Análisis de los Casos de Pago

The Juan Férnandez Ridge morphology consists of a gentle swell crested by isolated large seamounts. It was formed by a hotspot that is now ~900 km west of the Chile Trench. The chain of aligned seamounts trends ~85°E, and a 250-km-wide gap between seamounts separates the distant younger group of sea- mounts and the older ones near the trench. There, the O’Higgins seamounts have ages of ca. 8 Ma and sit on ocean crust ca. 39 Ma in age (Fig. 5). The ridge fi rst collided with the Chile margin in the north at ca. 22 Ma, after which it migrated to its current colli- sion point located at roughly 32–33°S (Yañez et al., 2002). Sub- duction of this ridge has a spatial correlation with a fl at slab and a lack of active volcanism. Its current area of subduction contains Valparaiso Basin, which is the only signifi cant deep-water forearc basin along the central Chilean margin (Laursen et al., 2002).

Observations

Several surveys to acquire multibeam bathymetry have resulted in a high-resolution map of morphotectonic and sedi- mentary structure developed during ridge subduction (Fig. 5). Structure associated with the ridge strikes NE and is accentuated by vertical displacement on long faults where the ocean crust bends into the trench. The faults parallel the broad swell of the ridge and are assumed to have originated from development of the ridge. The ridge structural trend continues beneath the trench axis and then across the entire continental slope to the shelf. Structure of the collision, subduction of Juan Fernández Ridge, and slope readjustment as the ridge moved southward are also imaged seismically (Laursen et al., 2002; Ranero et al., 2006). However, Juan Fernandez Ridge is not associated with an anoma- lously thick crust, but only with a small local area of thicker than normal crust (Kopp et al., 2004).

At the current area of collision, the Juan Fernández Ridge erodes the frontal prism and uplifts the margin as it subducts beneath the continent (von Huene et al., 1997). The ridge is a topographic barrier to northward sediment transport along the trench axis (Fig. 5). This barrier separates a trench with more than 2.5-km-thick turbidites to the south from one with turbidites <1 km thick the north (cf. Schweller et al., 1981; Ranero et al., 2006). Thick trench sediment is associated with subduction accre- tion, and thin sediment is associated with subduction erosion.

Seaward of the trench axis, the structural fabric imparted to the oceanic crust during generation at a spreading ridge, and the superimposed hotspot intrusion, are revealed in multibeam

bathymetry (Fig. 5). The morphology of the oceanic plate on either side of the ridge and its seamounts has a NW fabric. Near the trench axis, this seafl oor-spreading fabric is crossed locally by faults that formed during bending into the trench prior to sub- duction and that roughly parallel the trench. In contrast, the area affected by hotspot magmatism has few short trench-parallel scarps. The ridge has numerous small volcanoes in addition to the two large O’Higgins seamounts and a NE-elongated swell adjacent to the trench axis. The multibeam bathymetry shows how hotspot volcanism can modify the seafl oor-spreading fabric and structurally overprint older crust (Fig. 5).

Bend faulting of the ridge begins ~100 km from the trench axis, and vertical displacement is up to 1 km at the trench axis (Fig. 5). The swath of hotspot-modifi ed lithosphere is ~150 km wide and trends ~55° from the trench axis. On the slope where the ridge axis has subducted, its trend is distinct and appears to heighten after subduction (von Huene et al., 1997). Seamounts, including one subducted seamount that is clearly defi ned by mag- netic anomalies at the lower slope, are aligned with the diagonal ridge trend (Yañez et al., 2002). Microseismicity on the ridge’s diagonal trend projects inland from the coast (von Huene et al., 1997; Engdahl et al., 1998).

The ridge has been a barrier to northward axial turbidite transport, which was most abundant during glaciations of the past 1.5–3 m.y. (Oncken et al., 2006). The infl uence of trench-sedi- ment distribution, shallow subduction angle, and southward ridge migration has dominated tectonism across the continental slope (Fig. 5). Trench sediment abundance south of the ridge promotes accretion, whereas the meager fi ll north of the ridge promotes subduction erosion.

The subduction of Juan Fernandez Ridge is spatially associ- ated with the fl at slab and a lack of active arc volcanism. The cen- tral valley of Chile begins south of the subducted ridge beneath the continent. The geology of the fl at slab is discussed elsewhere in the volume, and it is clear that collision is important in the structure of the margin.

Ideas Regarding Tectonism

The association of ridge subduction, fl at slab subduction, and termination of volcanism is the basis for various hypotheses discussed in this volume by others. Development of the central valley of Chile is also attributed to subduction of the Juan Fer- nandez Ridge.

As the hotspot-modifi ed Mesozoic ocean crust bends into the trench, the NE-trending faults are reactivated and break into scarps as much as 1 km high. That relief erodes the frontal prism along the lower slope of the continental margin. The uplifted ridge across the margin clearly marks the subducted relief (Fig. 5), and increased lower-plate displacement once it subducts is suggested (von Huene et al. 1997). Bend faulting continues in the subduc- tion zone, and there the fl uid fl ow is impeded by the upper plate. The elevated pore pressure is likely to reduce friction and facili- tate lower-plate fault displacement after subduction. The faults

-4000 0 meter Uplifted Seafloor above JFR Valparaiso Ba_sin O'Higgins Seamount Juan Fernández Ridge turbidite flooded trench accreted pris_m seafloor spreading fabric 74°00_′W 73°40_′W 73°20_{′W 73°00} ′W 72°40_′W 72°20_′W 72°00_′W 71°40_′W 34°00_′S 33°40_′S 33°20_′S 33°00_′S 32°40_′S 32°20_′S 32°00_′S 31°40_′S 31°20_′S 31°00_′S

Figure 5. Multibeam bathymetry of the continental margin near Valparaiso, Chile, where the Juan Fernandez Ridge (JFR) collides and subducts. The ridge blocks axial transport of sediment, causing ponding of trench turbidite. Across the slope, the subducting ridge uplifts the overlying seafl oor. The zone of collision migrates slowly to the south, where thick sediment is accreted, whereas in the north, the thin trench sediment is associated with subduction erosion.

are also conduits for migration of water into the crust (Kopp et al., 2004).

Disruption of axial sediment transport by the ridge has resulted in a long sediment-starved margin, along with its small sediment supply because of regional aridity since Miocene time. Axial sediment ponding supplies the trench axis south of the ridge with thick turbidite sediment originating from the southern Andes. This division corresponds with accretion in the south and erosion in the north. The erosional margin character of the north is an example of the margin confi guration prior to Quaternary accre- tion south of the Juan Fernandez Ridge barrier. In pre-Quaternary time, the entire Chilean margin is likely to have been erosional.

In the area of collision, interpretation of tectonic history is problematical without temporal control. Ranero et al. (2006) pro- posed that an accretionary prism was destroyed by Juan Fernan- dez Ridge collision. After collision and 3–4 m.y. of erosion, the slope morphology and structure refl ect the removal of a middle- slope prism (Ranero et al., 2006). A broad diagonal ridge across the margin above the subducted Juan Fernandez Ridge changed structure and morphology on either side as it migrated southward. Since the ridge controlled accretion, the margin has only been accretionary south of the ridge during the past 1.5–3 m.y. Here, middle- and lower-slope morphology consists of margin-parallel ridges that are inferred to consist of trench sediment added to the margin upper plate, forming a 40-km-wide frontal and mid- dle prism. North of the diagonal ridge, the accreted middle- and lower-slope prisms are missing, and the depressed middle slope, its normal faults, and slope failure display a mature erosional margin morphology (Ranero et al., 2006). Alternatively, Laursen et al. (2002) proposed a period of compressional tectonism from subduction of seamounts that reactivated an older landward- dipping sequence of faults to form the outer fl ank of Valparaiso Basin. Subduction of the ridge also eroded the front of the margin framework. An area along the plate interface eroded and caused subsidence to form the basin depocenter. These scenarios differ between a long or short tectonic history that formed the basin, but a common feature to both scenarios is concentrated deformation over the subducting ridge and subduction erosion.

The modest crustal thickening of the crust beneath Juan Fer- nandez Ridge is inadequate to provide the signifi cant buoyancy that has been proposed to help in developing the fl at slab (Kopp et al., 2004). Wide-angle seismic data indicate an anomalously low upper-mantle velocity that is restricted to the area of reacti- vated NE-trending fault scarps. The low velocities may indicate mineral alteration by hydration of the upper mantle from water migrating down faults to mantle depths. Up to 20% of the mantle rock may be serpentinized (Kopp et al., 2004), and that would contribute the buoyancy necessary in conversion of steeply dip- ping subduction zones to a fl at-slab confi guration.