Main Andean Research Articles

Pliocene lahar deposits in the Coastal Cordillera of central Chile: Implications for uplift, avalanche deposits, and porphyry copper systems in the Main Andean Cordillera

Lahar deposits occur within a shallow marine sedimentary succession of the Pliocene La Cueva Formation in the Coastal Cordillera of central Chile (33°40′–34°15′S). Provenance studies of the abundant volcanic material in the lahar deposits suggest that they derive from denudation by mass wasting of Oligocene–Miocene volcanic rocks on the western slopes of the Main Andean Cordillera at the same latitude. Pliocene rock debris deposits preserved in the region of El Teniente (∼34°S) and scattered along the westernmost part of the Andes of central Chile indicate catastrophic erosive events related to the rapid uplift of the cordilleran block. This rock debris was deposited by avalanches and transformed further downslope into lahars by dilution with stream water. Lahars were channeled along the ancient drainage system that reached a shallow Pliocene sea at the site of the present Coastal Cordillera. The exceedingly rapid exhumation of active porphyry systems during the Early Pliocene in this part of the Andes may have played a role in affecting hydrothermal processes, brecciation, and diatreme formation at the porphyry systems of El Teniente and Río Blanco–Los Bronces.

Postmagmatic cooling and late Cenozoic denudation of the North Patagonian Batholith in the Los Lagos region of Chile, 41°−42°15′S

Zircon and apatite fission track (FT) thermochronology was applied to investigate the history of cooling and denudation of the Southern Andes between 41° and 42°15′S in relation to the late Cenozoic activity of the Liquine-Ofqui fault zone (LOFZ) and the northward migration of the Chile Triple Junction (CTJ). Fifty-six zircon and 51 apatite FT ages, plus 37 apatite confined track-length distributions were obtained mainly from plutonic rocks of the North Patagonian Batholith (NPB) in the main Andean Cordillera. Apatite FT ages and track lengths indicate a stage of rapid cooling at ∼5--3 Ma along both sides of the LOFZ, whereas older Miocene ages with monotonous cooling histories were obtained further away from the fault. Zircon FT ages range from Cretaceous to Pliocene, with marked differences observed along and across the LOFZ. Three different types of temperature-time histories characterise the post-magmatic cooling of the NPB in the region: deep intrusions with moderate and steady cooling rates, intrusions in the upper crust with very slow cooling rates following a stage of initial rapid cooling, and rapidly cooled and exhumed shallow intrusions, the latter with younger ages towards the fault zone. The most prominent denudation episode along the LOFZ is late Miocene to Pliocene, coeval with plate tectonic reconstructions for the arrival and subduction of the Chile Rise beneath the Taitao Peninsula.

Low electrical resistivity associated with plunging of the Nazca flat slab beneath Argentina.

Beneath much of the Andes, oceanic lithosphere descends eastward into the mantle at an angle of about 30 degrees (ref. 1). A partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material. This wedge is the ultimate source for magma erupted at the active volcanoes that characterize the Andean margin. But between 28 degrees and 33 degrees S the subducted Nazca plate appears to be anomalously buoyant, as it levels out at about 100 km depth and extends nearly horizontally under the continent. Above this 'flat slab', volcanic activity in the main Andean Cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge. But it is unknown where slab volatiles go once this happens, and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward. Here we present results from a magnetotelluric profile in central Argentina, from which we infer enhanced electrical conductivity along the eastern side of the plunging slab, indicative of the presence of partial melt. This conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth, or that melt is supplied from the 410 km discontinuity, consistent with the transition-zone 'water-filter' model of Bercovici and Karato.

Geomorphic and geologic evidence of ongoing uplift and deformation in the Mérida Andes, Venezuela

The Mérida Andes is a mountain chain that trends SW–NE in western Venezuela, extending for some 350 km from the Colombian–Venezuelan border to the city of Barquisimeto. Its highest peaks reach 5000 m in elevation in the central portion, near Mérida. This chain appears to be the northeastward topographic prolongation of the Eastern Cordillera of the Colombian Andes. The latter belongs to the main Andes chain that extends along the Pacific coast of South America. However, there is no direct genetic relationship between the Mérida Andes and Eastern Cordillera. The Mérida Andes uplift is not produced by convergence across a conventional type-B subduction, as most of the South American Andes are. In addition, the Mérida Andes and Eastern Cordillera are separated by the southern termination of the left-lateral strike-slip Santa Marta-Bucaramanga fault, and the NW–SE trending Santander Massif. The present Mérida Andes chain build-up results from Pliocene–Quaternary transpression due to oblique convergence between two continental blocks: South America and Maracaibo Triangular Block. The present geodynamic setting is responsible for ongoing strain partitioning along the Mérida Andes where the foothills and the mountain belt have been shortened transversely in a NW–SE direction whereas the Boconó fault, roughly located in the core and along the Mérida Andes axis, accommodates dextral slip. Tectonic inheritance in the Mérida Andes plays a major role, since the chain growth partly results from inversion of Jurassic (half-) grabens, exposing Precambrian and Paleozoic rocks of the South American continental crust along the chain core. Evidence of very different kinds attests to the ongoing growth of the chain. From the geomorphic viewpoint, the axial valleys display well-preserved Quaternary staircase terrace systems with more than 500 m of vertical drop between the oldest terrace and present river beds. Rivers cutting across the structural grain of the chain show very distinct transverse “wine cup” profiles, attesting to the rapid uplift of the chain. In some cases, rivers have downcut over a 1000 m of relief, where the lower 200–300 m of these profiles exhibit subvertical walls. Other features that support the significant uplift of the chain and its youth are synorogenic mollasic deposits along both flanks of the chain, deposited in flexural basins, whose thicknesses reach 8 and 3 km on the northwest and southeast of the Mérida Andes, respectively. These continental deposits of essentially Plio–Quaternary age are arranged in up-dip convergence growth wedges, which are being deformed or destroyed by basin-vergent intracutaneous wedges, triangular zones and/or flat-and-ramp structures rooted under the Mérida Andes. The Mérida Andes core comprises igneous and metamorphic rocks formed at depths of 8–10 km, that are cropping out at its highest summits at about 5000 m elevation, implying a total uplift of the order of 12–15 km in the last 3–5 Ma (average uplift rate of 2–5 mm/a). Activation age and amount of uplift match rather well with those derived from fission-track thermochronology. The entire mountain belt exhibits widespread seismicity, essentially restricted to upper crustal levels (seismogenic layer). As well, focal mechanism solutions confirm the strain partitioning taking place in the Mérida Andes at present: right-lateral strike-slip along the sub-axial Boconó fault and basinward thrust solutions on both flanks. Underfed/under-destruction pull-apart basins (Las González and Apartaderos) along the Boconó fault appear to be complementary evidence of rapid uplift of the Mérida Andes.

The Pampean flat-slab of the Central Andes

Late Cenozoic Andean deformation in the Pampean segment of Argentina and Chile (27°00′–33°30′S) provides an exceptional opportunity to study the orogenic effects of flat subduction in an active convergent margin. At these latitudes, constraints derived from oceanic geomorphic structures indicate that collision of the Juan Fernandez Ridge in the Nazca plate with the Chilean trench propagated from north to south from ca. 18 to ca. 11 Ma. In this region, Cenozoic tectonics have resulted in the development of the Principal and Frontal Cordilleras, the Cuyo Precordillera, and the associated Sierras Pampeanas in the eastward foreland region. The analyses of different structural transects across the Sierras Pampeanas between 27 and 33°S latitudes show a southward diachronic beginning of basement block uplift from 7.6 to 6 Ma in the Sierra de Aconquija (27°S), 4.5 to 4.19 Ma at Sierra de Famatina (29°S), 5.5 to 4.7 Ma at Sierra de Pocho (31°S), and ca. 2.6 Ma at Sierra de San Luis (33°S). Uplift times are constrained by the age of synorogenic deposits, fission-track data on apatite, and hydrogen isotopes in alunite, where available. These time constraints are compared with the migration and expansion of the magmatic activity into the foreland. The relation of these two data sets shows a striking coincidence, after a residence time of approximately 4–2.6 Ma, between magmatic activity and failure of the crust that resulted in basement block uplift. Thermal weakening of the crust associated with eastward migration of arc magmatism acted to elevate brittle–ductile subsurface décollments, thus leading to thick-skinned basement uplift of the Sierras Pampeanas. The evolution of the Sierras Pampeanas is linked to that of the main Andes at 33°S latitude. The main deformation phases and uplift of the thin- and thick-skinned fold and thrust belts of Principal Cordillera occurred between 20 and 8.6 Ma. Arc volcanism migrated eastward at this latitude between 16 and 15.8 Ma. This expansion was accompanied by deformation and uplift with the development of a foreland basin. Soon after the shallowing of the subduction zone, orogenic shortening and basin formation were followed by a migration of the volcanic front to the foreland. Propagation rates of the thrust front from 2.5 mm/a in the early stage (15–9 Ma) accelerated to 13.3 and 13.7 mm/a during the middle (9–6 Ma) and late (5–2 Ma) stages of shallowing. Uplift of the Frontal Cordillera overlapped the last stages of Principal Cordillera deformation at 9 Ma. This time is also constrained by magnetostratigraphic studies of the foreland deposits that record a rapid increase in subsidence rates at the same time that arc magmatism expanded to the Sierras Pampeanas, between 9.5 and 6.4 Ma. This rapid propagation of the thrust front and migration of the arc magmatism occurred 2 m.yr later than the 11 Ma collision of the Juan Fernandez Ridge at this latitude. Uplift of the Precordillera by tectonic inversion of pre-existing normal faults and basement uplifts of the Sierras Pampeanas in the vicinity of 33°S occurred from 2.6 Ma to the present. Neotectonic studies, as well as GPS measurements, indicate active deformation in the thrust front of the Precordillera, as well as in the Sierras Pampeanas. Magmatic activity ended in the Principal Cordillera at approximately 8.6 Ma and in the Sierras Pampeanas at 1.9 Ma. The last subduction-related volcanism occurred more than 750 km east of the trench. Analyses performed along four different transects show close relations between eastward propagation of the magmatic arc, formation of new brittle–ductile transition zones as a function of heat input, and orogenic deformation. Uplift and deformation in Sierras Pampeanas, thus clearly follows the eastward propagation of arc magmatism. This relation implies that uplift and deformation are more likely related to thermal weakening and crustal anisotropy than to fluctuations in horizontal compressive stress. Eastward propagation of magmatism to the foreland over the last 6 m.yr plays an important role in the thermal crustal weakening and development of brittle–ductile transitions that allowed uplift of Sierras Pampeanas basement-cored blocks.

Lithostratigraphy, petrology and 40Ar- 39Ar geochronology of the Crucero Supergroup, Puno department, SE Peru

The Cordillera de Carabaya region of the southern Peruvian Andes preserves a collage of Tertiary igneous suites, including extrusive and hypabyssal units, that are herein assigned to the Crucero Supergroup and Crucero Intrusive Supersuite, respectively. The supergroup consists of two petrologically and temporally distinct assemblages, the Picotani and the Quenamari Groups, both with silicic hypabyssal representatives. Stratigraphic, petrologic and 40Ar- 39 Ar geochronologic studies of these rocks demonstrate that they are exposed in four localities, the Quenamari, Antauta, Cayconi and Picotani volcanic fields. The Picotani Group incorporates a diverse assemblage of lavas and pyroclastic rocks, including lamprophyres (minette), medium- to high-K calc-alkaline basalts, shoshonites, and S-type rhyodacites and rhyolites, as well as commingled and mixed associations of these, constituting ten mappable formations and twelve delimited hypabyssal units. These were emplaced over a brief interval from ca. 22 -to- 26 Ma. The Quenamari Group, in contrast, comprises entirely silicic, strongly peraluminous suites including, biotite + sillimanite ± muscovite ± andalusite ± tourmaline-bearing pyroclastic units and cogenetic epizonal intrusions. These constitute three distinct volcanic formations and at least 8 intrusive bodies. Rocks of the Quenamari Group span a broad temporal interval, from ca. 6.5 to 17 Ma. The rock associations of the Crucero Supergroup are clearly distinct from time-equivalent igneous units of the calc-alkaline, Main Andean arc in southern Peru, thereby implying differing geodynamic conditions during their genesis.

Alkaline magmatism within the segment 38°–39°S of the Plio‐Quaternary Volcanic Belt of the southern South American Continental Margin

In the segment 38°–39°S of the southern South American volcanic belt, the subalkaline central‐vent volcanoes that form the late Quaternary volcanic front of the Andean orogenic arc are located in the Main Andean Cordillera 30–80 km west of the centers that formed the Pliocene/early Pleistocene volcanic front. The Pliocene/early Pleistocene volcanic front was located along a precordilleran uplift that diverges southeast from the Main Andean Cordillera. During the Pliocene the centers along this uplift erupted subalkaline magmas similar to those currently being erupted along the late Quaternary front. In the early Pleistocene the centers at the southeast end of this uplift began to erupt alkaline magmas. By the late Pleistocene, magmatic activity in these centers had diminished, and the volcanic front had migrated westward to its current location, but alkaline back arc basaltic volcanism continued in the late Quaternary within the valleys flanking the precordilleran uplift on which the Pliocene/early Pleistocene arc had been situated. The early Pleistocene alkaline orogenic arc lavas and the late Quaternary alkaline back arc basalts have La/Yb higher than, but Ba/La and 87Sr/86Sr similar to, the subalkaline magmas erupted from centers of both the Pliocene and late Quaternary volcanic fronts. Their TiO2 and La/Nb are intermediate between subalkaline magmas erupted from both the Pliocene and late Quaternary volcanic fronts and alkaline basalts of the Patagonian plateau lavas south of 39°S which have intraplate oceanic island petrochemical characteristics. Compared with the subalkaline lavas of the current volcanic front, Quaternary alkaline arc and back arc lavas between 38° and 39°S formed by lower degrees of partial melting of a similar mantle source. This mantle had been modified by source region contamination processes related to dehydration of subducted oceanic lithosphere such as occur below the current volcanic front in the southern Andes.

Andean‐age structure of Eastern Cordillera (Province of La Paz, Bolivia)

A Moho root beneath the Bolivian Andes, 40 km deep, is consistent with 230 km of overlap of Neogene age on a single, trenchward dipping transcrustal thrust fault with 10° of finite ramp cutoff (Main Andean Thrust (MAT)). Only 10% of the Andean crustal volume is ascribable to magmatic addition. The MAT is located within South American crust of full thickness. It intersects the basement top 450 km inland from the Neogene crustal margin. It is not a collision suture as shown by persistent pre‐Neogene facies continuity. Thrusting is not accompanied by terrane accretion. The present bilaterally symmetrical thrust belt responds to elastic line loading and to Coulomb rheology. In the hanging wall of the MAT, a deep high‐stress wedge base builds a steep critical slope. In the footwall, the foredeep response is fast subcritical growth by progradation and blind thrusting on a low‐stress decollement. Interaction is maintained by out‐of‐sequence renewal of movement on the MAT.

Probing the evolving Andean Lithosphere: Mid‐Late Tertiary magmatism in Chile (29°–30°30′S) over the modern zone of subhorizontal subduction

In the mid‐late Tertiary (27–6 m.y. ago), volcanism diminished, then ceased in the main Andean Cordillera in Chile between 29° and 30°30′S. Over the same time span the subduction angle shallowed under this region, which today lies in the modern shallowly dipping, nonvolcanic segment of the south central Andes (28°–33°S). The trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crustal thickening that accompany this major tectonic transition. Volcanic changes include, in addition to a decreased rate of volcanism, a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks, a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks, and an increase in crustal assimilation. These volcanic changes also correlate in time with deformational events that help to account for crustal thickening and the space‐time pattern of deformation and volcanism in the foreland region. Although the model stands alone, support for the tectonic setting and magmatic source region of these Tertiary volcanic rocks can be found in comparisons with recent volcanic rocks erupting over more steeply dipping modern Andean segments. The older Doña Ana Formation (27–18 m.y.) is similar to volcanic rocks in the Southern Volcanic Zone, while the younger Cerro de las Tortolas (14–10 m.y.) and Vallecito (6 m.y.) formations are similar to volcanic rocks over regions of thicker crust in the Central Volcanic Zone and the northern part of the Southern Volcanic Zone.

Geological manifestations of ridge collision: Evidence from the Golfo de Penas‐Taitao Basin, southern Chile

Recent geological and geophysical studies in the southern Andes adjacent to the intersection of the Chile Rise with the Peru‐Chile Trench (ANT‐NAZ‐SAM triple junction) have revealed a number of features and a Neogene geologic history that are unique along the Pacific margin of South America. This history includes (1) development of a Tertiary‐Quaternary marine basin with up to 3 km of sediment infill (Golfo de Penas‐Taitao basin, GTB), (2) disruption of the region by a series of faults with both normal and strike slip movements, and (3) localization of silicic, near‐trench volcanism and epizonal plutonism and related hydrothermal activity. The northern portion of the GTB began to subside in the Late Miocene (possibly earlier), and has subsequently been deformed, uplifted, and exposed. Gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today (Golfo de Penas). Subsidence and uplift have thus occurred diachronously in the region, although it is unclear when subsidence began in the Golfo de Penas. Tectonic disruption of the region is likely related to the Liquiñe‐Ofqui fault (LOF), a major, NS‐trending, crustal shear zone that curves westward and terminates in the Golfo de Penas. The LOF has both down‐to‐the‐west and right lateral offset and separates the main Andean Cordillera on the east from a large crustal block (the Chiloe block) on the west. We hypothesize that the GTB has developed as a pull‐apart basin in response to northward movement of the Chiloe block along the LOF. We propose a dynamic model whereby a stress gradient that decreases longitudinally away from the Chile Rise/Peru‐Chile Trench intersection is set up because the youngest, most buoyant, oceanic lithosphere is being subducted at the triple junction. The Chile Rise is viewed as a type of indenter which is acting to drive the Chiloe block northward in front of the northward‐migrating triple junction. This model explains the unique set of geologic features found in the region, and suggests that ridge‐trench interactions may be an important factor in orogenesis at active continental margins.

The Role of Competition in the Distribution of Andean Birds

The object of our study was to compare the distributions of bird species common to two Andean localities in order to improve our understanding of the factors involved as primary causes in imposing distributional limits. A previous paper (Terborgh 1971) had evaluated the importance of three mechanisms in the avifauna of a control locality: (1) factors that vary continuously and in parallel with the elevational gradient, (2) competitive exclusion, and (3) ecotones. As was discussed in that paper, the method, when applied to a single transect, leads to a systematic underestimation of the incidence of competition—mediated limits and to an overestimation of ecotone effects. By studying appropriately selected test localities, we could overcome many of these deficiencies and obtain results that are several steps closer to expressing reality. The control locality, the Cordillera Vilcabamba, is a sector of the Cordillera Oriental, or eastern chain of the main body of the Peruvian Andes. It carries what may be regarded as a complete avifauna, as the transect sliced through a continuous belt of forested terrain which, at any elevation, is exposed to invasion by appropriately adapted species from above and below as well as laterally. It differs in this respect from the test locality (Cerros del Sira), which is an isolated massif that rises out of the Amazonian plain some 100 km east of the main Andes. Because of this isolation, the upper portions of the Sira are exposed to invasion principally from below. An estimated 80%—82% of the species that would have occupied the summit zone of the Sira, had it been a portion of the main body of the Andes, were missing. Such a species deficit provides an ideal test of the efficacy of competition in limiting distributions, because it invites whatever species are available as colonists (in this case, those lower on the mountain) to invade until the community is effectively filled. Of the species that had the opportunity to expand their elevational ranges in the absence of high—elevation congeners that apparently excluded them on the control transect, a minimum of 71% did so, confirming the original assignments of these as competition—limited species in the control locality. In addition, a majority (58%) of the species that had no high—elevation congener on the control transect (assigned to the gradient mechanism), and that had the opportunity to expand into the species—deficit zone of the Sira, were found to do so. This reveals that diffuse competition (as distinguished from the directly observable exclusion of congeners) was really the primary limiting mechanism for most of the species that had been assigned to the gradient mechanism in the control study. Instead of accounting for approximately a third of all distributional limits, as was concluded in the earlier report, it now appears that competition, both direct and diffuse, accounts for something more than two—thirds of the distributional limits of Andean birds whose ranges end somewhere between the lowland plain and tree line.

Structural belts and mineral deposits of northwestern Argentina

Three structural belts paralleling the north-northeast trend of the main Andes chain in northwest Argentina differ in age and character of igneous activity and in mineralization. In the gently folded ranges of the eastern belt, where Precambrian-Permian, Cretaceous, and Miocene-Recent formations are represented, there are no post-Cambrian granites, few basic intrusive or volcanic rocks, and only limited occurrences of metallic ores. The sedimentary section is less complete in the strongly folded and thrust-faulted cordilleras of the central belt; late granites (probably late Tertiary) are numerous, and associated vein and replacement deposits contain lead, zinc, silver, copper, and other metals. In the high block-faulted plateau of the western belt (the Puna region), iron and copper are associated with Precambrian granite masses, molybdenum, columbite-tantalite, and some bismuth with pegmatites of similar age, and gold, silver, antimony, tin, and lead with widespread Miocene-Quaternary volcanic rocks.

Geology of Northwest Peru

The oil fields of northwestern Peru occur in an area of much faulted Tertiary rocks, extending from the southern border of Ecuador (about 3° 30^prime south latitude) south about 225 miles (to about 6° south latitude). It is the northern end of a desert belt extending along the entire Peruvian coast, but near the northern border, merging rapidly with the semi-arid and humid forest zone of southern Ecuador. The presence of oil in this region has been known from a very early date, and in Colonial and pre-Colonial times, several of the larger oil seeps were worked for their pitch or brea. To-day there are three general districts where commercial production is obtained: the Zorritos district in the northern part, where production is derived from the lower Miocene or upper Oligocene; farther south, the Cabo Blanco-Lobitos district; and the Negritos-La Brea district. In these latter and more important fields, the oil is derived principally from Eocene sediments. The Tertiary of this region, which at present is the source of the entire Peruvian production, has a total thickness of about 25,000 feet. This section is exceptionally complete, generally marine and fossiliferous, and on the basis of detailed stratigraphic and paleontologic studies, it has been subdivided into about 14 mappable units or formations. The beds range in age from early Eocene to Pliocene. They rest either upon an incomplete development of Cretaceous, or, more generally, on older mountain rocks such as Pennsylvanian slates. These Pennsylvanian metamorphics, closely associated with a granite intrusion, form most of the mountain region toward the east and a few other outlying areas. The major structural features of the region are the main Andean or mountain ranges on the east, and a Pacific fault which closely follows the edge of the continental shelf on the west. In the intervening area, the Tertiary rocks have been locally folded into smaller anticlines and synclines and extensively block faulted. The rocks are everywhere much faulted, and the local structure consists principally of many small fault blocks. Most of the faults are normal. End_Page 1------------------------------ Fig. 1A. Geological map of Northwest Peru. Fig. 1B. Profile showing continental shelf and adjoining land mass at Parinas Point. End_Page 2------------------------------