Fracture network, fluid pathways and paleostress at the Tolhuaca geothermal field
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Pérez Flores, Pamela
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Fracture network, fluid pathways and paleostress at the Tolhuaca geothermal field
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Abstract
In this study, we examine the fracture network of the Tolhuaca geothermal system located in the
Southern Andean volcanic zone that may have acted as a pathway for migration and ascent of deepseated
fluids under the far/local stress field conditions of the area. We collected the orientation, slipdata
and mineralogical content of faults and veins recovered on a ca. 1000 m deep borehole (Tol-1)
located in the NW-flank of the Tolhuaca volcano. Tol-1 is a non-oriented, vertical borehole that recovered
relatively young (<1 Ma) basaltic/andesitic volcanic rocks with subordinate pyroclastic/volcanoclastic
interbedded units of Pleistocene age. Here, we examined and measured the inclination, geometry,
texture, mineralogy, and relative sense of displacement of veins and faults. To determine the actual
azimuthal orientation of fault and veins we reoriented 66 segments (89 standard mini-cores) of Tol-1
using stable Characteristic remanent magnetization component (ChRM) obtained by thermal demagnetization
methodology. Paleo-declination of ChRM vectors was used to re-orient the borehole pieces, as
well as fault and veins, to a common anchor orientation value consistent with the Geocentric Axial
Dipole approximation (GAD). Inversion of RM-corrected fault-slip data reveals a local tensional stress
field with a vertically oriented s1 axis (083/74) and a subhorizontal, NS-trending s3 axis (184/03). Within
the topmost 400 m of the borehole, faults and veins are randomly oriented, whereas below 400 m depth,
faults and veins show preferential NE-to EW-strikes and steep (>50 ) dips. The EW-striking veins are
compatible with the calculated local stress field whereas NE-striking veins are compatible with the
regional stress field, the morphological elongation of volcanic centers, alignments of flank vents and
dikes orientation. Our results demonstrate that the paleomagnetic methodology proved to be reliable
and it is useful to re-orient vertical boreholes such as Tol-1. Furthermore, our data show that the bulk
transpressional regional stress field has local variations to a tensional stress field within the NE-striking
fault zone belonging to the Liqui~ne-Ofqui Fault System, favoring the activation of both NW- and NEstriking
pre-existent discontinuities, especially the latter which are favorably oriented to open under
the prevailing stress field. The vertical s1 and NS-trending subhorizontal s3 calculated in the TGS promote
the activation of EW-striking extensional veins and both NE and NW-striking hybrid faults,
constituting a complex fluid pathway geometry of at least one kilometer depth.
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Artículo de publicación SCOPUS
Identifier
URI: https://repositorio.uchile.cl/handle/2250/168812
DOI: 10.1016/j.jsg.2017.01.009
ISSN: 01918141
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Journal of Structural Geology 96 (2017) 134-148
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