Crustal structure of rifted and convergent margins : the U.S. East Coast and Aleutian margins

Lizarralde, D.

doi:10.1575/1912/5699

Cited by 5 publications

(10 citation statements)

References 42 publications

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“…The wedge offshore Nicoya Peninsula has higher velocities at comparable distances from the trench than any other well-constrained margin wedge. Margins that appear to have velocities most similar to the offshore Nicoya Peninsula wedge are located in the Shumagin Islands region of the Aleutian Trench[Lizarralde, 1997] and near the eastern Nankai Trough. The Shumagin Islands margin wedge is interpreted t6 be similar to the Prince William terrane, consisting of sedimentary rocks with interstratified volcanic rocks 6f both basaltic and andesitic composition [Lewis et al, 1988].…”

contrasting

confidence: 83%

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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contrasting

confidence: 83%

Structure of the Costa Rica convergent margin, offshore Nicoya Peninsula

Christeson¹,

McIntosh²,

Shipley³

et al. 1999

J. Geophys. Res.

108

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“…Finally, there is no clear expression of this feature in the gravity field. While all other portions of our velocity model can be easily explained with a simple velocity‐to‐density conversion and two‐dimensional gravity modeling [ Lizarralde , 1997], the calculated anomaly above the basement high is too large. There is no question that the structure exists beneath the profile, and so the gravity misfit suggests that the basement high is a local feature rather than one with infinite lateral extent.…”

Section: Discussionmentioning

confidence: 99%

“…The first arrival at far offsets on OBH19 may be a diving wave phase, as indicated in Figure 4. Travel time and synthetic seismogram calculations indicate that P 8 phases generally arrive very close to the observed first‐arrival branch and are quite weak, thus explaining the difficulty in identifying this phase [ Lizarralde , 1997]. This is commonly the case with lower crustal diving waves in relatively thick crust.…”

Section: Seismic Data and Velocity Modelmentioning

confidence: 99%

“…Models with a 6.90 km s −1 top velocity require the entire lower crust in this domain to be a low‐velocity zone and have RMS errors ∼10% greater than for the final model (0.133 s versus 0.119 s). The average velocity in this domain can be increased to ∼7.0 km s −1 only if the velocity in the forearc domain is reduced considerably [e.g., Lizarralde , 1997]. The requirements for the velocity in the forearc domain are strict, however, as described below.…”

Section: Seismic Data and Velocity Modelmentioning

confidence: 99%

“…Here we give a brief summary of these constraints. A more detailed discussion of the line BA1 data is given by Lizarralde [1997].…”

Section: Seismic Data and Velocity Modelmentioning

confidence: 99%

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Crustal construction of a volcanic arc, wide‐angle seismic results from the western Alaska Peninsula

et al. 2002

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[1] Results from the 1994 Aleutian Seismic experiment delineate basic oceanic arc crustal architecture, constrain magmatic flux rates and bulk arc composition, and address questions of continental crustal genesis. Here we present results from a transect across protocontinental crust of the westernmost Alaska Peninsula (line A3) and compare this structure to a purely oceanic arc transect farther west. Arc crustal structure is similar along these two transects. Magmatic accretion occurs at the top and bottom of preexisting oceanic crust as a 5-to 10-km-thick upper crustal carapace of low velocity (2-5.8 km s À1 ) volcaniclastics, flows and small plutons, and a mafic lower crustal underplate ($7.0 km s À1 ) of variable thickness, for a maximum arc crust thickness of $25-30 km. Lateral lower crustal velocity gradients and high velocities (>7.5 km s À1 ) beneath the forearc suggest dominantly vertical lower crustal accretion above a focused melt source and a forearc underlain by little magmatic crust but rather partially intruded and/or serpentinized mantle. The ratio of upper to lower crustal volume is $1, and the total arc crust volume implies a magmatic flux of $67 km 3 km À1 m.y. À1 , more than twice previous estimates for this arc and global productivity. The crust is thinner and more mafic than continental crust, and it lacks a massive tonalitic upper crust characteristic of the continents. An interpreted accumulation of upper crustal carapace material at midcrustal depths on line A3 has a velocity of $6.4 km s À1 , suggesting an intermediate composition. Accretionary complex terranes consisting of accumulations of this type material would thus have bulk compositions similar to continental crust.

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Segmentation of the Nazca and South American plates along the Ecuador subduction zone from wide angle seismic profiles

Gailler

Charvis

Flueh

2007

Earth and Planetary Science Letters

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International audienceWe describe the deep structure of the south Colombian-northern Ecuador convergent margin using travel time inversion of wide-angle seismic data recently collected offshore. The margin appears segmented into three contrasting zones. In the North Zone, affected by four great subduction earthquakes during the 20th century, normal oceanic crust subducts beneath the oceanic Cretaceous substratum of the margin underlined by seismic velocities as high as 6.0-6.5 km/s. In the Central Zone the subducting oceanic crust is over-thickened beneath the Carnegie Ridge. A steeper slope and a well-developed, high velocity, Cretaceous oceanic basement characterizes the margin wedge. This area coincides with a gap in significant subduction earthquake activity. In the South Zone, the subducting oceanic crust is normal. The fore-arc is characterized by large sedimentary basins suggesting significant subsidence. Velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing. Even if the distance between the three profiles exceeds 150 km, the structural segmentation obtained along the Ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation

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Crustal structure of rifted and convergent margins : the U.S. East Coast and Aleutian margins

Cited by 5 publications

References 42 publications

Structure of the Costa Rica convergent margin, offshore Nicoya Peninsula

Structure of the Costa Rica convergent margin, offshore Nicoya Peninsula

Crustal construction of a volcanic arc, wide‐angle seismic results from the western Alaska Peninsula

Segmentation of the Nazca and South American plates along the Ecuador subduction zone from wide angle seismic profiles

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