César R. Ranero

Quaternary convergent margin tectonics of Costa Rica, segmentation of the Cocos Plate, and Central American volcanism

Along Costa Rica, new geophysical data indicate considerable control of Quaternary convergent margin tectonics by the subducting lower plate. Three types of ocean crust enter the subduction zone: (1) Cocos Ridge with its underlying thick crust stands 2 km high, (2) on its north flank is normal crust covered 40% by seamounts, and (3) along the adjacent Nicoya margin the underthrust crust has a smooth sea floor. A 3- to 10-km-wide base of slope frontal prism varies little opposite different subducting crusts except where subducting seamounts eroded it. Once the breaching seamount has passed the prism it is quickly restored. The effect of oceanic crust on continental margin structure is most evident in the middle and upper slope. Where Cocos Ridge and its flanking seamounts subduct, erosion is pronounced relative to the stable slope where smooth lower plate subducts. Aligned upper plate features above lower plate segment boundaries extend more than 120 km landward of the trench axis and correspond in varying degrees with volcanic arc segmentation. The offset of volcanoes across the Costa Rica/Nicaragua border corresponds with a change in crustal structure and depth of the lava source. Subducted sediment shows little correlation with the slab signal in volcanic arc lavas but the magnitude of faulting associated with ocean plate flexure adjacent to the trench axis parallels it well. Thus fluids in ocean crust fractures and bound water in serpentinite may have a recognizable geochemical effect in arc lavas.

Revised tectonic boundaries in the Cocos Plate off Costa Rica: Implications for the segmentation of the convergent margin and for plate tectonic models

The oceanic Cocos Plate subducting beneath Costa Rica has a complex plate tectonic history resulting in segmentation. New lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the East Pacific Rise from lithosphere formed at the Cocos-Nazca spreading center. They also define two early phase Cocos-Nazca spreading regimes and a major propagator. In addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of Cocos Plate over the Galapagos hot spot. The subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab. Newly identified seafloor-spreading anomalies show oceanic lithosphere formed during initial breakup of the Farallon Plate at 22.7 Ma and opening of the Cocos-Nazca spreading center. A revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the Cocos-Nazca spreading center. At 19.5 Ma a major ridge jump reshaped its geometry, and after ∼14.5 Ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere. A suspected cause of ridge jumps is an interaction of the Cocos-Nazca spreading center with the Galapagos hot spot.

Magnetic anomaly interpretation across the southern central Andes (32°–34°S): The role of the Juan Fernández Ridge in the late Tertiary evolution of the margin

Marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of Chile between 32°–34°S. Three magnetic domains have been identified: oceanic, continental margin, and subaerial. The oceanic domain has seafloor spreading anomalies (16 (∼37 Ma) to 18 (∼39.5 Ma)) disturbed by anomalies of the Juan Fernandez hot spot chain. In the continental margin, the most prominent fabric are E-W anomalies in the upper slope corresponding to onshore E-W anomalies of large intrusive bodies. Onshore, a N-S lineament of short-wavelength anomalies defines the roots of a Cretaceous volcanic arc. A resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to ∼50 km landward of the trench. Absolute Cenozoic plate motion for Nazca and South American plates and dating of the Juan Fernandez chain provide a kinematic model of ridge-continent collision. The reconstruction indicates rapid southward migration of the collision point along ∼1400 km of the margin from 20 to 11 Ma (∼20 cm yr−1). From 11 Ma to present the collison point has migrated at a slower rate along ∼375 km of the margin (3.5 cm yr−1). The predicted location of the subducted portion of the Juan Fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship. In the last ∼10 Myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates. These observations indicate that subduction of the Juan Fernandez chain plays a major role in arc-forearc tectonics.

Generic model of subduction erosion

Erosion by high stress abrasion of convergent margins from horsts and grabens on the subducting plate is not shown in seismic images. In a proposed model, the frontal sediment prism is a dynamic mass that elevates pore-fluid pressure. Overpressured fluid invades fractures in the upper plate and separates fragments that are dragged into a subduction channel along the plate interface. Removed fragments are smaller than surface ship seismic techniques have resolved and beyond the reach of past scientific ocean drilling; however, current drill capability and downhole geophysics can test the model.

Subduction erosion and basal friction along the sediment‐starved convergent margin off Antofagasta, Chile

[1]xa0Subduction erosion is commonly associated with strong interplate coupling and a consequent abrasion of the upper plate. Northern Chile is an often cited example of a strongly coupled erosional margin. Its crystalline basement is inferred to form a strong upper plate, the trench axis contains little detectable sediment, and the subducting lower plate has a high-relief horst-graben topography. With little water-rich sediment to reduce interplate friction, the high relief of an igneous ocean crust thrust beneath continental basement should generate high friction interplate abrasion. However, a prestack depth-migrated seismic record images slope debris that collects in a frontal prism. This debris, including ∼30% pore fluid, fills subducting grabens and is subsequently incorporated into an ∼1.5-km-thick interplate reflective layer. The subduction zone thrust passes through the upper part of this layer. Interplate seismicity and taper analyses indicate basal friction at levels that are common in sedimented convergent margins. The continued growth of lower plate grabens after subduction probably accommodates upper plate material, a process that erodes the upper plate. Erosion is aided by weakening of the upper plate rock framework beneath the continental slope. This erosion undermines the upper plate and tips it seaward thereby steepening the continental slope which induces midslope gravity tectonics. Despite sediment starvation, a frontal prism constructed of remolded slope debris elevates pore pressure to reduce interplate friction. Coeval erosion and prism building control the size of the frontal prism. Processes other than high friction abrasion best explain subduction erosion along northern Chile.

Fluid expulsion related to mud extrusion off Costa Rica—A window to the subducting slab

A large number of mound-shaped structures that originated from mud extrusions is present along the convergent continental margin off Costa Rica and Nicaragua. Active fluid venting is indicated by the existence of CH4- and H2S-rich pore fluids as well as associated benthic fauna and authigenic carbonates. End-member fluid samples from all mounds are significantly depleted in dissolved Cl and other major elements, suggesting a general process of freshwater addition and thus a common source of the fluids. Our data clearly rule out dilution by gas hydrate dissociation as a dominant source of the freshwater. Enrichments of the fluids in B (up to 2 mmol/L) and inversely correlated δ18O vs. δD values point to clay-mineral dehydration as the cause for these anomalies. Calculations assuming a δ18O vs. δD equilibrium between the pore fluid and clay minerals at depth of formation indicate temperatures of dehydration between 85 and 130 °C. This temperature range is in agreement with the B enrichments and the presence of thermogenically formed CH4. Because temperatures above 50 °C are not reached within the sediment cover of the upper plate, the fluids most likely form within the subducted sediments and flow upward along deep-seated faults from ≥12 km depth. Mound-related fluid expulsion may contribute significantly to the recycling of mineral-bound water.

Tectonic control of the subducting Juan Fernández Ridge on the Andean margin near Valparaiso, Chile

Near the latitude of Valparaiso, Chile, a fundamental change in configuration of the Benioff Zone, volcanic arc activity, and the structure of the continental margin occurs opposite the subducting Juan Fernandez Ridge. Upper plate tectonics related to subduction of the ridge were studied by an international group of geoscientists in the two-degree segment offshore Valparaiso, extending from the shelf edge seaward across the eastern end of Juan Fernandez Ridge. Near the OHiggins group of seamounts, the Juan Fernandez Ridge strikes northeast rather than continuing its east-west trend across the Pacific Basin. The ridge uplifts the upper plate, and sediments of the Valparaiso Basin are deformed against its southern flank. This deformation is consistent with the southward migration required by an oblique trending ridge and the nearly trench-normal vector of plate convergence. In the trench axis, the ridge forms a basement barrier behind which sediments 2.5 km deep have ponded. The lower slope over the ridge appears eroded, whereas the margin not yet affected by ridge subduction is fronted by an accretionary prism about 25 km wide. Nazca Plate relief clearly influences tectonism of the margin where it subducts beneath thin continental crust; its relation to deeper processes segmenting the Andean Orogen appears to involve prior tectonic events.

Drowned 14-m.y.-old Galápagos archipelago off the coast of Costa Rica: Implications for tectonic and evolutionary models

Volcanic rocks were dredged from the Cocos and Fisher ridges and seamounts along a n250 km profile parallel to the Pacific coast of Costa Rica. The composition and laser 40Ar/39Ar nages of the Cocos Ridge and Seamounts are consistent with their formation above the Galapagos nhotspot 13.0–14.5 Ma. The reconstructed paleoenvironment and chemistry of the Fisher nRidge are consistent with it having originated at a mid-oceanic ridge system. Laser 40Ar/39Ar ndating of fresh basalt glass from the Fisher Ridge yielded isochron ages of 19.2 ± 0.3 Ma and n30.0 ± 0.5 Ma. The Fisher Ridge is along a lithospheric fault that may represent an extensional nfracture formed when the oceanic floor rode over the Galapagos hotspot. Even though the nyounger structures are currently at water depths of >1000 m, volcanological, geochemical, and ngeophysical observations indicate that they once formed an emerged archipelago very similar nin morphology to the Galapagos islands. The diversity of the biota on the isolated Galapagos nislands, as first described by Charles Darwin, has had an important influence on the development nof the theory of evolution. The existence of a now-drowned Galapagos archipelago n14.5 Ma considerably increases speciation times for the Galapagos biota and provides a ncomplete solution to a long-standing controversy concerning the divergence of the Galapagos nmarine and land iguanas from a single ancestral species.

Geophysical evidence for hydration of the crust and mantle of the Nazca plate during bending at the north Chile trench

Water transported in subducting oceanic plates plays a key role in a number of phenomena, including intraslab seismicity and arc magmatism. However, the locus of plate hydration and water distribution in crust and mantle of plates entering subduction zones is debated. We present evidence for anomalously low seismic velocities and densities of the crust and upper mantle of the Nazca plate at the north Chile trench. Crustal seismic velocities at the trench are lower than velocities of mature fast-spreading crust and even lower than velocities of highly extended slow-spreading crust. In addition, the Nazca plate at the north Chile trench may contain an ∼20-km-thick upper-mantle layer with ∼17% serpentine, which implies ∼2.5 wt% water. These results document pervasive rock alteration by water percolation linked to bending-related extensional faulting.

Fast rates of subduction erosion along the Costa Rica Pacific margin: Implications for nonsteady rates of crustal recycling at subduction zones

[1] At least since the middle Miocene (∼16 Ma), subduction erosion has been the dominant process controlling the tectonic evolution of the Pacific margin of Costa Rica. Ocean Drilling Program Site 1042 recovered 16.5 Ma nearshore sediment at ∼3.9 km depth, ∼7 km landward of the trench axis. The overlying Miocene to Quaternary sediment contains benthic foraminifera documenting margin subsidence from upper bathyal (∼200 m) to abyssal (∼2000 m) depth. The rate of subsidence was low during the early to middle Miocene but increased sharply in the late Miocene-early Pliocene (5-6.5 Ma) and at the Pliocene-Pleistocene boundary (2.4 Ma). Foraminifera data, bedding dip, and the geometry of slope sediment indicate that tilting of the forearc occurred coincident with the onset of rapid late Miocene subsidence. Seismic images show that normal faulting is widespread across the continental slope; however, extension by faulting only accounts for a minor amount of the post-6.5 Ma subsidence. Basal tectonic erosion is invoked to explain the subsidence. The short-term rate of removal of rock from the forearc is about 107-123 km 3 Myr -1 km -1 . Mass removal is a nonsteady state process affecting the chemical balance of the arc: the ocean sediment input, with the short-term erosion rate, is a factor of 10 smaller than the eroded mass input. The low 10 Be concentration in the volcanic arc of Costa Rica could be explained by dilution with eroded material. The late Miocene onset of rapid subsidence is coeval with the arrival of the Cocos Ridge at the subduction zone. The underthrusting of thick and thermally younger ocean crust decreased the subduction angle of the slab along a large segment of the margin and changed the dynamic equilibrium of the margin taper. This process may have induced the increase in the rate of subduction erosion and thus the recycling of crustal material to the mantle.