Exhumation of high-pressure rocks driven by slab rollback

International audienceRocks metamorphosed under high-pressure (HP) and ultra high-pressure (UHP) conditions in subduction zones come back to the surface relatively soon after their burial and at rates comparable to plate boundary velocities. In the Mediterranean realm, their occurrence in several belts related to a single subduction event shows that the burial­exhumation cycle is a recurrent transient process. Using the Calabria­Apennine and Aegean belts as examples, we show that the exhumation of HP rocks is associated in time and space with the subduction of small continental lithosphere blocks that triggers slab rollback, creating the necessary space for the exhumation of the buoyant continental crust that was deeply buried just before. The buoyancy force of the subducted crust increases until this crust detaches from the downgoing slab. It then exhumes at a rate that depends directly on the velocity of trench retreat to become part of the overriding plate. Heated from below by the asthenosphere that flows into the opening mantle wedge, the exhumed crust weakens and undergoes core-complex-type extension, responsible for a second stage of exhumation at a lower rate. The full sequence of events that characterizes this model (crust­mantle delamination, slab rollback and trench retreat, HP rock exhumation, asthenosphere heating and core-complex formation) arises entirely from the initial condition imposed by the subduction of a small continental block. No specific condition is required regarding the rheology and erosion rate of HP rocks. The burial­exhumation cycle is transient and can recur every time a small continental block is subducted

Subduction dynamics as revealed by trench migration

International audienceNew estimates of trench migration rates allow us to address the dynamics of trench migration and back-arc strain. We show that trench migration is primarily controlled by the subducting plate velocity V-sub, which largely depends on its age at the trench. Using the hot and weak arc to back-arc region as a strain sensor, we define neutral arcs characterized by the absence of significant strain, meaning places where the forces (slab pull, bending, and anchoring) almost balance along the interface between the plates. We show that neutral subduction zones satisfy the kinematic relation between trench and subducting plate absolute motions: V-t = 0.5V(sub) - 2.3 (in cm a(-1)) in the HS3 reference frame. Deformation occurs when the velocity combination deviates from kinematic equilibrium. Balancing the torque components of the forces acting at the trench indicates that stiff (old) subducting plates facilitate trench advance by resisting bending

Neogene to Quaternary evolution of the Calabrian Subduction System, (Central Mediterranean)

We construct an ESE striking to WNW geological cross-section across the Calabrian Subduction System (Central Mediterranean) using seismic near vertical profiles and field data. The interpreted profiles were time-to-depth converted, merged and translated in a geological section stretching from the Marsili Oceanic Basin (Southern Tyrrhenian Sea) to the Ionian accretionary complex . Moving toward the east, the resulting section through the Paola, Amantea, and Crati basins, the Coastal Chain and Sila Massif and Crotone basin. The maximum elongation of these basins change progressively moving toward the east: from NNW in the Paola to NS in the Crati to the NNE in the Crotone basins. Data we present suggest that: Across the Calabria Tyrrhenian Continental Margin (CTCM), top of Kabilian-Calabrian Unit (KCU) is laterally variable in depth forming basins, which are separated by major structures with contractional or transcurrent kinematics, filled by Oligo-Miocene clastic to evaporitic deposits up to 1500m thick. Plio-Quaternary deposits display a remarkable variation in thickness from 4.5 km in the Paola Basin to less than 400m in the central sector of the margin. Plio-Quaternary sediments are internally sub-divisible into four sub-units (namely D1-D4) separated by tectonics enhanced angular unconformities. W-ward vergent reverse faults with limited vertical displacement offset the top of KCU as well as the Oligo-Miocene sedimentary and evaporitic units in the eastern side of the Paola basin. On land (Amantea \u2013 Crati) and farther to the east (Crotone basin) below a Messinian-Pleistocene deposits the top of KCU is variable structured and covered by a Oligo-Miocene clastic deposits with different thickness. The Plio-Quaternary deposits, unconformably overlay the Messinian and older deposits, show the maximum thickness in the Crotone basin. Two main tectonic unconformities within the Plio-Quaternary deposits have been recognised allowing the separation of this unit into three sub-units. In the offshore portion of the Crotone basin, SE-ward reverse faults dissect the KCU and the Oligo-Miocene up to the Messinian deposits. While the pre-Messinian tectonic history across the Calabrian Subduction System seems to be quite similar, a main reorganization of the system occurring during the (?) early and (?) middle-Pliocene. Geometrical and stratigraphic relationship show that several W-ward and E-ward vergent reverse faults in the Paola and Crotone basins, respectively, cut and offset Messinian evaporites and older sedimentary units, controlling the geometry of the basins. In the Paola Basin the amount of subsidence gradually increase during deposition of subunits D2 and D3, which are probably Pliocene in age. On land, the evidence of the unconformities in the Crotone basin indicate that Pliocene deposition occurring during the uplift of the Sila Massif. Therefore uplift of the Sila range occurred during the strong subsidence of the Paola and Crotone basins. The evolution of the overall structure can be then divided in two different steps: 1)the onset of subsidence started in the Late Miocene and covered a large areas presently occupied by the Paola and Crotone basins. This basin, was probably already separated into sub-basins but evolved in a slowly subsiding and poorly deformed area located between the active accretionary prism and the volcanic arc. Therefore in the Middle-Upper Miocene this basin could be defined as forearc basin. 2)In the Pliocene the structure of this large basin was fragmented due to the uplift of a central range (Sila Massif) with an overall pop-up like structure. 3)Uplift of the belt producing subsidence along the flanks and simultaneously formation of two distinct basins: the Paola and Crotone basins. This process probably occurred during episodes of fast roll-back of the subducting slab, as attested by the opening of two ocean floor basins in the back-arc region

Recent extension driven by mantle upwelling beneath the Admiralty Mountains (East Antarctica)

Northern Victoria Land is located at the boundary between an extended, presumably hot, region (West Antarctic Rift System) and the thick, possibly cold, East Antarctic craton. The style and timing of Tertiary deformation along with relationships with the magmatic activity are still unclear, and contrasting models have been proposed. We performed structural and morphotectonic analyses at the NE termination of northern Victoria Land in the Admiralty Mountains area, where the relationship between topography, tectonics, and magmatism is expected to be well pronounced. We found evidence of two subsequent episodes of faulting, occurring concurrently with the Neogene McMurdo volcanism. The first episode is associated with dextral transtension, and it is overprinted by extensional tectonics during the emplacement of large shield alkaline volcanoes. Upper mantle seismic tomography shows that the extensional regime is limited to regions overlying a low-velocity anomaly. We interpret this anomaly to be of thermal origin, and have tested the role of largescale upwelling on lithosphere deformation in the area. The results of this integrated analysis suggest that the morphotectonic setting of the region and the magmatism is likely the result of upwelling flow at the boundary between the cold cratonic and the hot stretched province (WARS), at work until recent time in this portion of the northern Victoria Land

Slab disruption, mantle circulation, and the opening of the Tyrrhenian basins

Plate tectonic history, geological, geochemical (element and isotope ratios), and seismological (P-wave tomography and SKS splitting) data are combined with laboratory modeling to present a three-dimensional reconstruction of the subduction history of the central Mediterranean subduction. We fi nd that the dynamic evolution of the Calabrian slab is characterized by a strong episodicity revealed also by the discrete opening of the Tyrrhenian Sea. The Calabrian slab has been progressively disrupted by means of mechanical and thermal erosion leading to the formation of large windows, both in the southern Tyrrhenian Sea and in the southern Apennines. Windows at lateral slab edges have caused a dramatic reorganization of mantle convection, permitting infl ow of subslab mantle material and causing a complicated pattern of magmatism in the Tyrrhenian region, with coexisting K- and Na-alkaline igneous rocks. Rapid, intermittent avalanches of large amounts of lithospheric material at slab edges progressively reduced the lateral length of the Calabrian slab to a narrow (200 km) slab plunging down into the mantle and enhancing the end of the subduction process

Tectonics and seismicity of the Tindari Fault System, southern Italy: Crustal deformations at the transition between ongoing contractional and extensional domains located above the edge of a subducting slab

The Tindari Fault System (southern Tyrrhenian Sea, Italy) is a regional zone of brittle deformation located at the transition between ongoing contractional and extensional crustal compartments and lying above the western edge of a narrow subducting slab. Onshore structural data, an offshore seismic reflection profile, and earthquake data are analyzed to constrain the present geometry of the Tindari Fault System and its tectonic evolution since Neogene, including the present seismicity. Results show that this zone of deformation consists of a broad NNW trending system of faults including sets of right-lateral, left-lateral, and extensional faults as well as early strike-slip faults reworked under late extension. Earthquakes and other neotectonic data provide evidence that the Tindari Fault System is still active in the central and northern sectors and mostly accommodates extensional or rightlateral transtensional displacements on a diffuse array of faults. From these data, a multiphase tectonic history is inferred, including an early phase as a right-lateral strike-slip fault and a late extensional reworking under the influence of the subductionrelated processes, which have led to the formation of the Tyrrhenian back-arc basin. Within the present, regional, geodynamic context, the Tindari Fault System is interpreted as an ongoing accommodation zone between the adjacent contractional and extensional crustal compartments, these tectonic compartments relating to the complex processes of plate convergence occurring in the region. The Tindari Fault System might also be included in an incipient, oblique-extensional, transfer zone linking the ongoing contractional belts in the Calabrian-Ionian and southern Tyrrhenian compartments

Slab stiffness control of trench motion: Insights from numerical models

Subduction zones are not static features, but trenches retreat (roll back) or advance. Here, we investigate the dominant dynamic controls on trench migration by means of two- and three-dimensional numerical modeling of subduction. This investigation has been carried out by systematically varying the geometrical and rheological model parameters. Our viscoplastic models illustrate that advancing style subduction is promoted by a thick plate, a large viscosity ratio between plate and mantle, and a small density contrast between plate and mantle or an intermediate width (w ∼ 1300 km). Advancing slabs dissipate ∼45% to ∼50% of the energy in the system. Thin plates with relatively low viscosity or relatively high density, or wide slabs (w ∼ 2300 km), on the other hand, promote subduction in the retreating style (i.e., slab roll-back). The energy dissipated by a retreating slab is ∼35% to ∼40% of the total dissipated energy. Most of the energy dissipation occurs in the mantle to accommodate the slab motion, whereas the lithosphere dissipates the remaining part to bend and “unbend.” With a simple scaling law we illustrate that this complex combination of model parameters influencing trench migration can be reduced to a single one: plate stiffness. Stiffer slabs cause the trench to advance, whereas more flexible slabs lead to trench retreat. The reason for this is that all slabs will bend into the subduction zone because of their low plastic strength near the surface, but stiff slabs have more difficulty “unbending” at depth, when arriving at the 660-km discontinuity. Those bent slabs tend to cause the trench to advance. In a similar way, variation of the viscoplasticity parameters in the plate may change the style of subduction: a low value of friction coefficient weakens the plate and results in a retreating style, while higher values strengthen the plate and promote the advancing subduction style. Given the fact that also on Earth the oldest (and therefore probably stiffest) plates have the fastest advancing trenches, we hypothesize that the ability of slabs to unbend after subduction forms the dominant control on trench migration

Kinematics and Convergent Tectonics of the Northwestern South American Plate During the Cenozoic

The interaction of the northern Nazca and southwestern Caribbean oceanic plates with northwestern South America (NWSA) and the collision of the Panama-Choco arc (PCA) have significant implications on the evolution of the northern Andes. Based on a quantitative kinematic reconstruction of the Caribbean and Farallon/Farallon-derived plates, we reconstructed the subducting geometries beneath NWSA and the PCA accretion to the continent. The persistent northeastward migration of the Caribbean plate relative to NWSA in Cenozoic time caused the continuous northward advance of the Farallon-Caribbean plate boundary, which in turn resulted in its progressive concave trench bending against NWSA. The increasing complexity during the Paleogene included the onset of Caribbean shallow subduction, the PCA approaching the continent, and the forced shallow Farallon subduction that ended in the fragmentation of the Farallon Plate into the Nazca and Cocos plates and the Coiba and Malpelo microplates by the late Oligocene. The convergence tectonics after late Oligocene comprised the accretional process of the PCA to NWSA, which evolved from subduction erosion of the forearc to collisional tectonics by the middle Miocene, as well as changes of convergence angle and slab dip of the Farallon-derived plates, and the attachment of the Coiba and Malpelo microplates to the Nazca plate around 9 Ma, resulting in a change of convergence directions. During the Pliocene, the Nazca slab broke at 5.5°N, shaping the modern configuration. Overall, the proposed reconstruction is supported by geophysical data and is well correlated with the magmatic and deformation history of the northern Andes

Recent tectonic reorganization of the Nubia-Eurasia convergent 2 boundary heading for the closure of the western Mediterranean

: In the western Mediterranean area, after a long period (late Paleogene-Neogene) of Nubian northward subduction beneath Eurasia, subduction is almost ceased as well as convergence accommodation in the subduction zone. With the progression of Nubia-Eurasia convergence, a tectonic reorganization is therefore necessary to accommodate future contraction. Previously-published tectonic, seismological, geodetic, tomographic, and seismic reflection data (integrated by some new GPS velocity data) are reviewed to understand the reorganization of the convergent boundary in the western Mediterranean. Between northern Morocco, to the west, and northern Sicily, to the east, contractional deformation has shifted from the former subduction zone to the margins of the two backarc oceanic basins (Algerian-Liguro-Provençal and Tyrrhenian basins) and it is now active in the south-Tyrrhenian (northern Sicily), northern Liguro-Provençal, Algerian, and Alboran (partly) margins. Compression and basin inversion has propagated in a scissor-like manner from the Alboran (c. 8 Ma) to the Tyrrhenian (younger than c. 2 Ma) basins following a similar propagation of the subduction cessation and slab breakoff, i.e., older to the west and younger to the east. It follows that basin inversion is rather advanced in the Algerian margin, where a new southward subduction seems to be in its very infant stage, while it has still to properly start in the Tyrrhenian margin, where contraction has resumed at the rear of the fold-thrust belt and may soon invert the Marsili oceanic basin. GPS-derived strain rates higher in the Tyrrhenian margin than in the Algerian boundary suggest that this latter manner of contraction accommodation (contraction resumption at the rear of the orogenic wedge) is more efficient than subduction inception and basin inversion along newly-generated reverse faults (Algeria), but the differential strain rates may also be explained with the heterogeneous distribution of GPS stations. Part of the contractional deformation may have shifted toward the north in the Liguro-Provençal basin possibly because of its weak rheological properties compared with the area between Tunisia and Sardinia, where no oceanic crust occurs and seismic deformation is absent or limited compared with the adjacent strands of the Nubia-Eurasia boundary. The tectonic reorganization of the Nubia-Eurasia boundary in the study area is still strongly controlled by the inherited tectonic fabric and rheological attributes, which are both discontinuous and non-cylindrical along the boundary. These features prevent, at present, the development of long and continuous thrust faults. In an extreme and approximate synthesis, the evolution of the western Mediterranean is inferred as being similar to a Wilson Cycle in the following main steps: (1) northward Nubian subduction with Mediterranean backarc extension (since ~35 Ma); (2) progressive cessation, from west to east, of Nubian main subduction (since ~15 Ma); (3) progressive compression, from west to east, in the former backarc domain and consequent basin inversion (since ~8-10 Ma); (4) possible future subduction of former backarc basins