A slow-slipping active fold and thrust system at the SE corner of the Atacama basin, northern Chile
Date Issued
2011
Date
2011
Author(s)
Lin, Yen-Sheng
Abstract
Atacama Basin (~23°S, ~68°W) lies on the western edge of the Central Andes. It is a unique compressional depression in the forearc region of the South American plate boundary, which was formed by the oblique convergence between the Nazca and the South American plates. The Nazca plate is subducting northeastward beneath the South American plate at a rate of about 68~80 mm/yr based on different methods. However, this forearc region does not seem to absorb a lot of deformation at present.
In order to understand the characteristics and mechanisms of active forearc deformation related to the plate convergence, we chose to investigate the SE margin of the Atacama Basin, where active structures have been described previously. Since the hyper-aridity of the Atacama Basin results in extremely low erosion and sedimentation rates, we believe the present relief of land surface there is mostly produced by neotectonic activity, and can be used as a deformation marker.
Combining various remote-sensing data sets, such as an SRTM-DEM, Google Earth platform, and higher resolution QuickBird satellite images, several N-S trending ridges are mapped in this area. For further investigating tectono-geomorphic features, we performed detailed geomorphic surveys by real-time kinematic (RTK) GPS in the field to obtain high resolution topographic profiles across these features. These ridges are generally several tens of meters high, with their height decreasing northward. These ridges are interpreted to minor duplex fault-bend folds which grew on the backlimb of one larger asymmetric anticline. This major fold with steep forelimb facing east is likely formed by a shear fault-bend fold, and may be associated with an underlying west-dipping thrust fault. According to the principle proposed by Suppe (2004), the model without changes in depth, ramp angle, shear angle, and backlimb angle is selected to reconstruct the geometry of the structure beneath. We also suggest that this fault would merge at depth with the major active thrust system as a branch.
We also performed 40Ar/39Ar and U-Th dating of deformed strata. The surface ignimbrites mostly deposited in late Pliocene (3.0~3.2 Ma), and part of them are covered by a thin layer of lake deposits during ~440ka. If the structures have been active since the deposition of the lake deposits, the total deformation would yield a very low long-term slip rate of the faults, in an order of 10-1 mm/yr.
This result is similar to other researches in the forearc region of Central Andes, but is distinctly different from the Northern and Southern Andes. This very low slip rate of active structures may thus play important roles in the evolution of the forearc deformation belt, as well as the landscape development in this area.
In order to understand the characteristics and mechanisms of active forearc deformation related to the plate convergence, we chose to investigate the SE margin of the Atacama Basin, where active structures have been described previously. Since the hyper-aridity of the Atacama Basin results in extremely low erosion and sedimentation rates, we believe the present relief of land surface there is mostly produced by neotectonic activity, and can be used as a deformation marker.
Combining various remote-sensing data sets, such as an SRTM-DEM, Google Earth platform, and higher resolution QuickBird satellite images, several N-S trending ridges are mapped in this area. For further investigating tectono-geomorphic features, we performed detailed geomorphic surveys by real-time kinematic (RTK) GPS in the field to obtain high resolution topographic profiles across these features. These ridges are generally several tens of meters high, with their height decreasing northward. These ridges are interpreted to minor duplex fault-bend folds which grew on the backlimb of one larger asymmetric anticline. This major fold with steep forelimb facing east is likely formed by a shear fault-bend fold, and may be associated with an underlying west-dipping thrust fault. According to the principle proposed by Suppe (2004), the model without changes in depth, ramp angle, shear angle, and backlimb angle is selected to reconstruct the geometry of the structure beneath. We also suggest that this fault would merge at depth with the major active thrust system as a branch.
We also performed 40Ar/39Ar and U-Th dating of deformed strata. The surface ignimbrites mostly deposited in late Pliocene (3.0~3.2 Ma), and part of them are covered by a thin layer of lake deposits during ~440ka. If the structures have been active since the deposition of the lake deposits, the total deformation would yield a very low long-term slip rate of the faults, in an order of 10-1 mm/yr.
This result is similar to other researches in the forearc region of Central Andes, but is distinctly different from the Northern and Southern Andes. This very low slip rate of active structures may thus play important roles in the evolution of the forearc deformation belt, as well as the landscape development in this area.
Subjects
Neotectonics
active faults
Atacama Basin
Central Andes
slow slip rates
Type
thesis
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