Giuseppe Bettelli

Geological record of fluid flow and seismogenesis along an erosive subducting plate boundary

Tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the Earth’s subduction zones. In situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone. Until now, a fossil erosive subduction channel—the shear zone marking the plate boundary—has not been recognized in the field, so that seismic observations have provided the only information on plate boundary processes at erosive margins. Here we show that a fossil erosive margin is preserved in the Northern Apennines of Italy. It formed during the Tertiary transition from oceanic subduction to continental collision, and was preserved by the late deactivation and fossilization of the plate boundary. The outcropping erosive subduction channel is ∼500 m thick. It is representative of the first 5 km of depth, with its deeper portions reaching ∼150 °C. The fossil zone records several surprises. Two décollements were simultaneously active at the top and base of the subduction channel. Both deeper basal erosion and near-surface frontal erosion occurred. At shallow depths extension was a key deformation component within this erosive convergent plate boundary, and slip occurred without an observable fluid pressure cycle. At depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast (co-seismic) and slow (inter-seismic) slip. In the deepest portions of the outcropping subduction channel, extension is finally overprinted by compressional structures. In modern subduction zones the onset of seismic activity is believed to occur at ∼150 °C, but in the fossil channel the onset occurred at cooler palaeo-temperatures.

On the nature of scaly fabric and scaly clay

Abstract Scaly clay, deriving from the Italian argille scagliose , is a term that has been used with a range of meanings, from stratigraphic to genetic, and across many scales of observation. Moreover the diagnostic feature of scaly clay—scaly fabric—has a variety of associated expressions used differently in different geological or structural settings. In an attempt to clarify and rationalise these confused terminologies, we have analysed a wide range of scaly clays of clearly contrasting origin. We here describe the appearance and nature of the fabrics at different scales of observations and interpret the mechanisms responsible for their development. Importantly, mesoscopic similarities may well not be reflected at the microscopic scale. As a result, we recommend that the term scaly fabric should only be used for description at the hand-specimen scale, although the fabric can be sub-classified microscopically according to the shape and arrangement of the rock components. Because scaly fabric defines the tendency of the rock to break along specific surfaces and has a morphological expression, we characterise it as a variety of rock cleavage.

Structural style of the offscraped Ligurian oceanic sequences of the Northern Apennines: new hypothesis concerning the development of mélange block-in-matrix fabric

Shaly dismembered formations crop out extensively on the eastern side of the Northern Apennines, as overthrusted Cretaceous oceanic units. The lack of metamorphism and the unconformably overlain epi-Ligurian sequence, which represents the trench/slope-basin sedimentary sequence, suggest that these units are accreted sequences accomplished by offscraping and imbrication. These units are characterized by stratal disruption, lack of internal coherence and complete destruction of the original stratigraphic sequence. Block-in-matrix fabric results in symmetric boudins and systematic presence of isolated or intrafolial hinges of isoclinal folds. Two orthogonal sets of tight-to-isoclinal folds are always recognizable, involving the break-up and the boudinage of the competent layers, now transposed parallel to the axial planes of the disrupted isoclinal folds. The superposition of two generations of orthogonal tight-to-isoclinal folds may be locally observed either in single bed fragments or in some preserved coherent units. Disruption and block-in-matrix fabric is a consequence of polyphase folding and intense shortening. This geometry and kinematic interpretation well constrain the tectonic environment where these rocks were deformed to the proximity of the prism toe and within the prism itself. Thus, the Apennine broken formations result from the disruption of a sedimentary pile lying on the subducting plate of the Cretaceous to Eocene trench fore-arc system, which generated the Ligurian accretionary wedge.

Tectonic and sedimentary evolution of the frontal part of an ancient subduction complex at the transition from accretion to erosion: The case of the Ligurian wedge of the northern Apennines, Italy

Subduction can be associated either with accretion or with removal of material from the overriding plate. These two processes can either coexist or alternate in time along the same margin. Their inception has the potential to change the dynamic equilibrium of a marginal wedge resulting in the development of out-of-sequence thrusts, normal and strike-slip faults, or large submarine landslides in the frontal part of the subduction zone. In this work we investigate the effects of the transition from frontal accretion to frontal erosion on the stability of a subduction complex through the study of an ancient example from the northern Apennines (Ligurian subduction complex). New structural data suggest that in the early Neogene (Aquitanian), the removal and underthrusting of the toe of the wedge, formed by both the accreted oceanic sediments and the overlying wedge-top basin fill (i.e., the Subligurian units), implied a process of frontal tectonic erosion. The presence, on top of the subduction complex, of a complete succession of mid-late Eocene to late Miocene slope-apron sediments—i.e., the Epiligurian succession—facilitates a reconstruction of the sedimentary response to this event. In the Aquitanian, large areas of the wedge were denuded from the lower-slope sedimentary cover through extensive gravitational mass movements. The subsequent deposition of a thick body of submarine debris flows is documented. The mass-wasting deposits are interpreted as the sedimentary response to the underthrusting of the frontal part of the Ligurian subduction complex formed by the Subligurian units.

Internal structure and tectonic evolution of an underthrust tectonic mélange: the Sestola-Vidiciatico tectonic unit of the Northern Apennines, Italy

The Sestola-Vidiciatico Tectonic Unit (SVTU) in the Northern Apennines is an underthrust tectonic mélange presently sandwiched between the Tuscan-Umbrian foredeep units and the overlying Ligurian/Subligurian thrust-nappe. The SVTU has been generated during the collision between the European and the Adria plates and now it separates the former oceanic accretionary wedge Ligurian/Subligurian thrust nappefrom the underlying fold–and-thrust belt formed by Adria sedimentary units. The collision caused an eastward migrating foredeep basin and the overthrusting of the frontal part of the Ligurian/Subligurian thrust-nappe on the subducting Adria margin. Part of the inner lower-slope sediments of the migrating foredeep basin have been unconformably deposited on a frontal prism formed by material already accreted in the Ligurian/Subligurian prism gravitationally and tectonically reworked. The frontal prism and its sedimentary cover have been progressively dragged down along the plate boundary zone generating the SVTU. The lower-slope sediments have been incorporated in the mélange as they were not completely lithified, and they show a long deformation history ranging from continuous and pervasive soft-sediment deformation to discontinuous brittle deformation concentrated along faults and mainly controlled by cycles of fluid pressure as testified by the presence of crack-and-seal texture and implosion breccia in the veins.

Mechanisms of subduction accretion as implied from the broken formations in the Apennines, Italy

Mesoscale structural analysis of the Ligurian broken formations of the Northern Apennines and the regional occurrence of subduction-related structures indicate polyphase folding as the mechanism responsible for progressive disruption. The evidence for direct correlation between regionally extensive noncylindrical folds and tectonic disruption has a strong bearing on determining how the Ligurian accretionary prism evolved. For example, the block-in-matrix structure of the Ligurian tectonic melanges is produced through extension developed along fold limbs in a compressive stress field, as expected from offscraping and intraprism deformation. The model implied by the mesoscale and macroscale analyses of the Ligurian tectonic melanges provides one mechanism of accretion reconciling two deformation styles, i.e., stratal disruption and folding, commonly seen in ancient accretionary prisms. Furthermore, this model could be relevant in active accretionary prisms, especially in the intraprism part, where the seismic and drilling data are sparse.

Myths and recent progress regarding the Argille Scagliose, Northern Apennines, Italy

Argille scagliose (scaly clay) is a geological term first used in 1840 to describe rocks in the Northern Apennines of Italy. The term was originally created to stress the mesoscopic scaliness of a type of rock that commonly outcrops in this area. The rock is also typified by a chaotic assemblage of blocky components that are embedded within the scaly matrix. Before the advent of plate tectonic concepts, the extreme complexity of these rocks posed an extreme challenge to interpret with then-standard concepts of deposition processes in sedimentary basins. Similar rocks were recognized in many other mountain belts, thus the term became widely used. At the same time, the emphasis of the term changed from a description of the matrix to a term with multiple, intensely debated, genetic associations. Only after the discovery of plate tectonics was it accepted that these rocks are formed at subduction boundaries, and that the multiple types of embedded blocks can have an origin from both slope-instabilities within an accretionary prism, and from tectonic reworking/deformation processes near the base of an active accretionary prism. This paper reconstructs the scientific evolution of thinking about argille scagliose during the years when it was one of the supreme challenges to sedimentary geologists. The often strong debate between competing hypotheses for the origin of these rocks led to many myths that still outcrop in the geological literature. We explore why this happened, the apparent future of modern research on chaotic rocks in the Apennines and other fossil accretionary prisms, and what is and should productively remain from the long geological controversy over argille scagliose.

Chapter 3 Aseismic-Seismic Transition and Fluid Regime along Subduction Plate Boundaries and a Fossil Example from the Northern Apennines of Italy

This chapter reviews observations and theories for the aseismic-seismic transition in the megathrust between the incoming and overriding plates at a subduction zone. The temperature of the aseismic-seismic transition appears to be quite similar at erosive and accretionary margins, despite large differences between them in the lithology of the seismogenic subduction channel that composes the “megathrust” plate interface. This fact, and the recent laboratory demonstration that both smectite and illite are velocity-strengthening in creep, suggests that the oft-postulated change in mechanical behavior of the megathrust due to a smectite-illite clay mineral transformation at ∼150°C is not the cause of the onset in seismogenesis at these temperature conditions within the subduction channel. Field observations from fossil megathrust zones suggest that a temperature-dependent change in the availability of in situ fluid is likely to play a key role in the onset of seismogenesis. Perhaps the causal link is to the smectite-illite transformation and other metamorphic dewatering reactions that liberate water at ∼150°C, under conditions where these reactions are an important local source of hydrous fluids. Field studies of fossil megathrusts support the hypothesis that fluids “control” seismogenesis, and indicate that there are large fluid pressure variations during the seismic cycle. In the fossil erosive megathrust system preserved in the Apennines, two decollements are simultaneously active at the roof and base of the subduction channel. The uppermost (nonseismogenic) portion of the megathrust even appears to alternate between tensional and compressional modes of failure during the seismic cycle along the deeper portions of the megathrust.

Early exhumation of underthrust units near the toe of an ancient erosive subduction zone: A case study from the Northern Apennines of Italy

Apatite fission-track (AFT) analyses were performed on 16 sandstone samples from a tectonic melange unit exposed in three tectonic windows located near the inferred front of the early Miocene subduction system of the Northern Apennines of Italy. The tectonic windows display a block-on-block tectonic melange present under the Ligurian Units. The melange is formed by portions of the upper plate incorporated in the plate boundary shear zone as a consequence of a mechanism of frontal tectonic erosion during the Aquitanian (early Miocene). AFT and structural data, together with stratigraphic constraints, allow the reconstruction of a complete deformation cycle with a phase of underthrusting followed by underplating and early exhumation of the tectonic melange. The exhumation, in particular, took place at ∼10–20 km from the original subduction front. Moreover, the analysis suggests that multiple faults were active at the same time in the frontal part of the subduction zone.

Lateral variability of the erosive plate boundary in the Northern Apennines, Italy

The early to middle Miocene erosive plate boundary preserved in the Northern Apennines NW of the Sillaro Line is formed by two distinct units – the Sestola-Vidiciatico Tectonic Unit and the Subligurian Units. These two units occupy, respectively, the SE and the NW portions of the studied area. Upon closer examination, the features that distinguish these mapped units do not reflect differing plate boundary processes, but rather the incorporation or non-incorporation of forearc-toe mass-wasting deposits into the active subduction channel. In other respects, these two units document similar subduction channel processes, including the contemporaneous activity of multiple sub-parallel slip surfaces. This mode of subduction channel deformation leads to the ‘laminar’ incorporation of distinct stacked slices within the channel.