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Geophysical Research Abstracts Vol. 15, EGU2013-8197, 2013 EGU General Assembly 2013 © Author(s) 2013. CC Attribution 3.0 License. Structuralcontrolonkarstcollapsesinkholeformation Antonio Santo (1), Alessandra Ascione (2), Stefano Mazzoli (2), and Nicoletta Santangelo (2) (1) Dipartimento di Ingegneria Idraulica, Geotecnica e Ambientale, Università Federico II, Naples, Italy (santo@unina.it), (2) Dipartimento di Scienze della Terra, Università Federico II, Naples, Italy (alessandra.ascione@unina.it) Collapse sinkholes owing their formation to erosion and deformation phenomena caused by subsurface karstificationarewidespreadinthecarbonatemassifsofpeninsularItaly.Incontrastwithsolutiondolines,whicharedensely distributedonthesubplanartopsurfacesofthecarbonatemassifs,thecollapsesinkholes(hereinafterlabelledkarst collapse sinkholes) generally occur as isolated landforms and mostly affect the slopes and piedmont areas. In the latterinstances,thesinkholesalsoaffectalluvialfanconglomerates,orslopedebris,overlyingthecarbonaterocks. Weinvestigatedthekarstcollapsesinkholesofthesouthern-centralApenninesmountainbelt(Italy),whichisrepresentativeofayoungorogenicsystem,characterisedbyrecenttectonicactivityandstrongseismicity.Theaimofthe studyistheidentificationofthecausativefactorswhichcontroltheoccurrenceofsuchhazardousphenomena.The study was based on a regional scale analysis on sinkhole distribution in relation to the local geological-structural, geomorphological and hydrogeological contexts, and was paralleled with field analysis of some selected areas. The regional scale analysisindicates thatthe karstcollapse sinkholesare not the mere response tothe concurrence of the climatic and lithological conditions which commonly favour the development of karst processes, the occurrence of such landforms appearing strongly influenced by distinctive structural and hydrogeological conditions. In particular, a close relationship between the karst collapse sinkholes and the main extensional faults showing evidence of late Quaternary activity may be envisaged. This is inferred from the spatial distribution of the karst collapsesinkholes,whichisstrikinglyuneven,thesinkholesgenerallyoccurringinalignmentsfollowinglargelate Quaternary fault zones, or being clustered at the terminations of those faults. In addition, areas affected by the occurrence of groups of sinkholes, are also characterised by the presence of H2S and/or CO2 mineral springs, gas vents and of travertine deposits. Such co-presence points to interrelations linking dissolution phenomena, and carbonate precipitation, to the rise of aggressive fluids. In particular, this suggests enhanced deep-seated dissolution, and carbonate deposition at surface, in presence of ascending mineral waters. These evidences point to the important role played by extensional fault zones in the migration of deeply derived fluids, thus suggesting that active faults, in particular, represent preferential pathways for fluid rising and mixing with shallow groundwater. In addition, a focus on the relationships between karst collapse sinkholes and extensional fault zone properties has shown how the fault length and depth, and the dimension of the damage zone, influence the sinkhole formation and evolution.

Composite tectonic emplacement of the Ronda peridotites - the largest outcropping body of subcontinental mantle rocks on Earth -- occurred as a result of a series of geodynamic events including: (i) Mesozoic break-up of Pangaea and opening of the Tethyan Ocean, (ii) Oligocene back-arc lithospheric extension, and (iii) Early Miocene continental subduction associated with oblique plate convergence. Top-to-the-hinterland shear along the upper contact of the peridotites during stage (iii) above is consistent with the kinematics expected for an extrusion wedge consisting of subcontinental mantle rocks. On the other hand, coeval strike-parallel extension and thinning of the crustal rocks overlying the peridotites confirms that, similarly to further Alpine-Mediterranean examples, partitioned transpression resulted in the development of a complex deformation pattern, with kinematically linked shear zones aiding exhumation. Partitioning of transpressional deformation between coeval orogen-parallel wrenching and orogen-perpendicular, pure thrusting components is recorded by shear zone kinematics and dynamothermal metamorphism in the footwall to the ultramafic rocks. Left-lateral shear, characterizing the deeper, high-pressure (eclogitic) portions of the continental subduction system, propagated through the mantle into the overlying continental crust of the overriding plate, while top-to-the foreland frontal thrusting dominated at the leading edge of the hot peridotite body. In this latter area, strongly heterogeneous deformation and extreme metamorphic gradients characterize the dominantly carbonate Nieves Unit in the footwall to the peridotites. A well-developed foliation and mineral lineation, together with isoclinal intrafolial folds, occur in silicate-bearing, calcite/dolomite marbles within a c. 1.5 km-thick metamorphic aureole underlying the peridotites. For the inferred maximum pressure of 300 MPa, petrological investigations allow to define temperature ranges for the main zones of the metamorphic aureole: forsterite zone (> 510 °C; probably c. 700 °C), diopside zone (510-430 °C), tremolite zone (430-360 °C), and phlogopite zone (360-330 °C). Field structural analysis integrated with petrological, microstructural and EBSD textural data document large finite strains consistent with general shear within the metamorphic aureole, associated with NW-ward thrusting of the peridotites. On the other hand, post-kinematic silicate growth suggests that heat diffusion from the high-temperature peridotites continued after the final emplacement of the Ronda mantle extrusion wedge, leading to final zoning of the metamorphic aureole and to local partial annealing of calcite marble textures, particularly in the highest-temperature zone of the thermally softened footwall carbonates. Following substantial cooling, renewed crustal shortening affected the whole Nieves Unit, resulting in widespread development of NE-trending meso-scale folds.

Finite element modelling by means of MSC. Marc software has been used to gain new insights into the deep structure of the southern Apennines, particularly concerning the geometry and kinematics of fault systems controlling active deformation and seismicity. The Apennine accretionary wedge forms an allochthonous mass overlying foreland strata continuous with those exposed in the Apulian promontory. The tectonically buried portion of the Apulian Platform was involved in the final shortening phases, giving rise to reverse-fault- related, open, long-wavelength folds that form the hydrocarbon traps for the oil discoveries in southern Italy. Recent geophysical studies have provided evidence that deep-seated reverse faulting involves the basement, while interpretation of high-quality seismic profiles and cross-section balancing favour an inversion tectonics model involving reactivation of pre-existing (Permo-Triassic) basement normal faults, the related deformation being characterized by limited horizontal displacements. Our work suggests an important role of the mélange zone interposed between the buried Apulian Platform carbonates and the surface allochthnous units in producing effective decoupling between deep and shallow structural levels. While the former are dominated by deeply rooted major faults, surface units are dissected by numerous brittle structures that formed at various stages during Apennines evolution. As a result of late Quaternary extensional reactivation of the deep-seated major faults, the inherited fault network characterizing the shallow rigid lid (decoupled by the ductile mélange zone at its base) is reactivated in a complex fashion. This results in a highly segmented and articulated fault pattern. On the other hand, major brittle structures in the buried Apulian Platform carbonates and underlying basement represent mature fault zone that cumulated displacements of up to a few kilometres over geologic time. As such, they are capable of nucleating large earthquakes as a result of late Quaternary extensional reactivation. The role of these important discontinuities, as well as of thickness variations of different crustal layers, is emphasized by our finite element modelling, which effectively outlines stress perturbations and strain field around main active structures.

Field data and seismic reflection profiles of various resolutions, calibrated by deep well logs, have been used to unravel the tectonic evolution of the Crati Basin (southern Italy). The study area is located in the northern portion of the Calabrian Arc, a well-developed arc-shaped feature of the circum-Mediterranean belts, consisting of a series of ophiolite-bearing tectonic units and overlying basement nappes. NW–SE oriented left-lateral strike-slip faults exerted a major control on the tectonic evolution of northern-central Calabria, from Middle Miocene to Lower Pleistocene times. Such faults, arranged in an en-echelon geometry and dissecting the pre-existing Late Oligocene–Early Miocene orogenic belt, led to a structural setting including major N-S striking synforms – as the offshore Paola Basin and the Crati Basin are interpreted based on our results – separated by a broad antiformal ridge. Since the Middle Pleistocene, both E- and W-dipping normal faults developed in the southernmost sector of the Crati Basin, probably as a consequence of both uplift of the orogenic edifice and Tyrrhenian back-arc extension. The pre-existing regional strike-slip faults became inactive in this sector of the belt. However, working as persistent barriers, it is envisaged here that they inhibited the southern propagation of the newly formed normal faults, which therefore propagated towards the north. A minimum value of cumulative displacement of ca. 600 m has been unraveled for the central sector of the Crati Basin since Middle Pleistocene times. This yields a vertical strain rate of ca. 0.9 mm/y during the last 700 ka.

A focal mechanism study was carried out for seismic events that occurred in the central-northern Marche region in years 2003 to 2009. The study is based on earthquake data, having magnitude M ≥ 3.5, recorded by the seismic network of the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The results show that events tend to cluster into different sectors, each being characterized by a roughly homogeneous seismological behaviour. The new data confirm the occurrence of an extensional stress regime in the axial zone of the mountain belt, generally characterized by vertical or sub-vertical P axes. On the other hand, a different stress regime appears to dominate in the outer Apennine sector extending from the high valley of the Marecchia River to the foothills west of Ancona. Here, the well-known extensional domain of the axial zone of the Apennine mountain belt gives way, through an area characterized by oblique-slip faulting and variably plunging P axes, to a regime dominated by roughly horizontal P axes along the Adriatic onshore. Roughly horizontal P axes characterize also the adjacent Adriatic Sea sector, although some variability in the maximum compression direction occurs in the offshore area.

The object of this study is the estimation of the long-term erosion rate in the southern portion of the Apennines chain (southern Italy). This was done with the aim of validating the hypothesis, proposed by MAZZOLI et al.. (2008), that thin-skinned extensional tectonics played a major role in the exhumation of formerly deeply buried tectonic units. More in detail, the study aimed at providing data on the erosional component of the rock exhumation which, based on Apatite Fission Tracks data reported in MAZZOLI et al. (2008), has affec- ted in the last 10 Ma the deepest units in the southern Apennines fold–and-thrust belt. The erosion rate was estimated based on the evaluation of the volume of deposits delivered by a large hydrographic basin to a conti- nental environment sedimentary basin. The accumulation basin is the Sant’Arcangelo basin (hereinafter BSA), which developed as a marine environment wedge-top basin (originally connected to the Apennines foreland basin) during the Middle Pliocene and evolved as an alluvial-lacustrine basin in the late part of the Early Pleistocene in response to the thrusting related uplift of a ridge located to the E of it (VEZZANI, 1966, 1967; CARBONE, 1991; PIERI, 1994; ZAVALA, 2000; PATACCA & SCANDONE, 2001; GIANNANDREA & LOIACONO, 2003; BENVENUTI et al., 2006). The drainage basin correlated to the BSA covers a large portion of the eastern slope of the Southern Apennines, and includes elevations formed of formerly deep-seated structural units (namely, Lagonegro units and Apulian platform unit) exhumed during Pliocene-Quaternary times. Literature data and field surveys allowed the identification of the sedimentary unit (Castronuovo-San Lorenzo unit, CSL, composed of lacustrine deposits passing laterally and upwards into alluvial plain sediments) which was deposited within the BSA when the basin was isolated from the neighboring foreland basin. The CSL ranges in age from around 1 to 0,7 Ma (PATACCA & SCANDONE, 2001; MAT- TEI et al., 2004; SABATO et al., 2005). G.I.S. elaborations allowed the reconstruction of the top and bottom surfaces and volume of the CSL. The calculated volume value was corrected accounting for the erosion which affected the CSL top surface, for the original width of the deposition area, for the suspended load loss, and for the sediment porosity. The paleo-hydrographic basin coeval to the CSL was reconstructed based on geomorphological/geological evidences, which include the compositional analysis of pebbles of different units of the BSA succession. The obtained 0,25 ± 0,07 mm/a erosion rate value is comparable to the values, averaged over the last 0,7 Ma and spanning from 0,2 to 0.3 mm/a, estimated for the eastern chain margin-foreland basin area in both the southern and northern Apennines (AMATO et al., 2003; CYR & GRANGER, 2008). These values, which can be considered as representative of erosion which has affected the Apennines over the Quaternary, are much lower than the Pliocene-Quaternary rock exhumation of formerly buried units estimated by AFT data, and this points to the important contribution of the extensional tectonics in the exhumation processes in the Southern Apennines.

The monumental complex of the Historical Sanctuary of Machu Picchu, declared as World Heritage Site by UNESCO in 1983, is located in the Andean chain at approx. 80 km from Cuzco (Peru) and at an elevation of 2430 m a.s.l. along the Urubamba River Valley. From a geological point of view, the Machu Picchu granitoid pluton, forming part of the larger "Quillabamba granite", is one of a series of plutons intruded along the axial zone of the high Eastern Cordillera Permo-Liassic rift system including a variety of rock types, dominantly granites and granodiorites. The most evident structures at the outcrop scale consist of planar joint sets that may be variably reactivated and exhibiting 4 main orientations. At present, the site is affected by geological risk due to frequent landslides that threaten security and tourist exploitation. In the last years, the international landslide scientific community has promoted a multi-discipline joint programme mainly finalised to slope deformation monitoring and analysis after the warning, launched in 2001, of a potential collapse of the citadel, caused by a huge rock slide. The contribute of the Italian research team was devoted to implement a landslide risk analysis and an innovative remote sensing techniques. The main scope of this work is to present the implementation of a geo-structural modelling aimed at defining present and potential slope stability conditions of the Machu Picchu Citadel. Data have been collected by geological, structural and geomechanical field surveys and laboratory tests in order to reconstruct the geomorphological evolution of the area. Landslide types and evolution are strictly controlled by regional tectonic uplift and structural setting. Several slope instability phenomena have been identified and classified according to mechanism, material involved and state of activity. Rock falls, debris flows, rock slides and debris slides are the main surveyed landslide types. Rock slides and rock falls may produce blocks with dimensions variable from 10-1 to 102m3 that form the toe accumulation on steeper slopes. The area of the citadel has also been interpreted as affected by a deep mass movement (>100m) that, if confirmed by the present day monitoring systems, could be referred to a deep-seated gravitational slope deformation (DSGSD), probably of the type of the compound bi-planar sagging (CB) described by Hutchinson (1988). The analysis of active strain processes (e.g. tension cracks) along with the damage pattern surveyed on archaeological structures (e.g. sinking, swelling, tilting) suggest that the potential failure of a large rock slide may be located at a depth of ca. 30m. The various data sets have been integrated in order to obtain a general geo-structural and geotechnical model (strength and deformation parameters, seismic input) of the citadel at the slope scale. This represents a first step in implementing a slope stability analysis capable of reconstructing present and potential landslide evolution under static and dynamic conditions. This multi-discipline study, based on geological and structural analysis integrated with geotechnical and geomechanical interpretation, will aid defining actual landslide hazard and risk levels, indispensable for the design of low impact mitigation measures to be applied at Machu Picchu Citadel.

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