MODELLO

The transform western boundary between the North American and Caribbean plates is constituted by a complex system of fault arrays, partitioned since Eocene time along the 1000-km-long Polochic, Motagua, and Jocotan E-W left-lateral fault segments. The Polochic fault runs with trend from the Pacific to the Atlantic Ocean through Messico and Guatemala along the Cuilco-Chixoy-Polochic river valleys, with. The Motagua fault system extends from Central Guatemala to the Caribbean coast describing a relatively smooth gentle arc with a southern convexity. The southernmost Jocotan fault is partitioned in different sectors by cross-cutting N-S-trending graben structures in NE Honduras. The Polochic fault system is partially hidden by the Lago Izabal, a large asymmetric basin filled by more than 3000 m of Tertiary lacustrine and fan-shaped deltaic facies sediments. In the Motagua valley the thickness of lacustrine facies is unknown. The two fault system are separate by the Sierra del Las Minas, where Paleozoic low-grade metamorphic rocks cropout. The seismicity of these strike-slip faults is concentrated in the easternmost Motagua-Polochic segment, where large historical earthquakes occurred. In the eastern Guatemala, the Motagua and Polochic faults show a well-defined lineation patterns and a clear morphological expression which are easily identifiable on the aerial photographs, satellite-derived images as well as in the field. These lineaments define a complex system of drainage network. The field geological features consist of scattered slickenside on fault surfaces outcropping at several localities that commonly record strike-slip movements. The fault segments define a ENE-WSW and E-W general trend that splits in several splays, and is complicated by severals jogs, bends and interconnected step-overs. The geometry and the kinematic evolution of the area inferred integrating in a geological cross-section field structural data, seismic reflection profiles and modelled gravimetric measurements. The basins developed at the left step array segments of transtensional faults where the north and south strands of each evolve with different offset and activity since the Early Miocene. The Izabal basin, pertain the Polochic fault, evolved in a transtensional regime, where the activity of the different faults segment progressively shiffiting the sedimentary depocenter through SW and fold in a positive flower structures the sedimentary sequence.