
The Atacama Fault Zone (AFZ) is an extensive system of
faults cutting across the
Chilean Coastal Cordillera in Northern
Chile
Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...
between the
Andean Mountain range and the Pacific Ocean. The fault system is north–south striking and runs for more than 1100 km North and up to 50 km in width through the Andean forearc region.
The zone is a direct result of the ongoing subduction of the Eastward moving
Nazca plate
The Nazca plate or Nasca plate, named after the Nazca region of southern Peru, is an oceanic list of tectonic plates, tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru– ...
beneath the
South American plate
The South American plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African plate, with which it forms the southern part of the Mid ...
and is believed to have formed in the
Early Jurassic
The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic� ...
during the beginnings of the Andean
orogeny
Orogeny () is a mountain-mountain formation, building process that takes place at a convergent boundary, convergent plate margin when plate motion compresses the margin. An or develops as the compressed plate crumples and is tectonic uplift, u ...
. The zone can be split into 3 regions: the North, Central and South.
Tectonic history and formation
The AFZ has gone through periods of inactivity and reactivation since its inception in the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
.
The fault series was formed through a complex series of tectonic regimes dating back to the Early Jurassic, when the Andean back-arc basin separated from the Pacific Ocean.
Intra-arc ductile deformation occurred in the
Late Jurassic
The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...
, creating north-striking
mylonitic shear zones. A belt formed through a compressive regime in the early Cretaceous, followed by a compression of the Andean basement in the mid-Cretaceous. There was a regime of
extension from the
Oligocene
The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
to
Miocene
The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
, and finally large brittle reactivations took place from the Miocene to Present.
Regional geology
The fault zone penetrates the Chilean Coastal Cordillera through north–south striking elongate terranes of Jurassic to early Cretaceous igneous rocks.
[ The region was formed through arc-magmatism and is composed of mostly ]andesitic
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...
s and lavas with large diorite
Diorite ( ) is an intrusive rock, intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is Intermediate composition, inter ...
batholith
A batholith () is a large mass of intrusive rock, intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate ...
s.[ In the eastern branches of the fault Cretaceous-aged ]porphyry copper deposit
Porphyry copper deposits are copper ore bodies that are formed from hydrothermal fluids that originate from a voluminous magma chamber several kilometers below the deposit itself. Predating or associated with those fluids are vertical dikes of ...
s are associated to diorite and dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...
stocks-type intrusions. These porphyry copper deposits are poorly known constituting thus possible new targets for copper mining.[ Certain areas of the mentioned igneous province are overlain by ]terrane
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its d ...
s of continental clastic rocks and marine limestone
Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
s.[ Past and recent fault activity has reworked some of the surrounding rock, producing regions of ductilely deformed rocks, which can be categorized into two types: Cretaceous ]pluton
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
ic rocks (tonalites) deformed under greenschist
Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as Chlorite ...
conditions and metamorphic rocks
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...
from Jurassic volcanics, which includes intrusive rocks (diorites, gabbro
Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...
s and tonalite
Tonalite is an igneous rock, igneous, plutonic (Intrusive rock, intrusive) rock (geology), rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali fe ...
s), and Paleozoic
The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...
greywacke
Greywacke or graywacke ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and Sorting (sediment), poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size Lith ...
s formed in mid-amphibolite facies
Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (fl ...
conditions.
From Chañaral to the south the fault system coincides in extent with the Chilean Iron Belt, a collection of iron ore
Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the f ...
deposit running all the way to El Romeral next to La Serena. It is thought that the Atacama Fault acted as a "transcrustal" fault that allowed for molten iron ore magma migrate from its place of origin in the Earth's mantle
Earth's mantle is a layer of silicate mineral, silicate rock between the Earth's crust, crust and the Earth's outer core, outer core. It has a mass of and makes up 67% of the mass of Earth. It has a thickness of making up about 46% of Earth's ...
to shallow of the crust event reaching surface in volcanic eruptions of iron oxide. The resulting rock after the cooling of these magmas is iron oxide-apatite.[
Near ]Huasco
Huasco () is a Chilean city and commune, in the Huasco Province, Atacama Region. It is the central town of a mining district dominated by medium-scale mining.
The port city of Huasco is located 50 km west of Vallenar and close to the tow ...
the western branch of the Atacama Fault System is the Infiernillo Shear Zone (Spanish: Zona de Cizalla de Infiernillo) which runns in NNE-SSW direction and makes the limiti between the Early Cretaceous
The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
Infiernillo Plutonic Complex and the formations of La Negra (Jurassic) and Canto del Agua (Triassic) as well as some metamorphic rock
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...
s.[Arévalo et al., 2003]
The Atacama Fault System in the Huasco Province, southern Atacama Desert, Chile
/ref>
Fault mechanics
Due to the extensional regime shaping the region, the system is dominated by normal faulting, most of which is north–south striking and dipping around 60 degrees to the East.[ Though the system is mostly dip-slip, there are regions of strike-slip formed mylonites in the East providing evidence for past sinistral strike-slip motion.][
The general strike of the fault system is north–south, though the fault activity varies between the 3 regions of the AFZ. The Northern region encompasses the Salar del Carmen major fault which splits the region into a Western domain with large active faults striking N160 to N170 and an Eastern domain with mostly inactive faults overlain by ]Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...
deposits. In the Central region the Coastal Range is bounded by the N–S striking Remiendo Fault with ancient fault scarps in the eastern area of the region. In the Southern region the Coastal Range is bounded by the El Salado Fault, which trends North and is cut to the north by TalTal N130 striking faults.
References
* Pia, V., et al. 2010, 'Monitoring seismic and silent faulting along the Atacama Fault System and its relation to the subduction zone seismic cycle: a creepmeter study in N-Chile', EGU general assembly, pg.4298.
{{Major South American geological formations
Seismic faults of Chile
Strike-slip faults
Atacama Desert
Geology of the Chilean Coast Range
Geology of Antofagasta Region
Geology of Atacama Region
Geology of Tarapacá Region