The Andean orogenic period began in the Albian and consisted of at least 6 discrete compressional phases which are Albian to late Miocene in age. Deformation migrated progressively toward the Amazonian foredeep. The major Andean structures in central and northern Peru are related to the Eocene (Incaic), early Miocene (Quechua 1) and late Miocene (Quechua 3) phases. The Maranon thrust and fold belt (MTFB) which bounds the western Cordillera to the NE rose in the Eocene. It was reactivated partly by the Quechua 1 phase, which also folded the Altiplano farther E and possibly part of the eastern Cordillera. The sub-Andean thrust and fold belt (STFB) is related to the Quechua 3 phase. In both MTFB and STFB the imbricate thrusts merge at depth in large décollement faults. Gravity sliding does not play a dominant role, for these faults have to be rooted westward in the pre-Andean basement. The corresponding crustal shortening played a significant part in the creation of the sialic root of the Andes.

Field studies in the Andes of southern Peru show that in the High Andes and Pacific Lowlands, Quaternary and Recent faults are normal. This extensional tectonics postdates compressional deformations of Pliocene‐early Quaternary age. In the sub‐Andes the observed deformations are compressional; they affect early Quaternary deposits. Some of the faults separate Quaternary deposits from the bedrock and thus are clearly of tectonic origin and not landslide effects. Striations on the fault planes indicate N–S trending extension in the High Andes and Pacific Lowlands. The total amount of crustal stretching is small, probably of the order of 1% during the last 1–2 m.y. In the sub‐Andes, folds and faults affecting Neogene and early Quaternary deposits result from N–S shortening. Nevertheless, it is supposed that this N‐S shortening is of early quaternary age. The present‐day compression probably strikes E‐W, judging from focal mechanisms in the sub‐Andes of central Peru, southern Bolivia, and northwest Argentina. Data from structural analysis of faults and from earthquake focal mechanisms allow us to surmise the state of stress in the Andes of southern Peru. The High Andes and Pacific Lowlands, subjected to N‐S trending extension, are bounded by two zones of E‐W trending compression: the sub‐Andes to the east, and the contact between the convergent Nazca and South America plates to the west. In our model the maximum horizontal compressive stress trajectory σ Hmax is roughly parallel with the E‐W convergence between the two plates; σ Hmax corresponds to σ 1, in the sub‐Andes and to σ 2 in the High Andes. The latter situation is caused by the elevated mass of the High Andes, where σ zz (the vertical stress) is inferred to be σ 1. Thus the third principal stress axis, being orthogonal to the other two axes, it is oriented N‐S, allowing extension to occur in that direction. On the other hand, in the sub‐Andes σ zz is σ 3, and horizontal E‐W shortening occurs. The state of stress in the Andean continental crust above the 30° dipping slab appears to be different from that in the Andes of Central Peru situated above the flat subducting segment. In this region, compressional deformantion affect a wider part of the Cordillera.

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GEOLOGIC SETTING Pre-collision Cretaceous-lower Tertiary volcanic rocks of western Ecuador from the coastal area (Piii6n and San Lorenzo Formations) and Western Cordillera (Macuchi and Celica Formations) consist of three rock suites. The first suite includes tholeiites of oceanfloor affinities that have distinct light REE-depleted patterns and low La/Nb, La/Hf, and Th/Wf ratios. The island-arc tholeiites of the second suite have lower contents of Ti, Zr, Hf, and Nb. The third suite comprises calc-alkalic rocks, mainly continental-margin andesites and dacites. The continental volcanic-arc rocks crop out in an area east of the tholeiitic rocks. The boundary between the two areas is the major suture separating continental South America from accreted terranes to the west. A model is suggested for the geodynamic evolution of what is now western Ecuador during the Late Cretaceous. It includes two east-dipping subduction zones of Turonian to Santonian ages and the b accretion of the oceanic island arc to the continent. 1977, Effects ofalteration on element distributions in Archean tholeiites from thc Barberton greenslone belt, South Africa: ConlributioN Io Miiieralogy and Petrology. v. 64, p. 75-89. DaMel, I.

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