8
J.N. Kellogg and W.U. Mohriak
axis because abundant precipitation and mountainous terrain on land provided large volumes of detritus. Major rivers, which continue as large submarine canyons across the shelf and slope, carry considerable sediment
directly to the trench axis.
Onshore steep slopes rise to the Andean Continental Divide. The deforming Andean margin reaches a maximum width of about 800 km at 20° S
where the Altiplano Plateau correlates spatially and temporally with Andean arc magmatism (Fig. 2). The plateau was uplifted primarily because of
crustal thickening produced by horizontal shortening of a thermally softened lithosphere (Allmendinger et al. 1997). The plateau correlates with a
30° east-dipping segment of the subducted Nazca Plate. To the north and
south, where the mountain belt narrows considerably, the subducted plate
shallows and is nearly horizontal. Post-Pliocene volcanism is absent where
the plate is nearly flat, and well developed in the plateau where the plate is
steeper. To the north (Ecuador and Colombia) and south (33° S) of the flat
segments, the subducted Nazca Plate steepens to 30° and active volcanism
resumes. Total horizontal shortening estimates for the Central Andes are as
high as 320 km (Sheffels 1990; Schmitz 1994). Space geodetic data show
that roughly half of the overall Nazca-South America plate convergence in
Peru and Bolivia, about 30 to 40mm year- 1 , accumulated on the locked
plate interface and can be released in future earthquakes (Norabuena et al.
1998). About 10 to 15 mm year- 1 of crustal shortening occurred inland at
the sub-Andean foreland fold and thrust belt, indicating that the Andes are
continuing to build. Trench-parallel motion of coastal forearc slivers (5 to
lOmm year- 1 ) may be related to oblique convergence.
Collision and subduction of oceanic ridges along convergent margins
have long been associated with established or suspected vertical movements of arc and forearc areas and with the disruption of magmatic activity, seismicity, and trench continuity (e.g., Barazangi and !sacks 1976;
Greene and Wong 1989). Major ridges that are presently colliding with the
Andean margin and which might contribute to margin fragmentation are,
from north to south, the Carnegie Ridge off Ecuador, the Nazca Ridge off
Peru, and the Juan Fernandez Ridge and the Southern Chile Rise off Chile
(Fig. 2).
The Carnegie Ridge is a broad (250 km) aseismic ridge that stands about
1,500 m above the surrounding seafloor. It was produced by volcanism at
the Galapagos hot spot. The Carnegie Ridge has been colliding with the
Andean margin since at least 2 Ma based on examination of the basement
uplift signal along trench parallel transects (Gutscher et al. 1999a). Increased seismic coupling due to the colliding Carnegie Ridge may be correlated with five great (MW >7.8) earthquakes this century and the northeastward displacement of the North Andes block or microplate. The Nazca
J.N. Kellogg and W.U. Mohriak
axis because abundant precipitation and mountainous terrain on land provided large volumes of detritus. Major rivers, which continue as large submarine canyons across the shelf and slope, carry considerable sediment
directly to the trench axis.
Onshore steep slopes rise to the Andean Continental Divide. The deforming Andean margin reaches a maximum width of about 800 km at 20° S
where the Altiplano Plateau correlates spatially and temporally with Andean arc magmatism (Fig. 2). The plateau was uplifted primarily because of
crustal thickening produced by horizontal shortening of a thermally softened lithosphere (Allmendinger et al. 1997). The plateau correlates with a
30° east-dipping segment of the subducted Nazca Plate. To the north and
south, where the mountain belt narrows considerably, the subducted plate
shallows and is nearly horizontal. Post-Pliocene volcanism is absent where
the plate is nearly flat, and well developed in the plateau where the plate is
steeper. To the north (Ecuador and Colombia) and south (33° S) of the flat
segments, the subducted Nazca Plate steepens to 30° and active volcanism
resumes. Total horizontal shortening estimates for the Central Andes are as
high as 320 km (Sheffels 1990; Schmitz 1994). Space geodetic data show
that roughly half of the overall Nazca-South America plate convergence in
Peru and Bolivia, about 30 to 40mm year- 1 , accumulated on the locked
plate interface and can be released in future earthquakes (Norabuena et al.
1998). About 10 to 15 mm year- 1 of crustal shortening occurred inland at
the sub-Andean foreland fold and thrust belt, indicating that the Andes are
continuing to build. Trench-parallel motion of coastal forearc slivers (5 to
lOmm year- 1 ) may be related to oblique convergence.
Collision and subduction of oceanic ridges along convergent margins
have long been associated with established or suspected vertical movements of arc and forearc areas and with the disruption of magmatic activity, seismicity, and trench continuity (e.g., Barazangi and !sacks 1976;
Greene and Wong 1989). Major ridges that are presently colliding with the
Andean margin and which might contribute to margin fragmentation are,
from north to south, the Carnegie Ridge off Ecuador, the Nazca Ridge off
Peru, and the Juan Fernandez Ridge and the Southern Chile Rise off Chile
(Fig. 2).
The Carnegie Ridge is a broad (250 km) aseismic ridge that stands about
1,500 m above the surrounding seafloor. It was produced by volcanism at
the Galapagos hot spot. The Carnegie Ridge has been colliding with the
Andean margin since at least 2 Ma based on examination of the basement
uplift signal along trench parallel transects (Gutscher et al. 1999a). Increased seismic coupling due to the colliding Carnegie Ridge may be correlated with five great (MW >7.8) earthquakes this century and the northeastward displacement of the North Andes block or microplate. The Nazca