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.

International audienceNew GPS measurements in Chiapas (Mexico), Guatemala and El Salvador are used to constrain the fault kinematics in the North America (NA), Caribbean (CA) and Cocos (CO) plates triple junction area. The regional GPS velocity field is first analysed in terms of strain partitioning across the major volcano-tectonic structures, using elastic half-space modelling, then inverted through a block model. We show the dominant role of the Motagua Fault with respect to the Polochic Fault in the accommodation of the present-day deformation associated with the NA and CA relative motion. The NA/CA motion decreases from 18-22 mm yr−1 in eastern Guatemala to 14-20 mm yr−1 in central Guatemala (assuming a uniform locking depth of 14-28 km), down to a few millimetres per year in western Guatemala. As a consequence, the western tip of the CA Plate deforms internally, with ≃9 mm yr−1 of east-west extension (≃5 mm yr−1 across the Guatemala city graben alone). Up to 15 mm yr−1 of dextral motion can be accommodated across the volcanic arc in El Salvador and southeastern Guatemala. The arc seems to mark the northern boundary of an independent forearc sliver (AR), pinned to the NA plate. The inversion of the velocity field shows that a four-block (NA, CA, CO and AR) model, that combines relative block rotations with elastic deformation at the block boundaries, can account for most of the GPS observations and constrain the overall kinematics of the active structures. This regional modelling also evidences lateral variations of coupling at the CO subduction interface, with a fairly high-coupling (≃0.6) offshore Chiapas and low-coupling (≃0.25) offshore Guatemala and El Salvador

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