Latest Deformation Phase Research Articles

Reconstructing the Evolution of Foreland Fold‐And‐Thrust Belts Using U‐Pb Calcite Dating: An Integrated Case‐Study From the Easternmost Jura Mountains (Switzerland)

AbstractThis case‐study from the Jura Mountains in the foreland of the European Alps demonstrates how the coupling of subsurface analysis and U‐Pb carbonate dating can provide absolute timing constraints and shortening rate estimates of fold‐and‐thrust belts. It is confirmed that the initial Late Cenozoic foreland deformation driving the formation of the easternmost Jura Mountains in Switzerland was predominately thin‐skinned with contractional deformation largely restricted to the Mesozoic succession above a sub‐horizontal basal décollement. Thereby, the localization and structural style of related deformation structures was strongly guided by the characteristics of underlying Late Paleozoic half grabens. The main thin‐skinned thrust front formed at ∼12 Ma, followed by further deformation in the hinterland and locally continued foreland‐directed thrust propagation. The major deformation zones exposed at surface were established at ∼8 Ma but shortening continued until at least ∼4 Ma. Thick‐skinned contraction associated with the inversion of basement structures only played a subordinate role during the latest deformation phase after 8 Ma. Based on cumulative shortening values derived from balanced cross sections, our U‐Pb ages of syn‐tectonic calcite slickenfibres allow to estimate thin‐skinned deformation rates for the easternmost Jura Mountains between ∼0.9 and ∼0.1 mm/year, decreasing toward the eastern tip of the arcuate belt. Moreover, deformation rates seemingly decreased over time with rates of initial thin‐skinned thrusting being significantly higher than the later deformation north of the main thrust front. These new findings from a classical foreland setting highlight the potential of integrating U‐Pb dating in regional fold‐and‐thrust belt investigations elsewhere.

Selective reactivation of inherited fault zones driven by stress field changes: Insights from structural and paleostress analysis of the Pocuro Fault Zone, Southern Central Andes (32.8°S)

This study aims to explain the selective reactivation of normal faults during the Andean orogeny at the Southern Central Andes western flank. We conducted a structural mapping and paleostress field reconstruction in the regional-scale Pocuro Fault Zone (PFZ) at 32.8°S. Results reveal that the architecture of the PFZ results from at least two deformation phases, each revealing an individual progressive and gradual evolution. The earliest deformation phase is recorded by two ∼ NS-striking normal faults involving a 5 km wide damage zone characterized by quartz-laumontite and calcite veins that were developed under an extensional regime with a WNW-ESE-trending σ3-axis. The latest deformation phase is recorded by one NS-striking reverse-dextral fault with goethite-hematite syn-tectonic precipitation and two NW-striking reverse-sinistral faults. Reverse faults were developed under a compressional/transpressional regime characterised by an ENE-WSW-trending σ1-axis with a σ2-/σ3-axis permutation. From a geophysical data reassessment, we inferred that observed faults in the surface within the PFZ are regional-scale deep-seated structures. Considering previous geochronological data, we correlated the earliest and latest phases with the Abanico Basin extension (middle Eocene-early Miocene) and its subsequent inversion (Miocene). Given the neotectonic evidence (geomorphic markers and deformation of unconsolidated deposits), the latter phase likely remains active. Quartz-laumontite cementation of the fault core's cataclastic material promotes mechanical strengthening leading to negative feedback for the reactivation of inherited normal faults as reverse ones. Conversely, the concentration of fractures in the damage zone between the normal faults promotes mechanical weakening resulting in a preferential area for the propagation of reverse fault during the compressive/transpressional phase.

Metavolcanic rocks and orthogneisses from Porongos and Várzea do Capivarita complexes: A case for identification of tectonic interleaving at different crustal levels from structural and geochemical data in southernmost Brazil

The Dom Feliciano Belt (DFB), part of the Mantiqueira Province in southern Brazil and Uruguay, results from multiple Neoproterozoic collisions leading to the construction of the Gondwana supercontinent. Therefore, complex structural relations are expected, as it is the case for the Porongos Metamorphic Complex (PMC) and Várzea do Capivarita Complex (VCC). Several papers have described structural features from both complexes, and some evolution models were proposed for them. However, very few papers have tried to correlate them, and recent geochronological and provenance studies carried out in both complexes pointed out some inconsistencies in these models. Such studies demonstrate that part of the protoliths of metasedimentary and metaigneous rocks of PMC and VCC, of different metamorphic grades, could have been deposited in the same sedimentary environment. The geological meaning of such data is still poorly explained and requires a better understanding of the original geometrical relations between PMC and VCC. This paper focuses on the metavolcanic rocks from two subareas near the northeastern tip of the Porongos Metamorphic Complex area, and on the previously published data for the VCC. In order to investigate the structural and kinematic history of this part of PMC, detailed structural mapping, petrography, whole-rock and mineral chemistry studies were carried out in selected subareas. Metavolcanic rocks from Subarea 1 and Subarea 2 are metadacites and metarhyolites bearing a geochemical imprint of arc magmatism at ca. 790 Ma. Structural investigation of the area reveals that the main deformation phase D1 is mostly compressive and reached the metamorphic peak at amphibolite facies conditions. D2 and D3 are late deformation phases which develop open folds and axial plane cleavages. The most conspicuous D1 structure is an alternation of non-mylonitic S1 and mylonitic-S1, developed over an originally horizontal foliation, which suggests strain concentration in sub-horizontal shear zones interpreted as related to a fold and thrust belt evolution. The scattering of stretching lineation (L1) measurements along a great circle in stereoplots is interpreted to result from shearing along S1 planes, similar to what is described for the progressive deformation of VCC nearby. The present dataset supports the interpretation that PMC and VCC have shared a single volcano-sedimentary basin which subsequently achieved different crustal levels. A W-directed collisional event at 650 Ma metamorphosed the rocks and generated thrust-folds in both complexes. As indicated by recent studies in the local literature, deformed metavolcanics rocks of ca. 578 Ma magmatic age form concordant sheets in the PMC. Such data suggest a metamorphic-deformational event younger than ca. 578 Ma, which, in our view, is probably related to the reactivation of the contractional structures generated at ca. 650 Ma. This younger event must have folded the eastern part of the PMC generating shallow-level folds, and thrusted the rocks onto the eastern side to place them on top of the PMC western-side rocks. Thus, it is possible that PMC rocks register two thrusting episodes, one during peak metamorphic conditions (ca. 650 Ma) and one under retrograde metamorphic conditions (ca. 578 Ma).

What do fault patterns reveal about the latest phase of extension within the Northern Snake Range metamorphic core complex, Nevada, USA?

The Northern Snake Range is a classic example of a metamorphic core complex, Basin-and-Range province, United States. It is composed of a plastically deformed footwall and a brittlely deformed hanging wall, separated by the Northern Snake Range low-angle detachment (NSRD). Brittle deformation, however, is not confined to the hanging wall.This paper focuses on exposures in Cove Canyon, located on the SE flank of the Northern Snake Range, where penetrative, homogeneous faults are well exposed throughout the hanging wall, footwall and NSRD, and overprint early plastic deformation. These late-stage fault sets assisted Eocene-Miocene extension. Detailed analysis of the faults reveals the following: (1) The shortening direction defined by faults is similar to the shortening direction defined by the stretching lineation in the footwall mylonites, indicating that the extensional kinematic history remained unchanged as the rocks were uplifted into the elastico-frictional regime. (2) After ∼17 Ma, extension may have continued entirely within elastic-frictional regime via cataclastic flow. (3) This latest deformation phase may have been accommodated by a single, continuous event. (3) Faults within NSRD boudins indicate that deformation within the detachment zone was non-coaxial during the latest phase of extension.

Structural evidence for strike-slip deformation in the İzmir–Balıkesir transfer zone and consequences for late Cenozoic evolution of western Anatolia (Turkey)

The İzmir–Balıkesir transfer zone (İBTZ) is a recently recognized strike-slip dominated shear zone that accommodates the differential deformation between the Cycladic and Menderes core complexes within the Aegean Extensional System. Here, we present new structural and kinematic data obtained from field observations and 1/25,000 scale mapping of Miocene to Recent units within the İBTZ around İzmir Bay. The results point out that the İBTZ is a transtensional brittle shear zone that affects the pre-Neogene basement rock units, the early-middle Miocene volcano-sedimentary units and the Plio–Quaternary continental units.The analysis of large-scale structures and fault kinematic data indicate that three different deformation phases prevailed in the İzmir Bay region during the late Cenozoic. The first phase (Phase 1) is characterized by N–S directed extension and E–W contraction that gave way to the development of strike-slip faults with normal components and likely took place during the early (?) to late Miocene. This transtensional phase, forming the volcano-sedimentary basin of deposition was overprinted by the second phase (Phase 2) which is characterized by variable extension and contraction directions indicating wrench- to extension-dominated transtension. The structures related to Phase 2 are observed all around the İzmir Bay and indicate a distributed nature of the deformation that probably took place during the early Pliocene, coeval with the end of the activity of the Mid-Cycladic Lineament and the last exhumation of the central Menderes Massif. The latest deformation phase (Phase 3) is characterized by an association of NW–SE trending left-lateral and NE–SW trending right-lateral strike-slip faults and E–W trending normal faults forming transtensional deformation. During Phase 3, the İBTZ evolved from a wider shear zone into a relatively narrow discrete fault zone by the late Pliocene, during which the strike-slip and extensional deformation were completely decoupled from each other. The field-based evidence for strike-slip deformation from the region has only recently been recognized, but has very important implications for understanding the deformation styles and coupling of Aegean–West Anatolian extensional deformation system along the İBTZ. We conclude that the İzmir Bay Basin developed during the Plio–Quaternary within the dextral İBTZ.

Tectonics and quaternary evolution of the Northern Apennines watershed area (upper course of Arno and Tiber rivers, Italy)

AbstractThis work examines the connection between Quaternary tectonics and erosion/incision processes in the primary Tuscan‐Romagna watershed of the Northern Apennines, which essentially coincides with the topographic culmination of the Nero Unit structural ridge. Tectonic and geomorphic information were collected in the area where this ridge is crossed by the upper Tiber River course forming a deep gorge. Structural analysis and field mapping have revealed that the region experienced polyphase tectonics with superposed thrust folding events identifiable both at the map and mesoscopic scales. Hinterland‐SSW‐verging thrusts and thrust‐related folds deformed the whole thrust pile during the latest deformation phase. Backthrusts/backfolds controlled the development of intermountain basins nearby the main watershed during the Early Pleistocene and seemingly deformed, in the Tiber gorge, a low‐relief landscape developed in the Early Pleistocene (ca. 1.1 Ma). Successively, the upper Tiber River course area and Apennines axial zone underwent a generalized uplift, which is manifested by the deep incision of palaeo‐morphologies. This proposed sequence of events correlates well with the major geodynamic change of the Apennines revealed by an acceleration of uplift rates in the Middle–Late Pleistocene. This latter event may also correlate with increased rates of river incision recorded in Europe as a consequence of uplift and/or climate change. Copyright © 2008 John Wiley & Sons, Ltd.

Two subduction events in a polycyclic basement: Alpine and pre‐Alpine high‐pressure metamorphism in the Suretta nappe, Swiss Eastern Alps

This study is essentially based on coupling macrostructures, microstructures and metamorphic petrology in polymetamorphic mafic rocks from the Swiss Eastern Alps (Suretta nappe, Penninic domain). Petrographic criteria are used in conjunction with structural analysis and microprobe work to define crystallization/deformation relationships and to establish a relative but precise sequence of tectono‐metamorphic events. A first eclogite facies overprint and related exhumation occurred before emplacement of late Palaeozoic intrusives. During the Alpine cycle, the Suretta nappe was part of the thinned European continental margin. The Tertiary burial due to subduction and collision is responsible for D1 ductile thrusting and blueschist facies metamorphism. Late deformation phases, related to exhumation, are responsible for the development of extensional structures under greenschist facies conditions. Quantitative metamorphic petrology based on Gibbs free energy minimization (DOMINO by de Capitani) gives a constraint on the P–T conditions during the polymetamorphic and polycyclic evolution. The first high‐P metamorphic event related to pre‐Alpine structures occurred at c. 700 °C and at least 2.0 GPa. These conditions are compatible with pre‐Alpine high‐P re‐equilibration already described in several Alpine units. The Alpine high‐P metamorphism occurred under blueschist facies conditions at c. 400–450 °C and 1.0 GPa. Similar high‐P, low‐T conditions have already been described in the Mesozoic and Permian rock types. The two high‐P events are clearly related to two different geothermal regimes and geodynamic environments.

A decompressional P–T path, Reinbolt Hills, East Antarctica

Granulites exposed in the Reinbolt Hills, East Antarctica, are part of the extensive Late Proterozoic granulite complex of East Antarctica, which includes the Rauer Group to the east and the northern Prince Charles Mountains to the west. The deformation history includes three pervasive deformation phases. No chemical or mineralogical distinction between these phases has been detected and this is interpreted to be the result of complete re‐equilibration at the end of the third deformation phase. Two late deformation phases post‐date the metamorphism and record a medium‐temperature cooling path. A short segment of the P–T path of these rocks was inferred from mineral reactions that occurred during these late deformation phases. The path passes from 800°C, 7 kbar to 690°C, 5 kbar, indicating strong decompression, which is typical of a thrust‐dominated crustal thickening followed by rapid erosion or extensional collapse.

The Lufilian Arc: a microplate in the Pan-African collision zone of the Congo and the Kalahari cratons

The Mwembeshi Fault Zone in Zambia separates the Lufilian Arc and the Zambezi Belt with divergent tectonic structures, and marks the suture of the colliding cratons. The Mwembeshi Fault Zone is correlated with the Okahandja Lineament of the intracontinental branch of the Damaran Orogen in Namibia. Crustal shortening across the Lufilian Arc is larger at its western extremity, where large-scale thrust sheets are present in the Kolwezi area. The crustal block underlying the Lufilian Arc is largely fault bounded. It rotated clockwise as the blocked collision zone of the Congo and the Kalahari cratons was subject to sinistral strike-slip translations in the late deformation phases of the Pan-African orogeny. It is postulated that the western margin of the Lufilian Arc is formed by a transcurrent fault zone which probably evolved from a transform fault separating the Damaran and the Lufilian-Zambezi sections during the spreading phase of the Pan-African mobile zone that traverses southern Africa.

Seismo-tectonics in Finland

Abstract During the Middle Proterozoic orogenic period, the earlier structures of the Finnish bedrock underwent such intense deformation that also the later, differentiated, crustal movements, which were epeirogenic in nature, took place mainly along the deep fractures produced at this time. Recent seismicity, manifested by the latest deformation phase, is likewise associated with these crustal zones of weakness of ancient origin. An apparent, seismogenic, cause-and-effect relation is to be found in both the internal and external tectonics of the Fennoscandian Shield.