The rise and demise of deep accretionary wedges: A long-term field and numerical modeling perspective

Several decades of field, geophysical, analogue, and numerical modeling investigations have enabled documentation of the wide range of tectonic transport processes in accretionary wedges, which constitute some of the most dynamic plate boundary environments on Earth. Active convergent margins can exhibit basal accretion (via underplating) leading to the formation of variably thick duplex structures or tectonic erosion, the latter known to lead to the consumption of the previously accreted material and eventually the forearc continental crust. We herein review natural examples of actively underplating systems (with a focus on circum-Pacific settings) as well as field examples highlighting internal wedge dynamics recorded by fossil accretionary systems. Duplex formation in deep paleo–accretionary systems is known to leave in the rock record (1) diagnostic macro- and microscopic deformation patterns as well as (2) large-scale geochronological characteristics such as the downstepping of deformation and metamorphic ages. Zircon detrital ages have also proved to be a powerful approach to deciphering tectonic transport in ancient active margins. Yet, fundamental questions remain in order to understand the interplay of forces at the origin of mass transfer and crustal recycling in deep accretionary systems. We address these questions by presenting a suite of two-dimensional thermo-mechanical experiments that enable unravelling the mass-flow pathways and the long-term distribution of stresses along and above the subduction interface as well as investigating the importance of parameters such as fluids and slab roughness. These results suggest the dynamical instability of fluid-bearing accretionary systems causes either an episodic or a periodic character of subduction erosion and accretion processes as well as their topographic expression. The instability can be partly deciphered through metamorphic and strain records, thus explaining the relative scarcity of paleo–accretionary systems worldwide despite the tremendous amounts of material buried by the subduction process over time scales of tens or hundreds of millions of years. We finally stress that the understanding of the physical processes at the origin of underplating processes as well as the forearc topographic response paves the way for refining our vision of long-term plate-interface coupling as well as the rheological behavior of the seismogenic zone in active subduction settings.

Tectonic Transport Directions, Shear Senses and Deformation Temperatures Indicated by Quartz c-Axis Fabrics and Microstructures in a NW-SE Transect across the Moine and Sgurr Beag Thrust Sheets, Caledonian Orogen of Northern Scotland

Moine metasedimentary rocks of northern Scotland are characterized by arcuate map patterns of mineral lineations that swing progressively clockwise from orogen-perpendicular E-trending lineations in greenschist facies mylonites above the Moine thrust on the foreland edge of the Caledonian Orogen, to S-trending lineations at higher structural levels and metamorphic grades in the hinterland. Quartz c-axis fabrics measured on a west to east coast transect demonstrate that the lineations developed parallel to the maximum principal extension direction and therefore track the local tectonic transport direction. Microstructures and c-axis fabrics document a progressive change from top to the N shearing in the hinterland to top to the W shearing on the foreland edge. Field relationships indicate that the domain of top to the N shearing was at least 55 km wide before later horizontal shortening on km-scale W-vergent folds that detach on the underlying Moine thrust. Previously published data from the Moine thrust mylonites demonstrate that top to the W shearing had largely ceased by 430 Ma, while preliminary isotopic age data suggest top to the N shearing occurred at ~470–450 Ma. In addition, data from the east coast end of our transect indicate normal-sense top down-SE shearing at close to peak temperatures at ~420 Ma that may be related to the closing stages of Scandian deformation, metamorphism and cooling/exhumation.

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

The Borborema Province, northeastern Brazil, exhibit an extensive framework of shear zones in spatial proximity with syn-tectonic magmatism that makes it a perfect place to understand their relationship. In the eastern portion of this province an important dextral shear zone, that divides into two terranes, was originated during an escape tectonics after an oblique collision after a tectonic transport to NW. The recrystallization of quartz and feldspar shows a remarkable increase towards the shear zone, interpreted as a temperature increase during deformation. Thermodynamic modelling coupled with field relationship shows that high strain migmatitic textures such as stromatic structure was formed at ~650 °C and ~0.9 GPa prior to the shear development. Whereas low strain migmatites with schollen texture was formed at ~750 °C and ~0.7 GPa. We propose that the presence of melt during an oblique collision facilitated the emplacement of shear structures due to a thermal anomaly during the emplacement of syn-tectonic plutons.

The Alps-Apennines Interference Zone: A Perspective from the Maritime and Wester 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.

U-Pb dates measured in fracture-filling calcites from the SE Pyrenees: syn- or post-kinematic mineral growth?

<p>Recently, U-Pb dating of fracture-filling carbonates has revealed as a powerful tool to constrain the absolute timing of deformation in fold and thrust belts. However, geochronological studies of these minerals have to be combined with petrological observations and geochemical analyses to decipher if measured dates document fluid flow synchronously to deformation or post-kinematic events.</p><p>The Pyrenean compressional belt formed from Late Cretaceous to Oligocene due to the stacking of three thrust sheets and a deformed foreland basin. From top-and-older to bottom-and-younger, these consist of the Bóixols-Upper Pedraforca, Lower Pedraforca and Cadí thrust sheets and the Ebro foreland basin. Here, we quantify the duration of thrust sheet emplacement and shortening rates in the SE Pyrenees using U-Pb dating of 43 calcites filling fractures and interparticle porosity.</p><p>Four fracture sets related to compressional tectonics and one set related to extension are identified. The compressive sets include: 1) N-S, NNW-SSE and NNE-SSW trending veins; 2) E-W trending folding-related veins; 3) E-W trending reverse faults; and 4) NW-SE and NE-SW trending strike-slip faults. Fractures related to extension are NNW-SSE and NW-SE trending normal faults.</p><p>Elongated blocky, blocky and bladed calcite textures of the dated cements are observed. Elongated textures are observed in reverse, strike-slip and normal faults and occasionally in N-S, NNW-SSE and NNE-SSW and E-W veins. In these fractures, calcite crystals are arranged parallel, oblique, or perpendicular to fracture walls and provide evidence for syn-kinematic growth. Blocky and bladed textures have been identified in N-S, NNW-SSE and NNE-SSW veins, E-W folding-related veins, reverse and strike-slip faults and in calcite precipitated between sedimentary breccia clasts. Although these textures indicate precipitation after vein opening or at lower rates than vein opening, their presence in crack-seal veins and in stepped slickensides also indicates syn-kinematic growth. Moreover, clumped isotope temperatures measured in several blocky and bladed calcites precipitated in veins and faults indicate that most of them precipitated from fluids in thermal disequilibrium with host rocks, revealing rapid fluid flow and precipitation just after fracturing. Contrarily, low temperatures measured in blocky and bladed calcite precipitated in the interparticle porosity of sedimentary breccias indicate late fluid migration.</p><p>U-Pb dating applied to fracture-filling calcites in the SE Pyrenean fold and thrust belt yielded 46 ages from 70.6 ± 0.9 Ma to 2.8 ± 1.8 Ma (Cruset et al., 2020). The results reveal minimum durations for the emplacement of each thrust sheet (18.7 Myr for the Bóixols-Upper Pedraforca, 11.6 Myr for the Lower Pedraforca and 14.3 Myr for the Cadí), and that piggy-back thrusting was accompanied by post-emplacement deformation of upper thrust units above the lower ones during tectonic transport. These estimated durations, combined with the minimum shortening established for the Bóixols-Upper Pedraforca, Lower Pedraforca and Cadí thrust sheets by other methods, allows calculating shortening rates of 0.6 mm/yr, 3.1 mm/yr and 1.1 mm/yr, respectively. Finally, the results also reveal the development of local normal faults at late Oligocene times during the final stages of compression and exhumation.</p><p><strong>References:</strong></p><p><strong>Cruset et al. (2020)</strong>. Geological Society of London. 177, 1186-1196.</p>

Petrogenesis and emplacement depths of the Petite Pluton during the closure of the Rocas Verdes basin, southern Patagonia - preliminary results

<p>The Petite Pluton is a Cretaceous intrusion covering an area of nearly 136 km<sup>2 </sup>located in Isla Capitán Aracena, southernmost Patagonia, Chile. This pluton and other stocks are located outside of the margins the Early Cretaceous-Paleogene Fuegian Batholith. The Petite Pluton intrudes the Capitán Aracena ophiolitic complex, interpreted as supracrustal remnants generated during the rifting stage of the Rocas Verdes marginal basin (Late Jurassic- Early Cretaceous; cf. Calderón et al., 2013, Geochem. J.) overlain by hemi-pelagic sedimentary basin infill (Yahgan Formation). These units are locally deformed and exposed in the southern limit of the NW-SE-trending Magallanes fold-and-thrust belt. The satellite plutons consist of amphibole-bearing diorites and quartzdiorites (48-55 wt.% SiO<sub>2</sub>) with calc-alkaline compositional trends consistent with their generation in a subduction environment. On N-MORB normalized incompatible elements pattern, the rocks show peaks in LILE (Rb, Ba, Sr) and subtle throughs in Ti, Zr, Nb, Ta and Y. Chondrite-normalized REE pattern is concave upwards with enrichment of LREE relative to HREE without Eu anomaly. The mineral compositions of diorites of Petite pluton consist of amphibole (magnesio-hornblende and tschermakitic hornblende), plagioclase is labradorite and andesine (An<sub>44-59</sub>), with Ca-rich composition in small grains included within poikilitic amphibole, biotite (annite), quartz, minor contents of K-feldspar, titanite, magnetite-ilmenite pairs and traces of apatite and zircon. Amphibole composition can be used as a proxy of the amount of H<sub>2</sub>O-rich fluids involved in magma evolution and could potentially be used to constrain the crustal depths of pluton emplacement in magmatic plumbing systems (Yavuz & Döner, 2017, P. di Mineralogia; Torres García et al., 2020, Lithos). The calculated pressure and temperature of 3 kbar and 800-850°C, indicate the emplacement and crystallization of magma batches in the upper crust. Oxygen fugacity [log (ƒO<sub>2</sub>)] varies between -9.9 and -10.7 (NNO), indicating amphibole crystallization from basaltic-andesitic melts under moderately oxidizing conditions. The moderately Mg# (60-72) of amphibole is consistent with their crystallization from mafic-intermediate melt-dominated crystal mushes with residual melts generated after the fractionational crystallization of olivine and clinopyroxene at deeper crustal depths. The amphibole composition constraint an amount of 6 wt% of H<sub>2</sub>O in the residual melts. The subtle negative Eu anomaly in amphibole indicates its partially simultaneous fractionation with plagioclase, suggesting rapid undercooling. The emplacement of the Petite Pluton at ~10 km depth occurred during and/or lately after the tectonic emplacement of ophiolitic complexes within an accretionary wedge, governed by a NE tectonic transport (Muller et al., 2021, Tectonophysics). Late Cretaceous satellite plutons suggest a continentward migration of the magmatic arc, related to the flattening of the subducted oceanic lithosphere of the proto-Pacific Ocean.</p><p>Acknowledgements. The study is supported by  Fondecyt grant 1161818.</p>

Control of obliquity directions on structural development, from rifting to inversion: Examples from the Tethyan domain

<p>Oblique rift systems form when the axis of rifting is not orthogonal to the direction of tectonic extension, normally due to pre-existing zones of weakness that influence the location and orientation of new faults. Irrespective of the regional-scale obliquity, most individual extensional faults will tend to nucleate according to the orientation of the tectonic stress orientations, and therefore normal to the direction of maximum extension. Transfer faults in oblique systems will tend to form parallel to the direction of extension and, in contrast to orthogonal rifting, will play a major role in the architecture and development of the rift and its sedimentary basins.</p><p>An intriguing feature in oblique rift systems is the formation of reverse structures evocative of wrench tectonics during the syn-rifting stage. This stems from the orientation of geological structures relative to the direction of tectonic extension. Even slight changes in tectonic transport direction or stress orientations during the development of the rift system can lead to events of transpression or transtension along transfer structures. Because of the relevance of transfer structures in oblique systems, transpression can result in the appearance of discontinuities in the sedimentary record that are often interpreted as, somewhat incongruent, inversion events.</p><p>Oblique structures also play a crucial role during the full inversion of the rift system during convergence, particularly so because tectonic shortening will strike at an angle to the orientation of faults. Irrespective of the evolution of oblique rifting and inversion, the initial fault pattern is also normally preserved in fully inverted systems involved in fold-and-thrust systems. In many of cases, when the original rift obliquity is not well understood, the characteristic rhomboidal pattern is interpreted to relate to wrench tectonics.  In this presentation we will review evidence from Iberia, Northwestern Africa and the Eastern Alps to discuss the role that obliquity plays in rift development and its inheritance in fold-and-thrust belts with different degrees of inversion.</p>

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.

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 >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

The upper Messinian-lowermost Pliocene out-of-sequence event in the southern Apennines (Italy): a study about the kinematics of the major thrust faults

<p>The southern Apennines are a fold-and-thrust belt formed since the lower Miocene until the middle Pleistocene (e.g., Vitale and Ciarcia, 2013). Although a wide literature exists about the geology of this orogenic chain, few are the studies about the kinematics of the major thrusts. With this in mind, this work is aimed to investigate the out-of-sequence regional thrust system exposed in the Campania region. This system is characterized by a frontal ramp exposed along the N-NE side of the platform carbonate ridge forming the regional mountain backbone. The main structure is also exposed as a flat thrust in the Campagna and Giffoni tectonic windows located in the internal sector of the chain. We analyzed several outcrops; in some of them, we observed the Mesozoic carbonates superposed onto the upper Miocene wedge-top basin deposits of the Castelvetere Group. The kinematic analysis of major and minor structures suggests the occurrence of two thrust fault sets: (i) the oldest indicates an eastward tectonic vergence, whereas (ii) the youngest, and more developed, toward the north. In the external zones, the N-S shortening was synchronous with the deposition of the upper Messinian-lowermost Pliocene Altavilla Fm. The origin of this out-of-sequence regional deformation is still matter of debate (e.g., Vitale et al., 2017). In our opinion it was the shallow expression of a deep-seated thrusting episode within the buried Apulian slab. It was dominated by thrust ramps (thick-skinned tectonics) mainly verging to East, and by the N-verging structures associated to lateral ramps.</p><p><strong>References</strong></p><p>Vitale Stefano and Ciarcia Sabatino (2013) - Tectono-stratigraphic and kinematic evolution of the southern Apennines/Calabria-Peloritani Terrane system (Italy). Tectonophysics, 583, 164–182.</p><p>Vitale Stefano, Tramparulo Francesco d'Assisi, Ciarcia Sabatino, Amore F. Ornella, Prinzi Ernesto Paolo and Laiena Fabio (2017) - The northward tectonic transport in the southern Apennines: examples from the Capri Island and western Sorrento Peninsula (Italy). International Journal of Earth Sciences (Geologische Rundschau), 106, 97–113.</p>