Plate Tectonic Model for the Evolution of the Central Andes

DAVID E. JAMES Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, D. C 20015

ABSTRACT INTRODUCTION Data on the geophysics and geology of the The past decade has been one of revolution central Andes are interpreted in terms of plate in the earth sciences, for the advent and general theory and a model for Andean evolution is acceptance of the theory of sea-floor spreading presented. Analysis of upper mantle structure and its corollary, plate tectonics, have led to and seismicity shows that the underthrusting new insights into many fundamental processes Pacific plate is now about 50 km thick and the of geology and geophysics. Verification fol- overriding South American plate 200 to 300 lowed close upon the heels of the conceptual km thick. Underthrusting of the Pacific plate formulation of sea-floor spreading by Hess probably began in Triassic time and has con- (1962), and the essential elements of the theory tinued without substantial change to the pre- appear firmly established. More recently, the sent. Prior to underthrusting, the west coast of theory of plate tectonics has been advanced as South America was quiescent, and great thick- a unifying explanation of observable global tec- nesses of Paleozoic continental shelf deposits tonic phenomena associated with sea-floor were laid down in an area east of the present spreading (McKenzie and Parker, 1967; Mor- volcanic arc. In Late Triassic or Early Jurassic gan, 1968; hacks and others, 1968; McKenzie time, an incipient arc began to form at or west and Morgan, 1969). According to this theory, of the present coast of South America. Igneous the outer rind of the earth consists of rigid activity has since migrated eastward, culminat- plates in motion relative to one another. These ing in the Pliocene-Pleistocene volcanic epi- plates are generated at ocean ridges (or conti- sode. The crust beneath the volcanic cordillera nental rifts) and consumed at trenches, and is more than 70 km thick and probably consists most global seismic and orogenic activity oc- largely of rocks compositionally equivalent to curs at these plate junctures. The plates are per- those of the volcano-plutonic suites observed at haps as little as 0 to 10 km thick at the oceanic the surface. ridges and as much as 150 km thick or more far Increase in crustal volume of the volcanic arc from the ridges. Where two plates collide, one between Cretaceous time and the present im- is normally thrust under the other, and the up- plies either that the mantle above the under- per part of the descending plate is marked by a thrust plate has undergone 18 to 36 percent Benioff zone. Although the theory of plate tec- partial melting or that 1 to 2 km of rock has tonics is by no means as secure as that of sea- been melted from the underthrusting plate. floor spreading, it has been remarkably The intrusion of melt into the crust beneath the successful in clarifying the origins of many tec- volcanic cordillera and the resultant crustal dila- tonic features associated with continental mar- tation produced continentward compression of gins, island arcs, and orogenic belts. Recent the Paleozoic sedimentary rocks which form an applications of plate tectonics theory to conti- easterly belt of thrust and fold mountains. Here nental areas have led to important revisions in crustal shortening has produced crustal thick- concepts of the evolution of major geological nesses of 50 to 55 km. Few deposits of the type features of orogenic belts (Dewey and Bird, normally termed eugeosynclinal, and no ophio- 1970; Bird and Dewey, 1970; Hamilton, lites, are observed between trench and volcanic 1969a, 1970; Dickinson, 1970,1971; Atwater, arc; only in the intermontane foredeep behind 1970). Specifically, efforts to interpret the the arc has a clastic wedge of geosynclinal pro- structure and geology of both the Appalachian portions formed. orogen (Bird and Dewey, 1970) and the cor-

Geological Society of America Bulletin, v. 82, p. 3325-3346, 10 figs., December 1971

3325 3326 D. E. JAMES—PLATE TECTONIC MODEL, CENTRAL ANDES dilleran system of the western United States area shown in Figure 1. It is deepest (about 7.6 (Atwater, 1970; Hamilton, 1969a) have been km) and closest to shore (about 70 km) off the notably successful. coast of northern Chile. Farther north, the The present Andean orogenic belt appears to trench is less deep and locally as much as 200 be a modern active analogue of inactive cordil- km from the coast. The maximum topographic leran mountain belts elsewhere in the world relief between the deepest part of the trench (Hamilton, 1969b), and it is these inactive belts and highest Andean peaks is about 15 km. that are now interpreted in terms of continental margin plate tectonics. In this sense, then, the Geological Setting Andean orogenic belt provides direct observa- The eastern cordillera (Fig. 2) consists tional evidence concerning processes assumed primarily of Paleozoic dark shales, sandstones, to have played a major role in the evolution of and carbonates which probably were laid down continental margins through geological time. on the continental shelf and slope of that time. This paper examines available geophysical These deposits may aggregate 10 to 15 km in and geological data pertaining to the central thickness (Ahlfeld and Branisa, I960) and have Andes, with a view both toward interpreting been extensively folded and faulted eastward Andean evolution within the framework of by Cenozoic compression from the west. East- plate tectonics and toward outlining and eval- ward compression has also produced strong uating those features which bear upon plate tec- folding and thrusting of Mesozoic and Ceno- tonics in orogenic evolution. Some aspects of zoic continental deposits in the subandean older cordilleran systems will be examined in ranges at the eastern margin of the Andean the light of Andean structures; conversely, orogen (Ahlfeld, 1970). Little volcanism and older, eroded cordilleran terranes may provide no large-scale plutonism have occurred in the insight into lower structural levels beneath the eastern cordillera, although in the high moun- Andes. tains of northern Bolivia, the Paleozoic sedimentary rocks are intruded by granitic rocks of Physiographic Setting mid-Mesozoic and mid-Tertiary age; in addi- The area of this study includes southern tion, hypabyssal and volcanic rocks of Tertiary Peru, Bolivia, and northern Chile (Fig. 1). The age occur on the western flanks of the moun- Andean mountain belt between about 8° S. lat tains. consists essentially of a single chain of closely The western cordillera consists mostly of vol- spaced mountain ranges trending parallel to the canic and plutonic rocks of Mesozoic and Ceno- coast. South of about 12" S. lat, the mountains zoic age and associated continental sedimentary branch: the western belt continues parallel to rocks. Shallow-water marine deposits of Juras- the coast, while the eastern belt trends south- sic age are widespread and some marine deposi- east and forms a series of mountain ranges sev- tion persists into Cretaceous time. The crest of eral hundred kilometers from the coast. A the western cordillera is dominated by volcanic broad flat plateau about 4 km in elevation sepa- peaks formed during the culminating stages of rates the two mountain belts. The crests of both the most recent volcanic episode. The lavas of the eastern and western ranges are generally this episode appear mostly to be less than 10 above 5 km, although in the southern part of m.y. old (Rutland and Guest, 1965). Flanking the area, the eastern ranges are somewhat the volcanic crest to the west is the granitic lower. The western mountain belt (Cordillera Andean batholith. Although it had been widely Occidental) is here referred to as the western assumed that the batholith is Cretaceous or cordillera. The eastern belt consists of several early Tertiary in age (for example, Jenks, ranges, among which are Cordillera de Vil- 1956), recent K-Ar age dating indicates that cabamba, Cordillera de Vilcanota, Cordillera the picture is considerably more complex Real, Cordillera Oriental, and Cordillera Cen- (Giletti and Day, 1968; Farrar and others, tral, here referred to as the eastern cordillera. 1970). The study by Farrar and others for a The high plateau between these cordilleras, small area in northern Chile between about termed "Puna de Atacama" in the southern 27° S. and 28° S. lat demonstrates that the plu- part of the area and the "altiplano" elsewhere, tonic rocks range in age from Lower Jurassic to is here referred to as the altiplano. upper Eocene, with the older rocks to the west. The Peru-Chile trench runs parallel to and During latest Mesozoic and the Cenozoic, about 300 km west of the volcanic crest of the the altiplano has been a depositional basin for central Andes. The trench is continuous in the continental clastic sediments derived from both GEOPHYSICAL EVIDENCE 3327

24 METERS AIOVE SEA LEVEL METERS MIOW SEA LEVEL

66 64 Figure 1. Map of central Andes showing the morpho- 6. A-A' indicates line of structure cross section shown in logical provinces considered in this paper. Lines S1-S5 Figure 7. denote seismicity cross sections shown in Figures 5 and Cordilleras. These sediments hide the pre- GEOPHYSICAL EVIDENCE Cretaceous rocks whose history is unknown. The total volume of molasse (primarily conti- Most pertinent to an interpretation of the nental) deposition in the altiplano is enormous. central Andes are the spatial distribution of In the well-studied Lake Titicaca region, Ne- earthquakes and the structure of the crust and well (1949) reports that Cretaceous rocks may upper mantle. Earthquake distribution is here have an aggregate thickness of nearly 7 km. taken from the U.S. Coast and Geodetic Survey The Cenozoic sequence in Bolivian parts of the hypocenter compilation for the period 1964 to altiplano may aggregate as much as 15 km (Ev- the present. Many South American hypocenters ernden and others, 1966). If a Paleozoic sec- are not well determined; nonetheless, when tion comparable to that in the eastern cordillera compared to more precise locations obtained lies beneath the Cretaceous rocks of the Lake by local networks (James and others, 1969), it Titicaca region, the total sedimentary section is clear that the inaccuracy is not sufficient to could be more than 30 km. spoil the over-all picture of regional seismicity. 3328 D. E. JAMES—PLATE TECTONIC MODEL, CENTRAL ANDES

8 F

24-

64 Figure 2. Outline geological map showing distribu- logic Map of South America, published by the tion of major rock types in the central Andes (after Geo- Commission on the Geologic Map of the World, 1963).

Crustal and upper mantle velocity structure is unique earth models—layered models are con- based upon surface wave results presented venient for analysis, but there is no intent to previously (James, 1971) in which Rayleigh suggest that the crust and upper mantle are in and Love wave phase and group velocities were fact layered. The layers reflect on a gross scale determined for paths within the central Andes. what are probably gradational changes in elastic Analysis of phase and group velocities yields properties with depth. shear velocity, compressional velocity, and density as functions of depth. Phase and group Plate Geometry velocities of surface waves are controlled Both upper mantle structure and seismicity primarily by shear velocities, and both com- provide evidence as to the geometry of the pressional velocity and density are commonly South American and Nazca plates. The top of constrained to be simple functions of shear the low-velocity channel for shear waves is here velocity. The layered structures derived from assumed to correspond to the base of the litho- surface wave analysis are idealized and non- spheric plate; thus, the low-velocity zone is pre- GEOPHYSICAL EVIDENCE 3329

sumed to be a region of less mechanical La Paz, Bolivia. The La Paz records generally strength and greater S-wave attenuation than exhibit stronger shear wave attenuation for in- the overlying lithosphere. Upper mantle velo- termediate earthquakes than records of other cities and S-wave attenuation thus provide evi- permanent stations in the central Andes and dence as to the thicknesses of the two thus measure the approximate upper limit of converging plates. The Benioff zone is here attenuation in the mantle above the Benioff taken to correspond approximately to the upper zone. These observations and conclusions are part of the underthrust plate. Figure 3 shows supported by the more thorough studies of schematically the inferred structure of the plate Sacks (1971). junction. The absence of a clearly defined low-velocity Upper Mantle Structure. Surface-wave or high-attenuation channel in the upper man- results argue against a low-velocity channel tle beneath the cordillera prompts looking to beneath the central Andes to a depth of at least greater depths. Sacks (1963, 1971) showed 200 km in the upper mantle. Shear velocities in that the earthquake-free zone beneath South the uppermost mantle are about 4.4 to 4.5 America in the depth range 300 to 500 km is km/sec and do not decrease with depth. Addi- also a zone of low Q (high attenuation) and tional evidence for the absence of a low- probable low shear-wave velocity. This low- velocity channel above 200 km is suggested by velocity and high-attenuation zone places a the comparatively slight attenuation of S-waves maximum depth constraint on plate thickness; traveling between intermediate depth earth- accordingly, the lithosphere beneath the cen- quakes (h ~ 200 km) and stations situated in tral Andes is more than 200 but less than 300 the Cordilleras. Figure 4 shows P-wave and S- km thick. wave arrivals from intermediate depth earth- The structure of the upper mantle beneath quakes recorded at stations situated above the the Pacific basin adjacent to southern Peru and Benioff zone and back of the volcanic arc in northern Chile is distinctly different from that Tonga and South America. The comparison beneath the Cordilleras. Here, a major low- with the Tonga region is appropriate, because velocity channel for shear waves lies at a depth a major low-velocity zone exists there in the of only 50 to 60 km (James, 1971) indicating mantle above 200-km depth (Oliver and Isacks, a Nazca plate thickness of 50 to 60 km at the 1967). Despite differences in instrumental fre- juncture with the South American plate. quency response of the two examples shown in Seismicity. Patterns of global seismicity Figure 4 (the Fiji seismograph system peaks at have assumed particular significance in recent a higher frequency), it is clear that the shear years when examined in the light of plate tec- waves recorded at Fiji have suffered substan- tonics. Seismic activity in island arcs appears to tially greater attenuation than those recorded at be confined largely to rather narrow Benioff

CENTRAL ANDES 290 x/a 490 a)

J 100 l\

. 200 /! ASTHENOSPHERE

300 -300 ASTHENOSPHERE

Figure 3. Schematic cross section, showing juncture ing indicates region of moderate seismicity; square between South American and Pacific plates. Plate confi- hatching indicates regions of high seismicity. Letter "M" guration for Tonga arc is shown for comparison and is denotes M-discontinuity. adapted from Oliver and others (1968). Diagonal hatch- 3330 D. E. JAMES—PLATE TECTONIC MODEL, CENTRAL ANDES

Figure 4A. Recording at Figure 4B. Recording at Fiji World Wide Standard Station station Vunikawa for earthquake LPD (La Paz, Bolivia) for earth- at 208-km depth and 5.7" epicen- quake at 246-km depth and 6.1° tral distance (after Oliver and epicentral distance. Note that the hacks, 1967). Note the almost S-waves, while attenuated, do not complete attenuation of the S- exhibit the marked attenuation of wave arrival relative to the P- the S-waves in Figure 4B.

10 SEC zones (for example, Sykes, 1966), and this observation has played a major role in developing 10 SEC some of the fundamental tenets of plate tectonics. The seismicity of the central Andes is of Comparison with the Tonga-Fiji Island somewhat different style than that of island arcs Arc. Figure 3 compares the South American and sheds light on some important differences structure as it has been interpreted above with between cordilleran and island-arc tectonics. the structure of the Tonga-Fiji island arc as in- Earthquake hypocenters in the central Andes terpreted by a group of workers at the Lamont- were projected onto a series of vertical sections Doherty Geological Observatory of Columbia oriented perpendicular to the arc structure University (Sykes, 1966; Oliver and Isacks, (Fig. 5). The locations of the five sections are 1967; Isacks and others, 1968). Behind (west shown in Figure 1 and each projection contains of) the Tonga arc, there may actually be a gap only those events within 90 km of the section. in which there is no lithospheric plate, whereas The discussion that follows is limited to activity behind (east of) the Peru-Chile trench, the above 300 km. South American plate is continuous and of con- The depth distribution of earthquakes shown siderable thickness. This important contrast in Figure 5 provides for several important ob- may explain some principal differences. In the servations. First, there is a total cessation of Tonga-Fiji region, the downgoing lithospheric seismic activity at a depth of no more than plate is bounded both top and bottom by com- about 300 km; in some regions, activity termi- paratively mobile asthenosphere. Thus, at nates at depths of little more than 100 km. The depths greater than about 100 km, virtually all greater depth limits (200 to 300 km) at which of the earthquakes occur within the downgoing activity ceases correspond with what may be the lithospheric plate (Sykes, 1966). Moreover, top of a low-velocity zone. An interesting fea- overturn may occur in the asthenosphere above ture of the earthquake distribution is that al- the descending slab, producing secondary though seismic activity is most intense in a spreading behind the arc (Karig, 1970, 1971). rather narrow belt (the Benioff zone) some ac- In contrast, earthquakes are not confined to the tivity occurs within the entire region between Benioff zone beneath South America nor is the Benioff zone and the earth's surface. This there any evidence of spreading behind the arc. observation is especially important, as it con- These observations suggest that the thick litho- trasts sharply with those in the Tonga-Fiji re- spheric plate beneath South American pre- gion (Sykes, 1966; Sykes and others, 1969; cludes secondary spreading (or convection) Mitronovas and others, 1969) and other island and that, therefore, the stresses induced by the arcs where earthquake activity is confined to downgoing slab produce seismic activity rather comparatively narrow Benioff zones. Thus seis- than convective overturn above the Benioff mic activity occurs within the rigid South zone. American lithospheric plate as well as in the upper parts of the underthrusting Pacific plate. Crustal Structure If the South American plate is between 200 and The pertinent structure of the crust beneath 300 km thick, it is plausible that stress interac- the central Andes is shown in Figures 6 and 7. tion between the South American and Pacific Figure 6 is a somewhat speculative contour map plates would produce earthquake strain release of the M-discontinuity beneath the land area, within the brittle South American plate. and Figure 7 is a vertical cross section. The TRENCH W. CORD. DEPTH DISTANCE (KMI 1 IKMI 0 1 100 200 300 400 500 600 * ****** 5* 26511 1 21 1 25* 1 1 1 11 12 1 1 1 45* 11 11 1 1 65* 1 85* 1 1 1 21 1 105* 1 1 1 11 1 SI 125* 1 111 1 1 1 145* 1 1 11 165* 185* 1 205* LATITUDE LONGITUDE POINT 1 -8.00 -71.60 , 225* POINT 2 -9.40 -70.70 1 POINT 3 -13.90 -78.70 245* POINT 4 -12.50 -79.60 10.0 265* SCALE- 285' 305* TRENCH W. CORD. DEPTH 1 DISTANCE 1 KMI t I (KMI 0 100 200 300 400 500 600 700 BOO ****** 5* 1 1 1 1 25* 1 12 1111 1 1 45* 111 1 111 1 1 65* 1 211 1 1 11 1 85* 12 1 11 1 11 1 11 1 105* 111 11 1 1 S2 125* 11 145* 165* 1 LATITUDE LONGITUDE 185* POINT 1 -9.60 -70.50 POINT 2 -10.60 -69.20 205* POINT 3 -17.80 -75.00 POINT 4 -16.80 -76.30 225* SCALE- 10.0 245* 265* TRENCH W. CORD. DEPTH DI STANCE (KM! IKMI u 100 i 20U 300 400 500 600 • *•*•• 5* 1 25* 1 1 11 331 1 45* 1 2 11 2 11 65* 1 1 1 85* 11 1 11 1 1 1 105* 1 S3 122 1 1 1 1 125* 133 3 145* 1 2 1 1 165* 1 2 1 11 185* 1 111 205* LATITUDE LONGITUDE 1111 POINT 1 -12*10 -67*10 11 2 1 1 225* POINT 2 -13.40 -65.90 POINT 3 -IV. 60 -73*00 11. 1 245* POINT 4 -18.40 -74.20 " SCALE- 10.0 1 265* 285* 1 305* 3J5* 345* * • ••*•* U 100 200 3UO 400 500 6OO 900 DEPTH DISTANCE (KM) (KMI Figure 5A. Seismicity cross sections for lines SI 10-km square block. Data from U.S. Coast and Geodetic through S3 shown on Figure 1. Hypocenters projected Survey compilation for period January 1964 to June from distances within ^ 90 km of each cross section. 1970. The numbers denote number of hypocenters within each 3332 D. E. JAMES—PLATE TECTONIC MODEL, CENTRAL ANDES following summary is based on results given of more than 70 km beneath the western cor- elsewhere (James, 1971). dillera and western altiplano is among the The most striking aspect of the Andean struc- greatest yet measured anywhere in the world. ture is crustal thickness. Over a distance of little The crustal velocities (Fig. 7) are derived more than 500 km, it varies from about 11 km from surface-wave analysis, but they are consist- (including water layer) in the Pacific basin to ent with available refraction results (Woollard, 30 km along the coast to more than 70 km 1960; Ocola and others, 1971). The crust beneath the western Cordillera and western part beneath the Cordilleras can be approximated by of the altiplano. The crust thins rapidly east- a three-layer model; the crust beneath the ward, and beneath the eastern Cordillera is only coastal areas by a two-layer one. The petrologic about 50 to 5 5 km thick. The crustal thickness implications of these models bear critically

TRENCH W. CORD. EPTM DISTANCE (KM) I (KM) 0 100 200 300 400 • 500 600 * * * * * * * 5* 1 11 1 1 25* 1 111 1 1 1211 2 11 1 45* 1 1 1 1 1 1 65* 1 l 1 1 i 21 li !' i i 85* 11 11 3 1 1 1 22 111 1 11 1 105* 1 23 212 1 1 1 1 23311531 2311 125* 113 116 434222 1 1 31324124 11 1 1*5* 11111113 S4 1 1 l 165* 1 11 12 11 11 1 185* 21 2 Mi 122 11 205* 2 1 1 1 1 1111 ?25* 1 1 211 3 1 2 111 245* 12 1 LATITUDE LONGITUDE 265* POINT 1 -20.10 -72.50 POIN* 2 -20.50 -62.60 285* POIN1 3 -22.20 -62.70 POINT 4 -21.80 -72.60 305* SCALE- 10.0 1 J25* 1 3*5* 365* 385*

TRENCH W. CORD. EPTH DI STANCE (KM) 1 (KM) 0 100 200 4010 500 600 5* 11 1 11 21 25* 1 1221 1 13 1546373113 1 1 1 45* 13 2 111 3 121111 65* 1 12 1 1 112 85* 1 i i i 3 12 11 105* 11111 1 1 1 125* 145* 165* LATITUDE LONGITUDE POINT I -24.40 -72.40 S5 185* POINT 2 -24.90 -62.10 POINT 3 -26.60 -62.20 205* POINT 4 -26.10 -72.50 225* SCALE* 10.0 245* 265*

1OO 200 300 400 500 6OO 700 «0 «ao DISTANCE IK? IKMt Figure 5B. Vertical sections for lines S4 and S5. Explanation same as for Figure 5A. GEOPHYSICAL EVIDENCE 3333

Figure 6. Contour map of depth (in kilometers) to the M-discontinuity beneath the central Andes (from James, 1971). upon the evolution of the Andean arc, and it is must be characteristic of the lower crust be- appropriate to consider the crustal velocities in cause they exhibit the appropriate laboratory the context of the information and constraints velocities and densities. Ringwood and Green they provide upon rock types. Discussion be- (1966, p. 612) challenge this notion and assert low is confined to P-wave velocities, as they are that on the basis of chemical equilibrium well known and more easily related to rock studies, "the inference that the lower crust is types than shear-wave velocities. Only shear- composed of gabbroic or basaltic rocks is al- wave velocities, however, have been obtained most certainly wrong...." Their experimental explicitly. studies demonstrate that mafic rocks, when sub- The rocks in the lower parts of the crust, with jected to pressures and temperatures in the P-wave velocities consistently near 6.6 km/sec, range suggested for the lower parts of the crust, could be of gabbroic composition beneath the are metamorphosed to the eclogite facies or ocean basin, but may well be of intermediate garnet-clinopyroxene-granulite subfacies if the composition beneath the land area. It is a com- lower crust is anhydrous. Since these rocks mon misconception that gabbroic compositions should have substantially higher velocities and 3334 D. E. JAMES—PLATE TECTONIC MODEL, CENTRAL ANDES

densities than have been inferred for the lower tal rocks may be comagmatic with the granitic crust, they conclude that an anhydrous lower or intermediate rocks exposed at the surface; crust cannot be composed of mafic rocks but beneath the altiplano and the eastern cordillera, instead must consist of silicic to intermediate the 6.0 km/sec rocks could be slightly meta- high-grade metamorphic rocks. On the other morphosed sedimentary sequences, although hand, if the lower crust is hydrous and low igneous rocks may be mixed with these meta- temperature (T < 700° C), the rocks could be morphic assemblages. amphibolites with densities and seismic veloci- Within the uppermost crust beneath the alti- ties consistent with those observed. The likeli- plano and the western part of the eastern cor- hood of a hot lower crust and the absence of dillera, there is approximately 10 km of rock large volumes of exposed rocks of mafic compo- with a velocity of about 5.0 km/sec. In the sition give little encouragement to the notion of altiplano, the presence of this layer is confirmed a mafic lower crust, and it seems plausible that by explosion refraction studies (Ocola and oth- the lower crust is both anhydrous and composi- ers, 1971). The only rocks with measured velo- tionally similar to the intermediate and silicic cities consistently this low at depths greater volcanic and plutonic rocks which form the up- than a few kilometers are sedimentary and vol- per levels of the crust. canic (Press, 1966). This suggests, then, that at Rocks at intermediate to shallow depth with least the upper 10 km of the crust in this region P-wave velocity around 6.0 km/sec may be ei- consists of sedimentary rocks with possible as- ther granitic rocks or metamorphosed sedimen- sociated volcanic sequences. There is strong tary rocks. Beneath the western cordillera and evidence for a layer about 5 km thick with P- its western flanks, the intermediate-depth crus- wave velocity near 5.0 to 5.2 km/sec in the

0. Id Q 100 —

150 Figure 7. Cross section of crustal and upper mantle ing structure from James (1971). a denotes P-wave structure along line A-A' of Figure 1. Crustal structure velocity; ft denotes S-wave velocity. beneath trench from Fisher and Raitt (1962) and remain- GEOLOGY OF THE CENTRAL ANDES 3335 uppermost crust beneath the western cordil- The oldest rocks in the central Andes are of lera. This layer could consist of either volcanic Paleozoic age, mostly confined to a broad belt or sedimentary rocks; geological evidence sug- in the eastern cordillera. The only known pre- gests that it may be a combination of both. No Mesozoic rocks in the Peruvian western cordil- seismic velocities less than 6.0 km/sec have lera are some scattered orthogneisses and been observed in the coastal areas. paragneisses of dioritic to granitic composition In the zone situated between the trench and in a belt along the southern coast (Jenks, 1956). the shoreline, it is readily apparent from studies These rocks were metamorphosed more than by Fisher and Raitt (1962) and Scholl and oth- 400 m.y. ago (Aldrich and others, 1958). ers (1970) that this region is not a site of Other probable Precambrian or Paleozoic "eugeosynclinal" deposition of terrigenous metamorphic rocks have been reported in sediments. Indeed, basement rocks with velo- northern Chile, but the ages are not well estab- cities of 5 to 6 km/sec are nearly everywhere lished. found at depths of less than 1 km beneath a thin The Paleozoic sequences of the eastern cor- sedimentary cover. At the bottom of the trench dillera of the central Andes have been best off Peru, sediment accumulations appear to be studied in Bolivia (Ahlfeld and Branisa, 1960; less than 1 km (Fisher and Raitt, 1962) and off Ahlfeld, 1956) and in the Lake Titicaca region northern Chile may not be present at all (Scholl of southern Peru (Newell, 1949). Only meager and others, 1970, profile D-8, and Figs. 6 and information exists on the Paleozoic rocks of the 7). The seismic velocities in the basement rocks Cordillera Oriental of southern Peru (Stein- underlying the thin sedimentary cover are ap- mann, 1930; Newell and others, 1953; Hos- propriate for granitic or metamorphic rocks. mer, 1959). The following descriptions of Paleozoic stratigraphy pertain mostly to Bolivia GEOLOGY OF THE CENTRAL ANDES and unless otherwise referenced are based It is probably always somewhat hazardous to upon the work of Ahlfeld (1956, 1970) or Ahl- attempt to integrate a large amount of geologi- feld and Branisa (1960). cal information, the work of many people with The rocks of the eastern cordillera of Bolivia varying biases, into a coherent pattern suitable and southern Peru consist predominantly of for analysis in terms of broad and unquestiona- Ordovician, Silurian, and Devonian deposits bly over-simplified concepts of tectonic evolu- belonging to a single clastic facies of dark-gray tion. It should be stated at the outset that no shales and sandstones, and their low-grade attempt has been made to be complete, and metamorphic equivalents (Newell, 1949). emphasis is given only to those aspects of the These "miogeosynclinal" rocks are here inter- geology that seem to bear most critically on the preted as continental shelf deposits. Wide- erogenic evolution of the Andes. Moreover, spread deposition began in the Ordovician and much of the geology of this part of South continued into the Early Permian, and the ag- America has never been investigated; nearly gregate thickness of Paleozoic deposits is every aspect of the geology, from the details of thought to exceed 10 km. The only tectonic the Paleozoic stratigraphy to the chemistry of activity during this period appears to have been the recent volcanic rocks, is sadly riddled with gentle epeirogenic movements. unknowns. In spite of these limitations (al- Rocks of Ordovician age may aggregate 3 to though ignorance seldom hinders imagina- 5 km and consist largely of sandy shales, quart- tion), it is my intent here to place the major zitic sandstones, and occasional black shales geological features known into a workable (Ahlfeld, 1956). Silurian strata are poorly framework of plate tectonics. The simplicity of developed and consist mostly of nonfossilifer- the discussion that follows is not meant to deny ous sandstones. Devonian deposits 5 km or the complexity or even contradictions more more in thickness lie conformably on the older readily apparent on finer scale. Paleozoic sequences. The Devonian in south- A geologic map is shown in Figure 8. The ern Peru and Bolivia is an extremely uniform stratigraphic and tectonic development of the succession of dark shales, sandy shales, and central Andes is described below in more or quartzitic sandstones. Limestones are almost less chronological outline. The following sec- lacking, as in the Ordovician and Silurian strata. tion is devoted to a consideration of the igneous In the eastern Andes, the Devonian is rich in activity associated with the orogenic belt and to plant remains and bituminuous shales, suggest- the implications of magma source and genesis. ing near-shore environment. Carboniferous D. E. JAMES—PLATE TECTONIC MODEL, CENTRAL ANDES

BRAZIL

LEGEND

[ J CENOZOIC CONTINENTAL ::Sv3 CENOZOIC VOLCANIC

JURASSIC VOIC AN 1C

MESOZOIC SEDIMENTARY AND VOLCANIC

MESOZOIC- CENOZOIC INTRUSIVE

SU!ANDEAN CONTINENTAL RGENTI NA

80 78 76 74 64 Figure 8. Geologic map showing distribution of Bolivia, Bolivian Geol. Survey, 1968; und from Geologic principal rock units of the central Andes (adapted from Map of Peru, Peruvian Geol. Soc., 1956). Geologic Map of South America; Geologic Map of marine deposits are rare or absent; in Early Per- or Late Triassic K/Ar ages (Evernden and Kis- mian, the last of the shelf deposits were laid tler, 1970). Possibly the low-grade meta- down in Peru and Bolivia. Permian rocks are morphic phase that affected the Paleozoic strata predominantly limestones, carbonaceous of the Cordillera Real was contemporaneous shales, and arkosic and continental sandstones, with this plutonism. representing a littoral facies of alternating ma- Formation of the western cordilleran vol- rine and continental deposits (Newell, 1949). canic arc began in Late Triassic or Early Jurassic No rocks of mid-Permian to mid-Cretaceous time near the present coast of northern Chile age are reported in the eastern cordillera. Mi- and southern Peru. The rocks, consisting nor volcanism and hypabyssal intrusion oc- largely of pillow lavas, tuffs, breccias, and ag- curred both in Peru and Bolivia during this glomerates of basaltic to andesitic composition hiatus in deposition. Some small granitic bath- (Hosmer, 1959), probably represent subma- oliths in the Cordillera Real give Early Jurassic rine stages of development of an island arc GEOLOGY OF THE CENTRAL ANDES 3337 which formed immediately west of the Paleo- tal or volcanic in origin. The continental zoic continental shelf. These rocks occur only in deposits are of molasse type and derived from the coastal areas as shown in Figure 8. Along both western and eastern Cordilleras. The the southern Peruvian coast, the volcanic rocks Cretaceous rocks are predominantly red beds may attain a thickness of 4 km (Hosmer, 1959). with abundant conglomerates in both middle Middle and Upper Jurassic marine limestones, and Upper Cretaceous sections. The thickness shales, and sandstones occur abundantly in a of Cretaceous deposits varies greatly from place zone within and west of the present cordilleran to place, but northeast of Lake Titicaca, the crest. These deposits indicate that a shallow sea section may be 4 km thick (Newell, 1949); existed between the volcanic arc and the east- southeast of Lake Poopo, along the eastern ern ranges. Volcanism continued in Middle and edge of the altiplano, the Cretaceous section Upper Jurassic times along the coast of central may total more than 7 km (Kriz and others, Peru near Lima (Hosmer, 1959) and may have 1965). extended farther south at times. The Cenozoic rocks of the central Andes are The oldest plutonic rocks of the Andean almost entirely volcanic and continental. The batholith are at least as old as Lower Jurassic few marine deposits are confined to coastal (Farrar and others, 1970) and may be contem- areas. Cenozoic volcanic rocks, mostly Miocene poraneous with Triassic/Jurassic volcanism. or younger, have been most extensively studied The Lower Jurassic granitic rocks occur along in the western cordillera of northern Chile. the coastline of northern Chile (lat ~ 28° S.) Widespread Tertiary volcanism began here less than 100 km from the trench. The plutonic with pyroclastic eruptions. The areal extent of rocks in this area exhibit progressively younger the pyroclastics is variously estimated between ages eastward, and the small batholiths just west 110,000 km2 (Petersen, 1958) and 150,000 of the cordilleran crest are of Eocene age. The km2 (Zeil and Pichler, 1967) with an average age of the Andean batholith of central and thickness of nearly .5 km. Numerous K/Ar age southern Peru is Late Cretaceous or Early Terti- dates are published and most range from at ary (Giletti and Day, 1968), and here, too, the least 10 m.y. (upper Miocene) to 4.25 m.y. older rocks are to the west. The measurements (late Pliocene) (Rutland and Guest, 1965; cited are all K-Ar mica ages and are, therefore, Clark and others, 1967). The pyroclastics range measurements only of the final stage of cooling from andesite to rhyolite and are predomi- (or most recent metamorphism) of the rock and nantly dacites or rhyodacites (Guest, 1969). not necessarily of the times of emplacement. They were apparently erupted from fissures Volcanic rocks, predominantly andesites which occupied the approximate crest of the with some dacites and rhyolites, of Late Jurassic modern Andes, and it has been proposed that or Early Cretaceous age are found extensively the period of recent uplift in the Andes began along coastal central Peru (Hosmer, 1959). In- with the volcanic eruptions (Rutland and terlayered with them are continental and shal- Guest, 1965; Hollingworth and Rutland, low water marine deposits. In northern Chile, 1968). Quaternary eruptions in northern Chile volcanic and intercalated sedimentary rocks of produced the "andesitic" strato-volcanoes that Jurassic and Cretaceous age are as much as 15 now dominate the crest of the Andes (Pichler km thick (Zeil, 1964), but the rock types are and Zeil, 1969). The predominant rock type of poorly known; in general, they appear to be these flows is quartz-bearing latite-andesite. largely keratophyres and andesites (Zeil, The major Tertiary volcanism in southern 1964), probably laid down in submarine condi- Peru is contemporaneous with that in northern tions similar to those inferred for the equivalent Chile. Activity began in late Miocene time, in- rocks in southern Peru. Generally, volcanic creased in Pliocene/Pleistocene time, and con- rocks of Cretaceous age are comparatively rare tinued with waning intensity to historical time. (Hosmer, 1959), although Late Cretaceous or The extrusives have been best described for the early Tertiary rocks are especially widespread region around Arequipa, Peru (Jenks, 1948; in Peru south of Arequipa and in northern Jenks and Goldich, 1956), where andesite is by Chile. far the predominant rock type. Rhyolitic ignim- Except for a few thin marine limestone beds brites constitute less than 1 percent of the total of mid-Cretaceous age, Cretaceous and volume of the volcanic rocks. younger deposits within the altiplano and flank- Cenozoic volcanism occurs in the altiplano ing areas of the eastern cordillera are continen- and as far east as the western flank of the eastern 3338 D. E. JAMES—PLATE TECTONIC MODEL, CENTRAL ANDES cordillera. The volcanic rocks are variously and kinds of igneous associations are no more than vaguely described as dacitic or andesitic flows expressions of different structural levels in oro- and tuffs. The volcanic rocks are intercalated in genic belts. An essential tenet of these theories thick sections of continental debris. Molasse is that most of the plutonic and volcanic rocks deposition that began in the altiplano in Creta- of erogenic belts derive ultimately from a zone ceous time continued throughout Tertiary and near the top of the descending plate. The vol- Quaternary times. Near Lake Titicaca, a lower cano-plutonic complexes of the Andes exhibit Tertiary section about 7 km thick is followed by many examples of the close relations in time 4 km of younger Tertiary and Quaternary and space between volcanic rocks and small plu- rocks, a large part of which is volcanic (Newell, tons, and they support in a general way the 1949). The altiplano in Bolivia may be under- theory of consanguineous intrusive and extru- lain by even thicker Tertiary deposits. Evern- sive rocks. Without pursuing the argument fur- den and others (1966) estimate that Tertiary ther, I shall assume as a basis for discussion that continental deposits, lava flows, and pyroclastic the plutonic rocks and volcanic rocks are debris may exceed 15 km in the Bolivian alti- derived from magmas initially generated near plano; in the west-central Bolivian altiplano, the Benioff zone. Within this framework of as- Meyer and Murillo (1961) gave an average sumptions, then, there are a number of observa- thickness of 10 km for an incomplete Tertiary tions which seem especially to bear upon plate section. tectonics. This discussion and those that follow Miocene compression produced east- are not meant to deny the possibility that crustal directed folding and faulting for the first time differentiation and anatexis play a significant in the continental section of the altiplano and role in determining the nature and distribution eastern cordillera. This compression affected of crustal igneous rocks. the entire eastern ranges of the central Andes (Jenks, 1956) and was followed by much more Eastward Migration of Igneous Activity intense compression in Pliocene time (Ahlfeld Igneous activity in the central Andes has mi- and Branisa, I960), possibly coinciding with grated progressively eastward (or northeast- the Pliocene-Pleistocene volcanism. Continent- ward) with time, in a direction normal to the ward thrust faulting occurs throughout the east- trench. In a very general way, this migration is ern cordillera (Ahlfeld and Branisa, 1960). apparent in that the Andean batholith lies to the Newell (1949) believes that much of the uplift west of the present volcanic chain. But eastward of the eastern cordillera near Lake Titicaca has younging has been observed even in detail been accomplished by thrusting. within the large composite batholiths (Hosmer, 1959). DERIVATION OF THE IGNEOUS K-Ar ages yield a more concrete measure of ROCKS the rate of eastward migration (if we assume Theories of the genesis of arc-related mag- that these ages correspond roughly to periods mas have undergone considerable change in of emplacement and crystallization). The data the light plate tectonics. Recently it has been of Farrar and others (1970) for an area in proposed that andesitic melts derive ultimately northern Chile indicate that the oldest rocks are either from the descending lithospheric plates Lower Jurassic in age and crop out very near the themselves, or, by release of water from these coast. Since Lower Jurassic time, igneous ac- plates, by partial melting of the mantle above tivity has migrated eastward at a rate of about the plate {see, for example, Dickinson, 1970). 1 km/m.y. In central and southern Peru, results These propositions are now hotly debated, but presented by Giletti and Day (1968) yield a evidence is mounting that the melts are derived similar rate of migration. The oldest rocks in from near the Benioff zone (Dickinson, 1970). Peru are Late Cretaceous or early Tertiary, yet It is additionally presumed that the large com- these rocks and rocks of similar age in northern posite batholiths of erogenic belts are emplaced Chile are approximately equidistant from the in the upper crust beneath a cover of their own trench, suggesting that the plutonic equivalents volcanic ejecta (Hamilton and Myers, 1967; of the Lower Jurassic volcanic rocks of southern Hamilton, 1969b; Dickinson, 1970). The em- Peru lie beneath the continental shelf. Here, placement of plutonic rocks and the extrusion uppermost crustal velocities are appropriate for of volcanic rocks are assumed to be roughly granitic rocks. contemporaneous events and the parent mag- Migration of activity away from the trench mas cogenetic. By this hypothesis, the two has been observed in other island arcs (Kawano DERIVATION OF THE IGNEOUS ROCKS 3339 and Ueda, 1967), and it is perhaps appropriate the thickness or composition of the crust to speculate as to factors that might produce the through which the magma penetrated, lending migration: further support to the assertion that the magma 1. Progressive depression of isotherms near is neither derived from nor significantly con- the upper margin of the descending plate taminated by the crust. would cause the zone in which melting or water The crust beneath the Andean volcanic chain release occurs to migrate down the subduction is in excess of 70 km, and it is reasonable to zone. This isotherm depression might be pro- suppose that if crustal origin or contamination duced by continued underthrusting of a com- of andesitic magma is important, the Andean paratively cold plate into lower and hotter parts rocks will exhibit quite a different K-h relation of the mantle (see, for example, Oxburgh and than that found for island arcs. Some 60 ana- Turcotte, 1970). lyses of modern volcanic rocks in northern 2. If melt is derived from the mantle above Chile were used to obtain average potash con- the downgoing plate, the migration may repre- tent as a function of SiO2 content (Pichler and sent the time constant necessary to form suffi- Zeil, 1969; Zeil and Pichler, 1967; Guest, ciently large batches of magma to enable 1969; Jenks and Goldich, 1956). The depth to upward movement; in the zone where large the Benioff zone is taken beneath the eruptive quantities of water are released from the plate, center and not the sites of sample collection. partial melting of the mantle may occur more The depth to the Benioff zone beneath the rapidly than at greater depths along the subduc- eruptive centers is about 175 km. Figure 9 tion zone where less water is released and shows the northern Chile K-h values for three magma generation proceeds more slowly. Pre- SiC>2 contents. The lines on this figure repre- sumably, activity ceases when the adjacent man- sent approximate best fits for the K-h diagrams tle has been bled of its partial melt fraction. 3. The position of the subduction zone may change with time. In this event, the trench would have to move landward, implying stop- ing and underthrusting of the continental margin. The close approach (~ 70 km) of the Peru-Chile trench to the oldest plutonic rocks in northern Chile does suggest possible eastward migration of the trench. 4. The dip of the subduction zone changes, decreasing with time. Present data do not permit a clear choice between these various speculations. Some potash relations presented in the following section seem to indicate that the dip of the Benioffzone may not have changed much during Cenozoic time, but the data are not conclusive. Of the speculations given, perhaps the most plausible mechanism for eastward migration is progressive depression of the isotherms. K-h Relations 0 100 200 300 In a series of papers, Dickinson and Hather- h (km) ton have shown that the potash content of is- Figure 9. Averaged results of 60 analyses of northern land-arc andesites bears a simple functional Chile volcanic rocks plotted on a K-h diagram (Dickin- relation to the depth to the Benioff zone son, 1970, p. 832). The percentage of K2O for a given beneath the area in which they were erupted percentage SiO2 is obtained from a variation diagram for the volcanic suite. Dashed lines labeled K55, K«,, and K (Dickinson and Hatherton, 1967; Dickinson, 65 are approximate fits to Dickinson's (1970, Fig. 3) com- 1968, 1970). This correlation, termed K-h by bined K-h plots for 55,60, and 65 percent of SiO2. Depth Dickinson, suggests a genetic link between the to Benioff zone, h, is taken beneath probable source of inclined seismic zone and magma generation. eruption, not beneath sample locality. The single addi- tional value given for K55 (h = 80 km) is taken from two Dickinson (1970) states further that the K-h analyses of the Late Cretaceous/early Tertiary Acari plu- correlation exhibits little or no dependence on ton (see text). 3340 D. E. JAMES—PLATE TECTONIC MODEL, CENTRAL ANDES presented by Dickinson (1970). The Chilean cm/yr (Le Pichon, 1968) for 60 m.y., the volu- analyses fit well with and are easily within the metric requirements could be satisfied if 1 to 2 range of scatter of the empirical lines obtained km of the slab was melted. Possibly magma is by Dickinson. This simple observation does not generated both by melting in the slab and by prove, of course, that the magmas originated partial melting of the underlying mantle. near the Benioff zone, but it does pose an un- happy coincidence for those who would derive MODEL FOR THE EVOLUTION OF THE CENTRAL ANDES the rocks anatectically. Analyses are rare of unaltered older plutonic In this section, I propose a model for the rocks that could provide evidence of earlier evolution of the central Andes. The presenta- positions of the Benioff zone. Some recent ana- tion of a single model is not meant to exclude lyses given by Dunin-Borkowski (1970) of the multiple alternatives that exist. Rather, it is rocks of the Late Cretaceous or early Tertiary my intent here to present what seems, on the Acari pluton of southern Peru (~ 15.2° S., basis of the limited observations outlined 74.7° W.) include potash measurements in above, the most plausible of many possible sce- agreement with those predicted on the basis of narios. Against this backdrop of disclaimers, depth to the present Benioff zone (Fig. 9), sug- then, a schematic representation of events from gesting that the position and dip of the Benioff Paleozoic to Quaternary times is presented in zone have not changed greatly since the end of Figure 10. The evolution of the central Andean the Cretaceous. These few data are not defini- orogen is here envisioned to have resulted by tive but do point to a comparatively stable interaction between the South American and Benioff zone, at least during the Cenozoic. Nazca plates which has continued essentially unchanged from Mesozoic to the present time. Volumetric Considerations The model pertains principally to the central Some insight into the origin of the igneous Andes situated between about 14° and 22° S. rocks that make up the crust beneath the west- lat; although other areas, particularly in northern cordillera can be gained through the simple ern Chile, have provided missing elements to volume calculations. If we suppose that most of the synthesis. The cross section itself is roughly the western cordilleran crust consists of igneous along line A-A' of Figure 1. Geographic ter- rocks or their sedimentary derivatives, we can minology relates to present day. easily estimate the total volume of melt that was In late Paleozoic time, the ancestral continen- needed to produce the crust. For simplicity, I tal shelf and slope of western South America have here restricted the calculations to cross- were blanketed by undeformed Paleozoic sectional areas along the line A-A' (Fig. 1). The deposits totaling 10 to 15 km in thickness. To computations pertain to the vertical section the west of this shelf area lay a region of sialic shown in Figure 7 and include that part of the crust. Relicts of this crust are now exposed as crust to which igneous material has been added metamorphic rocks along the coast of southern since latest Mesozoic. Within this cross section, Peru and northern Chile. The origin of this the area of added crust is ~ 4000 to 8000 sialic crust remains something of an enigma. km2, depending upon whether the initial crust The rocks may have occupied their present po- was continental (h ~ 30 km) or oceanic (h ~ sition during the Paleozoic time or they may be 6 km, exclusive of water layer). The cross-sec- fragments swept into South America from the tional area of the underlying mantle between Pacific plate, probably near the beginning of the M-discontinuity and the top of the descend- subduction. The apparent absence of suture- ing plate is ~ 22,000 km2. Thus, if the magma zone rock assemblages seems to argue against which produced the Andean crust was derived the latter possibility, but evidence of suture entirely by partial melting of a noncirculating zones is commonly not obvious, and there are mantle above the Benioff zone, 18 to 36 per- not yet adequate data in the central Andes to cent partial melting is required. This would re- distinguish between the alternatives. The pic- quire an extraordinary amount of partial ture is further complicated by post-Paleozoic melting in the mantle and does not warmly sup- sedimentary deposition and volcanism which port the thesis that the mantle is the sole source have obliterated much evidence of the crust in of melt. If the magma is generated within the the region situated between the coastal meta- lithospheric plate, the source problem is less morphic rocks and the area of Paleozoic shelf severe. Assuming underthrusting of about 6 deposition. 0 I ST. (KM)

PRE-PALEOZOIC SEDIMENTARY

GNEISS CALC-ALKALINE ROCKS

Figure 10. Schematic sequence of cross sections isotherm" and indicates the presumed termperature (appropriate for line A-A' of Fig. 1) illustrating a at which melting or water release occurs in the li- model for the evolution of the central Andes. Letter thospheric slab. Arrows denote rising magma. "M" marks M-discontinuity. "M.I." denotes "magic 3342 D. E. JAMES—PLATE TECTONIC MODEL, CENTRAL ANDES

Marine deposition in the eastern cordillera pression were jammed up to form narrow high ended in mid-Permian time, and underthrust- mountain belts. The altiplano during the Ceno- ing of the Pacific plate began no later than Late zoic received some 15 km of molasse derived Triassic or Early Jurassic. The start of under- from both eastern and western mountain thrusting may have coincided with the begin- ranges. During late Quaternary time, the wan- ning of spreading on the East Pacific rise, but ing of igneous activity resulted in relaxation of definitely precedes the breakup of South the compressive stresses and graben structures America from Africa some 135 to 140 m.y. ago formed within the altiplano. (W. Pitman, 1971, personal commun.). During It is appropriate and instructive to examine Late Triassic-Early Jurassic time, an incipient the evolution of the crust in some detail. In the volcanic arc developed in the ocean west of the interpretation portrayed in Figure 10, virtually Paleozoic continental shelf (Fig. 10) with the the whole of the crust beneath the western cor- eruption of submarine basaltic and andesitic dillera consists of deep-seated equivalents of lavas near the coast of northern Chile and prob- the andesitic to silicic volcanogenic sequences ably along the present continental shelf area of and granitic batholiths that are exposed at the southern Peru. Approximately contemporane- surface. Two conclusions are implicit in this ous minor granitic plutonism occurred in the interpretation: (1) the lower crust beneath the Cordillera Real of the eastern cordillera. Shal- western cordillera is compositionally equiva- low-water marine deposition through mid- lent to the upper crust, and (2) the crustal sec- Jurassic time in the western cordilleran zone tion consists predominantly of rocks derived indicates that the crust beneath that area was from near the subduction zone. The first con- still comparatively thin. clusion derives from a consideration of the crus- A major phase of orogeny occurred during tal velocities and appropriate experimental Late Cretaceous or early Tertiary time. The work. Results of Ringwood and Green (1966) zone of this activity is clearly displaced eastward and Green (1970) have been summarized at from that of Triassic-Jurassic time (Fig. 10). It length above, and it remains here only to reiter- was during the Late Cretaceous-early Tertiary ate the important conclusion of their work. episode that the major part of the Andean bath- That is, that under the P-T conditions appropri- olith was emplaced. The crust in the zone of ate to the lower crust, the deep crustal velocit- orogeny was invaded by considerable volumes ies of the central Andes are representative of of intermediate to granitic melt; the resultant intermediate, not mafic, compositions. It is on dilatation of the western cordillera produced this basis that I am encouraged to conjecture compressive stresses which were transmitted that the lower and upper parts of the crust are laterally to the crust to the east, causing the compositionally similar. rocks of the eastern cordillera to be folded and The second conclusion, that most of the west- further uplifted. Molasse deposits in enormous ern cordilleran rocks are derived from the sub- quantities were dumped into the intervening duction zone or mantle beneath the crust, is altiplano basin, some 7 km in Cretaceous time partly conjecture but is supported by evidence alone (Newell, 1949). The absence of compa- of large volumetric additions to the crust. Spe- rable deposits to the west of the western cordill- cifically, well-documented tensional deforma- era suggests that even during the Cretaceous, tion of the western cordillera (Katz, 1970; desert conditions may have prevailed on the Kausel and Lomnitz, 1968) indicates that crus- western flank of the Andean chain. tal dilatation has accompanied volcanism and The volcanism and emplacement of granitic magmatic intrusion. In the eastern cordillera, plutons that began in Miocene time record the on the other hand, crustal shortening by folding beginning of the modern phase of igneous ac- and thrust faulting has probably been the major tivity in the Andes. This activity appears to have factor in crustal thickening. In the altiplano, been cyclical, and the Miocene stage was fol- crustal thickening may have been achieved lowed by the much stronger Pliocene-Pleisto- both by molasse deposition and crustal shorten- cene orogenic phase. Emplacement of magma ing, as well as by igneous intrusion in the west- into the crust beneath the present Andean crest ern altiplano. The crustal shortening in the produced great lateral compressive stresses that eastern ranges and altiplano is here interpreted caused intense folding and thrust faulting in the to have resulted from compression produced by altiplano and eastern cordillera. The eastern crustal dilatation of the volcanic arc, not by ranges during the Pliocene-Pleistocene com- compressive interaction between plates. DISCUSSION 3343

It is worth reviewing the implications of bears a clear relation to the Andean orogen. By deriving the crustal rocks of the western cordill- this revised view of geosynclines, the so-called era from the subduction zone. If, as shown ear- eugeosynclinal sequences within the western lier, the melt is derived by partial melting of the Cordillera have been derived from the volcanic mantle between the top of the underthrust plate arc itself as the arc has evolved. and the base of the crust, some 18 to 36 percent In the normal cordilleran arc setting, there of the mantle material must be melted. Partial are at least three distinct sedimentary assem- melting on this scale has been proposed for blages (Dickinson, 1971): (1) the complex me- areas such as Hawaii where magmas of lange of trench turbidites and ophiolites which predominantly tholeiitic composition are are ground under at the trench; (2) marine erupted; however, even 20 percent partial sedimentary and graywacke deposits that ac- melting of mantle rocks to produce intermedi- cumulate in sedimentary troughs situated be- ate to silicic melt seems improbable. Alterna- tween arc and trench; and (3) the volcaniclastic tively melting of 1 or 2 km of the downgoing wedge deposited in foredeeps behind the arc, plate could produce the necessary volumes of away from the trench. material. Possibly, crustal rocks are derived by Of these three types of "eugeosynclinal" as- a combination melting of downgoing plate and semblages, only the foredeep deposits are of overlying mantle. It may be also that some the great consequence in the Andes. This is source material derives from erosion of the probably due in part to the fact that the western South American plate. If the leading edge of slopes of the Andean volcanic chain are desert, the continental plate is progressively crumpled so that erosion proceeds slowly. These desert and underthrust at the trench, the crustal rocks conditions have been cited to explain the ab- dragged down could be melted to provide sence of turbidite deposits in the trench off magma for the continental crust. There are ob- northern Chile (Scholl and others, 1970). The vious problems, however, in transporting light remarkably high correlation between rainfall sialic material to great depths in the mantle. and thickness of turbidites in the trench appears too good to be coincidental and provides a sim- DISCUSSION ple explanation for an otherwise puzzling ob- The results and interpretations that have servation. Similarly, there appears to be little been presented in this paper leave many ques- sedimentation between trench and coast. No tions unresolved and pose a number of new serpentinite or ophiolite belt has been observed ones. Traditionally, the Andean orogenic belt in the central Andes. If ophiolites and related has been interpreted in terms of classical con- melange assemblages represent underthrust cepts of geosynclines. Major revision of ideas of oceanic crust and trench deposits, observation eugeosynclinal-miogeosynclinal evolution of of these rocks is presumably not likely until cordilleran belts (Dickinson, 1971) leads by subduction ceases and the underthrust assem- necessity to a re-evaluation of classical concepts. blages rebound isostatically. It is for this reason that I have avoided use of Because desert conditions preclude much geosynclinal terminology in this paper. It is use- erosion and deposition on the western flanks of ful, however, to examine briefly what insights the mountains, the ranges are asymmetrically the Andes may provide as to the nature of geo- eroded from their wet eastern flanks. The depo- synclines. sition of detritus derived from both western Classical concepts of geosynclines held that and eastern cordilleras has resulted in an abnor- elongate basins of deep subsidence were neces- mally thick "foredeep" clastic wedge in the in- sary precursors to later mountain belts which tervening altiplano trough. were presumed to have formed by deformation The "miogeosynclinal" deposits are here in- and anatectic melting of the thick eugeosyncli- terpreted to be the Paleozoic marine continen- nal sedimentary section. Although the Andean tal shelf deposits that accumulated during a orogenic belt has been interpreted in geosyncli- long period of quiescence at the western mar- nal terms, it has never fit readily into the mold, gin of the Paleozoic South American continent. for it is clear that no depositional eugeosyncli- A modern analogue of these deposits are the nal basin has ever existed. Dickinson (1971) thick sequences that lie on the continental shelf has reconstructed geosynclinal theories within of the eastern United States (Drake, 1966). the framework of plate tectonics and in so do- Finally, it is important to emphasize the east- ing has recast the concepts into a form that ward migration of igneous activity. If this mi- 3344 D. E. JAMES—PLATE TECTONIC MODEL, CENTRAL ANDES gration is characteristic of island arcs, polarities Dewey, J. F., and Bird, J. M., 1970, Mountain belts of dormant subduction zones can be deduced and the new global tectonics: Jour. Geophys. from the direction in which igneous rock ages Research, v. 75, p. 2625-2647. decrease. Moreover, it is possible that even the Dickinson, W. R., 1968, Circum-Pacific andesite rate of underthrusting can be determined from types: Jour. Geophys. Research, v. 73, p. 2261- 2269. the rate at which ages migrate. The younging of 1970, Relations of andesites, granites, and igneous rocks provides, together with such derivative sandstones to arc-trench tectonics: other empirical observations as K-h relations, Rev. Geophysics and Space Physics, v. 8, no. 4, yet another means of untangling the tectonic p. 813-860. histories of older orogenic belts. 1971, Plate tectonic model of geosynclines: Earth and Planetary Sci. Letters, v. 10, p. 165- ACKNOWLEDGMENTS 174. The interpretations and conclusions pre- Dickinson, W. R., and Hatherton, Trevor, 1968, sented in this paper are entirely my responsibil- Andesitic volcanism and seismicity around the Pacific: Science, v. 157, p. 801-803. ity, but many of the essential ideas and concepts Drake, C. L., 1966, Recent investigations on the would not have been possible without ex- continental margin of the eastern United States, tended discussions with many colleagues. I in Continental margins and island arcs: Canada especially thank S. R. Hart for innumerable Geol. Survey Paper 66-15, p. 33-47. stimulating discussions and suggestions and H. Dunin-Borkowski, E., 1970, Der Acari-Pluton L. James both for enthusiastic interest and dis- (Peru) als Beispiel der Differentiation des cussion and for pointing out that igneous ac- tonalitischen Magmas: Geol. Rundschau, v. 59, tivity commonly youngs with distance from the p. 1141-1180. trench. Special appreciation is due R. P. Morri- Evernden, J., Kriz, S., and Cherroni, C., 1966, Cor- relacion de formaciones terciarias de la cuenca son, who not only expressed active interest but altiplanica a base de edades absolutas deter- generously supplied preprints of parts of his minadas por el metodo potasio-argon: Hoja In- forthcoming book on South American geology. form. I, Serv. Geol. Bolivia. Finally, I wish to thank L. M. Dorman, O. B. Evernden, J. F., and Kistler, R. W., 1970, James, W. Hamilton, and W. R. Dickinson for Chronology of emplacement of Mesozoic bath- critically reading the manuscript and providing olithic complexes in California and western several important suggestions. Nevada: U.S. Geol. Survey Prof. Paper 623, 42 p. Farrar, E., Clark, A. H., Haynes, S. J., Quirt, G. S., Conn, H., Zentilli, M., 1970, K-Ar evidence REFERENCES CITED for the post-Paleozoic migration of granitic intrusion foci in the Andes of northern Chile: Ahlfeld, F., 1956, Bolivia, in Jenks, W. F., ed., Earth and Planetary Sci. 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