Emplacement, growth, and gravitational deformation of serpentinite seamounts on the Mariana forearc

Abstract

Serpentinite seamounts, representing some of the first material outputs of the recycling process that takes place in subduction zones, are found on the outer Mariana forearc. Multichannel seismic (MCS) and bathymetric data collected in 2002 image the large-scale structures of five seamounts, as well as the pre-seamount basement geometry and sediment stratigraphy. We present data from three edifices that provide insights into seamount growth and internal deformation processes and allow us to support the interpretation that serpentinite mud volcanoes are formed by the episodic eruption of mud flows from a central region. The presence of thrust faulting at the base of Turquoise and Big Blue Seamounts, along with the low surface slopes (5°-18°) of all the seamounts studied, lead us to infer that these edifices spread laterally and are subject to gravitational deformation as they grow. Numerical simulations using the discrete element method (DEM) were used to model their growth and the origins of features that we see in MCS sections, such as basal thrusts, inward-dipping reflections and mid-flank benches. The DEM simulations successfully reproduced many of the observed features. Simulations employing very low basal and internal friction coefficients (∼0.1 and ∼0.4, respectively) provide the best match to the overall morphology and structures of the serpentinite seamounts. However the simulations do not capture all of the processes involved in seamount growth, such as withdrawal of material from a central conduit leading to summit deflation; compaction, dewatering and degassing of mud flows; mass wasting in the form of sector collapse and growth upon a dipping substrate. A strong reflection beneath the summit of Big Blue, the largest serpentinite seamount on the Mariana forearc, represents the floor of a summit depression that has been partially in-filled by younger muds, supporting the idea that serpentinite seamounts grow by episodic mud volcanism. Boundaries of mud-flow units are visible in bathymetric data and as normal polarity, subhorizontal reflections on seismic profiles. Big Blue Seamount displays complex nesting relationships as it merges with other seamounts to form a large, composite edifice. Flank flows of serpentinite muds on Big Blue and Celestial Seamounts downlap pre-existing forearc substrate. The interface between serpentinite seamounts and the underlying forearc sediments is represented by a reverse polarity reflection beneath Big Blue and Celestial Seamounts, suggesting that the substrate is undercompacted/overpressured and may be a zone of fluid migration. DEM simulations suggest that this boundary represents a distinct décollement along which the seamounts slide laterally. In contrast, Turquoise Seamount grows laterally, not by stable sliding along the top of forearc sediments, but by incorporating them into large basal thrusts. © 2007 The Authors Journal compilation © 2007 RAS.