170 Scientific Prospectus

Figure Captions

Figure 1. Location of 2D and 3D seismic profiles from 1987 cruise of Fred Moore. Inset shows map of Central America, Cocos plate, and Caribbean plate. Nicoya Peninsula is located just below the "C" in Costa Rica on the inset drawing. The stippled rectangle shows the location of the 3D seismic grid. The longer, labeled lines are 2D profiles.

Figure 2. Simplified interpretation of a section through the lower and middle slope region of the Costa Rica continental margin.

Figure 3. Summary of solid rock volume balance calculations by plotting sediment accumulation vs. time. The diagonal lines represent different rates of accretion as a volume percent of the total incoming sedimentary section assuming zero porosity. Estimates of the accreted volume for the Costa Rica accretionary prism are between the two horizontal dashed lines labeled "observed accretion" This graph represents results pertaining only to the lower slope (from McIntosh, 1992).

Figure 4. A. 10Be (half-life = 1.5 m.y.) concentrations and lithology vs. depth (data from Zheng and Morris, unpubl. data) at DSDP Site 495, offshore of Guatemala. The solid 10Be profile emphasizes the high concentrations of 10Be measured in near-surface sediments and the exponential decay to unmeasurable values at about 200 m (approximately 12 Ma). For the volcanoes to contain 10Be, more than 90% of these uppermost sediments must subduct to the arc source region. The total amount (or inventory) of 10Be in a 1-cm-by-1-cm sediment column = 13.2 x 1012 a/cm2. The 10Be inventory is calculated by integrating beneath the solid 10Be-depth curve shown in the graph. The dashed curve on the graph indicates the distribution of 10Be in the sediments after correcting for radioactive decay (10Bcorr = 5.7 x 1012 a/cm2) during the roughly 2 m.y. required to subduct the sediments from the trench axis to a point beneath the present volcanic front. The bulk atom 10Be/9Be ratio = 460 x 10-11 . This ratio is averaged for the sediment column and uses the decay-corrected 10Becorrconcentration above and the total 9Be abundance. (See Brown et al., 1992; Tera et al., 1986; Monaghan et al., 1988; Morris and Tera, 1989; Morris et al., 1990; and Morris et al., in prep.; for discussion of the 10Be systematics and results from volcanic arcs and oceanic sediments). B. The equivalent diagram for Costa Rica, showing the approximate age-depth-lithology relations in the incoming sediment column. The absence of 10Be in the volcanoes indicates that the uppermost part of the incoming sediment is accreted and/or underplated. The column labeled "hypothetical 10Be conc. for Site CR-2" shows the distribution of 10Be concentrations (curved lines) expected with depth if the intraprism reflectors (horizontal lines) are gently folded décollements ("D"). The age of the packets decreases with depth, but the sediments within a packet are youngest at the top. The 10Be concentrations at proposed Site CR-2 will be at measurable levels if the sediments are younger than about 12 Ma. An absence of 10Be at proposed Site CR-2 implies older ages, which is consistent with episodic erosion or subduction erosion as discussed. The base of the underplated hemipelagic sediments is ~6-11 Ma, and the base of the carbonate is ~24 Ma. Stratigraphy for Costa Rica is inferred from seismic stratigraphy (Shipley and Moore, 1986) and by analogy with DSDP Site 495.

Figure 5. Correlation between Ba flux in subducted sediment and Ba enrichment of arc basalts, for the Northern Antilles (Ant), Marianas (Mar), Tonga (T), Mexico (Mex), Java, Aleutians (Al), and Guatemala (Guat) arcs (after Plank and Langmuir, 1993). These types of correlations, which are present for other elements as well, suggest that volcanic output is linked to sediment input in subduction zones. Line is best fit of solid points; vertical error bars are one standard deviation of the mean of volcanoes within each arc; horizontal error bars represent uncertainties in the thickness of sediment being subducted resulting from variable supply, underplating, and erosion (generally +100 m). Input fluxes do not include accreted material, however (see Plank and Langmuir, 1993). Open circles are for the Costa Rica subduction zone. Ba/Na ratios are for Arenal and Rincon volcanoes (data from Carr and Rose, 1987), which are the Costa Rican volcanoes closest to the proposed drilling transect (these northern Costa Rican volcanoes are also chemically distinct from the alkalic southern Costa Rica volcanoes, as noted by Carr et al., 1990). The Ba fluxes for Costa Rica are calculated from the composition and density of the sedimentary column at Site 495, off Guatemala. The arrow at the open circles shows the effect of frontal accretion on the subducted flux, where increasing amounts of the upper hemipelagic mud unit are removed (total meters of sediment subducted as indicated). Fifty meters of offscraping would bring the Ba flux for Costa Rica in line with the global trend. This is an illustrative example of how chemical fluxes might help to constrain accretionary processes. Accurate estimates require better control of the global trend (from improved constraints on the accretionary dynamics at other subduction zones) and Ba data on the incoming sedimentary column off Costa Rica (e.g., proposed Site CR-1).

To 170 Site CR-1

170 Table of Contents

Publications Home

ODP Home