Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision

The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakes M-w greater than or equal to 7.8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of Bat slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the influence of the Carnegie Ridge collision is investigated, Four distinct seismotectonic regions can be distinguished: Region 1 - from 6 degrees N to 2.5 degrees N with steep ESE-dipping subduction and a narrow volcanic are; Region 2 - from 2.5 degrees N to 1 degrees S showing an intermediate-depth seismic gap and a broad volcanic are; Region 3 - from 1 degrees S to 2 degrees S with steep NE-dipping subduction, and a narrow volcanic are; Region 4 - south of 2 degrees S with flat subduction and no modem volcanic are. The Carnegie Ridge has been colliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-parallel transects, The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed, broad volcanic are. These findings conflict with previous tectonic models suggesting that the Carnegie Ridge entered the trench at 1 Ma. Furthermore, the anomalous geochemical (adakitic) signature of the volcanoes in the broad Ecuador volcanic are and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant, shallowly subducting Carnegie Ridge from segments of steep subduction in Regions 1 and 3. It is further suggested that Carnegie Ridge supports a local 'flat slab' segment similar to that observed in Peru. The impact of the Carnegie Ridge collision on the upper plate causes transpressional deformation, extending inboard to beyond the volcanic are with a modem level of seismicity comparable to the San Andreas fault system. The pattern of instrumental and historical seismicity indicates (1) great earthquakes on the northern and southern flanks of the colliding ridge, (2) a slight reduction in observed seismicity at the trench-ridge intersection, (3) increased stress far into the continent, and (4) a NNE displacement of the N. Andes block, to be further effects of the collision. (C) 1999 Elsevier Science B.V. All rights reserved.