Extrusion of subducted crust explains the emplacement of far-travelled ophiolites

Continental subduction below oceanic plates and associated emplacement of ophiolite sheets remain enigmatic chapters in global plate tectonics. Numerous ophiolite belts on Earth exhibit a far-travelled ophiolite sheet that is separated from its oceanic root by tectonic windows exposing continental crust, which experienced subduction-related high pressure-low temperature metamorphism during obduction. However, the link between continental subduction-exhumation dynamics and far-travelled ophiolite emplacement remains poorly understood. Here we combine data collected from ophiolite belts worldwide with thermo-mechanical simulations of continental subduction dynamics to show the causal link between the extrusion of subducted continental crust and the emplacement of far-travelled ophiolites. Our results reveal that buoyancy-driven extrusion of subducted crust triggers necking and breaking of the overriding oceanic upper plate. The broken-off piece of oceanic lithosphere is then transported on top of the continent along a flat thrust segment and becomes a far-travelled ophiolite sheet separated from its root by the extruded continental crust. Our results indicate that the extrusion of the subducted continental crust and the emplacement of far-travelled ophiolite sheets are inseparable processes.

The Late Miocene-Early Pliocene out-of-sequence thrusting event: new insights into the tectonic evolution of the southern Apennines (Italy)

<p>We present a structural study on late Miocene-early Pliocene out-of-sequence thrusts affecting the southern Apennine chain. The analyzed structures are exposed in the Campania region (southern Italy). Here, leading thrusts bound the N-NE side of the carbonate ridges that form the regional mountain backbone. In several outcrops, the Mesozoic carbonates are superposed onto the unconformable wedge-top basin deposits of the upper Miocene Castelvetere Group, providing constraints to the age of the activity of this thrusting event. We further analyzed the tectonic windows of Giffoni and Campagna, located on the rear of the leading thrust. We reconstructed the orogenic evolution of this part of the orogen. The first was related to the in-sequence thrusting with minor thrusts and folds, widespread both in the footwall and in the hanging wall. A subsequent extension has formed normal faults crosscutting the early thrusts and folds. All structures were subsequently affected by two shortening stages, which also deformed the upper Miocene wedge top basin deposits of the Castelvetere Group. We interpreted these late structures as related to an out-of-sequence thrust system defined by a main frontal E-verging thrust and lateral ramps characterized by N and S vergences. Associated with these thrusting events, LANFs were formed in the hanging wall of the major thrusts. Such out-of-sequence thrusts are observed in the whole southern Apennines and record a thrusting event that occurred in the late Messinian-early Pliocene. We related this tectonic episode to the positive inversion of inherited normal faults located in the Paleozoic basement. These envelopments thrust upward crosscut the allochthonous wedge, including, in the western zone of the chain, the upper Miocene wedge-top basin deposits. Finally, we suggest that the two tectonic windows are the result of the formation of an E-W trending regional antiform, associated with a late S-verging back-thrust, that has been eroded and crosscut by Early Pleistocene normal faults.</p>

Strain partitioning within thrust sheets of tectonic windows: Insights from eastern Himalaya

<p>In a fold-thrust belt (FTB), penetrative strain within thrust sheets vary in its magnitude, orientation and type. Addressing variation in magnitude and orientation of strain from major thrust sheets in a FTB, both along the transport direction and along-strike, enable us to understand the complexity of strain partitioning during orogeny. Tectonic windows provide an opportunity to understand the impact of footwall structures on finite strain geometry and orientations of the overlying thrust sheets. In this study, we investigate how penetrative strain is partitioned from the internal to the external major thrust sheets in the Siang window in far-eastern Arunachal Himalayan FTB. We also compare these results with similar thrust sheets from well preserved tectonic windows in the eastern Himalaya, i.e., the Teesta window of the Sikkim and Kuru Chu window of the Bhutan Himalayan FTB.</p><p>We conduct finite strain analysis on quartz grains using R<sub>f</sub>-φ, normalized Fry and Shape Matrix Eigenvector methods. The studied lithologies are gneiss for the internal Pelling-Munsiari-Bomdilla thrust (PT) sheet, while quartzite and sandstone dominantly comprise the external Main Boundary thrust (MBT) and the Main Frontal thrust (MFT) sheets. The rocks north of the PT sheet are not accessible. Results from this study indicate that all the studied rocks record an overall flattening strain. Magnitude of the finite penetrative strain decreases from the internal PT sheet to the external MBT, MFT sheets in the Siang window. The long axes of the finite strain ellipsoids (X) generally have a low plunge and vary in bearing, irrespective of the structural positions of the different thrust sheets. Finite strain ellipses are folded along with the thrust sheets indicating that the penetrative strain developed prior to folding of the thrust sheets. The results also indicate that the footwall structures affect the strain geometry in the interior part of the Himalayan wedge. The grain scale shortening percentage is highest for internal PT sheet and it progressively decreases towards the external MFT sheet. The results indicate greater contribution of thrust-parallel stretch than thrust-perpendicular component, in both internal and external thrust sheets in the Siang window. Preliminary results also suggest that the strain magnitude and grain-scale shortening percentage are the lowest, and orientations of X-axes are more variable with respect to the regional transport direction in the far-eastern Siang window as compared to the other westerly lying regional transects of the Himalayan FTB.</p>

Extrusion of subducted crust explains the emplacement of far-travelled ophiolites

<p>Continental subduction below oceanic plates and associated emplacement of ophiolite sheets remain enigmatic chapters in global plate tectonics. Numerous ophiolite belts on Earth exhibit a far-travelled ophiolite sheet that is separated from its oceanic root by tectonic windows exposing continental crust, which experienced subduction-related high pressure-low temperature (HP-LT) metamorphism during obduction. However, the link between continental subduction-exhumation dynamics and far-travelled ophiolite emplacement remains poorly understood. We combine data collected from ophiolite belts worldwide with thermo-mechanical simulations of continental subduction dynamics to show the causal link between the extrusion of subducted continental crust and the emplacement of far-travelled ophiolite sheets. Our results reveal that buoyancy-driven extrusion of subducted crust triggers necking and breaking of the overriding oceanic upper plate. The broken-off piece of oceanic lithosphere is then transported on top of the continent along a flat thrust segment and becomes a far-travelled ophiolite sheet separated from its root by the extruded continental crust. Our results indicate that the extrusion of the subducted continental crust and the emplacement of far-travelled ophiolite sheets are inseparable processes.</p>

Structural and Stratigraphic Setting of Campagna and Giffoni Tectonic Windows: New Insights on the Orogenic Evolution of the Southern Apennines (Italy)

We present a structural study on the tectonic windows of Giffoni and Campagna, located in the western sector of the southern Apennines (Italy). We analyzed thrusts, folds, and related minor deformation structures. Here, a major in-sequence E-verging thrust fault juxtaposes Meso-Cenozoic successions of the Apennine Platform (Picentini Mts unit) and the Lagonegro-Molise Basin (Frigento unit). However, out-of-sequence thrusts duplicated the tectonic pile with the interposition of the upper Miocene wedge-top basin deposits of the Castelvetere Group. We reconstructed the orogenic evolution of these two tectonic windows, including five deformation phases. The first (D1) was related to the in-sequence thrusting with minor thrusts and folds, widespread both in the footwall and the hanging wall. A subsequent extension (D2) has formed normal faults crosscutting the D1 thrusts and folds. All structures were subsequently affected by two shortening stages (D3 and D4), which also deformed the upper Miocene wedge top basin deposits of the Castelvetere Group. We interpreted the D3–D4 structures as related to an out-of-sequence thrust system defined by a main frontal E-verging thrust and lateral ramps characterized by N and S vergences. Low-angle normal faults were formed in the hanging wall of the major thrusts. Out-of-sequence thrusts are observed in the whole southern Apennines, recording a crustal shortening event that occurred in the late Messinian–early Pliocene. Finally, we suggest that the two tectonic windows are the result of the formation of an E–W trending regional antiform, associated with a late S-verging back-thrust, that has been eroded and crosscut by normal faults (D5) in the Early Pleistocene.

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>

Oligocene turbidite fans of the Dukla Basin: New age data from the calcareous nannofossils and paleoenvironmental conditions (Cergowa beds, Polish–Slovakian borderland)

Calcareous nannofossils found in the Cergowa beds of the Dukla and Fore-Dukla tectonic units in the Outer Carpathians indicate a time of deposition in the range of the NP23–NP24 nannoplankton zones of the Lower Oligocene. Nannoplankton assemblages reflect the paleoecological changes at the Eocene–Oligocene transition from: (i) a greenhouse to an icehouse climate; (ii) brackish to normal salinity suggesting open sea conditions, which were controlled by the Paratethys Basin closure followed by opening and connection with the Tethyan Ocean. The absence of nannofossils of NP25 zone, but their presence in the tectonic windows between 40 and 80 km to the west, shows that deposition of the Cergowa beds in the western part of the basin lasted longer than in the east. Occurrences of nannofossils indicating zones NP16 and NP21, found in the uppermost mudstone-rich parts of studied sections, may prove the remobilization and redeposition of sediments of this stratigraphic age. Potentially, eroded material could be derived from some of the following lithostratigraphic units: NP16 — the Hieroglyphic beds, Przybyszów sandstones and Upper variegated shales; NP21 — the Globigerina marls, Mszanka sandstones and sub-Chert marls and shales and/or fine-grained equivalent of these units. Reworked specimens derived from the older Mesozoic strata occur occasionally in various samples.