Abstract:
At several localities around the world,
thrust belts have developed on both sides of
oceanic island arcs (e.g., Java-Timor, Panama,
Vanuatu, and the northeastern Caribbean).
In these localities, the overall vergence
of the backarc thrust belt is opposite to that
of the forearc thrust belt. For example, in
the northeastern Caribbean, a north-verging
accretionary prism lies to the north of the
Eastern Greater Antilles arc (Hispaniola and
Puerto Rico), whereas a south-verging thrust
belt called the Muertos thrust belt lies to
the south. Researchers have attributed such
bivergent geometry to several processes,
including: reversal of subduction polarity;
subduction-driven mantle fl ow; stress transmission
across the arc; gravitational spreading
of the arc; and magmatic infl ation within
the arc. New observations of deformational
features in the Muertos thrust belt and of
fault geometries produced in sandbox kinematic
models, along with examination of
published studies of island arcs, lead to the
conclusion that the bivergence of thrusting
in island arcs can develop without reversal of
subduction polarity, without subarc mantle
fl ow, and without magmatic infl ation. We
suggest that the Eastern Greater Antilles
arc and comparable arcs are simply crustalscale
bivergent (or “doubly vergent”) thrust
wedges formed during unidirectional subduction.
Sandbox kinematic modeling suggests,
in addition, that a broad retrowedge containing
an imbricate fan of thrusts develops only
where the arc behaves relatively rigidly. In
such cases, the arc acts as a backstop that
transmits compressive stress into the backarc
region. Further, modeling shows that when
arcs behave as rigid blocks, the strike-slip
component of oblique convergence is accommodated entirely within the prowedge and
the arc—the retrowedge hosts only dip-slip
faulting (“frontal thrusting”). The existence
of large retrowedges and the distribution of
faulting in an island arc may, therefore, be
evidence that the arc is relatively rigid. The
rigidity of an island arc may arise from its
mafi c composition and has implications for
seismic-hazard analysis.