Paleomagnetic Reorientation of Structural Elements in Drill Cores: an example from Tolhuaca Geothermal Field.

Abstract

Reorientation of mesoscopic faults, veins and fractures recovered from drillingis critical to construct reliable structural models that can account for their architecture and deformationregime. However, oriented cores are expensive and time consuming to drill. Some techniques achievereorientation by introducing tools into the borehole. Problems arise when boreholes are unstable orcollapse. One alternative technique allowing reorientation is to obtain reliable paleomagnetic vectorsto reorient each core piece after drilling. Here, we present stable and reliable remnant magneticvectors calculated from the Tol-1 core to analyze the geometry of the fracture network and itsrelationship to regional tectonic.Tol-1 core is a vertical, 1073 m deep geothermal well, drilled at the Tolhuaca Geothermal Field in theSouthern Volcanic Zone of the Andes by MRP Geothermal Chile Ltda (formerly GGE Chile SpA) in2009. The core consists of basaltic/andesitic volcanic rocks with subordinatepyroclastic/volcaniclastic units, with probable Pleistocene age. Fault planes with slickenlines andmineral fiber kinematic indicators are common in the upper 700 m of the core. Calcite, quartz andcalcite-quartz veins are recognized along of entire core, whereas epidote-quartz and calcite-epidoteveins occur in the last 350 m, minor chlorite, anhydrite and clay-minerals are present.Orientations of structural features in the core were measured with a goniometer using the core’s axisand a false north for each piece; hence, orientation data has a false strike but a real dip. To achievetotal reorientation of the pieces, we collected 200 standard-size paleomagnetic specimens, ensuringthat at least four of them were recovered from continuous pieces. Thermal (up to 700°C) andalternating field demagnetization (up to 90mT on steps of 2mT) methods were used to isolate a stableremnant magnetization (RM) vector, and each technique yielded similar results. RM vectors wererecovered between 0 to 25mT, and between 0 to 625°C. The declination of RM vectors was used tobring pieces to a common anchor orientation calculated through the Geocentric Axial Dipole Model(GAD).The paleomagnetic technique proved to be reliable to reorient the Tol-1 core. Structural analysesalong the core show N50-60E-striking preferential vein orientation. In addition, N40-50E- and N60-70W-striking preferential fault orientations were identified. Kinematic analysis of fault-slip data showsa N60E-striking bulk fault plane solution with normal strain regime. The veins and faults orientationshow strain axes compatible with published regional stress field (σmax N238E).