Observing the Structure and Effects of Terrane Accretion at Depth
through Patterns of Seismicity in Colombia's Cauca Cluster
Abstract
The Cauca Cluster of seismicity in western Colombia is the most
extensive and persistent collection of intermediate (>60
km) depth earthquakes that cannot be easily associated with a subducting
slab. The cluster stretches over an area of ~390 km by
~250 km from 2.5°N to 6°N and from 75.5°W to 77.75°W in
a wedge-like zone of seismicity thickening from ~25 km
in the west to ~125 km in the east. Prior tomographic
results suggest that the lower edge of this feature is contained within
the subducting Nazca plate’s oceanic lithosphere and corresponds to the
Wadati-Benioff zone (WBZ) seen in most slabs, however this cannot
explain the full thickness of the cluster. Large earthquakes within the
cluster have been reported since at least the 1960s-70s and a great
number of smaller events have been reported since the establishment of
the Red Sismológica Nacional de Colombia (RSNC)’s regional catalog in
1993. Here, we relocate more than 6,700 events from the RSNC’s catalog
beginning in 2010 and extending for ~10 years to dissect
the Cauca Cluster’s structure and relationship to seismicity below 10 km
depth. We find that while 40% of this seismicity can be associated with
the Nazca plate’s WBZ, 35% occurs as features completely or partially
within the overlying forearc mantle. These features include: 1) three
focused centers of seismicity at ~90,
~100, and ~120 km depth extending
~30 km perpendicular to the WBZ; 2) a feature dipping at
an angle shallower than the WBZ between 10 km and ~75 km
depth; and 3) a diffuse zone of seismicity extending from the WBZ to
~10 km depth. None of these features extend beneath the
active volcanic arc, and as such are limited to the stagnant corner of
the mantle wedge. We find that these features are also limited to an
area affected by the late Miocene accretion of the Panama-Choco terrane,
the top of which we associate with the dipping feature between 10 and 75
km depth. This accretion has likely cooled the deep forearc to a point
that allows for seismicity. The occurrence of these mantle wedge
earthquakes in an immobile part of the subduction system suggests they
are not produced directly by dehydration. They may instead be a result
of fracture induced by the upward movement of fluids from the slab or of
self-localizing thermal shear runaway triggered by reheating of the
cooled forearc along its arc-ward edge.