin Bay is partially underlain by oceanic crust with zones of variable width of extended continental crust along its margins. 3-D gravity inversions, constrained by bathymetric and depth to basement constraints, have generated a range of 3-D crustal density models that collectively reveal an asymmetric distribution of extended continental crust, approximately 25–30 km thick, along the margins of Baffin Bay, with a wider zone on the Greenland margin. A zone of 5–13 km thick crust lies at the centre of Baffin Bay, with the thinnest crust (5 km thick) clearly aligning with Eocene spreading centres. The resolved crustal thicknesses are generally in agreement with available seismic constraints, with discrepancies mostly corresponding to zones of higher density lower crust along the Greenland margin and Nares Strait. Deformation modelling from independent plate reconstructions using GPlates of the rifted margins of Baffin Bay was performed to gauge the influence of original crustal thickness and the width of the deformation zone on the crustal thicknesses obtained from the gravity inversions. These results show the best match with the results from the gravity inversions for an original unstretched crustal thickness of 34–36 km, consistent with present-day crustal thicknesses derived from teleseismic studies beyond the likely continentward limits of rifting around the margins of Baffin Bay. The width of the deformation zone has only a minimal influence on the modelled crustal thicknesses if the zone is of sufficient width that edge effects do not interfere with the main modelled domain.

Role of pre-existing structures in controlling the Cenozoic tectonic evolution of the eastern Tibetan plateau: New insights from analogue experiments

Pre-existing weakness due to repeated tectonic, metamorphic, and magmatic events is a fundamental feature of the continental lithosphere on Earth. Because of this, continental deformation results from a combined effect of boundary conditions imposed by plate tectonic processes and heterogeneous and anisotropic mechanical strength inherited from protracted continental evolution. In this study, we assess how this interaction may have controlled the Cenozoic evolution of the eastern Tibetan plateau during the India–Asia collision. Specifically, we use analogue models to evaluate how the pre-Cenozoic structures may have controlled the location, orientation, and kinematics of the northwest-striking Xianshuihe and northeast-striking Longmen Shan fault zones, the two most dominant Cenozoic structures in eastern Tibet. Our best model indicates that the correct location, trend, and kinematics of the two fault systems can only be generated and maintained if the following conditions are met: (1) the northern part of the Songpan–Ganzi terrane in eastern Tibet has a strong basement whereas its southern part has a weak basement, (2) the northern strong basement consists of two pieces bounded by a crustal-scale weak zone that is expressed by the Triassic development of a northwest-trending antiform exposing middle and lower crustal rocks, and (3) the region was under persistent northeast–southwest compression since ∼35 Ma. Our model makes correct prediction on the sequence of deformation in eastern Tibet; the Longmen Shan right-slip transpressional zone was initiated first as an instantaneous response to the northeast–southwest compression, which is followed by the formation of the Xianshuihe fault about a half way after the exertion of northeast–southwest shortening in the model. The success of our model highlights the importance of pre-existing weakness, a key factor that has been largely neglected in the current geodynamic models of continental deformation.

Emplacement of shallow dikes and sills beneath a small basaltic volcanic center – The role of pre-existing structure (Paiute Ridge, southern Nevada, USA)

Late Miocene sills and dikes in the Paiute Ridge area of southern Nevada were emplaced in an extensional setting beneath a small volume, alkali basaltic volcanic center. Dikes (400–5000 m long, 1.2–9 m wide) mostly occupy pre-existing E-dipping normal faults. Elastic deformation of the wall rocks alone cannot explain dike dimensions; inelastic deformation, wall rock erosion by flowing magma, and syn-emplacement extension of the host structural system also contributed to dike widths. After primarily subvertical emplacement, flow focused toward the southern end of one of the dikes to form a volcanic conduit. This dike and a fault-hosted radial dike subsequently were subject to high pressures due to transient volcanic processes. Three small sills (extending laterally up to ∼ 500 m, and 20–46 m thick) and two larger sills (each having lateral dimensions ∼ 1 km) locally branch off some dikes within ∼ 250 m of the paleosurface. Individual small sills extend only into the hanging wall blocks of the faults that host their parent dikes, and are connected to the dikes by stems that are only a few tens of meters wide; elsewhere along their strikes the parent dikes extend above the sills. This mode of sill emplacement was caused by local rotation of principal stresses related to the intersection of the dike-hosting fault planes with the complex contact between relatively strong Paleozoic carbonates and weak Tertiary tuffs. Orientation of bedding planes in the tuffs controlled the direction of sill propagation. The three most areally extensive sills formed lopoliths with sagging roofs, indicating interaction with the free surface.

The Role of Pre-existing Structures in the Origin, Propagation and Architecture of Faults in the Main Ethiopian Rift

Four major fault systems oriented N–S to NNE–SSW, NE–SW, E–W and NW–SE are identified from Landsat Thematic Mapper (TM) images and a high resolution digital elevation model (DEM) over the Ethiopian Rift Valley and the surrounding plateaus. Most of these faults are the result of Cenozoic - extensional reactivation of pre-existing basement structures. These faults interacted with each other at different geological times under different geodynamic conditions. The Cenozoic interaction under an extensional tectonic regime is the major cause of the actual volcano-tectonic landscape in Ethiopia. The Wonji Fault Belt (WFB), which comprises the N–S to NNE–SSW striking rift floor faults, displays peculiar propagation patterns mainly due to interaction with the other fault systems and the influence of underlying basement structures. The commonly observed patterns are: curvilinear oblique-slip faults forming lip-horsts, sinusoidal faults, intersecting faults and locally splaying faults at their ends. Fault-related open structures such as tail-cracks, releasing bends and extensional relay zones and fault intersections have served as principal eruption sites for monogenetic Plio-Quaternary volcanoes in the Main Ethiopian Rift (MER).

Upper Rhine Graben: Role of preexisting structures during rift evolution

The evolution of the Cenozoic Upper Rhine Graben was controlled by a repeatedly changing stress field and the reactivation of a complex set of crustal discontinuities that had come into evidence during Permo‐Carboniferous times. A comparison of the spatial and temporal thickness distribution of synrift deposits with preexisting fault patterns permits to infer a sequence of distinct basin subsidence phases that can be related to changes in the ambient stress field. Reactivation of a system of late Palaeozoic fault systems, outlining troughs and highs, controlled the nucleation of initially separated middle and late Eocene basins, the depocenters of which coincided with a preexisting WSW‐ENE trend. During Oligocene crustal extension the individual basins coalesced, resulting in the development of the SSW‐NNE striking Upper Rhine Graben. During the late Oligocene (Chattian) change in stress field, the Upper Rhine Graben was probably reactivated as a dextral strike‐slip system with the central graben segment forming a releasing bend. During the early Miocene (Aquitanian), a major reorientation of the regional stress field is held responsible for the main subsidence phase of the northern parts of the Upper Rhine Graben. This is reflected by a counterclockwise rotation and northeastward shift of the depocenter axis and later by the middle Miocene uplift and erosion of the southern parts of the Upper Rhine Graben. During the Plio‐Quaternary, the Upper Rhine Graben was reactivated as a sinistral strike‐slip system with the central graben segment forming a restraining bend.