Komplexe Lokalstruktur bei einfachem plattentektonischem Rahmen, Zentrale Anden
Final Report Abstract
Multiple phases of folding indicating varying shortening directions are common in the geologic histories of many mountain belts. Such variations are often attributed to changes in the driving forces, such as those induced by plate reorganizations. The southern central Andes have grown in an unchanged plate kinematic setting of east-directed subduction over the last 40 million years. This is reflected in their overall structure which shows the dominant effect of east-west shortening. Surprisingly, though, there are also folds and thrust faults indicating shortening perpendicular to the direction of subduction. We have studied several of these transverse structures in northwestern Argentina where they are particularly well exposed. They can be shown to have accommodated much less deformation than the main longitudinal structures, and appear to reflect a relatively young (9-2 Ma) and short-lived event postdating the main longitudinal structures but followed by renewed east-west shortening. The transverse structures comprise steeply dipping reverse faults that probably originated as normal faults in the continental Salta rift of Cretaceous to Paleogene age. They appear to be inversion structures which involved the reverse reactivation of the inherited faults. This is suggested by well-preserved, non-reactivated examples of east-trending synsedimentary normal faults occurring in our study area. A comparison of our own observations on the preserved and reactivated structures in the Salta rift with case studies from the literature indicates that the reactivation of a specific normal fault is facilitated by the following parameters: a trend roughly perpendicular to the shortening direction (contradicting inferences from analogue modelling), a relatively low dip angle, and low curvature in crosssection (i.e., strongly listric faults seem difficult to reactivate). The very existence of east-trending inherited faults provides no sufficient explanation for their reactivation in north-south shortening. Yet, general considerations on fault slip and stress states show that (1) due to their steep dips, normal faults must be mechanically weak (friction coefficient µ around 0.3 or less) to be reactivated in a thrust regime, and (2) when such weak faults are present, those trending parallel to the shortening direction (our transverse faults) need not be extremely weak to be reactivated as well. This holds true even for a nearly uniaxial stress state where the lesser horizontal stress is not much larger than the overburden stress. These relations suggest that just a slight increase in north-south horizontal stress may be sufficient to create the transverse structures in the former rift regions.
Publications
-
(2006): Orogen-parallel Contraction in the Central Andes: Changing Plate Kinematics or Intrinsic Causes? AGU 2006 Fall Meeting, San Francisco, Abstract T44A-07
Kley, J., Salazar, L., Monaldi, C.R.
-
(2006): Structure of the Cerro Colorado anticline interpreted from the 3-D integration of seismic and field data, northwest Argentina. GeoBerlin, Jahrestagung der Deutschen Gesellschaft für Geowissenschaften
Salazar, L., Kley, J.
-
(2007): Análisis estructural del anticlinal Cerro Colorado a partir de datos sísmicos y geología de campo en la sub-cuenca de Tres Cruces. noroeste de Argentina. IX Congreso Geológico Venezolano
Salazar, L., Kley, J., Rossello, E., Monaldi, C.R., Wiegand, M.
-
(2008): 3D structure of the Tres Cruces synclinorium from seismic data and serial balanced cross-sections, Eastern Cordillera, Argentina. 7th International Symposium on Andean Geodynamics (ISAG 2008, Nice), Extended Abstracts: 477-480
Salazar, L., Kley, J., Rossello, E.A., Monaldi, C.R., Wiegand, M.
-
(2008): Preserved extensional structures in an inverted Cretaceous rift basin, northwestern Argentina: Outcrop examples and implications for fault reactivation, Tectonics, 27, TC1011
Monaldi, C. R., J. A. Salfity, Kley, J.
