Rigid Basement Block Research Articles

Tectonostratigraphy, structural geometry and kinematics of the NW Iranian Plateau margin: Insights from the Talesh Mountains, Iran

The structural geometry and kinematic variations along curved fold and thrust belts and their interaction with neighboring rigid basement blocks are not well understood. In this study, we present a structural mapping and stratigraphic analysis of the Talesh Mountains, Iran, to document the timing and evolution of this curved orogen due to collision with the South Caspian Block. NE and SW verging oblique slip thrust faults are the main structural features in the northern and southern parts of the mountains. The N-trending central part of the range is connected to the curved ends of the Talesh Mountains by a system of high angle reverse faults with right lateral strike-slip sense of motion. Restored cross sections indicate greater horizontal shortening (25–30%) in the southern parts which gradually decreases toward the center (13–16%), and then increases to the north (22%). A compilation of stratigraphic sections across the region indicates three major phases of regional deformation in the early Oligocene, middle Miocene and early Pliocene. The middle Miocene phase, in good agreement with other data sets across the Iranian Plateau, is the onset time of deformation for the Talesh Mountains. The regional convergence direction has changed during this major phase of deformation in the NW margins of the Iranian Plateau. The observed kinematic variation along the major faults across the Talesh Mountains is likely resulted from a regional change in the shortening direction in middle Miocene. Variations on the measured Late Cenozoic horizontal shortening across the northern, central and southern parts of the Talesh Mountains resulted from their different orientations with respect to the regional convergent direction as well as interaction with the rigid south Caspian block. These results highlight that the orientation and timing of deformation was strongly influenced by the neighboring Caspian block and regional variations in the convergence direction associated with evolution of the Iranian Plateau.

Tectonic evolution of the Cape and Karoo basins of South Africa

The Cape and Karoo basins formed within the continental interior of Gondwana. Subsidence resulted from the vertical motion of rigid basement blocks and intervening crustal faults. Each basin episode records a three-stage evolution consisting of crustal uplift, fault-controlled subsidence, and long periods of regional subsidence largely unaccompanied by faulting or erosional truncation. The large-scale episodes of subsidence were probably the result of lithospheric deflection due to subduction-driven mantle flow. The early Paleozoic Cape basin records the combined effects of a north-dipping intra-crustal décollement (a late Neoproterozoic suture) and a right-stepping offset between thick Rio de la Plata craton and Namaqua basement. Following the Saldanian orogeny, a suite of small rift basins and their post-rift drape formed at this releasing stepover. Great thicknesses of quartz sandstone (Ordovician–Silurian) and mudstone (Devonian) accumulation are attributed to subsidence by rheological weakening and mantle flow. In contrast, the Karoo basin is a cratonic cover that mimics the underlying basement blocks. The Permian Ecca and lower Beaufort groups were deposited in a southward-deepening ramp syncline by extensional decoupling on the intra-crustal décollement. Reflection seismic and deep-burial diagenetic studies indicate that the Cape orogeny started in the Early Triassic. Deformation was partitioned into basement-involved strike-slip faults and thin-skinned thrusting. Uplift of the Namaqua basement resulted in erosion of the Beaufort cover. East of the Cape fold belt, contemporaneous subsidence and tilting of the Natal basement created a late Karoo transtensional foreland basin, the Stormberg depocentre. Early Jurassic tectonic resetting and continental flood basalts terminated the Karoo basin.

Early rotation and late folding in the Pennsylvania salient (U.S. Appalachians): Evidence from calcite-twinning analysis of Paleozoic carbonates

Calcite-twinning analysis of Paleozoic limestones from 42 sites reveals that the change in regional strike along the frontal edge of the Pennsylvania salient is accompanied by an equal-magnitude rotation of paleostress directions of up to 60 degrees. The rotations, recorded at 22 reliable sites, show no discernible difference between sites with rocks of Cambrian-Ordovician and Silurian-Devonian age. Evidence for similarly fanned orientations is not present in foreland sites. Scatter in the data is attributed to grain-scale rotations and compaction overprinting, as demonstrated in prior studies, and it was reduced by data-cleaning methods as well as by the use of contouring and data-averaging methods. Comparisons of paleostress directions within the belt reveal only minor rotations in the southwest region of the salient, and the bulk of rotation is accommodated by the northern limb. We hypothesize that these rotations resulted from convergence in the thrust wedge against a northerly bounding, rigid basement block around Pennsylvanian times. This created a structural anisotropy in the evolving belt that guided the post-rotational formation of folds in Early Permian times and produced the current pattern of the salient. This model of decoupled thrusting/rotation and folding explains a variety of previously conflicting observations in the area while offering a kinematically consistent scenario that may also apply to other curved orogenic belts.

Geometric and kinematic model for basement-involved backthrusting at Diamante River, southern Andes, Mendoza province, Argentina

At the confluence of the Las Aucas stream with the Diamante River (69°30′W Long.; 34°40′S Lat.), kilometer-scale structures affecting basement rocks were surveyed. Andean deformation involved both Cordillera Principal and Cordillera Frontal rocks, whose structures probably were locally contemporaneous and intimately related. The Cordón del Carrizalito, a huge structure interpreted as a rigid basement block raised by a reverse fault on the eastern edge, which produced intense deformation in the sedimentary cover, can be explained by a geometric and kinematic model for basement-involved structures in the upper crustal levels. The Lomas Bayas structure is considered a west-vergent anticline associated with a backthrust, which branches out from the basement block's eastern edge. A new kinematic quantitative model, based on the Lomas Bayas structure, explains basement-involved backthrusts. Shearing of the basement rocks produced progressive rotation of the backthrust's hangingwall block, whose upper toe is bent but sedimentary cover is delaminated and detached. These retrodeformable models are applied to two cross-sections to test their viability and calculate the structural shortening.

Cretaceous collision and indentation in the West Carpathians: View based on structural analysis and numerical modeling

A model of indentation of a rigid promontory into weak metasedimentary rocks during Cretaceous convergence is suggested for tectonic evolution of southern part of West Carpathians. An early arcuate cleavage fan has developed in front of northward moving southern rigid basement block. The interaction of the moving indenter with western stationary basement promontory is responsible for development of the boundary‐parallel shear zone along which the main southern indenter is shifted to the east. This results in development of a new steep transpressional cleavage overprinting the early fabric. Eastward displacement of the southern indenter causes the development of a thrust zone parallel to the margin of the eastern stationary promontory. A proposed numerical model of the deformation of a thin viscous sheet in front of oval rigid indenter reliably simulates the development of the observed deformation pattern. Modeled discrete partitioning between the western promontory and the indenting block fully agrees with the observed secondary cleavage associated with the transpressional shear zone. Our numerical model interconnects this complex kinematic frame with finite strain pattern, which was to date possible only for simple boundary conditions. In addition, the model explains the polyphase cleavage patterns in terms of complex shapes of promontories and changes in movements of indenting blocks.

Late Cenozoic deformation in the South Caspian region: effects of a rigid basement block within a collision zone

Active deformation in the South Caspian region demonstrates the enormous variation in kinematics and structural style generated where a rigid basement block lies within a collision zone. Rigid basement to the South Caspian Basin moves with a westward component relative both to stable Eurasia and Iran, and is beginning to subduct at its northern and western margins. This motion is oblique to the approximately north–south Arabia–Eurasia convergence, and causes oblique shortening to the south and northeast of the South Caspian Basin: thrusting in the Alborz and Kopet Dagh is accompanied by range-parallel strike–slip faults, which are respectively left- and right-lateral. There are also arcuate fold and thrust belts in the region, for two principal reasons. Firstly, weaker regions deform and wrap around the rigid block. This occurs at the curved transition zone between the Alborz and Talysh ranges, where thrust traces are concave towards the foreland. Secondly, a curved fold and thrust belt can link a deformation zone created by movement of the basement block to one created by the regional convergence: west-to-east thrusts in the eastern Talysh represent underthrusting of the South Caspian basement, but pass via an arcuate fan of fold trains into SSW-directed thrusts in the eastern Greater Caucasus, which accommodates part of the Arabia–Eurasia convergence. Each part of the South Caspian region contains one or more detachment levels, which vary dependent on the pre-Pliocene geology. Buckle folds in the South Caspian Basin are detached from older rocks on thick mid-Tertiary mudrocks, whereas thrust sheets in the eastern Greater Caucasus detach on Mesozoic horizons. In the future, the South Caspian basement may be largely eliminated by subduction, leading to a situation similar to Archaean greenstone belts of interthrust mafic and sedimentary slices surrounded by the roots of mountain ranges constructed from continental crust.

Mechanical analysis of the geometry of forced-folds

A mechanical model of forced-folding, comprised of an anisotropic cover overlying displaced, rigid basement blocks, is used to investigate the influence of various parameters on theoretical fold form: shape and dip of basement fault, strength of basement-cover contact, and degree of anisotropy of the cover. We show that the degree of anisotropy in the cover largely influences the geometry of the forelimb of the forced-fold. Folds produced in isotropic cover display forelimbs that taper from large dip angles near the basement-cover contact to low dip angles at the ground surface. In contrast, dips in the forelimbs of folds in anisotropic cover are nearly uniform with depth. We show that the basement-cover contact and the shape of the basement fault largely influence the geometry of the backlimb. Backlimb rotation occurs in cover welded to the basement and in cover underlying curved basement faults. In addition, the kinematic features of the theoretical folds are compared with the fold geometry generated by parallel kink and trishear models. Folds in isotropic cover overlying straight basement faults closely resemble the fold forms produced by the trishear kinematic model while fold forms in anisotropic cover more closely resemble folds produced by parallel kink geometric constructions.

Mechanical models of trishear-like folds

Previous workers have formulated velocity descriptions of the trishear kinematic model of fault propagation-folds, which are inherently non-unique. We present two complete mechanical models of fault-related folding and assess the validity of the assumptions used in the assignment of velocities in the trishear description and to eliminate the untenable situation of an infinite number of possible solutions for velocity fields. The mechanical model of forced-folding, Forced Fold, based on viscous folding theory, is used to derive the velocity fields in an anisotropic sedimentary cover overlying faulted and displaced rigid basement blocks. The solution of displacements around a stress-free fault in an elastic body is used to model fault-arrest folds that form around a fault imbedded in a deformed medium. The results indicate that the velocity fields assumed in the trishear model more closely resemble the velocities derived in the mechanical Forced Fold model than the mechanical model of fault-arrest folding. The Forced Fold model produces a triangular region of concentrated deformation similar to the trishear region assumed in the kinematic models, while the deformation produced by the fault-arrest model is not concentrated within a triangular zone.

Pliocene and Pleistocene stratigraphy and tectonics, Ohara Depression and Wakarara Range, North Island, New Zealand

Abstract The Ohara Depression ‐ Wakarara region is situated at the arcward margin of the forearc basin near Hawke's Bay on the North Island of New Zealand. Deformation of late Pliocene and Pleistocene sediments in approximately the last 2.5 Ma is a product of oblique convergence of the Australian and Pacific plates. The region lies directly to the east of Mesozoic greywacke of the axial ranges of the North Island, and deformation reflects the interaction of this rigid backstop with the inboard part of the forearc. Deformation has resulted in the differential movement of several relatively rigid basement blocks. A veneer of late Neogene and Quaternary sediments on these blocks records the timing and style of deformation. An important observation derived from the Quaternary tectonic record is that strain is partitioned between two northeast‐trending domains that parallel the margin. The western domain that abuts the axial ranges is a domain of strike‐slip faulting. Directly east of the strike‐slip domain, a...

Metallogeny of the northeastern Kibaran belt, Central Africa

The evolution of the intracontinental Kibaran beit spans the period from about 1400 to 950 Ma. Its earlier history involved the deposition of a very thick psammitic–pelitic sedimentary pile with some intercalated volcanic rocks, which were folded around 1200 Ma ago. This deformation created essentially upright structures, which were controlled by rigid basement blocks. Metamorphism related to igneous granite intrusions were more intense in anticlinal structures. From about 1000–950 Ma a renewed compressive phase equivalent to the Lomanian orogeny was associated with the emplacement of LIL-element enriched ‘tin’ granites. They formed sheets at basement/sediment and granite/sediment contacts, with cupolas at tectonic highs. Pegmatites and quartz veins with Sn, W, Nb/Ta, Be and Li are derived from the ‘tin’ granites. Generally, this mineralization shows a pronounced structural control with a clear symmetry relation to N–S trending folds and thrusts. Gold-rich quartz veins occurring in districts marginal to the Sn–W mineralization, are a distal and lower temperature phase. Mafic–ultramafic layered intrusions with Ni, Co, Cu, platinum-group metals and Fe/Ti deposits form a belt parallel to the eastern margin of the Kibaran towards the Tanzanian craton. These rocks may be related to the Bukoban/Malagarasian basalts with an age of 1000–850 Ma. Subvolcanic alkaline and carbonatite complexes which were the source for small Ree deposits are clearly post-Kibaran in age. Mineralization.

Tectonic Evolution of Val Verde-Delaware Basin, Texas: ABSTRACT

The Val Verde-Delaware basin is a marginal foreland basin genetically related to the Ouachita overthrust belt and its associated subduction zone. Proximal structures to the overthrust belt evidence compressional folding and faulting indicative of a horizontal maximum principal stress, but the dominant principal stress of the more distal and productive structures is vertical. A similar tectonic style is evidenced for the Permian basin as a whole. Isopachs indicate that structural growth began early and continued intermittently throughout the Paleozoic. Maximum End_Page 420------------------------------ instability occurred during the Mississippian, Pennsylvanian, and Wolfcampian with concurrent uplift of the producing structures and subsidence of the intervening grabens. The period of instability is coincident with the period of maximum activity along the Ouachita subduction zone. Thermal and isostatic activity related to the subduction zone may have caused the differential vertical uplift and subsidence. The stress system appears to be caused by fluid movements in the crust or subcrust. As lighter material was subduced to mantle depths there was some partial melting and diapiric rise of these lighter materials. The complete process is not fully understood. Rigid basement blocks were tilted and uplifted along basement faults. The overlying sediment behaved plastically, and basement faults die out abruptly upward in the section. Faulting is rarely found in the borehole, but steep to overturned beds are common. Minor faulting in the producing fields appears to have served as a conduit for rising hydrothermal fluids which cause some secondary cementation and loss of porosity and permeability. This has caused some structurally well-located wells to be dry or noncommercial. The tectonic style of the Val Verde-Delaware basin is similar to that of many foreland basins throughout the world. End_of_Article - Last_Page 421------------