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Allochthonous Terranes in the Andes of Ecuador and Northwestern Peru

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Allochthonous Terranes in the Andes of Ecuador and Northwestern Peru All~chthon~usterranes in the Andes of Ecuador and n~rthwesternPeru1 TOMASFEININGER Geological Survey of Cmada, 1 Observatory Crescent, Ottawa, Ont., Canada KIA OY3 Received December 3, 1985 Revision accepted August 21, 1986 In Ecuador and northwestern Peru the Andes and adjacent country, particularly on the Pacific side, are colnposed of at least five distinctive geologic terranes. The teranes are distinguished from one another and from cratonic South America to the east by dissimilar basements, cover rocks, intrusive rocks, and Bouguer gravity anomaly fields. The Pii6n terrane, occupying most of coastal Ecuador, has a basaltic basement characterized by the largest known on-land positive Bouguer anomalies in the western hemisphere. The Tahuin terrane occupies most of northwestern Peru and the southwestern comer of Ecuador. The terrane has an especially complex basement and is the site of generally positive Bouguer anomalies. The small Bir6n terdne has an unusual basement composed in part of cordierite gneiss and amphibolite that give consistent Late Cretaceous K- Ar mineral ages. The wedge-shaped Chaucha terrane lies in part on the western Andean slope. between the oceanic Piii6n terrane on the north and the continental Bir6n terdne on the south. The vast Santiago terrane com- poses the high Andes of southern Ecuador and northwestern Peru. It is the site of the unique Santiago Fornlation, a thick succession of Lower Jurassic limestones found nowhere else in the region. Geologic and geophysical evidence supports the view that the five terranes a= parautochthonous or allochthonous fragments emplaced against cratonic South America from Middle Jurassic to Late Eocene time. Continental-border subduction alone (at the so-called "Andean margin") may have keen an inadequate engine for orogeny. Additional allochthonous terdnes perhaps await identification at other places along the Andes. Whether the emplacement of allochthonous terrdnes has been an important process elsewhere in the tectonic development of the Andes remains to be established. Geologic mapping on the oceanward western border of the Andean orogen, studies of basement petrology and chronology, and paleoinagnetic studies are particularly needed. The distribution of mineral deposits (including petroleum) in Ecuador and northwestern Peru is not unifoml but is instead related spatially to the five terranes and cratonic South America. This relationship can be useful to prospectors. Dans 1'6~uateuret le nord-ouest pCruvien, surtout du c6tk du Pacifique, au moins cinq terdnes gkologiquement distinctifs composent les Andes et les regions adjacentes. On peut distinguer chaque terrane de ses voisins et du craton de 1'Amkrque de Sud 21 l'est sur la base des caractkristiques du socle, des roches intrusives, de la couverture et du champ des anomalies gravi- mitriques Bouguer. Le terrane Pii6n couvre la plupart de la c6te de lYfiquateur.I1 a un socle basaltique qui se caractCrise par des anomalies positives Bouguer terrestres qui sont les plus fortes connues dans l'hkmisphkre occidental. Le terrane Tahuin couvre la rnajeure partie du nod-ouest pCruvien et le coin sud-ouest de 1'Equateur. Le socle du terdne est particulikrement complexe et s'accorde 2I des anomalies Bouguer gCnCralement positives. Le petit terrane Bir6n a un socle peu commun composC en partie de gneiss h For personal use only. cordikrite et amphibolite dont les minkraux donnent des Ages K - Ar indiquant la fin du CrCtacC. Le terrane Chaucha, en fomle d'un coin serrC entre le terrane ockanique Pifi6n au nord et le terrane continental Bir6n,au sud, est situk partiellement sur le flanc andin occidental. L'immense terrane Santiago compose le coeur des Andes de 1'Equateur mCridional et du nod-ouest pCruvien. I1 est le site de la formation de Santiago, qui est unique puisque composCe d'une succession Cpaisse de roches calcaires du Jurassique infirieur qui ne se trouvent nulle part ailleurs. L'Cvidence gCologique et gCophysique soutient 19idCeque les cinq teranes ynt des fragments parautochthones ou alloch- tones acscolCs contre le craton de 1'AmCrique du Sud du Jurassique moyen 2I 1'Eocltne tardif. Cette observation affaiblit l'idke que seule la simple subduction 21 la marge continentale (souvent nommk wmarge andine*) pourdit avoir CtC suffisante pour pro- nlouvoir 190rogCnltse.D'autres terranes allochtones peuvent &re pdsents dans les Andes. I1 reste 2I ktablir si la mise en place des terranes allochtones a jouC un r6le important ailleurs dans le dCveloppement tectonique des Andes. Pour ce, on a besoin surtout de la cartographie gCologique de la marge occidentale de l'orogbne andin, des ktudes de la pktrologie et de la chrono- logie du socle et des Ctudes palComagnCtiques. La rkpartition des gisements minCraux et pktroliers en ~~uateuret dans le nord-ouest ptruvien n'est pas homogbne mais est fonction d'un rapport spatial entre les cinq terrane et le craton de 1'AmCrique du Sud. Ce qui est d'une gmnde aide pour le pro- specteur. Can. J. hrth Sci. 24, 266-278 (1987) Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIVERSITY OF MICHIGAN on 10/10/14 Introduction (1) The crust under western South America changes dramati- The geology of Ecuador and northwestern Peru has been cally at the Gulf of Guayaquil, from oceanic in the north to interpreted more or less comfortably within the classical geo- continental in the south (Lonsdale 1978). This change, noted synclinal framework for more than half a century (Steinmann by Gansser (1973) as one of the outstanding enigmas of Andean geology, implies a nonunifornl geologic history from 1929; Sauer 1965, 1971 ; Faucher and Savoyat 1973). In recent years, however, the application of plate-tectonic theory and place to place along the Andes. (2) Regionally metamorphosed rocks in Ecuador and north- topical studies of metamorphic phase petrology, gravity surveys, and isotopic dating have raised questions regarding western Peru are diverse. Rather than constituting a uniform the validity of prevailing geological interpretations. Three basement of great antiquity, these rocks range widely in observations in particular merit discussion. composition, metamorphic history, and age (Feininger 1982). .- Major faults commonly separate regions of unlike basement 'Geological Survey of Canada Contribution 22686. rocks . Pnnted in Canada 1 11nprilnCau Canada FEININGER For personal use only. FIG. 1. Distinctive geologic temnes (bold capital letters) and their bounding faults (lower-case letters) in Ecuador and northwestern Peru. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIVERSITY OF MICHIGAN on 10/10/14 Faults are dashed where located approximately. Heavy line with saw teeth is axis of the Peru-Chile trench. (3) High-grade sillimanite- and cordierite-bearing meta- of square kilometres in area and commonly bounded by major morphic rocks that give consistent Late Cretaceous K - Ar ages faults, which in its geology and gravity anomalies differs sub- (Feininger and Silberman 1982) crop out nearly to the Pacific stantially from neighboring regions and from cratonic South shore in southwestern Ecuador. These young metamorphites America to the east. Using classical methods of stratigraphic, lie to the west of the Andes, in places inore than 75 krn ocean- lithologic, or metamorphic correlation, it is difficult if not ward of a coeval magmatic arc. impossible to tie one terrane to another using a comnlon geo- The purpose of the present article is to identify and logic history or a simple paleogeographic reconstruction. It characterize five distinctive geologic terranes that compose seems probable that the terranes have been brought to their much of Ecuador and northwestern Peru (Fig. 1). The term present sites by tectonic processes. Some may be far-travelled "terrane" is used here to refer to a discrete region, thousands fragments - allochthonous-whereas other may be parauth- CAN. J. EARTH SCI. VOL. 24, 1987 For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIVERSITY OF MICHIGAN on 10/10/14 FIG.2. Physiographic map of Ecuador and northwestern Peru. Specific physiographic features are in light type; provincial capital cities are in bold type. Triangles are active and dormant volcanos. Heavy lines are terrane boundaries, dashed where located approximately. Heavy lines with saw teeth is axis of the Peru - Chile trench. ochtsnsus, having been displaced limited but unknown dis- vince, composed on the Andes and their foothills with a medial tances by regional wrench faults. depression, the Inter-Andean valley, in the north. To the A brief overview of the three major physiographic provinces south, the general elevation sf the Andes decreases progres- that constitute continental Ecuador and northwestern Peru sively into northwestern Peru. Foothills flank the Andes on the (Fig. 2) will help orient the reader. Dominant is the Sierra pro- east nearly without a break, but on the west they are discon- TABLE1. Summary comparisons of five distinctive geologic teranes and cratonic South America in Ecuador and northwestern Peru Pificin temne Tahuin temne Birdn terane Chaucha terane Santiago terdne Cmtonic South America Basement Composition Basaltic Granitic on basaltic Granitic Granitic(?) Granitic(?) Granitic Rock types Diabase and pillow basalt Sillimanite -garnet Cordierite gneiss Schist and quartzite Gneiss, schist, Felsic and intermediate gneiss. migmatite. with interlayered quartzite, and granulite schist, quartzite amphibolite, schist phyllite
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