Kinematics of the North American–Caribbean‐Cocos plates in Central America from new GPS measurements across the Polochic‐Motagua fault system

The Polochic‐Motagua strike‐slip fault system in Guatemala marks the on‐land plate boundary between the North American (NA) and the Caribbean (CA) plates. GPS observations in 1999 and 2003 show that the far‐field velocity across the system (NA‐CA relative velocity) is ∼20 mm/yr. This is significantly higher than the NUVEL‐1A velocity but is consistent with the GPS based CA‐NA velocity proposed by DeMets et al. (2000). The observations are modeled by a fault centered on the Motagua fault, locked at a depth of 20 km, with a slip‐rate decreasing from eastern to central Guatemala from 20 to 12 mm/yr towards the NA‐CA‐Cocos triple junction. This decrease is accommodated by ∼8 mm/yr of E‐W extension in the westernmost part of CA south of the Motagua fault. About 10 mm/yr of dextral slip is observed across the Mid‐American Volcanic Arc. The NA‐CA‐Cocos triple junction is thus a complex, ∼400 km‐wide wedge‐shaped area.