Francesca Meneghini

Large-scale pseudotachylytes and fluidized cataclasites from an ancient subduction thrust fault

In the Kodiak accretionary complex, Kodiak Island, Alaska, pseudotachylyte occurs in black, locally vitreous ultrafine-grained fault rock. Microscopic observations show that the pseudotachylytes are composed of glass, with vesicles, amygdules, microlites, and flow structures, indicating a frictional melt. The pseudotachylyte is gradational to cataclasite and shows outcrop-scale injection and ductile deformation structures. The cataclasite was ductily mobile (i.e., fluidized) simultaneous with the formation and emplacement of pseudotachylyte melt. The pseudotachylytic rocks postdate the stratal disruption fabric of associated shear-zone melanges and show similar direction of thrust transport, and have undergone limited subsequent deformation. We interpret the stratal disruption as resulting from underthrusting of the subducting plate and pseudotachylyte development as the final activity of this thrust surface. The gradational contacts between pseudotachylyte and cataclasite demonstrate that the cataclasite also formed as a seismic product and may represent paleoseismic rupture zones, possibly of very great earthquakes, with or without accompanying pseudotachylytes. The pseudotachylytes are voluminous, and many are spatially disconnected from generation surfaces. This style is distinct from pseudotachylytes described in other environments, and this may explain the rarity of documented examples of subduction-thrust pseudotachylyte.

Deformation and hydrofracture in a subduction thrust at seismogenic depths: The Rodeo Cove thrust zone, Marin Headlands, California

We have investigated the fabric and the deformational processes of an exhumed subduction zone thrust active at seismogenic depths. The Rodeo Cove thrust zone, which outcrops north of the Golden Gate Bridge of San Francisco, imbricates two basalt-chertsandstone sequences belonging to the Marin Headlands terrane (Franciscan Complex). The thrust outcrop is a 200-m-thick complex zone that displays a range of stratal disruption from incipient deformation to a broken formation in the central part of the outcrop, dominated by basaltic lithologies, where zones of concentration of deformation have been mapped. Disruption is made by variably dense discrete fault systems synthetic to the main thrust (R and P fractures). These faults are marked by cataclasites with a shaly matrix that shows a scaly foliation defi ned by chlorite and pumpellyite, which also constrain the depth of faulting (8‐10 km, T = 200‐250 °C) within the seismogenic zone. The central part of the fault also features the densest system of carbonate-fi lled veins. Veins occur in the broken formation matrix and fragments, in both cases parallel to the foliation. The veins are either folded, truncated, or pressure-solved along the cleavage. Cementation and hardening of shear surfaces of the fault core may have caused the distribution, as opposed to localization, of subsequent slip events. The fault core may have developed in basaltic rocks because of their inherently high permeability and propensity to transmit overpressure from deeper levels of the subduction zone. Our analysis has shown that accretionary deformation is strongly controlled by injection of overpressured fl uids occurring through systems of multiple dilatant fractures grossly parallel to the decollement zone. The crosscutting relationships between veining and foliation suggest that fl uid injection is cyclic and, consequently, that large transient variations in permeability and cohesion may occur. The repeated injection of veins parallel to the fault zone may be explained by cyclic changes of the stress, or by difference in tensional strength parallel to and perpendicular to the foliation, both of which would require extremely high fl uid pressure. We interpret the features of the Rodeo Cove thrust zone as evidence of the seismic cycle and hypothesize a compressional stress fi eld in the interseismic phase and an extensional stress fi eld in the immediately postseismic phase.

Anatomy of the Ligure-Piemontese subduction system: evidence from Late Cretaceous–middle Eocene convergent margin deposits in the Northern Apennines, Italy

In the Northern Apennines, in contrast to the Western Alps and Alpine Corsica, upper structural levels of the Late Cretaceous–middle Eocene subduction complex are still preserved and well exposed. This subduction complex developed in the Ligure-Piemontese basin since the Late Cretaceous time as a consequence of convergence between the Eurasia and Adria plates. Representative successions of this ancient subduction complex are well preserved in the Ligurian units of the Northern Apennines, where turbidite and mass-gravity deposits showing pristine stratigraphic features are present. Three main domains, represented by different groups of tectonic units, can be identified, each delineating a different domain of the subduction zone. In this article, we first present a brief history of geological research in the Northern Apennines during the last half of the twentieth century and then a comprehensive picture of the stratigraphy and tectonics of the Ligurian units. A new interpretation of the related tectonostratigraphic units is proposed within the conceptual modern geodynamic framework of convergent margins.

Fingerprints of late Neoproterozoic ridge subduction in the Pan–African Damara belt, Namibia

Subduction of mid-ocean ridges is a common feature in recent convergent margins, but is rarely documented in Proterozoic to Paleozoic orogenic belts. Here we describe evidence for ridge-trench interaction in the deeply eroded late Neoproterozoic Damara orogenic belt, central Namibia. The earliest interaction is indicated by primary intrusive contacts between amphibolite facies mid-ocean ridge metabasalts and trench metasediments. U-Pb zircon ages of 550–540 Ma from syntectonic granites in the forearc indicate the timing of partial melting and mafic underplating of the prism in response to ridge subduction. The thermal peak in the Damara belt, associated widespread granitic and alkalic plutonism, and hydrothermal activity coincide with the waning stages of tectonism at 530–520 Ma and are interpreted to indicate slab window widening and slab delamination. We suggest that the proposed two-stage thermal evolution of the Damara belt, comprising latest Neoproterozoic ridge subduction and early Cambrian slab delamination, represents a fingerprint of ridge subduction in ancient orogens.

From accretion to exhumation in a fossil accretionary wedge: a case history from Gottero unit (Northern Apennines, Italy)

The Gottero unit of the Northern Apennines, Italy, is representative of the sedimentary cover of the Ligure-Piemontese oceanic lithosphere. This unit consists of a thick sedimentary sequence that includes Valanginian-Santonian pelagic deposits and Campanian-early Paleocene turbiditic deposits. The latter are overlain by early Paleocene trench deposits related to frontal tectonic erosion of the accretionary wedge slope. This sequence is interpreted as recording trenchward motion of the oceanic lithosphere. The Gottero unit records a pre-Late Oligocene, complex deformation history related to subduction and accretion events. This deformation history has developed through underthrusting (D1a), underplating (D1b and D1c) and later exhumation (D2a and D2b) episodes. The folding phase related to the main underplating sub-phase (D1b) is predated by a sub-phase (D1a) connected to rapid fluid escape and followed by a sub-phase dominated by the development of shear zones (D1c). The D1b sub-phase is characterized by similar folds and a slaty cleavage developed under P/T conditions of 0.4GPa/210°-270 °C. The D1c sub-phase, characterized by west-verging thrusts, is particularly signficative in understanding the dynamics of the Ligure-Piemontese accretionary wedge because it testifies active shortening of the Gottero unit also after its transfer to the prism. In addition, sub-phase D1c represents the transition from the sub-phases connected to accretion and the tectonics dominated by extension, characterized by parallel folds and low-to high-angle normal faults. The gravity driven extension is represented by the D2a and D2b sub-phases and can be interpreted as the result of the thicknening of the Ligure-Piemontese accretionary wedge, produced by continuous underplating at its base but also by shortening of the previously underplated units. These final tectonic events resulted in the exhumation of the Gottero unit to the surface during the Early Oligocene, when this Unit became one of the source areas of the conglomerates deposited in the Tertiary Piedmont basin. This deformation history suggests the occurrence of a complex sequence of deformations during the transition from accretion to exhumation, even in the intermediate levels of the accretionary wedge.

Oligocene-Miocene foredeep deposits in the Lake Trasimeno area (Central Italy): insights into the evolution of the Northern Apennines

In the Trasimeno Lake area (Umbria Region), several thrust sheets belonging to the Sansepolcro-Monte Filoncio and Rentella Units, are interposed between the Tuscan Nappe (in the hanging-wall) and the Umbria-Romagna Unit (in the footwall). The thrust sheets succession of the Sansepolcro-Monte Filoncio Unit is made up of Rupelian-Chattian Scaglia toscana Fm. and Chattian-Aquitanian foredeep deposits of Macigno Fm.; the base of the siliciclastic succession becomes gradually younger eastward. The compositional mode of the fine-grained rock fragments in the arenaceous turbidites is comparable with that of the upper part of the Macigno Fm. in the Tuscan Nappe. The succession of the Rentella Unit includes Rupelian-Aquitanian Monte Rentella Fm. and Aquitanian-Burdigalian foredeep siliciclastic turbidites of the Montagnaccia Fm.. In the lower portion of the Montagnaccia Fm. a level of black cherty horizons is present, as detected in all Aquitanian-Burdigalian successions of the Northern Apennines. The compositional mode of the fine-grained rock fragments in the turbidites is comparable with that of Marnoso-Arenacea Fm.. All these data allow an Oligocene-Miocene paleogeographic reconstruction of the Northern Apennines foredeep. Based on the age, the compositional mode and structural position, the Rentella and Sansepolcro-Monte Filoncio Units can be compared to the Carigiola and Acquerino Units cropping out in the Tuscan-Emilian Apennines.

Quartz vein formation by local dehydration embrittlement along the deep, tremorgenic subduction thrust interface

Hydrothermal quartz veins are ubiquitous in exhumed accretionary complexes, including the Namibian Damara belt. Here, subduction-related deformation occurred at temperatures ≤550 °C, and vein geometry is consistent with plate interface shear, low effective normal stresses, and mixed-mode deformation. Quartz vein δ18O values relative to Standard Mean Ocean Water (SMOW) range from 9.4‰ to 17.9‰ (n = 30), consistent with precipitation from metamorphic fluids. A dominant subset of quartz veins away from long-lived high-strain zones and basaltic slivers have δ18O values in a smaller range of 14.9‰ ± 1‰, requiring precipitation from a fluid with δ18O of 12‰ ± 1‰ at 470–550 °C. This uniform fluid isotope value is consistent with progressive local breakdown of chlorite allowing extensive hydrofracture at temperatures typical of the plastic regime. In active subduction zones, brittle deformation within the plastic regime is inferred from observations of tectonic tremor, a noise-like seismic signal including overlapping low- and very low-frequency earthquakes, which occurs below the seismogenic zone. Both tremor and hydrothermal veins correlate with zones of inferred high fluid pressure, could represent a mixture of shear and dilatant failure, and may therefore be controlled by episodic hydrofracturing within a dominantly plastic and aseismic regime.

A Revised Subduction Inception Model to Explain the Late Cretaceous, Double‐Vergent Orogen in the Precollisional Western Tethys: Evidence From the Northern Apennines

The Meso-Cenozoic alpine belts of the Mediterranean area are characterized by complex architectures, result of a complex subduction and collision evolution that preserve also a legacy of the rifting-related configuration of the continental margins. The Northern Apennines is a segment of these belts originated during closure of the Ligure-Piemontese ocean, and collision between the Europe and Adria plates. The different configuration of the Adria and Europe margins, inherited from asymmetric rifting, is recorded in the Ligurian Units, that preserve incorporation into the subduction factory of fragments of the oceanic domain (Internal Ligurian Units), and portions of the Ocean-Continent Transition Zone (OCTZ) toward Adria (External Ligurian Units). We provide here unpublished data on the stratigraphy and sedimentology of these units, together with a review of what is already established in literature. Both datasets combined testify that at 80 M.a., an accretionary prism was growing between the deposition basins of the two groups of units, and fed both basins with clasts from the ocean realm, the continental crust and the subcontinental mantle. We propose that closure of the Ligure-Piemontese ocean occurred through subduction that nucleated at the transition from the oceanic plate and the thinned Adria margin, and developed a double-vergent prism by accreting oceanic material and continental extensional allochthons from the OCTZ. We believe the revised site of subduction initiation, and the pre-collisional architecture, inherited from the rifting and spreading phases, allow reconciling most of the discrepancies between the various interpretation proposed in literature for the pre-collisional evolution of the Apennines.

Structural and Thermal Constraints to the Tectonic Evolution of Foredeep Successions in the Northern Apennines, Italy

A multi-method investigation (structural analysis, organic matter thermal maturity and clay mineralogy) was carried out on highly deformed foredeep successions in the Northern Apennine with special regard to shear zones in the Lago Trasimeno area (Rentella Unit) to unravel the main structural mechanisms and the thermal condition of the Miocene accretionary processes of the Northern Apennines belt. The Rentella Unit consists of a foredeep succession deposited in a paleogeographic domain between the Tuscan and the Umbria-Marche Domains. Meso- and microstructural data indicate shear planes compatible with C-planes (N160°, 45°SW). C-planes veins show dilational jogs and a staircase shape coherent to a top-to-the-NE sense of shear. Type II calcite twins in the shear zone associated veins suggest temperatures of 150-200°C. Temperature-dependent clay minerals and vitrinite reflectance indicate lower maximum paleo-temperatures (<100-110°C) in the early diagenesis and in the immature to early-mature stages of hydrocarbon generation. Thermal parameters suggest the development of the shear zone in the foredeep deposits at shallow depths (<3 km). Hot fluids coming from deeper structural levels within the collisional prism caused higher temperatures recorded in calcite twins of the shear zone veins.