CPT Q005 – Are sediments in trenches deformed and contorted as expected?

5. Question: Is there evidence that sediments in trenches are deformed and contorted as one should expect if subduction is genuine?

Response: There are hundreds of articles in the peer-reviewed literature that document mélange structure in subduction zone accretionary wedges. I have already described the complex and contorted structure observed both by drilling and seismic profiling of the accretionary wedge that lies within the Nankai Trough where the Philippine Plate is actively subducting beneath southwestern Japan. Similarly, I have already described the classic example of a fossil subduction zone, the Franciscan terrane along the central and northern California coast, which displays strongly contorted sediments and ocean crustal rocks that have been subducted to depths of 20-30 km, metamorphosed to blueschist grade, and then amazingly have risen up the subduction channel back to the surface. One of many other examples I could point to is in Guatemala, similar in many ways to the Franciscan, and described in the recent paper by Marroni, M., et al., “Deformation history of the eclogite- and jadeitite-bearing mélange from North Motagua Fault Zone, Guatemala: insights in the processes of a fossil subduction channel,” Geological Journal, 44, 167-190, 2008, whose abstract I reproduce as follows.

In Guatemala, along the northern side of the Motagua Valley, a mélange consisting of blocks of eclogite and jadeitite set in a metaserpentinitic and metasedimentary matrix crops out. The metasedimentary rocks display a complex deformation history that includes four tectonic phases, from D1 to D4. The D1 phase occurs only as a relic and is characterized by a mineral assemblage developed under pressure temperature (P-T) conditions of 1.00-1.25 GPa and 206-263°C. The D2 phase, characterized by isoclinal folds, schistosity and mineral/stretching lineation, developed at P-T conditions of 0.70-1.20 GPa and 279-409°C. The following D3 and D4 phases show deformations developed at shallower structural levels. Whereas the D1 phase can be interpreted as the result of underplating of slices of oceanic lithosphere during an intraoceanic subduction, the following phases have been acquired by the mélange during its progressive exhumation through different mechanisms. The deformations related to the D2 and D3 phases can be regarded as acquired by extrusion of the mélange within a subduction channel during a stage of oblique subduction. In addition, the structural evidences indicate that the coupling and mixing of different blocks occurred during the D2 phase, as a result of flow reverse and upward trajectory in the subduction channel. By contrast, the D4 phase can be interpreted as related to extension at shallow structural levels. In this framework, the exhumation-related structures in the mélange indicate that this process, probably long-lived, developed through different mechanisms, active in the subduction channel through time.

While mélange formation is expected in places like the Nankai Trough today under the slow rates of convergence assumed in UPT because the sediments are inherently soft, many of the other striking features of the fossil accretionary wedge deposits, such as large volumes of blueschist rocks returned to the surface, are not readily explained in the framework of UPT, but instead seem to require the catastrophic conditions associated with CPT.

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