Abstract
The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakesMw ½ 7:8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strikeof the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction. Thissegmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the influence of the Carnegie Ridgecollision is investigated. Four distinct seismotectonic regions can be distinguished: Region 1 – from 6ºN to 2.5ºN withsteep ESE-dipping subduction and a narrow volcanic arc; Region 2 – from 2.5ºN to 1ºS showing an intermediate-depthseismic gap and a broad volcanic arc; Region 3 – from 1ºS to 2ºS with steep NE-dipping subduction, and a narrowvolcanic arc; Region 4 – south of 2ºS with flat subduction and no modern volcanic arc. The Carnegie Ridge has beencolliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-paralleltransects. The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested bythe seismic gap and the perturbed, broad volcanic arc. These findings conflict with previous tectonic models suggestingthat the Carnegie Ridge entered the trench at 1 Ma. Furthermore, the anomalous geochemical (adakitic) signature of thevolcanoes in the broad Ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tearsseparating the buoyant, shallowly subducting Carnegie Ridge from segments of steep subduction in Regions 1 and 3. Itis further suggested that Carnegie Ridge supports a local ‘flat slab’ segment similar to that observed in Peru. The impactof the Carnegie Ridge collision on the upper plate causes transpressional deformation, extending inboard to beyond thevolcanic arc with a modern level of seismicity comparable to the San Andreas fault system. The pattern of instrumentaland historical seismicity indicates (1) great earthquakes on the northern and southern flanks of the colliding ridge, (2) aslight reduction in observed seismicity at the trench–ridge intersection, (3) increased stress far into the continent, and (4) aNNE displacement of the N. Andes block, to be further effects of the collision. 1999 Elsevier Science B.V. All rightsreserved