Is the Gastre Fault System (central Patagonia) a transcontinental strike-slip fault?

ZAFFARANA C. B.

Vienna, Austria

Congreso; EGU General Assembly 2008; 2008

European Geophysical Union

Large scale intracontinental strike-slip faults and ductile shear zones constitute prominentcrustal structures. Assessing the timing, geometry, length and kinematics of thesedeformation zones is of great tectonic interest since large displacements may be responsiblefor both the juxtaposition of terranes with contrasting histories and thedevelopment of localized zones of either extension or shortening. Recent paleogeographicmodels for Western Gondwana consider the Gastre Fault System (GFS) incentral Patagonia as a major NW-SE intercontinental shear zone that accommodatedcirca 500 km of dextral motion between Patagonia and cratonic South America duringLate Triassic-Jurassic times. Furthermore, it was proposed that the GFS has beenthe precursor of the Agulhas-Malvinas fracture zone in the South Atlantic Ocean, andthat its activity was responsible for the large clockwise rotation paleomagnetically detectedin circa 190 Ma dolerites from the Malvinas/Falkland Islands. Plutonic suitesof the Triassic-Early Jurassic Central Patagonian Batholith (CPB) constitute the dominantoutcrops in the Gastre area, where the GFS was defined and is assumed to havecontrolled the emplacement and distribution of the plutons.However, the tectonic relevance attributed to the GFS in the paleogeographic and tectonicmodels is being questioned by the growing geological information in the area.As part of ongoing mapping and anisotropy of the magnetic susceptibility (AMS)sampling on the CPB for my Ph.D. study, two “classical” localities in Gastre showinggreen-schist facies mylonites and cataclasites were revised. Results from a western localityindicate the presence of subvertical, NW-SE foliation and subvertical lineationin both a 272 Ma (U-Pb zircon conventional) megacrystic granite and its matavolcanichost rock. Aligned feldspar megacrystals in the granite define a magmatic foliationthat parallels the mylonitic foliation, whereas microstructural analyses revealthe presence of a high-temperature deformation which is not observed in the host.These observations strongly suggest that magmatic foliation and high-temperature deformationin the granite record Permian strain, with late development of mylonitesrecording similar strain kinematics at lower temperatures. Further east, near Gastre,field observations and magnetic fabric in another “classical” locality allow finding theoccurrence of N-S subvertical foliations and moderate to steep lineations in variablysized mylonite and cataclasite stripes.Despite the presence of several high-strain localities as those mentioned above, mostof the visited outcrops in the CPB reveal the predominance of magmatic structureswith the subordinate occurrence of a slight subsolidus overprint. This, andthe between-locality dispersion shown by both magmatic foliation and lineation atbatholith scale, suggests that deformation in the bulk of the Gastre area could be ascribedto the emplacement history of the CPB.Thus, the preliminary results of this study indicate the predominance of variably orientedmagmatic fabrics at regional scale, steep lineations in high-strain localities, andthe presence of Permian deformation, which taken together seem to diminish the paleogeographicsignificance attributed to the GFS as a major, transcontinental strike-slipdeformation zone during breakup of Western Gondwana. As an alternative, the observationscould be reconciled with a protracted history of pluton emplacement followedby development of high and low temperature deformation during the cooling process,the latter locally deriving in mylonites and cataclasites.