Active Tectonics and Earthquake Geology Along the Pallatanga Fault, Central Andes of Ecuador

Abstract

Based on new geological data and the analysis of a 4 m spatial resolution Digital Elevation Model (DEM), we provide a detailed and comprehensive description of section of the Chingual Cosanga Pallatanga Puna Fault System, a major active fault system in Ecuador. This work allows estimating new slip rates and large earthquakes parameters (displacement, recurrence) along a ∼100 km-long section of the continental-scale dextral shear zone that accommodates the extrusion of the North Andean Sliver with respect to the South America continental Plate. We focus on the NE-SW Pallatanga strike-slip fault zone and related contractional and transcurrent features that extend to the north in the Inter-Andean valley and the Cordillera Real, respectively. The detailed analysis of the available DEM allowed mapping a series of lineaments at the regional scale and along the entire fault system. Field studies on key areas show valley deflections, aligned and elongated hills of Tertiary or Quaternary sediments, as well as faulted Holocene deposits and even preserved coseismic free-face ruptures in some places. Such morphological anomalies strongly suggest that those landscape scars represent long-living (Holocene to historical times) earthquake faults. Altogether, these new data confirm that very large crustal earthquakes (M∼7.5) have been generated along the fault system, probably during multiple segment ruptures. This conclusion agrees with reports of large earthquakes during historical times (post-1532 CE) in 1698, 1797, and 1949. They all occurred in the vicinity of the Pallatanga fault, causing catastrophic effects on environmental and cultural features. Based on new sample dating of both soils and volcanic series, we infer that the NE-SW dextral Pallatanga fault slips at rates ranging from ∼2 to 6 mm/yr for southern and central strands of the studied area, respectively. Further north, surface faulting is distributed and the deformation appears to be partitioned between sub-meridian fault-related folds (∼2 mm/yr) and NE-SW strike-slip fault(s), like the ∼1 mm/yr Pisayambo Fault that ruptured the surface in 2010. All this information offers the opportunity to size the earthquake sources for further seismic hazard analyses.

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

“…After the study done by Pennington (1981), this Andean sector was considered as a proper microplate: the Northern Andean Block (NAB in Figure 1a), undergoing 0.6 cm/yr NNE‐ward (Trenkamp et al., 2002) independent drift with respect to nearby plates (Cediel et al., 2003; Egbue & Kellogg, 2010; Ramos, 2009). 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 (Figure 1; Baize et al., 2020; Egbue & Kellogg, 2010; Jiménez et al., 2014; Nocquet et al., 2014; Pousse‐Beltran et al., 2017; Trenkamp et al., 2002). Active seismicity along both types of faults has been recorded and inferred for historical time (e.g., Beauval et al., 2010; Dimate et al., 2003; Paris, 2000).…”

“…Active seismicity along both types of faults has been recorded and inferred for historical time (e.g., Beauval et al., 2010; Dimate et al., 2003; Paris, 2000). In addition, a compilation of dextral fault displacements from field constraints suggests variable slip rates from 0.4 to 1 cm/yr during the last 1.8 Ma (Baize et al., 2020; Egbue & Kellogg, 2010; Eguez et al., 2003; Pousse‐Beltran et al., 2017). Dextral faults are assumed to have formed as Late Cretaceous suture zones, as oceanic terranes occur along the western NAB, and subsequently reactivated during Cenozoic times.…”

Effects of crustal assimilation on 238U-230Th disequilibria in continental arc settings

“…This trend requires a spatially restricted, non-radiogenic Pb source such as mid-upper crustal Jurassic intrusions emplaced in the Sub-Andean zone (Ancellin et al, 2017;Chiaradia et al, 2004;this study). This distinct Pb isotopic signature could also be due to displacement of crustal blocks with non-radiogenic Pb along the Cosanga Fault, part of the NE-SW running Chingual-Cosanga-Pallatanga-Puna shear zone (CCPP; Figure 1A) (Baize et al, 2020).…”

Timing of Quaternary volcanism and its relationship with tectonics in the central segment of the Ecuadorian Andes

“…1c), which traverses the Ecuadorian Andes and extends northward into Colombia (Witt and Bourgois, 2010;Nocquet et al, 2014;Alvarado et al, 2016). Although several studies have been carried out on the slab structure (e.g., Gutscher et al, 1999;Michaud et al, 2009;Yepes et al, 2016) and the kinematics along crustal faults (e.g., Fiorini and Tibaldi, 2012;Alvarado et al, 2014Alvarado et al, , 2016Baize et al, 2020;Jomard et al, 2021), their relationship with the volcanism is under discussion.…”