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Beautiful fault scarps in Northern Ecuador (Billecocha region)

Source info:

Author: Stéphane Baize
Date: 2018-10-09 22:56:00
Blog: Stephane on Blogger Earthquakes, geology and related topics... but not only
URL: http://stephaneonblogger.blogspot.com/2018/10/beautiful-fault-scarps-in-northern.html

Summary:

Seventy kms north of Quito and 25 km west of Ibarra, Northern Ecuador, the Western Cordillera is cut by beautiful fault scarps, close to the mountain range edge  (~ 4000 asl) with the Inter-Andean valley. Here is what we can see in the field: The car is parked within the sagpond, where fine sediments are expected to accumulate at the toe of the fault scarp. We excavated this scarp and expected (fine) sediments to evidence the potential successive offsets along this fault. This other picture nicely illustrates the morphologic signature of the fault, several hundred meters to the north.  Actually the area is sliced by a series of N30°-trending lineaments within a ~3 km-wide band extending over ~20 km (as of our current state of analysis), as you can see on the snapshot (below) of the DEM available on Sigtierras Geoportal. 4m resolution DEM of the Billecocha area. Yellow arrows locate the first picture. Surprisingly, they have not been studied in details since a paper by Ego et al. in 1996, who concluded that "the Billecocha normal faults are unlikely to have a tectonic origin. Various local processes (e.g. gravity gliding, "sackung") that may be possible are contemplated for triggering the normal faulting. (...) normal faulting occurs by shearing along plane of weakness (bedding planes) in response to the elastic rebound of the fluid-saturated plateau".The recent advances in the seismotectonic knowledge (e.g. Alvarado et al., 2016) of the area suggest that there could however accommodate part of the tectonic deformation in the North Andean Sliver, in a remote continuation of the Quito fold system. This system would then be a good candidate for having generated the devastating earthquake of Ibarra (M~7+) in 1868.We therefore decided to start a paleoseismological study on those geological objects, in the framework of the ANR-REMAKE and the LMI SVAN (Instituto Geofisico de la Escuela Politecnica Nacional, Institut de Recherche pour le Développement, and Institut de Radioprotection et Sûreté Nucléaire).Other views (to the south of the trenched sagpond) of the fault scarps, including the lower slopes of the Cotacachi volcano. In the distance, we observe fault strands breaking the lowest slopes of the Cotacachi volcano The northern flank of the Cotacachi volcano holds several moraines which are cross-cut by Billecocha fault strands We excavated one large trench (T1) across the highest section of fault scarp (see on the left), to get the largest sedimentary recording and determine the 'long-term' slip.The stratigraphy includes an old alluvium at the base (red colors, footwall of the trench) (probably associated with the Late Glacial Maximum), then a series of black organic soils and whitish volcanic falls coming from surrounding volcanoes (Cuicocha, Chachimbiro).After a first quick examination, all these last deposits are probably younger than 6 ka.The whole pile is displaced by fault planes and both the arrangement of layers (fault terminations) and the stratigraphic record (colluvial wedges) suggest the occurrence of 2-3 earthquakes before the very last one which breached the current ground surface (1868?).We excavated two other small trenches (T2 and T3) across the lowest parts of the scarp, which we suspect to be the latest event(s). We wanted to check the age of the last deformed layers, and actually could not find post-tectonic layers. Actually, even in morphology, one can notice that there is a small step damming the pond, still today. In addition to those few events of offset, we observed syn-depositional soft-sediment deformation features, such as load casts or convoluted laminations. Those features could be due to shaking. The upper part of this "lacustrine" deposit (fault hangin wall) is embedded into the lower part (load casts?).  View of the thirs trench (looking towards the west in the sense of drainage) where we could check that the upper "lacustrine" layers are offset by the fault. Note the tiny step blocking the exit of drainage. We know that the most recent layers are offset in the trenches and, in several places, we found that the last rupture is very recent, leaving a fresh and preserved free face like appearing on the picture below;  Is that the evidence of surface rupturing during the 1868 Ibarra earthquake that destroyed large fractions of the cities in the Inter Andean Valley (Coticocha, Ibarra, etc)? It's hard to be sure, but this fault is a good candidate. The dark fringe along the cumulative scarp is a free-face scarp made of black soil. It could be the trace of a very recent (historic?) surface-rupturing earthquake. Besides the strike-slip indices in the trenches, we have collected several morphological and outcrop evidences proving that those faults are not simple normal faults. Actually the observations are included in a complex network of 'tectonic' faults with a strike-slip component. A view towards the sagpond where the trench were excavated, from the lower reach of the Cotacachi slope. The morphological signature strongly supports strike-slip kinematics. The moraine edge at the base of the Cotacachi slope is laterally offset (1-2 m?). Close to the trench site (sagpond), we captured this strike-slip fault strand that cut the Holocene soil In that place, only 100-200 m from the trench, the basement and Holocene soil are displaced by a clear reverse/thrust fault Obviously, this study is at its first steps and the elements proposed here will be revised thanks to analyses of the trench logs and morphological data, sample analysis (chemistry, dating), etc. 

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