Pleistocene slip rates on the Boconó fault along the North Andean Block plate boundary, Venezuela

Abstract

The Boconó fault is a strike‐slip fault lying between the North Andean Block and the South American plate which has triggered at least five Mw > 7 historical earthquakes in Venezuela. The North Andean Block is currently moving toward NNE with respect to a stable South American plate. This relative displacement at ~12 mm yr−1 in Venezuela (within the Maracaibo Block) was measured by geodesy, but until now the distribution and rates of Quaternary deformation have remained partially unclear. We used two alluvial fans offset by the Boconó fault (Yaracuy Valley) to quantify slip rates, by combining 10Be cosmogenic dating with measurements of tectonic displacements on high‐resolution satellite images (Pleiades). Based upon a fan dated at >79 ka and offset by 1350–1580 m and a second fan dated at 120–273 ka and offset by 1236–1500 m, we obtained two Pleistocene rates of 5.0–11.2 and <20.0 mm yr−1, consistent with the regional geodesy. This indicates that the Boconó fault in the Yaracuy Valley accommodates 40 to 100% of the deformation between the South American plate and the Maracaibo Block. As no aseismic deformation was shown by interferometric synthetic aperture radar analysis, we assume that the fault is locked since the 1812 event. This implies that there is a slip deficit in the Yaracuy Valley since the last earthquake ranging from ~1 to 4 m, corresponding to a Mw 7–7.6 earthquake. This magnitude is comparable to the 1812 earthquake and to other historical events along the Boconó fault.

Significance of Northern Andes Terrane Extrusion and Genesis of the Interandean Valley: Paleomagnetic Evidence From the “Ecuadorian Orocline”

“…Complex plate interaction yields a wide deformation zone, extending from Ecuador to Colombia and Venezuela, characterized by both submarine/onshore accretionary wedges and seismically active dextral faults (Trenkamp et al, 2002;DeMets et al, 2010, Mora et al, 2017Alvarado et al, 2016;Pousse-Beltran et al, 2017). After Pennington (1981) this Andean sector was considered as a proper microplate: the Northern Andean Block (NAB in Fig.…”

“…The boundary between South America and NAB is assumed to coincide with both regional dextral strike-slip and reverse fault systems that extend from the Gulf of Guayaquil (Ecuador) to the gulf Triste (Venezuela), following main topographic ranges and changing strike and deformation style along their length (Fig. 1; Trenkamp et al, 2002;Egbue and Kellogg, 2010;Nocquet et al, 2014;Jimenez et al, 2014;Pousse-Beltran et al, 2017;Baize et al, 2020). Active seismicity along both types of faults has been recorded and inferred for historical time (e.g.…”

“…Besides Quaternary-to-present kinematics, strike-slip tectonics is inferred to have occurred since the Late Cretaceous, along dextral faults that bounded oceanic-affinity terranes parallel to the Andean chain of Ecuador and Colombia (Daly 1989;Egbue and Kellogg 2010;Alvarado et al, 2014;Jimenez et al, 2014;Pousse-Beltran et al, 2017). The basement of the Ecuador Coastal forearc (Fig.…”

“…GPS data (Alvarado et al., 2014; Nocquet et al., 2014; Pérez et al., 2018; Trenkamp et al., 2002; Villegas‐Lanza et al., 2016; Figure 1) document a 0.6 cm/yr oblique NE‐ward drift of the Northern Andes with respect to South America that is considered to be accommodated by orogen‐parallel dextral strike‐slip faults (Alvarado et al., 2016; Egbue & Kellogg, 2010; Jiménez et al., 2014; Nocquet et al., 2014; Pousse‐Beltran et al., 2017; Trenkamp et al., 2002). This is consistent with an average 0.76 cm/yr Quaternary slip rate obtained from field investigation along the main strike‐slip faults (Pallatanga and Cayambe‐Afiladores faults; Egbue & Kellogg, 2010 and references therein).…”

“…Besides Quaternary‐to‐present kinematics, strike‐slip tectonics is inferred to have occurred since the Late Cretaceous, along dextral faults that bounded oceanic‐affinity terranes parallel to the Andean chain of Ecuador and Colombia (Alvarado et al., 2014; Daly, 1989; Egbue & Kellogg, 2010; Jiménez et al., 2014; Pousse‐Beltran et al., 2017). The basement of the Ecuador Coastal forearc (Figure 1c) is made by upper Cretaceous, 88.8 ± 1.6 Ma (Ar 39 /Ar 40 dating by Luzieux et al., 2006), oceanic crustal terranes that were accreted to South America during a 70° clockwise (CW) rotation constrained at 70 Ma (Luzieux et al., 2006; Roperch et al., 1987).…”