JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. B4,
PAGES 4439-4459, APRIL 10, 1992
Select options 'fulled paper': Shipley-CR3D-JGR92.pdf
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Three-Dimensional Seismic Imaging of the Costa Rica Accretionary Prism
Structural Diversity in a Small Volume of the Lower Slope
THOMAS H. SHIPLEY
KIRK. D. MCINTOSCH AND ELI A. SILVER AND PAUL L. STOFFA
ABSTRACT
Conventional two-dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms.
We undertook a three-dimensional seismic reflection survey of a small part of a prism arcward of the Middle America Trench off Costa Rica to more correctly image structure and to use the improved structural information to examine the processes of accretion.
This survey reveals small features, with dimensions of hundreds of meters, while also defining features thousands of meters in lateral extent, both of which were underappreciated in conventional two- dimensional data from the same area.
We have imaged active offscraping at the trench and both duplexing and out-of-sequence faulting a few kilometers arcward of the trench.
Fault spacing and reflector geometry vary dramatically over a space of several hundred meters. Some of these variations are related to visible changes in morphology of the underlying oceanic basement, but others are not so easily documented.
Fault surface reflections define an architecture which may control gross fluid motion through the prism. This architecture is apparently formed by duplexing and out-of-sequence faulting and has been maintained by periodic motion on some of the out-of-sequence faults.
The slope sediment apron records multiple phases of deformation. Abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism. A primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism.
…CONCLUSIONS
Sediments are actively accreted to the prism at the trench by offscraping the upper portion of the henripelagic section. Just a few kilometers arcward of the trench, prism growth and thick-ening are accomplished by a combination of both duplex and out-of sequence thrusting.
The observed rates of accretion are consistent with steady state growth of the lower slope portion of the prism for the last 5 to 10 m.y. as constrained by DSDP site 565.
Duplex and out-of-sequence thrusting a re occurring at only a few kilometers beneath the seafloor, very early in the deformation history.
Thus adjustments to maintain critical taper begin very early in the accretion processes, must be fairly continuous, and may help explain the complexity of structure in the very young accreted section and overlying slope apron.
Clear relationships frequently exist between basement with respect to fault spacing, length and shape. In other cases the overlying structure has no apparent relationship to basement morphology.
However the high convergence rate and frequent basement structures suggests a similar, if not directly identifiable, cause for the overlying structures. At the largest scale, intra prism fault zone reflections have greater extent than any known, unfaulted, stratigraphic horizons.
These structural features were not previously recognized to be spatially significant surfaces on individual lines in this area. We believe that these are evidence for a fairly well-defined structural architecture that potentially controls prism-wide fluid flow pathways and influences continuing prism deformation.
The amplitude of the fault reflections are fairly high and become highest where they intersect the top of the prism, suggesting that they have at times influenced fluid motion and mineralization.
Where these surfaces splay upwards near the base of slope the reflections become particularly bright and reversed in polarity. In addition, bright spots in local closures at the top of the prism indicate that upward fluid movement is impeded given the right structural trap.
The fault surfaces mainly result from the nature of the thickening at or near the bottom of the prism, accompanied with later continued (episodic?) motion on out-of-sequence faults, which help maintain the fault reflection continuity over time.
The boundary between the mainly slope sediments and underlying prism is the most prominent feature in the seismic data, for both its consistently high amplitude and broken and discontinuous nature.
The amplitudes are largely the result of the low-velocity slope sediments (about 1700 m/s) overlying higher-velocity accreted and more deformed sediments (1800- 2400 m/s) but enhance displaces by fluid accumulations at the boundary.
Numerous faults break the surface into small segments, most much less than1 km2 in map view. The faulting which disrupted this surface extends into the overlying slope sediment apron which records these multiple deformation phases and complexly juxtaposes the primarily accreted rocks and slope apron rocks.
In addition, the numerous faults with scarps delineated on the amplitude map of the seafloor further disrupt the slope structures.
These high-angle, small offset faults are important because they define recent active shortening of a broad region of the underlying prism. This implies a fairly weak theology but broad coupling of the stress into the overriding plate.