Tectonic Transport Research Articles

Provenance of lower Palaeozoic metasediments of the East Odenwald (Mid-German-Crystalline Zone, Variscides)\u2014a correlation with the East European Platform (Poland)

U–Pb age spectra of detrital zircons related to the East European Platform could be traced in paragneiss through the whole Mid-German-Crystalline Zone (Variscides, Central Europe) from the Odenwald via the Spessart to the Ruhla crystalline forming an exotic unit between Armorica and Laurussia. The depositional ages of the paragneiss are defined by the youngest age of the detrital zircons and the oldest intrusion ages as Ordovician to Silurian. The Ediacaran dominated age spectrum of detrital zircons from the paragneiss of the East Odenwald suggests the latter to be derived from the shelf of the East European Platform (Baltica), which was influenced by the 1.5 Ga old detritus delivered from a giant intrusion (Mazury granitoid, Poland). The detrital zircon age spectrum of the lower Palaeozoic paragneiss of the East Odenwald and sandstone of the northern Holy Cross Mountains are identical. The pure Sveconorwegian spectrum of the lower Palaeozoic quartzite from the Spessart, (Kirchner and Albert Int J Earth Sci 2020) and the Ruhla (Zeh and Gerdes Gondwana Res 17:254–263, 2010) could be sourced from Bornholm and southern Sweden. A U–Pb age spectrum with 88% Palaeozoic detrital zircons from a volcano-sedimentary rock of the East Odenwald is interpreted to be derived from a Silurian magmatic arc (46%), which was probably generated during the drift of the Mid-German-Crystalline Zone micro-continent to the south. A tentative plate tectonic model of Mid-German-Crystalline Zone is presented taking into account (a) the East European Platform related age spectra of the detrital zircons (b) the Ordovician to Silurian depositional age of the metasediments (c) the Silurian and Early Devonian intrusion age of the plutonic and volcanic rocks and (d) the U–Pb ages of the Middle Devonian high-grade metamorphism. The East European Platform-related part of the Mid-German-Crystalline Zone is interpreted as a micro-continent, which drifted through the Rheic Ocean to the south and collided with the Saxothuringian (Armorican Terrane Assemblage) during the Early Devonian. Such large-scale tectonic transport from the northern continent to the southern continent is also known from the SW Iberia, where Laurussia-related metasediments of the Rheic suture zone are explained by a large scale tectonic escape (Braid et al. J Geol Soc Lond 168:383–392, 2011).

Syn-tectonic Dipilto batholith (NW Nicaragua) linked to arc-continent collision: High- and room-temperature AMS evidence

The northern Central America region (Honduras and Nicaragua) is characterized by geotectonic units delimited by strike-slip fault systems and shortening zones of poorly constrained ages. The eastern portion of the Chortís block, named the “Dipilto micro-block” or “Patuca” is one of these units that besides with the Siuna Serpentinite Mélange, provides evidence for Early Cretaceous arc-continent collision. Nevertheless, within the Dipilto micro-block is the Early Cretaceous Dipilto batholith (NW Nicaragua), an Ilmenite-Series granitoid suites whose emplacement tectonic setting is unknown.We document the room- and high-temperature anisotropy of magnetic susceptibility (RT-AMS and HT-AMS) and hysteresis cycles from 31 sites in the Dipilto batholith and cross-cutting dikes as well as the oxide mineral microscopy of main lithologies. This is to investigate the magmatic fabric in the granitoids and dikes, and the relation to the emplacement dynamics. The RT-AMS and HT-AMS results show magnetic lineation and foliation (Kmax and Kmax–Kint plane) dominated by paramagnetic minerals (biotite and hornblende). The central parts of the intrusion show a NE-SW Kmax distribution and sites near to boundary of the country-rock a Kmax–Kint plane sub-parallel to the contact. These results and previous structural data in the surrounding Nueva Segovia Schist country-rock support an intrusion emplaced in a contractional setting concurrent with the batholith's NE-SW regional geometry. The Dipilto batholith age (119–112 Ma) implies a syn-collisional origin that together with our results indicate an Early Cretaceous progressive shortening in the region associated with the Siuna Intraoceanic Arc collision (~134–113 Ma). Furthermore, they indicate that the shortening azimuth (i.e., the direction of tectonic transport) is ~127°–137° in its current geographical position, but was 227°–237° in its Early Cretaceous paleo-position, probably related to the final stage of shortening and collision events of the southwestern Mexican paleocontinent.

RELAÇÕES ENTRE CISALHAMENTO, MAGMATISMO E ANATEXIA: EXEMPLO DA ZONA DE CISALHAMENTO CRUZEIRO DO NORDESTE, PROVÍNCIA BORBOREMA, BRASIL

A extensa trama de zonas de cisalhamento presentes ao longo da Província Borborema, nordeste do Brasil, faz desta região um dos locais ideais para estudar processos crustais que envolvem a formação dessas estruturas, em especial sua relação com corpos magmáticos. As relações de campo, análise microtectônica e modelamento termodinâmico de gnaisses migmatíticos próximos à Zona de Cisalhamento Cruzeiro do Nordeste, no limite entre os Terrenos Rio Capibaribe e Alto Moxotó, mostram que a evolução tectono-termal apresenta relação direta com o alojamento de corpos ígneos alinhados segundo a zona de cisalhamento. A evolução da região inicia-se por um processo de encurtamento crustal associado a um transporte tectônico para NW. Há um notável aumento da temperatura para norte, que culmina no início da anatexia em cerca de ~650°C e ~0.9 GPa. A transição para o regime transcorrente resultou do escape tectônico facilitado pelo enfraquecimento reológico da crosta devido a presença de bolsões de melt associado a uma leve descompressão em aproximadamente 750°C e 0.7 GPa.

Cenozoic deformation in the Tethyan Himalaya, SE Tibet: Insights from magnetic fabrics and structural analysis of Upper Triassic flysch

Anisotropy of magnetic susceptibility (AMS) analyses and structural observations of Triassic flysch in the Tsetang area, SE Tibet, were carried out to assess the Cenozoic tectonic evolution and kinematics of deformation of the eastern Tethyan Himalaya Thrust Belt. We analysed a total of 202 standard-sized specimens from four sample sites across the Renbu–Tsetang Thrust from north to south. Rock magnetism and the detailed mineralogy demonstrated that ferromagnetic and paramagnetic minerals both contribute to the AMS. However, the paramagnetic contribution is dominant in the study area. The AMS results, together with field-based structural investigations and analyses of microstructures, revealed that layer-parallel shortening (LPS) was accommodated mainly by isoclinal folds, cleavage, and thrust faults. Shear fabrics (K1 axes oriented parallel to the direction of local tectonic transport) were found mainly in the north thrust fault zone and were linked to fault movement. Microstructural analyses of quartz and metamorphic minerals such as muscovite (sericite) and chlorite in slates and sandstones imply that the peak temperature during the deformation did not exceed 300 °C. The shear AMS fabric in north Qonggyai, near the thrust zone, shows that the strain variation is related to the main activity (11–19 Ma) of the Renbu–Tsetang Thrust. The magnetic lineation that plunges towards the east in the southern Qonggyai area indicates extension that was linked to an E–W extensional event during the early Miocene. Based on the available geochronological data, two stages of tectonic deformation can be defined: (a) syncollisional layer-parallel shortening at ~40 Ma ago, and (b) E–W extension and continuing compaction resulting in shearing at 11–19 Ma.

The Alps-Apennines Interference Zone: A Perspective from the Maritime and Western Ligurian Alps

In SW Piemonte the Western Alps arc ends off in a narrow, E-W trending zone, where some geological domains of the Alps converged. Based on a critical review of available data, integrated with new field data, it is concluded that the southern termination of Western Alps recorded the Oligocene-Miocene activity of a regional transfer zone (southwestern Alps Transfer, SWAT) already postulated in the literature, which should have allowed, since early Oligocene, the westward indentation of Adria, while the regional shortening of SW Alps and tectonic transport toward the SSW (Dauphinois foreland) was continuing. This transfer zone corresponds to a system of deformation units and km-scale shear zones (Gardetta-Viozene Zone, GVZ). The GVZ/SWAT developed externally to the Penninic Front (PF), here corresponding to the Internal Briançonnais Front (IBF), which separates the Internal Briançonnais domain, affected by major tectono-metamorphic transformations, from the External Briançonnais, subjected only to anchizonal metamorphic conditions. The postcollisional evolution of the SW Alps axial belt units was recorded by the Oligocene to Miocene inner syn-orogenic basin (Tertiary Piemonte Basin, TPB), which rests also on the Ligurian units stacked within the adjoining Apennines belt in southern Piemonte. The TPB successions were controlled by transpressive faults propagating (to E and NE) from the previously formed Alpine belt, as well as by the Apennine thrusts that were progressively stacking the Ligurian units, resting on the subducting Adriatic continental margin, with the TPB units themselves. This allows correlation between Alps and Apennines kinematics, in terms of age of the main geologic events, interference between the main structural systems and tectonic control exerted by both tectonic belts on the same syn-orogenic basin.

Meso to Neoproterozoic polyphase rifting and tectonic inversion: The example of the São João del Rei region at the southern border of the São Francisco Craton, Brazil

The up to 3000 m thick Mesoproterozoic and Neoproterozoic sedimentary succession exposed at the southern border of the São Francisco Craton, São João del Rei region (southern Minas Gerais State, Brazil), records the stacking of five sedimentary basins. These basins evolved due to multiple rifting episodes related to the southernmost Espinhaço rift system and were deformed around 600 Ma during the evolution of the Ribeira Orogen. These basins are the 1- São João del Rei Basin with quartzite units with maximum depositional age of ca. 1.5 Ga preserved in a half graben associated to the younger rift phases, 2- Carandaí Basin with rift related fault bounded debris flow deposits with maximum depositional age of ca. 1.38 Ga, and a sag carbonate infill recording the post-rift thermal relaxation, 3- Prados Basin, an NE-SW aulacogen linked to the passive margin Andrelândia Basin (<1.04 Ga) and 4- a foreland basin of the Brasiliano Orogeny with early syn-collisional plagioclase wacke deposits with maximum depositional age of ca. 650 Ma. The present paper discusses the role of the faults at the São João del Rei region in the evolution of these basins and the subsequent orogenic deformation. Five phases of fault activity were recognized. The first one is recorded by a few E-W faults and a single N–S fault and associated growth strata within the São João del Rei Basin. The second phase is recorded by activity of NE-SW, E-W and minor NW-SE faults during the Carandaí rifting (1.53–1.38 Ga). The main faults are the NW-SE Lenheiro and São João del Rei faults and the E-W São José Fault. Minor NW-SE faults are interpreted as release faults associated with the last two faults. NE-SW mafic dikes occurring along the faults suggest precursor/coeval magmatism. The third phase is characterized mainly by the reactivation of the NW-SE Lenheiro Fault leading to the development of the NW-SE Prados aulocogen. The fourth phase is inferred to be a reverse reactivation of NW-SE faults related to the NW tectonic transport direction of the Ribeira Orogen. The NW verging D1 folds and steeply SE dipping cleavage contribute to this interpretation. However, the reverse reactivation was insufficient to overcome the cumulative previous normal throw. During the fifth phase NE-SW faults were reactivated as dextral shear zones. Subhorizontal stretching lineation and mylonitic foliation occur along the Lenheiro Fault for at least 60 km. These structures are related to the E-W compression that generated other NE-SW regional dextral shear zones, such as the Três Corações and Caxambu shear zones during the late stages of the Brasiliano Orogeny. Structural data also reveals that during the Carandaí and/or Prados rifting, prior to the orogenic contraction, rotational oblique tilting in what was to become the northern limb of the Lenheiro Anticline generated the difference between the trends of the Beta axis and L1 in the northern limb of the anticline in relation to the other structural domains. This also explains the ENE-WSW trend of the Lenheiro Anticline contrasting with the NE-SW trend of the Águas Santas Syncline and of the other D1 regional folds south of the studied area. A forward model based on a cross section through the Águas Santas Syncline reveals that the NE-SW Lenheiro Fault acted as a major listric fault responsible for the development of a rollover anticline and the half-graben in which the São João del Rei basin deposits are preserved. Rotation and partial erosion of the hanging wall resulted in the deposition of the Carandaí diamictites in angular unconformity on the São João del Rei basin deposits. The model indicates 41% of total extension with a cumulative displacement of 4.2 km produced by the Carandaí and Prados rifting phases. The minimum shortening of the orogenic compressive phase is estimated to be about 11% with 1.57 km of displacement. These values may be underestimated since the model does not take into account parasitic folds, crystalplastic deformation and volume loss produced by layer parallel shortening and metamorphic reactions.

Core‐Log‐Seismic Integration in Metamorphic Rocks and Its Implication for the Regional Geology: A Case Study for the ICDP Drilling Project COSC‐1, Sweden

AbstractContinental collision causes deformation in the crust along shear zones. However, the physical and chemical conditions at which these zones operate and the deformation processes that enable up to hundreds of km of tectonic transport are still unclear because of the depth at which they occur and the challenges in imaging them. Ancient exhumed collision zones allow us to investigate these processes much better, for example at the COSC‐1 borehole in the central Scandinavian Caledonides. This study combines data from the COSC‐1 borehole with different seismic measurements to provide constraints on the spatial lithological and textural configuration of the Seve Nappe Complex. This is one of the few studies that shows that core‐log‐seismic integration in metamorphic rocks allows to identify the spatial distribution of major lithological units. Especially gamma ray logs in combination with density data are powerful tools to distinguish between mafic and felsic lithologies in log‐core correlation. Our results indicate that reflections along the borehole are primarily caused by compositional rather than textural changes. Reflections in the Seve Nappe Complex are not as distinct as in greater depths but continuous and several of them can be linked to magmatic intrusions, which have been metamorphically overprinted. Their setting indicates that the Seve Nappe Complex consists of the remnants of a volcanic continental margin. Our results suggest that ductile‐deformed middle crustal reflectivity is primarily a function of pre‐orogenic lithological variations which has to be considered when deciphering mountain building processes.

Transition from contractional to transpressive tectonics: evidence from the Feira Nova Region, Rio Capibaribe Domain, Borborema Province, NE Brazil

Differences in tectonic style and kinematics in orogenic belts can result from either multiphase or progressive deformation. In eastern Borborema Province, a regional shallow dipping foliation is multiply folded and crosscut by transcurrent shear zones. Here, we investigate if these fabrics resulted from diachronous deformational events or from a single-phase progressive deformation. The study area, the Feira Nova region, mostly comprises metasedimentary rocks from the Surubim Complex, and is bounded by the NE-trending sinistral Gloria do Goitá (GGSZ) and the sinistral contractional Paudalho (PSZ) shear zones, which separate it from Paleoproterozoic basement rocks. Structures can be grouped into a contractional and a transpressional stage, both related to the Brasiliano Orogeny. The contraction-related structures are represented by a gently dipping foliation (S2) related to a top-to-the-NW tectonic transport. The contact between the metasedimentary and basement rocks is parallel to S2, indicating the fabrics in both lithotypes are of Brasiliano age, with strain localization having produced the PSZ. NW-verging macroscopic folds are consistent with this regime. The following transpression generated steep mylonitic foliation (S3) mainly along the GGSZ, and caused refolding. The data here presented are consistent with a progressive deformation history comprising a gradual transition from contraction to transpression.

Structural diversity within a thrust complex reflecting progressive overprinting during a protracted orogenic process of terrane accretion, Natal belt, South Africa

The Tugela Terrane, forming the northern segment of the Mesoproterozoic Natal belt, South Africa, represents an example of accretionary tectonics that were initiated by closure of the Tugela Ocean basin, and culminated in accretion of magmatic arcs against the southern margin of the Archaean Kaapvaal Craton during the assembly of Rodinia. A detailed structural analysis of the Madidima thrust sheet, part of the Tugela Terrane, reveals five distinct deformation events, D1 to D5. Deformation varied between dominantly non-coaxial during D2, D3 and D5, and dominantly coaxial during D4, the latter being interpreted as a phase of gravitational collapse. While D2 ductile shearing characterizes the most pervasive event that defines the tectonic grain of the sheet, most, if not all, of the structural imprints can be understood as representing consecutive stages of deformation overprinting during a protracted orogenic process of arc-continent convergence. The observed structural complexity is partly due to this multiple overprinting in the course of orogeny, but also due to the broadly synchronous formation of different types of folds within the thrust sheet, and the corresponding variation of foliation orientations. The persistently north-east-directed tectonic transport during at least three of the deformation events is accompanied by intense transposition of early-formed structures, multiple foliation reactivation and formation of transpression-related structures.

Structural Evolution of the Rio das Velhas Greenstone Belt, Quadrilátero Ferrífero, Brazil: Influence of Proterozoic Orogenies on Its Western Archean Gold Deposits

The Quadrilátero Ferrífero region is located in the extreme southeast of the Brasiliano São Francisco craton, Minas Gerais state, Brazil. It is composed of (i) Archean TTG granite-gneaissic terranes; (ii) the Archean Rio das Velhas greenstone belt; (iii) the Proterozoic metasedimentary and metavolcano-sedimentary covers. The Rio das Velhas rocks were deposited in the synformal NW–SE-directed Nova Lima basin. The Archean deformation converted the Nova Lima basin into an ample synclinorium with an eastern inverted flank. Archean orogenic gold mineralization within the Rio das Velhas greenstone belt rocks is controlled by NNW–SSE-directed, Archean regional shear zones subparallel to the strata of the Nova Lima synclinorium borders. Transamazonian and Brasiliano orogenies are superposed onto the Archean structures that control gold mineralization. In the eastern domain, Brasiliano fold-and-fault belts prevail, whereas in the western domain Archean and Transamazonian structures abound. The present study focus mainly is the western domain where the Cuiabá, Morro Velho, Raposos, Lamego and Faria deposits are located. Gold orebodies plunge to the E–NE and are tectonically controlled by the Archean D1–D2 deformation. The D3 Transamazonian compression—Which had a SE–NW vector sub-parallel to the regional mineralized Archean foliation/bedding—Buckled these structures, resulting in commonly open, synformal and antiformal regional folds. These are well documented near the gold deposits, with NE–SW axial traces and fold axes plunging to E–NE. Such folds are normal to inverted, NW-verging, with an axial planar foliation dipping moderately to the SE. The Transamazonian compression has only been responsible for the reorientation of the mineralized Archean gold ores, due to coaxial refolding characterized by an opposite tectonic transport. It has therefore not caused any other significant changes. Thrust shear zones, sub-parallel to the strong Transamazonian foliation, have given rise to localized metric segmentation and to the dislocation of gold orebodies. Throughout the region, along the towns of Nova Lima to Sabará, structures pertaining to the Brasiliano Araçuaí orogeny are represented only by gentle folding and by a discrete, non-pervasive crenulation cleavage. Thrust-shear zones and small-scale normal faults have caused, at most, metric dislocations along N–S-oriented planes.

Grain-shape controlled strain in quartz grains in high ductile flow regime: Observations from the Main Central Thrust Zone of the Kumaun Himalaya, India

In ductile shear zones, the strain shown by the rocks depends much on the composition and shape of the mineral constituents. Under simple shear, quartz grains commonly reorient themselves in the direction of tectonic transport or flow. In ductile shear zones, quartz grains are elliptically stretched in the direction of mylonitic foliation to accommodate the imposed ductile strain. Our observations on the rocks of a crustal scale shear zone, the Main Central Thrust (MCT) of the Himalaya, however, reveal that at several places of the shear zone the quartz grains are polygonal and show planar boundaries. The fabric of rocks at such places is not compatible with that of the prevailing fabric of rocks, and can be described as strain insensitive fabric. Following the Panozzo (J. Struct. Geol. 6:215–221, 1984) method, we have estimated strain from quartz grains that show planar boundaries. Our results show that in the MCT zone, the areas of high ductile strain, as existing near the trace of the MCT, the amount of strain shown by such grains of quartz is low, while in areas of low strain, as existing in areas away from the MCT, the amount of strain is relatively higher. As such, the method holds importance in those cases where grain shapes (i.e., planar boundaries) put constraint on estimation of strain because the conventional methods of strain estimation require elliptical shape of objects. This is possibly the first application of the Panozzo method on deformed rocks from India.

Quantifying deformation processes in the SE Pyrenees using U–Pb dating of fracture-filling calcites

It is difficult to quantify the timing of the deformation processes in brittle fold–thrust belts because minerals suitable for dating and well-preserved growth strata sediments are scarce or absent. Here, we quantify the duration of thrust sheet emplacement and shortening rates in the SE Pyrenean thrust sequence using U–Pb dating of fracture-filling calcites. The obtained U–Pb dates reveal a minimum duration for the emplacement of each thrust unit (18.7 Ma for the Bóixols–Upper Pedraforca, 11.6 Ma for the Lower Pedraforca and 14.3 Ma for the Cadí thrust sheets) and show that piggy-back thrusting was accompanied by post-emplacement deformation of the upper thrust sheets above the lower sheets during their south-directed tectonic transport. We calculated shortening rates of 0.6, 3.1 and 1.1 mm a −1 from the older to younger emplaced thrust sheets. Our results also reveal the formation of local normal faults during the late Oligocene as a result of the late stages of compression and exhumation in the SE Pyrenees. We observed that temperatures &gt;110 °C could be a limiting factor when applying the U–Pb dating method. Supplementary material: U-Pb analytical results, concordia plots and fracture data are available at https://doi.org/10.6084/m9.figshare.c.5078862

From Crustal Thickening to Orogen‐Parallel Escape: The 120‐Myr‐Long HT‐LP Evolution Recorded by Titanite in the Paleozoic Famatinian Backarc, NW Argentina

AbstractExposed sections of accretionary orogens allow reconstruction of their tectonic evolution. Most commonly, orogens are characterized by two‐dimensional shortening perpendicular to the orogenic front. We describe the midcrustal section of the backarc of the early Paleozoic Famatinian accretionary orogen, exposed in the Sierra de Quilmes. Here crustal deformation evolved from a typical two‐dimensional shortening with tectonic transport toward the west, to a noncoaxial constrictional strain with a southward tectonic transport parallel to the orogen. During the early phase of deformation, high‐temperature and low‐pressure (HT‐LP) metamorphic complexes were juxtaposed by west directed thrusting on remarkably thick shear zones forming a thrust duplex. Deformation of the buried footwall complex continued after the exhumed hanging wall ceased to deform. We suggest that the thermally weakened footwall complex responded by initiating a phase of south verging thrusting, parallel to the orogen, associated with strong constriction, associated with L‐tectonites, and sheath folds. This late phase of deformation defines a noncoaxial constrictional regime characterized by simultaneous east‐west and vertical shortening and strong north‐south, orogen‐parallel stretching. Titanite ages and Zr‐in‐titanite thermometry demonstrate that this backarc remained above 700°C for 120 Myr between 500 and 380 Ma. Combined with regional geology, the new data suggest that west verging thrusting interrupted an early, back‐arc extensional phase, and lasted from ~470 to 440 Ma and that footwall constriction and south verging thrusting continued for another 40 to 60 Ma. The Famatinian backarc exposed in Sierra de Quilmes thus is an example of how shortening and orogenic growth in a hot orogen was counterbalanced by lateral flow.

Protracted Shearing at Midcrustal Conditions During Large‐Scale Thrusting in the Scandinavian Caledonides

AbstractDuring continental collision, large tracts of crust are mobilized along major shear zones. The metamorphic conditions at which these zones operate, the duration of thrusting, and the deformation processes that facilitated hundreds of km of tectonic transport are still unclear. In the Scandinavian Caledonides, the Lower Seve Nappe displays a main mylonitic foliation with thickness of ~1 km. This foliation is overprinted by a brittle‐to‐ductile deformation pattern localized in C‐ and C′‐type shear bands proximal to the tectonic contact with the underlying Särv Nappe. Thermobarometry of amphibolites and micaschists suggests a first high‐pressure stage at 400–500°C and 1–1.3 GPa recorded in mineral relics. The main mylonitic foliation developed under epidote amphibolite facies conditions along the retrograde path from 600°C and 1 GPa to 500°C and 0.5 GPa. Age dating of synkinematic titanite grains in the amphibolites indicates that this mylonitic fabric formed at around 417 ± 9 Ma but older ages spanning 460–430 Ma could represent earlier stages of mylonitization. The shear bands developed at lower metamorphic conditions of 300–400°C and ~0.3 GPa. In the micaschists, the recrystallized grain size of quartz decreases toward the shear bands. Monomineralic quartz layers are eventually dismembered to form polyphase aggregates deforming by dominant grain size sensitive creep accompanied by slip in muscovite and chlorite. Plagioclase zoning truncations suggest that the shear bands originated by fracturing followed by ductile deformation. The results suggest protracted and long‐lasting shearing under amphibolite to greenschist facies conditions during the juxtaposition, stacking, and exhumation of the Lower Seve Nappe.

A case study of superposed structures in the tectonic interference zone between the Southern Brasília and Ribeira orogens, southeastern Brazil

The superposition of the Ribeira Orogen (590-560 Ma) on the southern segment of the Southern Brasília Orogen (630-605 Ma) generated a region known as the tectono-metamorphic interference zone. This zone is characterized by the almost orthogonal superposition of structures and metamorphic isograds attributed to the Ribeira Orogen on older structures and metamorphism belonging to the Southern Brasília Orogen.A flat-lying nappe stack characterizes the Southern Brasília Orogen, and it presents top to the east tectonic transport with intense ductile deformation inside the nappes, visible by the development of a strong E-W stretching lineation and related sheath folds. In the Ribeira Orogen more open upright folds, with axial planes dipping steeply to SE predominate, often with a down dip stretching lineation.In the study area, located in this interference zone, descriptive criteria permit grouping of three sets of structures, attributed to three deformation phases, Dn, Dn+1 and Dn+2, here understood as either separated in time or parts of a progressive deformation. Dn produced the main schistosity, Sn, dipping steeply to the south/southeast; associated tight folds and a strong stretching lineation (Ln), plunging shallowly to E/NE, are also ascribed to this phase. Dn+1 generated open to tight regional folds with NE-SW axes and axial planes dipping moderate to steeply to SE. Gentle to open Dn+2 folds with subvertical ENE-WSW trending axial planes refolded older structures locally. Superposed structures between these phases, of type I (Dome and Basin), type II (Mushroom) and type III (Walking Stick) were recognized. The attitude of the structures of these three phases and the type of interference patterns allow to interpret Dn as related to the northeastward tectonic transport in the southern part of the Southern Brasília Orogen and Dn+1 and Dn+2 as reflecting the NW-SE shortening typical for the Ribeira Orogen deformation. The folds and the interference pattern between Dn and Dn+1 are predominantly coaxial. Our understanding of this coaxiallity is that the strong NE-SW linear anisotropy related to thrusting to NE during the Southern Brasília Orogen (Dn) was used as fold axes by the NW-SE shortening related to the Ribeira Orogen.

Microstructural and metamorphic evolution of the Carrancas Klippe, interference zone of the Neoproterozoic southern Brasilia and Ribeira orogens, SE Brazil

The Carrancas Klippe is the basal structure of a nappe system, at the southwestern edge of the São Francisco Craton, in the southern part of the Southern Brasília Orogen. The klippe comprises three metasedimentary units: from base to top, 1) a micaceous quartzite (São Tomé das Letras Unit), 2) chloritoid phyllite grading to staurolite schist with quartzite and gray phyllite intercalations (Campestre Unit), and 3) biotite schist (Santo Antônio Unit), all belonging to the Neoproterozoic Andrelândia Group. This paper describes the results of detailed mapping, structural and metamorphic analyses, including microtectonics, mineral chemistry and related geothermobarometry, and Sm–Nd dating of garnet. Deformational structures were grouped into three sets, D1, D2 and D3. D1 generated thrusting to the east-southeast with associated stretching lineation and tight recumbent folds. D2 produced tight to close asymmetrical folds with axial planes dipping to the SSW, S and SE, and subhorizontal axes. Steep north-south trending axial planes and shallow fold axes characterize open D3 folds and crenulations. In the northwestern part of the klippe garnet chloritoid phyllites formed during upper greenschist facies metamorphism of D1 age with syn-D1 snowball garnet. Towards the southeast the phyllites grade into amphibolite facies schists with syn-D2 garnet and staurolite. Geothermobarometric calculations based on mineral chemical compositions resulted in the following average values for D1 equilibria: 470 ± 45 °C and 12.3 ± 1 kbar. For D2 the following average values were calculated: 580 ± 35 °C and 8.4 ± 1.4 kbar. An age of 562 ± 24 Ma was obtained by Sm–Nd dating of D2 garnet growth. In conclusion, D1 is characterized by east to southeastward thrusting, including the emplacement of the klippe, related to the Southern Brasília Orogen tectonics, at upper greenschist facies conditions with relatively high pressures. During D2, the direction of tectonic transport changed to north, under greenschist facies conditions in the northwest and medium pressure amphibolite facies conditions in the southeast, producing staurolite and kyanite isograds that cut the klippe structure. This phase is attributed to the NNW-SSE shortening in the Ribeira Orogen. D3 reflects late E-W shortening also recorded in major NE-SW dextral shear zones under upper greenschist facies conditions with lower pressures.

Tectonic Evolution of the Eastern Moroccan Meseta: From Late Devonian Forearc Sedimentation to Early Carboniferous Collision of an Avalonian Promontory

AbstractThe deformed Paleozoic succession of the Eastern Moroccan Meseta crops out in relatively small and isolated inliers surrounded by Mesozoic and Cenozoic rocks. Two of the largest inliers (Mekkam and Debdou) are characterized by a monotonous succession of slates and greywackes affected by polyphasic folding that occurred at low‐ to very low grade metamorphic conditions. New U‐Pb ages on detrital zircon grains from the Debdou‐Mekkam metasediments constrain the maximal depositional age as Late Devonian, interpreted to be close to the true sedimentation age. Furthermore, the εHf values of the Devonian detrital zircons, together with the presence of a series of scattered zircon grains with ages between c. 0.9 and c. 1.9 Ga, suggest provenance from a subduction‐related magmatic arc located on the Avalonian margin. The Debdou‐Mekkam massif is characterized by an Early Carboniferous first deformational event (D1), which gave way to a pervasive cleavage (S1) associated with plurikilometric‐scale, tight to isoclinal, overturned to recumbent folds. Later events (Dc) occurred at Late Carboniferous time and generated variably developed crenulation cleavages (Sc) associated with variously oriented metric‐ to kilometric‐scale folds, which complicate the pattern of both D1 intersection lineations (L1) and axial traces. The restoration of this pronounced curved pattern yields originally SW‐NE‐oriented D1 fold axes with regional SE‐vergence. This important Early Carboniferous shortening and SE‐directed tectonic transport can be explained by closure of the Rheic Ocean and the first phases of the collision between the northern passive margin of Gondwana and an Avalonian promontory.

Supradetachment to rift basin transition recorded in continental to marine deposition; Paleogene Bandar Jissah Basin, NE Oman

AbstractA transition from supradetachment to rift basin signature is recorded in the ~1,500 m thick succession of continental to shallow marine conglomerates, mixed carbonate‐siliciclastic shallow marine sediments and carbonate ramp deposits preserved in the Bandar Jissah Basin, located southeast of Muscat in the Sultanate of Oman. During deposition, isostatically‐driven uplift rotated the underlying Banurama Detachment and basin fill ~45° before both were cut by the steep Wadi Kabir Fault as the basin progressed to a rift‐style bathymetry that controlled sedimentary facies belts and growth packages. The upper Paleocene to lower Eocene Jafnayn Formation was deposited in a supradetachment basin controlled by the Banurama Detachment. Alluvial fan conglomerates sourced from the Semail Ophiolite and the Saih Hatat window overlie the ophiolitic substrate and display sedimentary transport directions parallel to tectonic transport in the Banurama Detachment. The continental strata grade into braidplain, mouth bar, shoreface and carbonate ramp deposits. Subsequent detachment‐related folding of the basin during deposition of the Eocene Rusayl and lower Seeb formations marks the early transition towards a rift‐style basin setting. The folding, which caused drainage diversion and is affiliated with sedimentary growth packages, coincided with uplift‐isostasy as the Banurama Detachment was abandoned and the steeper Marina, Yiti Beach and Wadi Kabir faults were activated. The upper Seeb Formation records the late transition to rift‐style basin phase, with fault‐controlled sedimentary growth packages and facies distributions. A predominance of carbonates over siliciclastic sediments resulted from increasing near‐fault accommodation, complemented by reduced sedimentary input from upland catchments. Hence, facies distributions in the Bandar Jissah Basin reflect the progression from detachment to rift‐style tectonics, adding to the understanding of post‐orogenic extensional basin systems.

STRUCTURE OF SUPER QUARTZITE AT THE BURAL-SARDYK DEPOSIT, EAST SAYAN MOUNTAINS

The article presents the results of geological and structural studies of the northwestern part of the Gargan quartziferous province, wherein the Bural-Sardyk deposit is located. The early views on this deposit of high-quality siliceous raw materials have been amended using the new data. This deposit is associated with quartzites of the Meso-Proterozoic Irkut formation and discordantly overlaps the Gargan block. According to the detailed study results on the cover of the Oka-Urik interfluve area, its structure is predetermined by physical and mechanical properties of the rocks, and the degree of rock dislocation depends on the rock viscosity. Most of its section is composed of viscous quartzites that create the structural framework of the study area. Such viscous rocks are not prone to folding. An alternative is detachment taking place concordantly to layering. The only interlayer is composed of limestones that underwent intense plastic deformation reflected in multi-folding. Actually, limestones mark a zone of plastic detachment. The authors propose an interpretation of the formation of the deposit structure. The main deformation event was preceded by the sediment transformation stage in conditions of diagenesis and catagenesis due to the influence of a lithostatic load. Primary recrystallization under lithogenesis led to the formation of high-purity silicon rocks. At the next stage, the rocks were subjected to longitudinal compression in the PT conditions of green-shale metamorphism. This stage is mainly evidenced by sublayer detachments. In parallel with that process, quartzites experienced super-plastic flow and recrystallization, which led to the formation of super-quartzites. The bodies of super-quartzites tend to be layered and less discordant. Such a structural position suggests the possibility of detecting latent manifestations of super-quartzites at different levels in the quartzites of the Irkut formation. Tectonic detachments are observed in the sedimentary cover and do not reach the foundation. Based on these features, there are reasons to use the thin-skinned tectonic model. Rock deformation occurred under stress oriented from the northwest to the southeast. This is indicated by the position of the axial surfaces of the folds, as well as the orientations of the maximum compression axis and the crystallographic axes of quartz. Macro- and microstructural data indicate that the tectonic transport of matter was directed to the southeast. The research results were used for compiling a new geological map of the deposit.

Metamorphic Response to Alpine Thrusting of a Crustal-scale Basement Nappe in Southern Calabria (Italy)

Abstract Structural, geophysical and age data indicate that the tilted cross-section of the Variscan continental crust exposed in the Serre of southern Calabria forms the uppermost Alpine nappe (‘Serre nappe’) of three Variscan basement slices derived from the southern European margin. This Alpine nappe stack is a fragment of the western Mediterranean Alps and rests now, after Miocene emplacement, on top of the Apennine carbonate platform. We report for the first time a P–T path for prograde Alpine metamorphism, which is restricted to the two lower nappes (Castagna and Bagni nappes) that are squeezed between cooler tectonic units, the Serre nappe above and the Apennine platform below. Therefore, we attribute their metamorphism to tectonic loading and concomitant shear heating during Eocene south-directed overthrusting of the crustal-scale Serre nappe. In the underlying Castagna nappe, Alpine metamorphism is only locally recorded, mainly by new growth of garnet, forming at the expense of retrogressed Variscan biotite dated at 43 Ma. The local existence of Alpine besides relict Variscan mineral assemblages in the strongly but heterogeneously overprinted rocks allows for characterization of metamorphic evolutions during both the Alpine and Variscan orogeneses in the former intermediate level of the Variscan crust of Calabria. The metamorphic evolutions have been reconstructed through P–T pseudosection modelling for Al-rich metasediments. In the Castagna nappe, rarely preserved Variscan garnet–sillimanite–biotite–ilmenite–plagioclase–quartz (± K-feldspar ± Si-poor white mica) assemblages formed under amphibolite-facies subsolidus conditions (650 ± 60 °C, 4·0 ± 0·5 kbar). During subsequent decompression and cooling to greenschist-facies conditions garnet was replaced by biotite–sillimanite and later by white mica–chlorite intergrowths. Retrogression of Variscan biotite is evidenced by the exsolution of ilmenite along grain boundaries and cleavages, textures that were subsequently overgrown by Alpine garnet coexisting with Si-rich white mica, rare chloritoid (in metapelites), and hornblende (in metagreywackes). Alpine garnet shows prograde zoning and is Ca-rich, and thus is distinct from unzoned and Ca-poor Variscan garnet porphyroblasts. Estimated conditions (520 ± 40 °C, 8·0 ± 1·0 kbar) record elevated pressures during Alpine lower amphibolite-facies metamorphism. In the lowermost Bagni nappe, rare prograde-zoned, Ca-rich garnet in strongly retrogressed mylonitic quartz-phyllites allows isopleth thermobarometry, which indicates lower amphibolite-facies conditions (555 ± 10 °C, 7·4 ± 0·3 kbar) resembling those for Alpine garnet growth in the Castagna nappe. The similar clockwise P–T paths for prograde Alpine metamorphism and the consistent peak pressures of 7–9 kbar in the Castagna and Bagni nappes point to a joint short-lived metamorphism during overthrusting of the crustal-scale Serre nappe within the south European margin during the north-directed subduction of the Alpine Tethys. South-directed overthrusting of the now tilted Variscan crustal section of the Serre along the up to 500 m thick mylonite horizon of the Curinga–Girifalco Line is in agreement with seismic data indicating an extended, few kilometres thick low-velocity zone (Bagni and Castagna nappes and mylonites of the Curinga–Girifalco Line) below the exposed lower crustal section of the Serre nappe. Alpine tectonic transport direction, timing and metamorphic conditions described here are consistent with those reported from the Aspromonte area in southernmost Calabria, suggesting a coeval Alpine history characterized by metamorphism owing to nappe loading and concomitant shear heating. The Alpine subduction–erosion–accretion processes inferred here for the Calabrian basement nappes resemble those proposed for the Dent Blanche nappe system in the Western Alps.