Structure at the Toe of the Subduction Complex: Middle America Trench offshore Guatemala

Ambos, Elizabeth L.; Hussong, D. M.

doi:10.2973/dsdp.proc.84.139.1985

Cited by 7 publications

(6 citation statements)

References 15 publications

(24 reference statements)

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“…Costa Rican tapers associated with the erosive impact of the Cocos Ridge and its associated seamounts (three upper triangles, Figure 9) plot close by whereas those opposite a smooth subducting plate are in the stable wedge field [ Ranero and von Huene , 2000]. Of the two Guatemalan tapers, the one from the DSDP drill transect [ Ambos and Hussong , 1985] is without a significant frontal prism and perhaps was recently impacted by a subducting seamount explaining its greater taper. The grouping of intermediate tapers supports our inference that many conditions at the front of sediment‐starved and well‐sedimented segments of the Chilean margin are similar.…”

Section: Discussionmentioning

confidence: 99%

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

Huene¹,

Ranero²

2003

J. Geophys. Res.

204

226

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Subduction 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.

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Section: Discussionmentioning

confidence: 99%

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

Huene¹,

Ranero²

2003

J. Geophys. Res.

204

226

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“…The velocities in Table 3 do wedge. We note that velocities somewhat similar to ours were interpreted offshore Guatemala along the Middle America Trench [Ibrahim et al, 1979;Ambos and Hussong, 1985]. These results were based on very limited and largely nonreversed data not comparable to those surveys included in Table 3.…”

Section: Comparison With Other Marginsmentioning

confidence: 99%

Structure of the Costa Rica convergent margin, offshore Nicoya Peninsula

Christeson¹,

McIntosh²,

Shipley³

et al. 1999

J. Geophys. Res.

108

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Abstract. We present the results of a seismic refraction survey conducted offshore Costa Rica near the Nicoya Peninsula. A dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers. These data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the Middle America Trench onto the Nicoya Peninsula. The best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron. Velocities increase from 3.5 to 4.2 to 4.6 km/s at distances of 10, 20, and 30-50 km landward of the trench.These velocities are higher than observed within margin wedges at other well-studied convergent margins but lower than the velocities within the adjacent Nicoya Complex, which are --5.5 km/s at similar depths below the surface. We interpret the margin wedge velocities as indicating that material similar to the Nicoya Complex extends seaward to near the lower slope but that fracturing, alteration, or accretion processes have lowered the velocity of the margin wedge with respect to the Nicoya Complex. The seismic refraction data cannot constrain the exact thickness or velocity of a possible low-velocity zone (LVZ) overlying the subducting plate; however, geologically reasonable structures are only produced with a LVZ <400 m thick. Velocities in the upper part of oceanic layer 2 are-•3.5-4.0 km/s within the subducting slab. These velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust. The top of the subducted slab is well resolved, and deepens from 5 km depth at the trench to 15-16 km depth at the Nicoya Peninsula coastline. The dip angle of the subducting plate increases from 6 ø to 13 ø at a distance of-30 km from the trench. Interplate seismicity appears to become common-•55 km from the trench where the plate boundary is at-•14 km depth.

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“…We restricted our analysis to a horizontal range of 15 km from the trench axis since beyond this range the spatial sampling of the data is very poor and the amplitude information is not very reliable. Ambos & Hussong (1984) analysed this data set by tracing rays from a shot located at the OBS location up to the surface and matching the travel time with the data within the range of 5-15 km. They marked the arrivals of two distinct phases: (i) a refracted (turning ray) arrival discernible up to a horizontal range of approximately 17 km and (ii) a reflection (post-critical) continuing up to a horizontal range of approximately 11 km.…”

Section: K Sen and L N Frazermentioning

confidence: 99%

Kirchhoff-Helmholtz reflection seismograms in a laterally inhomogeneous multi-layered elastic medium - II. Computations

Sen

Frazer

1985

Geophysical Journal International

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High-frequency reflection and refraction seismograms for laterally variable multi-layered elastic media are computed by using the frequency domain elastic Kirchhoff-Helmholtz (KH) theory of Frazer and Sen. Both source and receiver wavefields are expanded in series of generalized rays and then elastic (KH) theory is applied to determine the coupling between each source ray and each receiver ray at each interface. The motion at the receiver is given as a series of integrals, one for each generalized ray. We use geometrical optics and plane wave reflection and transmission coefficients for rapid evaluation of the integrand. When the source or the receiver ray field has caustics on the surface of integration geometrical ray theory breaks down and this gives rise to singularities in the KH integrand. We repair this using methods suggested by Frazer and Sen.Examples of reflection seismograms for 2-D structures computed by elastic KH theory are shown. Those for a vertical fault scarp structure are compared with the seismograms obtained by physical modelling. Then OBS data obtained from the mid-America trench offshore Guatemala area are analysed by computing KH synthetics for a velocity model that has been proposed for that area. Our analysis indicates the existence of a small low-velocity zone off the trench axis.No head wave arrivals are obtained in our KH synthetics since we do not consider multiple interactions of a ray with an interface. The nearly discontinuous behaviour of elastic R / T coefficients near the critical angle causes small spurious phases which arrive later than the correct arrivals.

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Structure at the Toe of the Subduction Complex: Middle America Trench offshore Guatemala

Cited by 7 publications

References 15 publications

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

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

Structure of the Costa Rica convergent margin, offshore Nicoya Peninsula

Kirchhoff-Helmholtz reflection seismograms in a laterally inhomogeneous multi-layered elastic medium - II. Computations

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