Plate Interactions, Escape, And Surface Strain In Northwestern South America

SERGILOPEZ, S.A., ROSSELLO & HECTOR MORA

Buenos Aires

Congreso; AAPG - International Conference & Exhibition; 2019

AAPG - International Conference & Exhibition

65 moment-magnitude (Mw) 5.0 CMT solutions for subduction earthquakes were chosen, with hypocenter depths minors than 40 km that occurred in a period of ca. 38 years (1970 - 2017) along 1,000 km in the Ecuador - Colombia margin between latitudes S2º and N6º, for the analysis of Nazca plate convergence decoupling, and in force balance model derivations, to obtain theoretical estimations of forearc sliver translation, and to compare with high-quality GNSS velocities and strain rates from 65 continuous operation receivers distributed in the northwestern corner of South America. The results show that most of the obliquity of convergence Nazca-South America has been consumed in the subduction process, with only a not-explained lesser part related with the decoupling and transfer to the western Colombia forearc translation parallel to margin and faster than the central-eastern Andean region. The forearc escape was confirmed by GNSS velocities and surface strains, with the associated dextral shear component along the Romeral shear zone. The Cocos-Caribbean oblique convergence and ridge subduction mirror the Nazca- South America plates interaction. Owing to that, the continental Panama region has been extruded northeasterly and collides against western Colombia, generating (i) a buttress effect on the forearc sliver and on the north Andes escape, (ii) the progression of an oroclinal bending, and (iii) the development of the Eastern Range syntaxial orogen along a maximum compression zone around N5º. The Panama collision also pushed the northern Andes northeasterly with increased GNSS velocities, explaining the associated dextral shear strain component related to the strike-slip motion on the Bocono shear zone. Similarities between vectors on the Caribbean plate and on the northwestern Colombian lowlands seem to indicate the nonexistence of Caribbean subduction in this area, at least for the latter deformation event of the Andes. However, the stations located far north near the Colombian-Venezuelan margin fit the previously stated transpressional plates boundary, and also explain the translation of northern Colombia forearc eastwards, with the associated dextral shear component of strain along the Oca-El Pilar shear zone, and the positive syntaxis of the Santa Marta mountain range. The Nazca-South America oblique convergence and Carnegie ridge collision by themselves do not successfully appear to explain the total upper plate deformation and translation in the northwestern corner of South America. The recently documented asthenosphere flow, sub-parallel to the GNSS vectors, and the geometries of the Moho and the lithosphere-asthenosphere boundaries justify evaluating the role of subcrustal density anomalieson the surface strain rates.