The geometry of fault zones (e.g. fault surface roughness, fault rock distribution, network of secondary faults andfractures) is one of the main factors controlling earthquake nucleation, rupture speed and length of main shocks,foreshock and aftershock evolution, ground motion and seismic radiation pattern. Despite the pivotal role that faultgeometry bears on earthquake mechanics, little is known about the precise architecture of fault zones at seismo-genic depths. In fact, most exhumed faults, especially those capable of producing moderate to large earthquakes(faults > 20 km in length) are discontinuously and poorly exposed at the surface because of vegetation cover and/orweathering.Here we present a detailed, though preliminary, structural geology survey of a large exceptionally exposed, ex-humed seismogenic fault in the Atacama Desert (Chile), the Bolfin Fault Zone (BFZ). The BFZ is a sub-verticalNNW-striking, left-lateral strike-slip fault belonging to the∼1000 km long Atacama Fault System. The BFZ ishosted in amphibolites, granodiorites and quartzdiorites of the Coastal Cordillera and records Mesozoic (sinistral)and Cenozoic (normal) displacements.We integrated Unmanned Aerial Vehicle surveys with detailed mapping of the network of ductile shear zones andbrittle faults of the BFZ to produce high-resolution maps of the fault zone structure over four representative tran-sects along fault strike. We conducted mineralogical, microstructural and geochemical investigations of the faultzone rocks.From this field and microstructural study, four main deformation events were recognized, based on their crosscut-ting relationships observed in the field. An early deformation event (D0) consists of NW- to N- striking ductile(amphibolitic to granulitic facies) shear zones. The first brittle deformation event (D1, BFZ sensu-strictu) consistsof NW- to NE-striking sinistral strike-slip faults associated with greenschists to sub-greenschists (albite + chlo-rite + epidote + actinolite) foliated cataclasites and pseudotachylytes. The mineral assemblage and Al content inchlorite constrain deformation conditions at 6-8 km depth and 280-350◦C ambient temperature. The presence ofpseudotachylytes, formed in a fluid-rich environment, as attested by the abundance of amygdules in the matrix(only associated with the D1 event) documents the ancient seismicity of the fault zone. A N-, NW- and NE-strikingfault set consisting of dextral transtensional faults represents the intermediate brittle deformation event (D2) andis marked by hydrothermal epidote (+ quartz + calcite) precipitation. Lastly, two conjugate fault sets, a NNE-SSWto NE-SW strike dextral-normal and a ESE-WNW strike sinistral-normal transtensional faults filled with calcite(+ palygorskite±gypsum±halite), are interpreted as the youngest (Miocene to post-Moicene) deformation event(D3). The occurrence of palygorskite constrains D3 deformation conditions of < 150◦C to ambient temperature.Given the exceptional exposure and the relatively easy accessibility compared to other fault zones cropping out indesert areas, the Bolfin Fault Zone provides a unique opportunity to (1) examine the structure and the evolution ofseismogenic fault zones in the Earth’s continental crust and, (2) produce 3-D fault models for earthquake rupturenumerical simulations.

Gomila, R., Jensen, E., Fondriest, M., Di Toro, G., Mitchell, T., Bistacchi, A., et al. (2019). Architecture of an exceptionally exposed seismogenic source: The BolfinFault Zone (Atacama Desert, Chile). In EGU General Assembly 2019.

Architecture of an exceptionally exposed seismogenic source: The BolfinFault Zone (Atacama Desert, Chile)

Andrea Bistacchi;
2019

Abstract

The geometry of fault zones (e.g. fault surface roughness, fault rock distribution, network of secondary faults andfractures) is one of the main factors controlling earthquake nucleation, rupture speed and length of main shocks,foreshock and aftershock evolution, ground motion and seismic radiation pattern. Despite the pivotal role that faultgeometry bears on earthquake mechanics, little is known about the precise architecture of fault zones at seismo-genic depths. In fact, most exhumed faults, especially those capable of producing moderate to large earthquakes(faults > 20 km in length) are discontinuously and poorly exposed at the surface because of vegetation cover and/orweathering.Here we present a detailed, though preliminary, structural geology survey of a large exceptionally exposed, ex-humed seismogenic fault in the Atacama Desert (Chile), the Bolfin Fault Zone (BFZ). The BFZ is a sub-verticalNNW-striking, left-lateral strike-slip fault belonging to the∼1000 km long Atacama Fault System. The BFZ ishosted in amphibolites, granodiorites and quartzdiorites of the Coastal Cordillera and records Mesozoic (sinistral)and Cenozoic (normal) displacements.We integrated Unmanned Aerial Vehicle surveys with detailed mapping of the network of ductile shear zones andbrittle faults of the BFZ to produce high-resolution maps of the fault zone structure over four representative tran-sects along fault strike. We conducted mineralogical, microstructural and geochemical investigations of the faultzone rocks.From this field and microstructural study, four main deformation events were recognized, based on their crosscut-ting relationships observed in the field. An early deformation event (D0) consists of NW- to N- striking ductile(amphibolitic to granulitic facies) shear zones. The first brittle deformation event (D1, BFZ sensu-strictu) consistsof NW- to NE-striking sinistral strike-slip faults associated with greenschists to sub-greenschists (albite + chlo-rite + epidote + actinolite) foliated cataclasites and pseudotachylytes. The mineral assemblage and Al content inchlorite constrain deformation conditions at 6-8 km depth and 280-350◦C ambient temperature. The presence ofpseudotachylytes, formed in a fluid-rich environment, as attested by the abundance of amygdules in the matrix(only associated with the D1 event) documents the ancient seismicity of the fault zone. A N-, NW- and NE-strikingfault set consisting of dextral transtensional faults represents the intermediate brittle deformation event (D2) andis marked by hydrothermal epidote (+ quartz + calcite) precipitation. Lastly, two conjugate fault sets, a NNE-SSWto NE-SW strike dextral-normal and a ESE-WNW strike sinistral-normal transtensional faults filled with calcite(+ palygorskite±gypsum±halite), are interpreted as the youngest (Miocene to post-Moicene) deformation event(D3). The occurrence of palygorskite constrains D3 deformation conditions of < 150◦C to ambient temperature.Given the exceptional exposure and the relatively easy accessibility compared to other fault zones cropping out indesert areas, the Bolfin Fault Zone provides a unique opportunity to (1) examine the structure and the evolution ofseismogenic fault zones in the Earth’s continental crust and, (2) produce 3-D fault models for earthquake rupturenumerical simulations.
abstract + slide
Architecture , seismogenic source, BolfinFault Zone,Atacama Desert;
English
EGU General Assembly 2019
2019
EGU General Assembly 2019
2019
21
EGU2019-7505
https://meetingorganizer.copernicus.org/EGU2019/EGU2019-7505.pdf
none
Gomila, R., Jensen, E., Fondriest, M., Di Toro, G., Mitchell, T., Bistacchi, A., et al. (2019). Architecture of an exceptionally exposed seismogenic source: The BolfinFault Zone (Atacama Desert, Chile). In EGU General Assembly 2019.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/299857
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