The Polochic-Motagua fault systems (PMFS) are part of the sinistral
transform boundary between the North American and Caribbean plates. To
the west, these systems interact with the subduction zone of the Cocos
plate, forming a subduction-subduction-transform triple junction. The
North American plate moves westward relative to the Caribbean plate.
This movement does not affect the geometry of the subducted Cocos plate,
which implies that deformation is accommodated entirely in the two
overriding plates. Structural data, fault kinematic analysis, and
geomorphic observations provide new elements that help to understand the
late Cenozoic evolution of this triple junction. In the Miocene,
extension and shortening occurred south and north of the Motagua fault,
respectively. This strain regime migrated northward to the Polochic
fault after the late Miocene. This shift is interpreted as a
``pull-up'' of North American blocks into the Caribbean realm. To the
west, the PMFS interact with a trench-parallel fault zone that links the
Tonala fault to the Jalpatagua fault. These faults bound a fore-arc
sliver that is shared by the two overriding plates. We propose that the
dextral Jalpatagua fault merges with the sinistral PMFS, leaving behind
a suturing structure, the Tonala fault. This tectonic ``zipper''
allows the migration of the triple junction. As a result, the fore-arc
sliver comes into contact with the North American plate and helps to
maintain a linear subduction zone along the trailing edge of the
Caribbean plate. All these processes currently make the triple junction
increasingly diffuse as it propagates eastward and inland within both
overriding plates.