Abstract: Complex Andean Foreland Structures: Product of Two-Phase Tertiary Deformation

Everett, John R. and Ronald J. Staskowski - Earth Satellite Corporation

Acceleration and changes in direction of subduction during Eocene and Miocene times changed the kinematics of many Tertiary structural features in western South America. Northeast-directed subduction from less than 10cm/yr to more than 25cm/yr during the Eocene established many of the elements of the tectonic fabric throughout the Andes. Subduction rates decreased to ±5cm/yr during the Oligocene. During the Miocene, subduction rates accelerated to more than 15cm/yr and subduction became east-directed.

The Eocene episode of acceleration produced northwest-trending folds, thrusts, and uplifts; northeast-trending normal faults, north-northeast-trending right-slip faults and east-trending left-slip faults. The change in direction of convergence during the Miocene, coupled with acceleration, produced folds and thrusts along many of the old right-slip faults, left-slip along many of the former thrust and fold trends, right-slip on many early normal faults, and extensional faulting along many former left-slip faults.

The effects of these changes are easily seen in satellite imagery. Understanding the reorientation of stress vectors and the changes in convergence rates during the Tertiary has profound implications for the interpretation of geophysical data and the exploration for hydrocarbons.

Fig. 1 is a Landsat Thematic Mapper image of the Marañon basin of Peru. At the surface, abrupt changes in the axial trend of the folds from northwest to north-northeast suggest this two-phase deformation. In some of the deeply eroded folds, the axis of folding in the older rocks does not parallel that of the folding in the younger rocks.

In seismic sections, folding dies out in the post-Eocene rocks across the northwest-trending folds. These folds appear highly asymmetric with the steeper limb of the anticline associated with a steeply dipping fault. These steeply dipping faults appear to have a strike-slip sense of motion during recent reactivation. In these folds, pre-Eocene rocks are the principal targets.

In general, the north-northeast-trending folds appear to be less complex with the anticlinal axes in Miocene and older rocks essentially parallel (most axial planes of these folds dip toward the west). These folds appear to be thrust-tip or thrust-propagation folds and the thrusts may initiate at previously existing strike-slip faults.

In general terms, there appears to be an overlapping pattern of folds and thrusts that die out laterally into strike-slip faults. The older folds trend northwest and the younger folds trend north-northeast. By analogy to the Western Overthrust Belt of Wyoming (USA), the best petroleum exploration targets are at the bends in the fold axes or where the folds are swinging into thrusts. These are the areas where fracturing is likely to be the greatest and where culminations in the fold axes are likely to occur.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil