application of the ads method to predict a hidden basal detachment nw borneo fold and thrust belt

Abstract The NW Borneo margin is characterized by a complex deepwater fold-and-thrust belt. Despite previous studies, the definition of a univocal detachment level for folding and thrusting is still lacking. The area-depth-strain (ADS) method can be used to determine the location for a detachment in areas lacking data, and to balance geological cross sections. This study applies the ADS method to the central part of the NW Borneo fold-and-thrust belt to predict a structurally conclusive detachment level in an area lacking a seismic detachment reflection. Seismic interpretations were completed after the ADS-determination of the basal detachment, providing input for a 2D sequential restoration that delivered values on shortening distribution and shortening rate. The kinematic and mechanic analyses presented, document that the central part of the NW Borneo fold-and-thrust belt is affected by both, near- and far-field stresses, and that the far-field crustal shortening component becomes more important northward. This work demonstrates that the ADS method can be effectively applied in fold-thrust belt settings with limited information on the detachment, supports in a quantitative way the tectonic and stratigraphic interpretation of seismic-reflection data and provides a robust structural base for the restoration of balanced cross-sections, including the reconstruction of syn-kinematically eroded stratigraphy

IMAGING THE ACTIVE STRESS FIELD OF THREE SEISMOGENIC AREAS ALONG THE APENNINES AS REVEALED BY CRUSTAL ANISOTROPY

During the last decades, the study of seismic anisotropy has provided useful information for the interpretation and evaluation of the stress field and active crustal deformation. Seismic anisotropy can yield valuable information on upper crustal structure, fracture field, and presence of fluid-saturated rocks crossed by shear waves. Several studies worldwide demonstrate that seismic anisotropy is related to stress-aligned, filled-fluid micro-cracks (EDA model). An automatic analysis code, “Anisomat+”, was developed, tested and improved to calculate the anisotropic parameters: fast polarization direction (φ) and delay time (∂t). Anisomat+ has been compared to other two automatic analysis codes (SPY and SHEBA) and tested on three zones of the Apennines (Val d’Agri, Tiber Valley and L’Aquila surroundings). The anisotropic parameters, resulting from the automatic computation, have been interpreted to determine the fracture field geometries; for each area, we defined the dominant fast direction and the intensity of the anisotropy, interpreting these results in the light of the geological and structural setting and of two anisotropic interpretative models, proposed in the literature. In the first one, proposed by Zinke and Zoback, the local stress field and cracks are aligned by tectonics phases and are not necessarily related to the presently active stress field. Therefore the anisotropic parameters variations are only space-dependent. In the second, EDA model, and its development in the APE model fluid-filled micro-cracks are aligned or ‘opened’ by the active stress field and the variation of the stress field might be related to the evolution of the pore pressure in time; therefore in this case the variation of the anisotropic parameters are both space- and time- dependent. We recognized that the average of fast directions, in the three selected areas, are oriented NW-SE, in agreement with the orientation of the active stress field, as suggested by the EDA model, but also, by the proposed by Zinke and Zoback model; in fact, NW-SE direction corresponds also to the strike of the main fault structures in the three study regions. The mean values of the magnitude of the normalized delay time range from 0.005 s/km to 0.007 s/km and to 0.009 s/km, respectively for the L'Aquila (AQU) area, the High Tiber Valley (ATF) and the Val d'Agri (VA), suggesting a 3-4% of crustal anisotropy. In each area are also examined the spatial and temporal distribution of anisotropic parameters, which lead to some innovative observations, listed below. 1) The higher values of normalized delay times have been observed in those zones where most of the seismic events occur. This aspect was further investigated, by evaluating the average seismic rate, in a time period, between years 2005 and 2010, longer than the lapse of time, analyzed in the anisotropic studies. This comparison has highlighted that the value of the normalised delay time is larger where the seismicity rate is higher. 2) In the Alto Tiberina Fault area the higher values of normalised delay time are not only related to the presence of a high seismicity rate but also to the presence of a tectonically doubled carbonate succession. Therefore, also the lithology, plays a important role in hosting and preserving the micro-fracture network responsible for the anisotropic field. 3) The observed temporal variations of anisotropic parameters, have been observed and related to the fluctuation of pore fluid pressure at depth possibly induced by different mechanisms in the different regions, for instance, changes in the water table level in Val D’Agri, occurrence of the April 6th Mw=6.1 earthquake in L’Aquila.Since these variations have been recognized, it is possible to affirm that the models that better fit the results, both in term of fast directions and of delay times, seems to be EDA and APE models

Fault architecture and deformation mechanisms in exhumed analogues of seismogenic carbonate-bearing thrusts

4noreservedFaults in carbonates are well known sources of upper crustal seismicity throughout the world. In the outer sector of the Northern Apennines, ancient carbonate-bearing thrusts are exposed at the surface and represent analogues of structures generating seismicity at depth. We describe the geometry, internal structure and deformation mechanisms of three large-displacement thrusts from the km scale to the microscale. Fault architecture and deformation mechanisms are all influenced by the lithology of faulted rocks. Where thrusts cut across bedded or marly limestones, fault zones are thick (tens of metres) and display foliated rocks (S-CC' tectonites and/or YPR cataclasites) characterized by intense pressure-solution deformation. In massive limestones, faulting occurs in localized, narrow zones that exhibit abundant brittle deformation. A general model for a heterogeneous, carbonate-bearing thrust is proposed and discussed. Fault structure, affected by stratigraphic heterogeneity and inherited structures, influences the location of geometrical asperities and fault strain rates. The presence of clay minerals and the strain rate experienced by fault rocks modulate the shifting from cataclasis-dominated towards pressure-solution-dominated deformation. Resulting structural heterogeneity of these faults may mirror their mechanical and seismic behaviour: we suggest that seismic asperities are located at the boundaries of massive limestones in narrow zones of localized slip whereas weak shear zones constitute slowly slipping portions of the fault, reflecting other types of "aseismic" behaviour. © 2013 Elsevier Ltd.mixedTesei, T.; Collettini, C.; Viti, C.; Barchi, M.R.Tesei, T.; Collettini, C.; Viti, C.; Barchi, M. R

Exhumation patterns along shallow low-angle normal faults: An example from the Altotiberina active fault system (Northern Apennines, Italy)

A multi-method approach (palaeothermal and thermochronological analyses; thermal modelling) is applied to reconstruct the exhumation history of the Altotiberina Fault (ATF), a representative example of crustal-scale active low-angle normal faulting in the Northern Apennines (Italy). Thermal maturity and thermochronological data yield similar burial histories but different exhumation patterns for the sedimentary successions in the hangingwall and the footwall of the ATF. Since 3.8Ma, the ATF footwall has exhumed at rates of 0.90mma(-1). Exhumation led to bending and deactivation of the ATF uppermost portion as a result of tectonic unloading and isostatic adjustment, followed by migration of extension and the development of a set of domino-like, east-dipping normal faults, rooting on the buried portion of the ATF. ATF activity and isostatic rebound exhumed Triassic rock units from depths of about 4km. We suggest that isostatic instability is accommodated at shallow crustal levels, in a similar way to what is observed on larger structures at mid-low crustal levels

A multidisciplinary study of a natural example of a CO2 geological reservoir in central Italy

Storage of CO2 and, in particular, geological storage is one of the most promising solutions to counteract the continued increase of anthropogenic greenhouse-gas emissions. Here we present a multidisciplinary study of a natural CO2 geologic reservoir where fluid overpressure, measured at 85% of the lithostatic load, is trapped at similar to 4700 m depth in the Northern Apennines of Italy. Deep borehole data and seismic reflection profiles show that the observed CO2 overpressure: (a) is hosted in dome shaped structures that are the result of the interplay between the compressional and extensional tectonic phases of the area and (b) occurs within dolostones that are sealed by Ca-sulphates (anhydrites) horizons. Field studies on outcropping evaporites, that represent exhumed analogues of the lithologies found at depth in the pressurized reservoir, show dolostones affected by fracturing and faulting and Ca-sulphates characterized by ductile folding without macroscopic fractures. Borehole and laboratory P-wave velocities coincide for anhydrites (6.2-6.3 km/s) but are different for dolostones (6.2-6.3 km/s in situ and similar to 7.2 km/s in lab), confirming the different mode of deformation in the two rock types. Since anhydrites are not dominated by fractures, the very low permeability values, similar to 10(-19) to similar to 10(-22) m(2), recorded in laboratory experiments, can also be representative for in situ conditions. These data confirm the sealing ability of the anhydrites also for high values of fluid pressure. The integration of these datasets suggests that: (1) tectonics and structural position, and (2) lithology and associated mode of deformation, play a key role for channelling and trapping deep-seated CO2-rich crustal fluids. (C) 2012 Elsevier Ltd. All rights reserved

Tectono-stratigraphic evolution of a basin generated by transpression: the case of the Early Pliocene Lascari Basin (Northern Sicily)

In the present paper integrated stratigraphy and structural analyses are aimed at the description of a synsedimentary transpressional tectonic event, driving the onset and development of the Early Pliocene Lascari Basin, located NW of the Madonie Mountains(northern Sicily). Our data show that the transpressive tectonics generated a morphostructural high flanked by a deep and narrow tectonic depression, bounded by steep and tectonically controlled slopes. Within this depression an Early Pliocene fining and deepening upward sedimentary succession was deposited, unconformably overlying the already deformed substrate. The succession is made up of «base-ofslope» breccias, cross-stratified calcarenites, and bathyal limestones deposited in a high-energy palaeostrait setting. The growth geometry that characterizes the stratal pattern of the deposits suggests the activity of a synsedrimentary transpressional event during the Early Pliocene time. The roughly E-W trending Lascari Syncline, where the Early Pliocene syntectonic basin is hosted, belongs to a system of north verging folds, deforming the pre-existing, originally flat thrusts, generated in Langhian-Serravallian time. This paper presents an original interpretation of the stratigraphic and structural evolution of the study sector of the Sicily Fold and Thrust Belt. Our data contribute also to both define the structural style and constrain the timing of transpressional tectonic event in northern Sicily. A brief comparison performed between the data here presented and those coming from adjacent sectors of northern Sicily suggests that the transpressional tectonics in these sectors of the chain was active at least since the latest Tortonian till at least the Late Pliocene

Potential evidence for slab detachment from the flexural backstripping of a foredeep: Insight on the evolution of the Pescara basin (Italy)

The discrepancy between the size of the Apenninic chain and the depth of the Adriatic foredeep is investigated using 2D flexural backstripping on well‐constrained depth‐converted cross‐sections in the Pescara basin (Central Italy). The procedure consisted of removal, uplift, unfolding and unfaulting of the Pliocene–Pleistocene foreland deposits to produce a palaeogeographic map of the basin at the end of the Messinian and to constrain sedimentary rates since the Miocene. Results are found to support the contribution of an external load to the foreland evolution together with the Apenninic chain load. The interplay of the two types of loads resulted in spatial and temporal variations of the foredeep evolution that are quantified by palaeogeographic maps and sedimentation rates obtained through backstripping. Results are interpreted as representing the effects of a southward‐migrating wave linked to slab detachment beneath the Adriatic foredeep. This procedure can be useful to investigate similar problems on other chains worldwide