REARRANGEMENT OF A RANGE-TRANSVERSE DRAINAGE AFFECTED BY WRENCH TECTONICS IN GUATEMALA

Along the 50 km wide Sierra de Chuacús range, in Guatemala, numerous paleovalleys are preserved. They document the rearrangement of a range-transverse drainage, consecutive to its disruption by range-parallel strike-slip faults. Under steady conditions of wrenching, the investigated drainage appears to have evolved through episodes of protracted deformation and rapid rearrangements. These rearrangements operate by discrete river diversions that tend to occur in clusters. Both range-transverse and range-parallel motions have influenced drainage evolution. We use the paleovalleys as markers not only to track drainage rearrangement, but also bedrock deformation and erosion patterns. The ultimate period of rearrangement led to the formation of a 10⁴ km² catchment and was associated with the erosion of 1 km of crust on average, over the last 7 Myrs. The general architecture of the fluvial deposits, paleoflow and provenance analysis, ⁴⁰Ar -³⁹Ar dating of ignimbrites, and cosmogenic ¹⁰Be-²⁶Al burial dating permit to reconstruct the piecemeal growth of this catchment since the Late Miocene. The catchment experienced a tenfold increase in drainage area as a result of transpression during the Late Miocene and transtension since the Pliocene. Its growth is associated with the replacement of the range-transverse drainage by a range-parallel drainage. The driving force of the rearrangement is uplift along range-parallel strike-slip faults. However, strike-slip motion and periodic annealing of tectonic deflections by river diversions set the propensity for the streams to be involved in the captures, and then their fate to be captured or captors. The diversions are driven by a wealth of mechanisms such as regressive erosion, river avulsion, karstic flow, phreatic flow, and aridification. In essence these diversions actually are the ultimate manifestation of the tectonic defeat of rivers, but the aforementioned mechanisms, and especially groundflow, act as powerful catalysers, and allow rearrangement to proceed sufficiently quickly to overcome drainage deformation by wrenching.