Ekkehard Scheuber

Repeated crustal thickening and recycling during the Andean orogeny in north Chile (21°–26°S)

posttectonic granitoids, (2) lower crustal P wave velocities of 7.3–7.7 km s � 1 compatible with underplated mafic crust, and (3) results from recent experimental petrologic work showing garnet stability in mafic mineralogies � 12 kbar (� 40 km crustal thickness). Analogous to older Andean magmatic episodes in north Chile, newly underplated basaltic crust may account for the remaining deficit in Neogene crustal thickening. Similar evolutionary patterns in geochemistry and initial Sr and Nd isotopic characteristics of Andean (200 Ma to present) magmatic rocks suggest that the Andean orogeny in this region evolved by a combination of processes of repeated arc migration, tectonic and magmatic crustal thickening, and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust. INDEX TERMS: 1020 Geochemistry: Composition of the Crust, 1030 Geochemistry: Geochemical cycles (0330), 8159 Evolution of the Earth: Rheology– crust and lithosphere, 9360 Information Related to Geographic Region: South America; KEYWORDS: Andean Orogeny, Continental Crust, Recycling, Adakite, TTG, Crustal Thickening

Magmatic arc tectonics in the Central Andes between 21° and 25°S

Abstract In northern Chile, four magmatic arc systems, each displaced towards the east in relation to the previous one, developed in sequence since the Lias. This migration of the magmatic arc from the Coastal Cordillera to the Western Cordillera probably was a consequence of tectonic erosion of the continental margin. Within each of these systems, tectonic deformation took place contemporaneously to magmatic activity. As the continental crust of the upper plate has been weakened in the area of the magmatic arc by the igneous processes, it is relatively sensitive to the stresses introduced by plate convergence. A very important factor for the deformation in the magmatic arc is the angle of convergence obliqueness which may cause shortening if obliqueness is small (

Tectonic Development of the North Chilean Andes in Relation to Plate Convergence and Magmatism Since the Jurassic

Since the early Jurassic the magmatic arc of the north Chilean Andes has been displaced from the Coastal Cordillera to the Western Cordillera. This eastward migration happened stepwise and four successive, ± stationary arc systems can be distinguished. The deformation history of the arc systems in relation to plate convergence and igneous activity shows that the magmatic arc, a zone of relative crustal weakness, reacted very sensitively to changing conditions of plate convergence. Both long-term continuous deformations and short-term tectonic events are recognized. They reflect periods of more or less steady state conditions and relatively sudden changes of subduction parameters respectively. Two major periods of different deformational styles, related to differing plate configurations and accompanying convergence obliqueness, can be distinguished: (1) 200–90 Ma (sinistral convergence obliqueness >45°) with general (trans-)tension during tectonic phases and interphases, and (2) since 90 Ma (dextral convergence obliqueness <45°) with transpression during phases and slight extension in interphases.

The kinematic and geodynamic significance of the Atacama fault zone, northern Chile

Abstract The Atacama fault zone (AFZ) is the dominant feature in the structure of the North Chilean Coastal Cordillera. Left lateral displacement took place along its system of longitudinal faults during the Jurassic and early Cretaceous. This development was contemporaneous with arc magmatism and was later reactivated, resulting in a steep normal fault. Strike-slip movements along the AFZ consist of two sets of ductile shear zones of different ages: one Jurassic, formed under amphibolite-facies conditions; the other early Cretaceous, with greenschist-facies mylonites. Structural asymmetries point to a sinistral sense of shear in both sets. The AFZ can be interpreted as a magmatic arc structure which accommodated the oblique subduction of an oceanic plate (trench-linked strike-slip fault). The sinistral sense of shear is consistent with reconstructions of late Jurassic to early Cretaceous plate configurations in the SE Pacific.

Tectonics of the Jurassic‐Early Cretaceous magmatic arc of the north Chilean Coastal Cordillera (22°–26°S): A story of crustal deformation along a convergent plate boundary

The tectonic evolution of a continental magmatic arc that was active in the north Chilean Coastal Cordillera in Jurassic-Early Cretaceous times is described in order to show the relationship between arc deformation and plate convergence. During stage I (circa 195–155 Ma) a variety of structures formed at deep to shallow crustal levels, indicating sinistral arc-parallel strike-slip movements. From deep crustal levels a sequence of structures is described, starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions (Bolfin Complex). The high-grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike-slip and thrust kinematics with a NW–SE directed maximum horizontal shortening, i.e., parallel to the probable Late Jurassic vector of plate convergence. A kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds, thrusts, and high-angle normal faults which formed during the same time interval as the discrete shear zones. During stage II (160–150 Ma), strong arc-normal extension is revealed by brittle low-angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels. During stage III (155–147 Ma), two reversals in the stress regime took place indicated by two generations of dikes, an older one trending NE–SW and a younger one trending NW–SE. Sinistral strike-slip movements also prevailed during stage IV (until ∼125 Ma) when the Atacama Fault Zone originated as a sinistral trench-linked strike-slip fault. The tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates. The structures of stages I and IV suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations. However, decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages II and III.

The Precordilleran fault system of Chuquicamata, Northern Chile: evidence for reversals along arc-parallel strike-slip faults

Abstract The Chilean Precordillera, situated between the Longitudinal Valley and the Western Cordillera of Northern Chile, was the site of the Andean magmatic arc from the late Cretaceous to the Eocene-Oligocene boundary. Magmatism came to an end during the Incaic tectonic phase (38 Ma), which caused arc-normal shortening and the development of longitudinal dextral strike-slip faults (Precordilleran Fault System). This magmatic arc tectonism is also related to the formation of the Chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the Precordillera. Structural investigations in and around the Chuqicamata open-pit mine have shown that wrench tectonics determined the kinematics of the area. The NS-striking West Fissure, which separates a 35-Ma-old non-mineralised pluton to the west from a central late Paleozoic basement ridge containing the mineralization, became a sinistral fault along with other parallel faults in the area. The central part is separated from similar Paleozoic rocks to the east by the Messabi-Este fault and a narrow faulted and sheared syncline of Mesozoic-Cenozoic sediments. This fault bears structures indicating dextral movements, which probably are of an age that is similar to the mylonites (34.8 Ma) in the western pluton. The dextral movements preceded the sinistral shear. Thus, the fault system of Chuqicamata displays a reversal of arc-parallel shear movements. According to the orientation of quartz veins in the mineralized body, it is presumed that the sense of displacement of these strike-slip motions reversed, when mineralization started at about 32 Ma. During this time the stress field must have changed fundamentally. The Incaic Phase dextral transpression is supposed to have been induced by the oblique vector of plate motion. The following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling. As, however, the vector of plate motion remained unchanged during that time, oblique subduction cannot be used as an argument for arc-parallel sinistral shear movements.

The Time-Space Distribution of Cenozoic Volcanism in the South-Central Andes: a New Data Compilation and Some Tectonic Implications

The coincidence of late Paleogene to Neogene shortening and crustal thickening with vigorous volcanic activity in the central Andes has long invited speculation about a causal relationship between magmatism and deformation. In aid of understanding this and related issues, we present here a new compilation of radiometric ages, geographic location and dominant rock type for about 1450 Cenozoic volcanic and subvolcanic centers in the southcentral Andes (14–28° S). This paper describes variations in the timespace distribution of volcanism from 65 to 0 Ma, with emphasis on the post-30 Ma period where Andean-style shortening deformation and volcanism were most intense. The central Andes are unusual for the abundance of felsic ignimbrites and their distribution is shown separately from the intermediate to mafic volcanic centers which are here termed the “arc association”. Overall, the time-space patterns of volcanic activity for the ignimbrite and the arc association are similar but ignimbrite distribution is more patchy and more closely associated spatially with the plateau region.

Central and Southern Andean Tectonic Evolution Inferred from Arc Magmatism

Patterns of spatial distribution, and geochemical and isotopic evolution from subduction-related igneous rocks provide tools for scaling, balancing and predicting orogenic processes and mechanisms. We discuss patterns from two Andean key arc segments, which developed into fundamentally different types of orogens: (1) A plateau-type orogen with thick crust in the central Andes, and (2) a non-plateau orogen with normal crust in the southern Andes.

Structural evidence of orogen-parallel strike slip displacements in the Precordillera of northern Chile

ZusammenfassungDie nordchilenische Präkordillere, zwischen Längstal und Westkordillere gelegen, ist aus mehreren langgestreckten prämesozoischen Grundgebirgsrücken aufgebaut, die sich als Antiklinalkerne oder als aus diesen hervorgegangene, durch Aufschiebungen begrenzte Rücken über das mesozoisch-tertiäre Deckgebirge erheben. Das Gebiet ist durchzogen von einem System steiler, parallel oder in spitzem Winkel zu diesen Einengungsstrukturen verlaufenden Störungen. Folgende Strukturen zeigen, daß es an diesen Störungen Seitenverschiebungen gegeben hat: (a) asymmetrische en-echelon-Störungsanordnungen, (b) stratigraphische und strukturelle Diskontinuitäten an Störungen, (c) Gefüge der Störungsgesteine und (d) Schlingen. Ein dextraler Versatz von mehreren Zehnern von km ist wahrscheinlich. Orogenparallele Seitenverschiebungen und Orogen-normale Einengung können als Phänomene der Magmatic-Arc-Tektonik gesehen werden, da von der höchsten Oberkreide bis zum Höhepunkt der Deformation (45-30 Ma) die magmatische Aktivität der Anden in der Präkordillere lag. In diesem Zusammenhang stehen auch die Vererzungen der großen porphyry-copper-Lagerstätten des Gebietes. Schiefe, zu dextraler Transpression führende Subduktion wird für die Strukturbildung der Präkordillere verantwortlich gemacht.AbstractThe Chilean Precordillera, situated between the Longitudinal Valley and the Western Cordillera of northern Chile, is made up of several elongate basement ridges following the trend of the Andes. These ridges, which morphologically rise above the Mesozoic-Tertiary cover rocks, are developed as anticlinal cores or as pressure ridges bounded by reverse faults and, thus, show considerable orogennormal shortening. Several nearly vertical faults cut through the Precordillera, parallel or at a very low angle to the mountain ranges. From the following structures it can be inferred that orogen-parallel transcurrent movements took place along the faults: (a) asymmetric en echelon fault arrays, (b) stratigraphic and structural discontinuities across major faults, (c) fabrics in fault rocks, and (d) vertical folds. A dextral displacement of the order of tens of km is probable. The orogen-parallel strikeslip movements as well as the orogen-normal shortening are considered as phenomena of magmatic arc tectonics due to the focussing of central Andean igneous activity on the Precordillera from the Late Cretaceous until the paroxysm of deformation (45-30 Ma). Deformation along the Precordilleran Fault System is related with the development of the large porphyry copper ore deposits of that area. The structural evolution of the Precordillera can be explained by oblique subduction resulting in dextral transpression.ResumenLa Precordillera del norte chileno, situada entre el Valle Longitudinal y la Cordillera Occidental, esta constituida de uno o más dorsales de basamento, los que forman núcleos de grandes anticlinales o pilares en compresión que se elevan morfologicamente sobre las rocas mesozóicas y terciarias de su cobertura. Varias fallas más o menos verticales cortan a la Precordillera paralelamente o con angulo agudo a su rumbo. Las siguientes estructuras permiten deducir que se produjeron movimientos transcurrientes paralelos al orogeno en las fallas: (a) juegos asimétricos escalonados de fallas, (b) discontinuidades estratigráficas y estructurales ligados a la falla, (c) fábricas de las rocas de falla y (d) pliegues con ejes verticales. Un desplazamiento dextral de decinas de kilométras es probable. Las fallas de desplazamiento en el rumbo paralelas al orógeno asi como el acortamiento normal al orógeno se consideran como fenomenos de la tectónica de arco magmático, puesto que desde el Cretácico superior hasta el tiempo de la deformatión (45-30 Ma), la actividad magmática de los Andes Centrales estuvo ubicada en la Precordillera. La deformación en el sistema de fallas de la Precordillera está tambien relacionada con la mineralizatión de los grandes yacimientos de pórfidos cupriferos de la zona. La evolutión tectónica de la Precordillera puede haber sido originada por subductión oblicua produciendo transpresión dextral.Краткое содержаниеВ северной части Чиле, простирающейся межд у продольной долиной и западными Кордильер ами, возникли до горообра зовательного процес са Кордильер многочисл енные хребты из отложений до-мезозой ских основных гор, кот орые образовались в виде я дер антиклиналей, или в виде хребтов в ре зультате надвигов от дельных единиц на мезозойско-третичные покровные горы. Вся об ласть пронизана сист емой зон разрывов, залегаю щих по отношению к названным структур ам сдавления паралле льно, или под острым углом. Ч то при этих нарушения х имели место боковые с мещения видно: а) по асимметричным ку лисообразным разлом ам, б) по стратиграфическ им и структурным перерывам в них, в) по те кстуре пород деформа ции и г) по вертикальным ск ладкам. Весьма вероятно, что з десь имело место прав остороннее смещение на многие де сятки км. Смещение по крыльям п аралельно орогену и обычное сдавление ор огеном можно рассмат ривать, как феномен магматич еской дуговой тектоники, т.к. магмати ческая активность Ан д в пре-кордильерском г орообразовательном процессе продолжалс я от верхнего мела до кульминационной точ ки деформации (45–50 Ма). С этой активностью связаны и образовани я крупных меднопорфир овых месторождений этой области. Косая су бдукция, ведущая к трансграссии направ о, рассматривается, ка к отвественная за обра зование этих структу р докордильерского горообразовательно го процесса.

40Ar/39Ar and RbSr analyses from ductile shear zones from the Atacama Fault Zone, northern Chile: the age of deformation

Abstract The influence of deformation on the K-Ar and the Rb-Sr isotope system is investigated. It is assumed that, due to the diffusion processes involved, deformation has a similar effect on isotopic equilibrium as has temperature. In order to examine the influence of deformation on the K-Ar and the Rb-Sr isotope systems two shear zones from the Atacama Fault Zone (AFZ), situated in the north Chilean Coastal Cordillera, have been investigated. The AFZ, which was active as a sinistral strike-slip fault during the Mesozoic, has two sets of shear zones, one formed under amphibolite (SZ1), one under greenschist facies conditions (SZ2), Rb-Sr and 40Ar/39Ar age determinations were conducted on samples from cross sections of each set. In SZ1 the hornblendes and bioties from a weakly deformed sample reveal cooling ages of 153-152 and 150 ± 1 Ma, respectively. Biotite from the center of the shear zone of SZ1 gave an isochron of 143.9 ± 0.3 Ma (MSWD = 0.04) which is interpreted as the age of deformation which produced resetting of the mineral system. In SZ2 hornblendes yielded 40Ar/39Ar plateau (cooling) ages of ∼ 138 Ma. Biotites from undeformed samples gave Rb-Sr and 40Ar/39Ar total degassing ages of 130 ± 1 Ma, whereas biotite from the mylonitic rocks yielded 126-125 Ma which dates the time of deformation. Sr isotope homogenization occurred in the mylonitic rocks, and is most likely a result of deformation. The formation of SZ1 can be correlated to the Araucanian (= Nevadan) phase. The deformation in SZ2 is related to the onset of uplift and cooling of the Coastal Cordilleran magmatic arc.