The process of subduction erosion remains largely enigmatic. Yet, although 60% of the convergent margins are erosional and the 40% left correspond to accretionary, the scientific debate has mostly focused on the latter.
Former studies have correlated features characterizing several erosional margins to hypothesize about those parameters either controlling or triggering the process of subduction erosion. Those features include thinner blanket of sediments, thinner subduction channel (SC), larger tapers, subsidence at the upper-middle slope, etc) distinguish erosional from accretionary margins.
We attempt to identify potentially relevant parameters involved in or controlling mass-transfer modes in brittle, tectonically erosive forearcs by means of four series of sandbox experiments. Each series represents one potential parameter. During the sandbox experiments, we were able to track particle displacement by using Particle Image Velocimetry (PIV). The analysis was focused on internal and basal material transfer, mass transfer mode patterns, wedge geometry, SC evolution and frictional properties of the analogue granular material.
A wedge built with a Mohr Coulomb material, reproducing brittle behavior of rocks, is characterized by variable frictional strengths, conditioning mechanical interpretations to the spatio-temporal frictional strength variations. (What do you mean by “conditioning mechanical interpretations?”) This hypothetical erosional forearc (i. e. no incoming sand layer, high frictional basal detachment, strong granular material) was designed initially in a critical taper state and with rear material loss. First observations pointed to a strong interaction between SC segmentation and wedge deformation. The spatial distribution of the frictional strength within the wedge mainly controlled the loci and/or volumes of modes of mass transfer. Phases in the evolution of the components of the velocity field were observed, implying that subduction erosion may be an irregular process.
The variable behavior of the frictional strength also determined the flow inside the SC. The first experimental series, in which the amount of material loss at the wedge's rear was systematically varied, confirmed the strong influence of the SC on the wedge evolution. The second experimental series was designed to analyze the influence of different surface taper angles. This series showed that the temporal variability of the different frictional strength values controlled the geometry of the SC and the stability of the wedges. The third series, which evaluated the effect of internal friction properties of the granular material of the wedges, indicated that basal erosion was more favorable for strong wedges, which may be the case for wedges composed of crystalline basement (usually the case for erosional margins). The fourth series tested the response of the wedge to topographic highs and lows on the incoming plate. These bathymetric anomalies favored frontal erosion and basal erosion at the frontal segment of the wedge and inhibited basal erosion at the rear part of the wedge.
This study was the first investigating potential kinematic boundary conditions for subduction erosion in a systematic manner. For the subduction erosional process, the aperture at the box's rear, which allowed rearward material loss, was shown to have the largest influence on obtained results. If the amount of material leaving the system was larger than the amount of material subducted at the wedge's toe, the margin evolved as erosional. We found the surface slope to be the second important parameter, strongly controlling the amount of basally eroded material.
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