R Funiciello

Midcrustal shear zones in postorogenic extension: Example from the northern Tyrrhenian Sea

Metamorphic core complexes of the Aegean region have revealed midcrustal, shallow-dipping extensional shear zones. These shear zones display constant kinematic indicators over large regions (100–200 km). We analyze the example of the northern Tyrrhenian Sea and then compare it to the Aegean region. We first summarize our observations on ductile extension and metamorphic evolution in the northern Tyrrhenian Sea from Alpine Corsica to Tuscany. (1) Extension migrated from west to east from the early Miocene in Corsica to the Recent in the Apennines; (2) Extension is accommodated by shallow east dipping extensional shear zones at the depth of the brittle-ductile transition, from the early Miocene to the Pliocene. (3) West dipping normal faults accommodate extension on the eastern side of the volcanic arc. (4) Extension is preceded along the convergence front by the formation of a thrust wedge, where high-pressure and low-temperature conditions are recorded; maximum PT conditions decrease toward the east, and PT paths are systematically very cold, suggesting that a large part of the exhumation occurred during synorogenic extension. We discuss the possible mechanisms that account for constant shear sense over large domains. The model involves retreat of the slab and migration of the volcanic arc. Partially molten lower crust acts as a low strength zone where extensional strain is localized. Eastward motion of the upper mantle as a consequence of the migration of the slab induced a component of shear toward the volcanic arc at the base of the stronger upper crust. In the weak upper mantle and lower crust, to the west of the volcanic arc, extensional stresses are not transmitted; this produces a top-to-the-east sense of shear at the base of the upper crust that migrates eastward, following arc migration.

Magnetic fabric of weakly deformed clay-rich sediments in the Italian peninsula: Relationship with compressional and extensional tectonics

Abstract We present the results of anisotropy of magnetic susceptibility (AMS) analyses carried out in weakly deformed Neogene and Quaternary clay-rich sediments from different compressional and extensional settings of the Italian peninsula, discussing the relationships between the magnetic fabrics and the tectonic settings. A well defined magnetic lineation of tectonic origin was found in several structures. The studied cases indicate that the AMS analysis of fine-grained sediments constitutes a powerful method to better constrain the tectonic evolution of sedimentary basins, where strain markers are not available. In the extensional basins the magnetic lineation coincides almost always with the stretching direction, obtained from mesostructral analysis of faults and joints, and it is generally aligned with the bedding dip. This geometric relationship is different in the compressional basins, where the magnetic lineation is almost parallel to the bedding strike. In both the extensional and compressional environments the magnetic lineation was acquired during the early stages of deformation, when the bedding was still sub-horizontal and it was not modified by the subsequent tectonic phases.

Analogue models of collapse calderas and resurgent domes

Collapse calderas and resurgent domes are a common association related to inflation–deflation processes in volcanic systems. The structure of calderas and domes depends upon the permitted, relative movements of crustal volumes at depth (the so-called “space problem”). In order to study the structures of collapse calderas and resurgent domes and to take the space problem into account, several analogue models were made. Dry-quartz sand was used to simulate the rheology of the brittle crust, while newtonian silicone putty, located at the base of the sand-pack, simulated the ductile behaviour of the magma. A piston moved the silicone putty downward or upward, inducing collapse and doming within the sand. Three separate sets of experiments simulated: (1) caldera collapse; (2) resurgence; and (3) superimposition of resurgence on collapse and vice versa. Collapse experiments are characterized by the development of two concentric depressions; the first-formed depression is bordered by outward dipping reverse ring faults; the subsequent, outer concentric depression is bordered by inward dipping normal ring faults. The deformation pattern during resurgence is a function of the overburden thickness (T) and the dome diameter (D). For higher T/D ratios a dome forms, bordered by inward dipping high angle reverse ring faults; outward dipping normal ring faults develop at late stages. For lower T/D ratios, the dome shows, at late stages, a crestal depression accompanied by radial fractures; subsequently, an apical extrusion of silicone occurs. The superimposition of resurgence over collapse (and vice versa) is characterized by the complete reactivation, with opposite kinematics, of all the pre-existing ring faults during inversion. Both in caldera and resurgence, reverse ring faults form in the early stages due to differential uplift; extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults. The experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences.

The Albano Maar Lake (Colli Albani volcano, Italy): recent volcanic activity and evidence of pre-Roman age catastrophic lahar events

Abstract The evaluation of volcanic hazard in the Roman hinterland related to the quiescent Colli Albani Volcano has recently been the subject of renewed attention and several interpretations by many authors. However, very little was known of the recent history of the volcano, making such interpretations rather speculative. The most recent activity of Colli Albani Volcano originated from the Albano polygenetic maar lake, which erupted several phreatomagmatic units, the most recent of which, the Peperino Albano ignimbrite, has been dated at around 25 ka. An area of several square kilometers centered around Albano Lake is presently the site of shallow and frequent seismic activity and gaseous emission as well as hydrothermal activity and is therefore considered the most prone to geologic hazards. This paper presents new stratigraphic and geomorphologic data as well as age determinations that allow rejuvenation of the most recent activity of the Colli Albani Volcano, and particularly the Albano maar lake, to the Holocene. This study allows for the first time to identify a potential hazard related to the Albano maar lake withdrawal interpreted to be related to endogenous causes, namely CO 2 emission. The main results of the study are: (1) the Peperino Albano is not, as is generally believed, the last phreatomagmatic eruption from the Colli Albani Volcano; a previously unrecognized phreatomagmatic surge deposit has been identified overlying the paleosol at the top of the Peperino Albano and related lahar deposits; (2) two lahar deposits separated by paleosols top the stratigraphic succession and are dispersed only to the NW, corresponding to the lowest point of the maar rim, indicating that catastrophic hydrologic events occurred at the Albano Lake in recent times; rapid and substantial lake-level variations and lake withdrawal are reported by Roman historians and recorded by the stratigraphy of the Albano Lake lacustrine sediments; (3) microfracturing related to seismic energy release is linked to sudden variation of CO 2 flow and upwelling of hydrothermal fluids. These occurrences across the lake are the likely causes that triggered during Holocene several episodes of lake withdrawal, rising the water table and probably triggering convective rollover of the lake water.

Alpine structural and metamorphic signature of the Sila Piccola Massif nappe stack (Calabria, Italy): Insights for the tectonic evolution of the Calabrian Arc

Combined structural and petrographical investigations, coupled with 40Ar/39Ar geochronology, were carried out in the Sila Piccola Massif of the Calabrian Arc in order to define the structural geometry and map out the major structural and metamorphic breaks within the exposed nappe sequence. On the basis of the contrasting Alpine pressure-temperature (P-T) and structural signatures the nappe stack can be divided in two major tectonic complexes, bounded by a flat-lying ductile to brittle extensional shear zone. The upper complex consists of a nappe-like structure, where a major top to the east compressional shear is recorded. The lower tectonic complex consists of an ophiolite-bearing sequence showing typical high-P/low-T parageneses (Mg-carpholite and Na-amphibole). The 40Ar/39Ar geochronology on phengites in equilibrium with blueschist minerals provided a minimum age estimate for the blueschist event in the lower complex rocks at the Oligocene-Eocene boundary (around 35 Ma). Ductile to brittle top to the west extensional shear accompanied the nearly isothermal retrogression and exhumation of the lower complex rocks, reworking the previous nappe contacts with shear localization along the upper/lower tectonic complex discontinuity. The 40Ar/39Ar dating indicates that this postnappe stacking tectonic evolution took place from 30 Ma onward. It is proposed that exhumation of the deep-seated rocks occurred below a top to the west extensional detachment active during convergence and orogenic complex formation (synorogenic extension). The age of this detachment is bracketed between 30 Ma and the post-orogenic Neogene basin sedimentation (middle-upper Miocene). The revised structural and metamorphic scenario is here integrated into a new tectonic evolutionary reconstruction, which involves an early high-P/low-T top to the east crustal thickening episode during the construction of the Apennine orogenic wedge (Eocene-Oligocene), followed and overprinted by a top to the west extensional shear, probably active from the late Oligocene.

Magnetic fabric of clay sediments from the external northern Apennines (Italy)

Abstract We report on the anisotropy of magnetic susceptibility (AMS) analyses of fine-grained sediments deposited during the Messinian in foredeep basins at the front of the northern Apenninic chain. The data refer to 32 sampling sites, mostly distributed in the fine-grained intervals of the Laga and Colombacci formations, extending along the belt for a total length of about 300 km. Rock magnetism analyses indicate that the magnetic susceptibility and its anisotropy are in most cases dominated by the paramagnetic minerals of the clay matrix. In order to delineate the contribution of the ferrimagnetic fraction to the overall susceptibility fabric, the anisotropy of the anhysteretic remanent magnetisation was investigated at some representative sites. The magnetic fabric of the studied sediments mostly reflects the effects of compaction, showing a predominant magnetic foliation parallel to the bedding plane. At all the sites a well distinct magnetic lineation was also found, which is parallel to the fold axes and thrust fronts, both at local and regional scales. This feature is maintained in sequences that differ for sedimentological character and age, implying that the magnetic lineation was produced by a mild tectonic overprint of the primary sedimentary-compactional fabric. The relationship between the magnetic lineation trends and the vertical axis rotations detected by Speranza et al. [Speranza, F., Sagnotti, L., Mattei, M., 1997. J. Geophys. Res. 102, 3153–3166] indicates that the magnetic lineation formed during the compressive phases of the Messinian-early Pliocene, when the Apenninic front was almost rectilinear and oriented N320°.

An active margin across the Adriatic Sea (central Mediterranean Sea)

Abstract New seismological and structural data from the central Adriatic—Gargano promontory area demonstrate that the current models of an aseismic and slightly deformed Adriatic block have to be revised. A seismically active deformational belt is mapped along two main fault systems, the Tremiti Islands and Mattinata faults. Moreover, geologic and geophysical evidence suggests that a more extensive lithospheric boundary may cut across the central Italian peninsula to the Tyrrhenian basin.

Topographic lineament swarms: Clues to their origin from domain analysis of Italy

Regional-scale, subparallel linear topographic features characterize almost all planetary surfaces. This experiment concerns their tectonic meaning by focusing on map limits or domains through which individual swarms of these linear features are developed. These domain limits are contrasted with map boundaries of better-known structural features in a tectonically active region (Italy) to seek clues to stress environments and times of origin of the lineaments. The study uses regional raised relief maps to filter the lineament data to a very simple subset: those strictly topographic features of a length and prominence capable of retaining detectability through generalizations required by relief-map production. A total of 5,372 lineaments were drawn using 4 differently lighted images of 1:1,000,000 scale relief maps of Italy. Seven different tests were used for reliability and reproducibility of the data. Rose diagrams were prepared for 86 subareas by computer fitting of gaussians to azimuth-frequency histograms. Individual azimuthal “petals” of these roses were then correlated to delimit the general area over which a given azimuthal swarm is developed. The precise swarm boundaries were then located by computer contouring the population density of lines of each swarm on the original data set. In this way, 48 local swarms were mapped. Boundaries of these swarms correlate poorly with traditional litho-tectonic provinces. Instead, they seem to be associated with basin axes, broad arches, coastal flexures, areas of normal fault swarms, and projections of structural grain from adjacent sea floors. The 48 domains may be grouped into 8 noncontiguous but azimuthally compatible super-swarms covering much of Italy. The most prominent of the super-swarms are greatly expanded versions of regional structural grains: trend of the Po Basin, axis of the upper Adriatic Basin, zone of south Alpine underthrusting, and landward extensions of structural grain of the Tyrrhenian Sea. From these relationships, a model for lineament-swarm origins is proposed, involving very minor regional stretching of the thin, brittle carapace of extremely large, sometimes subtle, structures deforming by ductile mechanisms at depth.

Interaction and linkage of extension fractures and normal faults: examples from the rift zone of Iceland

Abstract Field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of Iceland. Each zone consists of a pair of extension fractures or a pair of normal faults, with lengths from metres to kilometres. These zones evolve from an underlapping stage, through an overlapping stage (the most common configuration) and, finally, to a linkage stage. Of all the zones, only 7% are underlapping, whereas 93% are overlapping, with hook-shaped fracture pairs. The length/width ratios of the overlapping zones have a mean value of 3.5. The preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures (length, overstep) and the development of the interaction. In the overlapping zones, most fracture pairs show moderate shear (strike-slip) components, related to local variations in the extension (opening) directions. Vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase; both, in turn, are proportional to the total lengths of the faults forming the pair. The geometrical and kinematic features of overlapping spreading centres at mid-ocean ridges show close similarities to those reported here. These similarities indicate that the architecture and evolution of overlapping zones are scale independent.

The interaction between regional and local tectonics during resurgent doming : the case of the island of Ischia, Italy

Abstract The volcanic island of Ischia is located on the Tyrrhenian margin of Central Italy, characterized by Plio-Quaternary NW–SE- and NE–SW-trending extensional fractures. Ischia displays a resurgent dome uplifted by at least 800 m in the last 33 ka. Remote sensing and field data have been collected to study the structural setting of the island, the deformation pattern associated with resurgence and the superimposition of the regional and the resurgence-induced stress fields. NW–SE and NE–SW extensional fracture systems predominate throughout the island and around the resurgent block, suggesting a relationship with the regional extensional structures. These systems were formed before resurgence and were partly reactivated during resurgence. The reactivation of pre-existing regional systems during resurgence confined the extent of the uplifted area. N–S- and E–W-trending systems have been found exclusively at the borders of the dome and are interpreted as being induced by resurgence. The topmost resurgent block shows an octagonal shape in map view and is tilted at an angle of 15° around a NE–SW-trending horizontal axis; the block is partly bordered by high-angle, inward-dipping regional faults. More than 90% in volume of the volcanic products coeval with resurgence on Ischia have been erupted outside the resurgent block area, suggesting that the resurgence process locally replaced volcanic activity in the last 33 ka.