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 the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the eas ...

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 tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru–Chile Trench, of the N ...

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-J ...

during the beginnings of the Andean

orogeny Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An '' orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted ...

. 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 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

. 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 of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the name ...

, 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 Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * Ext ...

from the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but t ...

Miocene The Miocene ( ) is the first 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 means "less recen ...

, 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 predomin ...

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 ...

s and lavas with large

diorite Diorite ( ) is an 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 in composition between low-sil ...

batholith A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, s ...

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 o ...

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 rhyo ...

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 ow ...

s of continental clastic rocks and marine

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

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 conditions and metamorphic rocks from Jurassic volcanics, which includes intrusive rocks (diorites,

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ...

s and

tonalite Tonalite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total ...

s), and

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...

greywacke Greywacke or graywacke (German ''grauwacke'', signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lit ...

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 ...

conditions. From

Chañaral Chañaral is a small coastal city and commune in the Atacama Region, Chile and capital of the Chañaral Province. It is largely unknown by tourists, who generally head straight for towns such as San Pedro de Atacama. The town has no high rise b ...

to the south the fault system coincides in extent with the

Chilean Iron Belt The Chilean Iron Belt is a geological province rich in iron ore deposits in northern Chile. It extends as a north-south beld along the western part of the Chilean regions of Coquimbo and Atacama, chiefly between the cities of La Serena and Talta ...

, 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 ...

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 rock between the crust and the outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up about 84% of Earth's volume. It is predominantly so ...

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.

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). It follows the Neogene Period and spans from 2.58 million year ...

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