Preliminary results of an on-going project aimed at deciphering the micromechanics of brittle deformation of Triassic dolostones are here presented. Samples collected from large, trastensional high-angle faults (several 100’s m-throw) crosscutting the Mesozoic carbonates of the Neo Tethys (Campanian-Lucanian Platform) are investigated by mean of a detailed field structural survey, optical microscopy and SEM methodologies, and image analysis. The goal is to characterize in detail both composition and, mainly, texture of the cataclastic dolomitic rocks in order to assess the 3D structural architecture of the fault cores. Moreover, the present study addresses the relative timing of different mechanisms such as intergranular extensional crushing, chipping and shear fracturing, which all caused grain comminution during grain rolling and crushing. Finally, based upon the absolute thickness of key texture types recognized in the study cataclastic fault rocks, a simple model is assumed to compute the amount of overburden acting at time of deformation on the exhumed fault cores now days cropping out nearby the Basilicata-Campania border of southern Italy. Study hand samples pertain to from well-exposed fault cores pertaining to high-angle faults oriented either NW-SE, parallel to the Apennine belt, or NE-SW, almost orthogonal to it. The faults crosscut the entire Meso-Cenozoic carbonate succession, and offset the low-angle thrust faults juxtaposing the aforementioned carbonates (fault hanging wall) against the Meso-Cenozoic, mixed carbonate-silicate Lagonegro succession (fault footwall). The amount of throw of individual high-angle faults was actually measured taking into account the displaced thrust faults as markers. Based upon large scale field mapping (1:10,000 scale) and 3D modelling of an about 100 km2 area (Move software), the NW-SE high-angle faults postdate the NE-SW ones, suggesting a different time of activity. In the field, hand samples were selected based upon their distribution within the fault cores and, mainly, distance from main slip surfaces. Microscale analysis of 66 oriented fault rock samples shows that, mostly, the study cataclastic rocks are made up of dolomite. In a few cases, some small portions of silicate grains deriving from the Lagonegro succession (Scisti Silicei Fm.) are found. In order to characterize the main textural types, a great attention is been paid to the following parameters: 1) grain-matrix ratio; 2) grain sphericity; 3) grain roundness; 4) grain sorting. Moreover, by suing an automatic box-counting technique (Image-J software), the fractal dimension of selected samples was computed. The preliminary results are consistent with four main textural types present within the study fault cores. The distinction is made on the basis of the grain-matrix ratio, which increases generally in the vicinity of the main slip surfaces. In particular, protocataclasites characterized by intense intergranular extensional fracturing are present adjacent to the fractured and fragmented dolomitic damage zones, whereas cataclasites and ultracataclasites in the inner portions of the fault cores. A negative correlation between grain-matrix and grain sphericity, roundness and sorting is documented for the latter rocks, which means that the more deformed ultracataclasites show more spherical and rounded smaller grains relative to cataclasites. The fourth type of texture, which is discontinuously present along main slip surfaces of the fault cores, consists on cataclasites and ultracataclasites cemented with calcite minerals. On the basis of the aforementioned box-counting analysis, the three uncemented textural types show dissimilar D-values (fractal dimensions): D protoclasites = -1.82 (+ 1 sigma standard deviation 0.02); D cataclasites = - 1.72 (+ 0,05); D ultracataclasites = -1,56 (+ 0.04). This result shows therefore a positive correlation between D-value and fault maturity, and it might be helpful to quickly assess the degree of cataclasis in subsurface faults from analysis of cores recovered from exploration wells. Moreover, by taking into account only the cumulative thickness of the ultracataclastic bands present within the study faults, the result of the present study is consistent with the cores of the NW-SE faults being subjected to a lithostatic load about 6 times higher than those pertaining to the NE-SW faults. Ongoing 3D modelling of the study area will shed more lights on the slip distribution along individual faults to assess the relationship between ultracataclastic band thickness and fault intersection geometries.

Brittle deformation of Triassic dolostones

TONDI, Emanuele
2014-01-01

Abstract

Preliminary results of an on-going project aimed at deciphering the micromechanics of brittle deformation of Triassic dolostones are here presented. Samples collected from large, trastensional high-angle faults (several 100’s m-throw) crosscutting the Mesozoic carbonates of the Neo Tethys (Campanian-Lucanian Platform) are investigated by mean of a detailed field structural survey, optical microscopy and SEM methodologies, and image analysis. The goal is to characterize in detail both composition and, mainly, texture of the cataclastic dolomitic rocks in order to assess the 3D structural architecture of the fault cores. Moreover, the present study addresses the relative timing of different mechanisms such as intergranular extensional crushing, chipping and shear fracturing, which all caused grain comminution during grain rolling and crushing. Finally, based upon the absolute thickness of key texture types recognized in the study cataclastic fault rocks, a simple model is assumed to compute the amount of overburden acting at time of deformation on the exhumed fault cores now days cropping out nearby the Basilicata-Campania border of southern Italy. Study hand samples pertain to from well-exposed fault cores pertaining to high-angle faults oriented either NW-SE, parallel to the Apennine belt, or NE-SW, almost orthogonal to it. The faults crosscut the entire Meso-Cenozoic carbonate succession, and offset the low-angle thrust faults juxtaposing the aforementioned carbonates (fault hanging wall) against the Meso-Cenozoic, mixed carbonate-silicate Lagonegro succession (fault footwall). The amount of throw of individual high-angle faults was actually measured taking into account the displaced thrust faults as markers. Based upon large scale field mapping (1:10,000 scale) and 3D modelling of an about 100 km2 area (Move software), the NW-SE high-angle faults postdate the NE-SW ones, suggesting a different time of activity. In the field, hand samples were selected based upon their distribution within the fault cores and, mainly, distance from main slip surfaces. Microscale analysis of 66 oriented fault rock samples shows that, mostly, the study cataclastic rocks are made up of dolomite. In a few cases, some small portions of silicate grains deriving from the Lagonegro succession (Scisti Silicei Fm.) are found. In order to characterize the main textural types, a great attention is been paid to the following parameters: 1) grain-matrix ratio; 2) grain sphericity; 3) grain roundness; 4) grain sorting. Moreover, by suing an automatic box-counting technique (Image-J software), the fractal dimension of selected samples was computed. The preliminary results are consistent with four main textural types present within the study fault cores. The distinction is made on the basis of the grain-matrix ratio, which increases generally in the vicinity of the main slip surfaces. In particular, protocataclasites characterized by intense intergranular extensional fracturing are present adjacent to the fractured and fragmented dolomitic damage zones, whereas cataclasites and ultracataclasites in the inner portions of the fault cores. A negative correlation between grain-matrix and grain sphericity, roundness and sorting is documented for the latter rocks, which means that the more deformed ultracataclasites show more spherical and rounded smaller grains relative to cataclasites. The fourth type of texture, which is discontinuously present along main slip surfaces of the fault cores, consists on cataclasites and ultracataclasites cemented with calcite minerals. On the basis of the aforementioned box-counting analysis, the three uncemented textural types show dissimilar D-values (fractal dimensions): D protoclasites = -1.82 (+ 1 sigma standard deviation 0.02); D cataclasites = - 1.72 (+ 0,05); D ultracataclasites = -1,56 (+ 0.04). This result shows therefore a positive correlation between D-value and fault maturity, and it might be helpful to quickly assess the degree of cataclasis in subsurface faults from analysis of cores recovered from exploration wells. Moreover, by taking into account only the cumulative thickness of the ultracataclastic bands present within the study faults, the result of the present study is consistent with the cores of the NW-SE faults being subjected to a lithostatic load about 6 times higher than those pertaining to the NE-SW faults. Ongoing 3D modelling of the study area will shed more lights on the slip distribution along individual faults to assess the relationship between ultracataclastic band thickness and fault intersection geometries.
2014
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11581/369006
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