Alex Geen
Introduction
The Atacama Fault Zone (AFZ), also referred to as the Atacama Fault System (AFS), is an extensive region of coastal Chile, with features extending in parallel and sub-parallel orientations to the continental margin from 20°30’S to 29°45’S (Riquelme, R., et al. 2002). The AFS is a region characterized by many overlapping faults that cover the approximately 1000 kilometer long region from the municipality if Iquique to Chanaral (Riquelme, R., et al. 2002). The AFS extends through several Chilean topographical regions, including the Coastal Cordillera, the Central Depression, the PreCordillera, The PreAndean Depression, and the Western Andean Cordillera, whose geographic positions are outline in Figure 1 (Riquelme, R., et al. 2002).
Introduction
The Atacama Fault Zone (AFZ), also referred to as the Atacama Fault System (AFS), is an extensive region of coastal Chile, with features extending in parallel and sub-parallel orientations to the continental margin from 20°30’S to 29°45’S (Riquelme, R., et al. 2002). The AFS is a region characterized by many overlapping faults that cover the approximately 1000 kilometer long region from the municipality if Iquique to Chanaral (Riquelme, R., et al. 2002). The AFS extends through several Chilean topographical regions, including the Coastal Cordillera, the Central Depression, the PreCordillera, The PreAndean Depression, and the Western Andean Cordillera, whose geographic positions are outline in Figure 1 (Riquelme, R., et al. 2002).
Geological Context
To understand the geology of the AFS, first consider the tectonic setting. Chile is on the western edge of South America, where the Nazca Plate is undergoing oblique subduction beneath the South American plate, giving rise the off-coast Peru-Chile Trench, as is illustrated on the video on the following site: http://goo.gl/TMfkIO. This trench is also shown in Figure 1. Presently, the Nazca Plate is subducting at a rate of 7 cm/year in the north and 8cm/year in the south (USGS, 2012). Given the AFS’s broad length, the geologic units through which the faults cut are diverse in composition and origin. For example, in the Coastal Cordillera gabbros, diorites and granodiorites are the prominent units (Riquelme, R., et al. 2002), in addition to andesitic tuffs (Scheuber, E., 1990).
In reference to Figure 2, a broader overview of the geologic units of the AFS can be determined by following the outlined traces of the faults from the aforementioned geographic coordinates. What follows is a list of some of the most prominent units and their ages.
--
Age (Approximate) / Unit(s)
Pleistocene-Holocene / Alluvial Deposits
Jurassic-Cretaceous / Granodiorites, Diorites, Granites
Precambrian-Ordivician / Mica Schists, Gneisses, Migmatites
Carboniferous-Permian (328-235Ma) / Granites, Granodiorites, Diorites, Tonalities
Jurassic / Volcanic Continental, Marine Sequences, Andesitic Lavas, Basaltic Agglomerates
Oligocene-Miocene / Continental Sedimentary Sequences, Conglomerates, Sandstones, Shales
Miocene-Pliocene / Basalts and Intermediate Pyroclastics
Devonian-Carboniferous / Phyllites, Metasandstones
Jurassic Superior / Orthogneiss Intrusive Protolith
Miocene Superior - Pliocene / Clastic Sedimentary Sequences, (Alluvial, Colluvial, Fluvial)
Pleistocene / Marine Coastal & Fluvial Sedimentary Sequences
--
Given the high diversity of geologic units in the AFS region, the above is not all encompassing. However, the list does emphasize that the region is composed of all igneous, metamorphic and sedimentary rock types. This diversity raises important questions as to the region’s genesis.
Processes such as the production of back-arc basins by volcanic arcs, followed by compressive regimes, and finally extensional regimes are important considerations. These regimes are related directly to the stresses of the subducting Nazca plate, which causes the deformations which yield the past and present faults in the AFS. Additionally, as the Nazca plate is subducted, material is effectively “scraped” off (on geology time scales) producing accretionary wedges (Geological Society, 2015), which is a plausible explanation for the pattern in the geological units seen of the coast of Chile. It should be noted the dip angle of the subducting plate is not constant beneath the South American plate, instead dipping more steeply at the earliest subducted regions (Cahill, T., et al., 1992). The difference in dip is extreme, changing from approximately horizontal subduction to a 30 degree dip (Cahill, T., et al., 1992).
To understand the geology of the AFS, first consider the tectonic setting. Chile is on the western edge of South America, where the Nazca Plate is undergoing oblique subduction beneath the South American plate, giving rise the off-coast Peru-Chile Trench, as is illustrated on the video on the following site: http://goo.gl/TMfkIO. This trench is also shown in Figure 1. Presently, the Nazca Plate is subducting at a rate of 7 cm/year in the north and 8cm/year in the south (USGS, 2012). Given the AFS’s broad length, the geologic units through which the faults cut are diverse in composition and origin. For example, in the Coastal Cordillera gabbros, diorites and granodiorites are the prominent units (Riquelme, R., et al. 2002), in addition to andesitic tuffs (Scheuber, E., 1990).
In reference to Figure 2, a broader overview of the geologic units of the AFS can be determined by following the outlined traces of the faults from the aforementioned geographic coordinates. What follows is a list of some of the most prominent units and their ages.
--
Age (Approximate) / Unit(s)
Pleistocene-Holocene / Alluvial Deposits
Jurassic-Cretaceous / Granodiorites, Diorites, Granites
Precambrian-Ordivician / Mica Schists, Gneisses, Migmatites
Carboniferous-Permian (328-235Ma) / Granites, Granodiorites, Diorites, Tonalities
Jurassic / Volcanic Continental, Marine Sequences, Andesitic Lavas, Basaltic Agglomerates
Oligocene-Miocene / Continental Sedimentary Sequences, Conglomerates, Sandstones, Shales
Miocene-Pliocene / Basalts and Intermediate Pyroclastics
Devonian-Carboniferous / Phyllites, Metasandstones
Jurassic Superior / Orthogneiss Intrusive Protolith
Miocene Superior - Pliocene / Clastic Sedimentary Sequences, (Alluvial, Colluvial, Fluvial)
Pleistocene / Marine Coastal & Fluvial Sedimentary Sequences
--
Given the high diversity of geologic units in the AFS region, the above is not all encompassing. However, the list does emphasize that the region is composed of all igneous, metamorphic and sedimentary rock types. This diversity raises important questions as to the region’s genesis.
Processes such as the production of back-arc basins by volcanic arcs, followed by compressive regimes, and finally extensional regimes are important considerations. These regimes are related directly to the stresses of the subducting Nazca plate, which causes the deformations which yield the past and present faults in the AFS. Additionally, as the Nazca plate is subducted, material is effectively “scraped” off (on geology time scales) producing accretionary wedges (Geological Society, 2015), which is a plausible explanation for the pattern in the geological units seen of the coast of Chile. It should be noted the dip angle of the subducting plate is not constant beneath the South American plate, instead dipping more steeply at the earliest subducted regions (Cahill, T., et al., 1992). The difference in dip is extreme, changing from approximately horizontal subduction to a 30 degree dip (Cahill, T., et al., 1992).
Deformation in the AFS & The Forms Observed
The AFS is not completely characterized by any one fault classification. Instead, it is composed of normal faults, sinistral strike-slip faults, and reverse faults (Lavenu, A., et al., 2000), where the class is a function of the age of the fault, since the stress regime has changed over time. As such, this region is characterized by non-coaxial deformation (Mitchell, T.M., et al., 2009).
The strain history is particularly observable in the sinistral (left-lateral) strike slip deformations, where the fault core can be clearly distinguished from the zone of damage surrounding it. Mitchell, T.M. et al. (2009) examined several different scales of sinistral strike-slip faults in the AFS, from kilometer to sub-meter, all with very different displacements. For example, the Caleta Coloso fault is 60 km in length, or approximately 6% the length of the AFS, with a fault core of 400m width and a displacement of 5 kilometers. As can be seen in similar examples in Figure 3, the fault core is a region of relatively little damage, while the damage zone (as the name would imply) has rock characterized by extensive deformation. At a width two orders of magnitude smaller than the Caleta Coloso fault, the Blanca fault still exhibits a similar damage zone. The type of deformation observable at macroscopic scale can also be observed on scales approaching microscopic, as seen in Figure 4.
Strong evidence for non-coaxial deformation in the AFS can be found by investigating the cross cutting relationships of the faults across certain rock units. For example Schueber, E., et al. (1990) successfully inferred the dominant deformation being N-S shear during the Jurassic period, as evidence by mylonitic foliations in the protoliths, on scales of millimeter to kilometer. This type of deformation occurred in the early Cretaceous as well, evidenced by ductilely deformed plutons from 135 to 122 Ma (Schueber, E., et al., 1990). In the post-Cretaceous period, the AFS became inactive, only to later reactivate in a different regime, the reasons for which will be discussed in the ‘Stress’ section. The strain observed is characteristic of normal faulting, where the vertical displacement is anywhere from tens to hundreds of meters (Riquelme, R., et al., 2002). The predominant normal faults in the AFS have their geometry characterized by a dip at approximately 60° to the east (Chorowicz, et al., 1996), which is in agreement with the principles of Andersonian faulting.
The AFS is not completely characterized by any one fault classification. Instead, it is composed of normal faults, sinistral strike-slip faults, and reverse faults (Lavenu, A., et al., 2000), where the class is a function of the age of the fault, since the stress regime has changed over time. As such, this region is characterized by non-coaxial deformation (Mitchell, T.M., et al., 2009).
The strain history is particularly observable in the sinistral (left-lateral) strike slip deformations, where the fault core can be clearly distinguished from the zone of damage surrounding it. Mitchell, T.M. et al. (2009) examined several different scales of sinistral strike-slip faults in the AFS, from kilometer to sub-meter, all with very different displacements. For example, the Caleta Coloso fault is 60 km in length, or approximately 6% the length of the AFS, with a fault core of 400m width and a displacement of 5 kilometers. As can be seen in similar examples in Figure 3, the fault core is a region of relatively little damage, while the damage zone (as the name would imply) has rock characterized by extensive deformation. At a width two orders of magnitude smaller than the Caleta Coloso fault, the Blanca fault still exhibits a similar damage zone. The type of deformation observable at macroscopic scale can also be observed on scales approaching microscopic, as seen in Figure 4.
Strong evidence for non-coaxial deformation in the AFS can be found by investigating the cross cutting relationships of the faults across certain rock units. For example Schueber, E., et al. (1990) successfully inferred the dominant deformation being N-S shear during the Jurassic period, as evidence by mylonitic foliations in the protoliths, on scales of millimeter to kilometer. This type of deformation occurred in the early Cretaceous as well, evidenced by ductilely deformed plutons from 135 to 122 Ma (Schueber, E., et al., 1990). In the post-Cretaceous period, the AFS became inactive, only to later reactivate in a different regime, the reasons for which will be discussed in the ‘Stress’ section. The strain observed is characteristic of normal faulting, where the vertical displacement is anywhere from tens to hundreds of meters (Riquelme, R., et al., 2002). The predominant normal faults in the AFS have their geometry characterized by a dip at approximately 60° to the east (Chorowicz, et al., 1996), which is in agreement with the principles of Andersonian faulting.
Stresses & Tectonics
It has been established that the AFS has strike-slip faults with sinistral displacement, some minor reverse faulting (such that the geometries work) and, most recently, normal faulting. Though it would seem reasonable for the AFS to be dominated by compressional stresses given its proximity to the subduction of the Nazca Plate, the AFS is instead characterized by an extensional regime (Delouis, B. et al., 1998). This is evidenced by numerous fresh ruptures that correlate to Late Quaternary earthquakes, as well as the uplift of the western blocks of the fault zone (Delouis, B. et al., 1998). The speculated causes for the seemingly contradictory presence of an extensional regime at the subduction zone are offshore subsidence, and onshore uplift (Delouis, B. et al., 1998). Offshore subsidence would be a function of a mechanism described earlier in the discussion of accretionary wedges, where material is physically 'scraped' off the Nazca Plate as it is subducted. The material would accumulate in that single region, inducing subsidence (Delouis, B. et al., 1998). Onshore, the process of 'underplating' where partial melts are induced in the overriding plate (in this case, the South American Plate), is responsible for uplift (Delouis B. et al, 1998). In conjunction, these two processes could give rise to the observed extensional regime (Deluois, B. et al., 1998). These two processes are illustrated clearly in Figure 6.
It has been established that the AFS has strike-slip faults with sinistral displacement, some minor reverse faulting (such that the geometries work) and, most recently, normal faulting. Though it would seem reasonable for the AFS to be dominated by compressional stresses given its proximity to the subduction of the Nazca Plate, the AFS is instead characterized by an extensional regime (Delouis, B. et al., 1998). This is evidenced by numerous fresh ruptures that correlate to Late Quaternary earthquakes, as well as the uplift of the western blocks of the fault zone (Delouis, B. et al., 1998). The speculated causes for the seemingly contradictory presence of an extensional regime at the subduction zone are offshore subsidence, and onshore uplift (Delouis, B. et al., 1998). Offshore subsidence would be a function of a mechanism described earlier in the discussion of accretionary wedges, where material is physically 'scraped' off the Nazca Plate as it is subducted. The material would accumulate in that single region, inducing subsidence (Delouis, B. et al., 1998). Onshore, the process of 'underplating' where partial melts are induced in the overriding plate (in this case, the South American Plate), is responsible for uplift (Delouis B. et al, 1998). In conjunction, these two processes could give rise to the observed extensional regime (Deluois, B. et al., 1998). These two processes are illustrated clearly in Figure 6.
Citations
Cahill, T., Isacks, B.L., 1992, Seismicity and Shape of the Subducted Nazca Plate, Journal of Geophysical
Research, v. 97, p.17,503-17,529. Doi: 10.1029/92JB00493.
Chorowicz, J., Vicente, J., Chotin, P., Mering, C., 1996, Neotectonic Map of the Atacama Fault Zone (Chile) From SAR ERS-1 Images, Andean
Geodynamics: Extended Abstracts, p. 165-168, ISBN: 2-7099-1332-1
Deluois, B., Philip, H., Dorbath, L., 1996, Extensional stress regime in the Antofagasta coastal area (northern Chile).
Delouis, B., Philip, H., Cisternas, A., 1998, Recent crustal deformation in the Antofagasta region (northern Chile) and the subduction process, v. 132, p. 302-338 doi: 10.1046/j.1365-246x.1998.00439.x
Gobierno de Chile – Servicio Nacional de Geologia Y Mineria: Subdireccion Nacional de Geologia, 2003,
Mapa Geologico de Chile: Version Digital
Geological Society, 2015, Oceanic/Continental: The Andes. URL: https://www.geolsoc.org.uk/Plate-Tectonics/Chap3-Plate-Margins/Convergent
/Oceanic-continental
Lavenu, A., Thiele, R., Machette, M.N., Dart R.L., Bradley, L., Haller, K.M., 2000, Maps and Database of
Quaternary Faults in Bolivia and Chile, May 2000 Version.
Mitchell, T.M., Faulkner, D.R., 2009, The nature and origin of off-fault damage surrounding strike-slip
fault zones with a wide range of displacements: A field study from the Atacama fault system,
northern Chile, v.31, p.802-816, doi: 10.1016/j.jsg.2009.05.002
Riquelme, R., et al., 2002, A geormorphilogical approach to determining the Neogene to Recent tectonic
deformation in the Coastal Cordillera of northern Chile (Atacama), v. 361, p. 255-275, doi:10.1016/S0040-1951(02)00649-2.
Scheuber, E., Andriessen, P., 1990, The kinematic and geodynamic significance of the Atacama fault
zone, northern Chile, v. 12, p. 243-257, doi: 10.1016/0191-8141(90)90008-M.
USGS, 2012, Poster of the Seismicity of the Nazca Plate and South America. http://earthquake.usgs.gov/earthquakes/eqarchives/poster/regions
/nazca.php
Cahill, T., Isacks, B.L., 1992, Seismicity and Shape of the Subducted Nazca Plate, Journal of Geophysical
Research, v. 97, p.17,503-17,529. Doi: 10.1029/92JB00493.
Chorowicz, J., Vicente, J., Chotin, P., Mering, C., 1996, Neotectonic Map of the Atacama Fault Zone (Chile) From SAR ERS-1 Images, Andean
Geodynamics: Extended Abstracts, p. 165-168, ISBN: 2-7099-1332-1
Deluois, B., Philip, H., Dorbath, L., 1996, Extensional stress regime in the Antofagasta coastal area (northern Chile).
Delouis, B., Philip, H., Cisternas, A., 1998, Recent crustal deformation in the Antofagasta region (northern Chile) and the subduction process, v. 132, p. 302-338 doi: 10.1046/j.1365-246x.1998.00439.x
Gobierno de Chile – Servicio Nacional de Geologia Y Mineria: Subdireccion Nacional de Geologia, 2003,
Mapa Geologico de Chile: Version Digital
Geological Society, 2015, Oceanic/Continental: The Andes. URL: https://www.geolsoc.org.uk/Plate-Tectonics/Chap3-Plate-Margins/Convergent
/Oceanic-continental
Lavenu, A., Thiele, R., Machette, M.N., Dart R.L., Bradley, L., Haller, K.M., 2000, Maps and Database of
Quaternary Faults in Bolivia and Chile, May 2000 Version.
Mitchell, T.M., Faulkner, D.R., 2009, The nature and origin of off-fault damage surrounding strike-slip
fault zones with a wide range of displacements: A field study from the Atacama fault system,
northern Chile, v.31, p.802-816, doi: 10.1016/j.jsg.2009.05.002
Riquelme, R., et al., 2002, A geormorphilogical approach to determining the Neogene to Recent tectonic
deformation in the Coastal Cordillera of northern Chile (Atacama), v. 361, p. 255-275, doi:10.1016/S0040-1951(02)00649-2.
Scheuber, E., Andriessen, P., 1990, The kinematic and geodynamic significance of the Atacama fault
zone, northern Chile, v. 12, p. 243-257, doi: 10.1016/0191-8141(90)90008-M.
USGS, 2012, Poster of the Seismicity of the Nazca Plate and South America. http://earthquake.usgs.gov/earthquakes/eqarchives/poster/regions
/nazca.php