Early Tertiary "Incaic" tectonism, uplift, and volcanic activity, Andes of central Peru

DONALD C. NOBLE Department of Geology and Geological Engineering, Michigan Technological University, Houghton, Michigan 49931

EDWIN H. McKEE U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 FRANÇOIS MÉGARD Institut Français d'Etudes Andines, Contralmirante Montero 141, Lima 18, Perú

ABSTRACT

Potassium-argon age determinations on volcanic strata that overlie an extensive postorogenic erosion surface in the Western Cordillera show that the "Incaic pulse" of compressive deforma-tion as well as subsequent uplift and erosion of the Andes of central Peru had taken place before 40 to 41 m.y. ago. Intense volcanic ac-tivity, which began before the end of uplift and erosion, continued for about 6 m.y. into early Oligocene time. The timing of these events suggests that uplift and volcanic activity, and perhaps also the preceding tectonic pulse, may have been initiated by the same global disturbance that produced the abrupt change in direction of movement of the Pacific lithospheric plate reflected by the bend in the Hawaiian-Emperor volcanic chain.

INTRODUCTION

It has long been recognized (for example, Steinmann, 1929) that the Andes of Peru underwent strong deformation during early and/or middle Cenozoic time. As pointed out by McLaughlin (1929), and later corroborated by workers such as Harrison (1951, 1956) and Megard (1978), there is abundant evidence for this period of deformation in the Western Cordillera of central Peru. There a thick sequence of volcanic and volcaniclastic strata, itself deformed later during Miocene time, unconformably overlies the coarse clastic sedimentary rocks of latest Cretaceous and probably early Tertiary age commonly termed the Casapalca Formation.

Steinmann (1929) assigned an early or middle Tertiary age to the deformation represented by the post-Casapalca unconformity, which he termed the "Incaic phase" of the Andean orogeny. Megard (1973), on the basis of charophytes of probable Oligocene age in rocks originally thought to belong to the Casapalca Forma-tion, but now known to postdate "Incaic" deformation, provi-sionally concluded that deformation took place during Oligocene time. Noble and others (1974) inferred an Eocene or older age for deformation from a single potassium-argon age of 41 m.y. obtained on tuff deposited after deformation, and Noble and others (1978), on the basis of additional field and geochronological information, concluded that deformation probably took place during early and/or middle Eocene time. Audebaud and others (1976) have ac-cepted a late Eocene time for deformation (their "tectonique fini-eocene) on the basis of the K-Ar age presented by Noble and others (1974), whereas Cobbing (1978) and his associates postulated that the compressive deformation probably took place during the Paleocene.

This paper presents new radiometric age, stratigraphic, and structural data that set a firm upper limit on the time of "Incaic" tectonism and later uplift and date the pulse of intense igneous ac-tivity that followed.

GEOLOGIC SETTING

The Western Cordillera of central Peru is underlain by intensely deformed rocks of Precambrian(P) and early Paleozoic age, less strongly deformed rocks of late Paleozoic age, folded and faulted carbonate and clastic strata of Middle Triassic to Late Cretaceous age, clastic and volcanic rocks of latest Cretaceous and Cenozoic age, and granitic rocks of Mesozoic and Cenozoic age. The youngest formation of the Mesozoic "miogeosynclinal" sequence, the Celendin Formation of Santonian (about 80 m.y.) age (Wilson, 1963; Megard, 1978) is overlain, in places conformably (Megard, 1978), by coarse conglomerate, sandstone, siltstone, and mudstone of the Casapalca Formation of latest Cretaceous and probably early Tertiary age. Strata of the same lithologic character and age as-signed to such units as the Chota, Huaylas, and Huanca Forma-tions are present elsewhere in Peru. The Casapalca Formation is composed largely of debris from the uplifted Mesozoic volcanic belt to the west into which the Coastal batholith had been emplaced (Megard, 1978; Bussell and others, 1976; Pitcher and Bussell, 1977).

A major erosion surface ("post-Incaic unconformity"), de-veloped on the Coastal batholith as well as on folded Mesozoic strata and the conformably overlying Casapalca Formation to the east, can be traced throughout much of the Western Cordillera of northern and central Peru (Megard, 1973, 1978; Cobbing, 1973b). Where it is cut on the rigid Coastal batholith, the erosion surface is now undeformed and nearly horizontal (Noble and others, 1978), whereas east of the Coastal batholith in the Western Cordillera the surface was strongly deformed during Neogene time by later pulses of compressive tectonism.

In most places in the Western Cordillera the erosion surface is overlain by coarse conglomerate that near Casapalca has been named the Carmen conglomerate (McKinstry and Noble, 1932). The conglomerate is overlain by 2 km or more of volcanic and vol-caniclastic rocks that include ash-flow and subaqueous tuff, re-worked tuff, volcanic sandstones of rhyolitic to dacitic composi-tion, volcanic breccia, conglomerate, and flows mostly of inter-mediate composition. Freshwater limestone and gypsum are pres-ent locally.

Many names have been applied to this largely volcanic sequence.

Geological Society of America Bulletin, Part 1, v. 90, p. 903 - 9 0 7 , 1 fig., 1 table, October 1979, Doc. no. 91004.

903

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904 NOBLE AND OTHERS

Steinmann (1929) included it within his "Rimac formation," which included rocks now assigned to the Casapalca Formation. McLaughlin (1929), recognizing the major unconformity between the Casapalca Formation and overlying volcanic and volcaniclastic rocks, proposed the term "Rio Blanco format ion" for the upper, dominantly volcanic sequence. This name was later accepted by McKinstry and Noble (1932), who, however, erroneously placed the Carmen conglomerate in the underlying Rimac formation. Harrison (1951, 1953, 1956) proposed the name "Serie Abigar-r ada" for rocks equivalent to McLaughlin's Rio Blanco formation. Hosmer (1959; see, for example, Coney, 1971a) and Petersen (1965) applied the term "Tacaza Volcanics," derived from south-ern Peru, to the deformed volcanic rocks that overlie the Casapalca Formation in central Peru. Cossio N. (1964) proposed the term "volcánico Calipuy" to cover all the Cenozoic volcanic strata in the Santiago de Chuco and Santa Rosa quadrangles of northern Peru. British geologists (for example, Cobbing and Pitcher, 1972; Cob-bing, 1973a, 1973b, 1978; Myers, 1975) more recently have applied the terms "Calipuy formation," "Fm. Calipuy," "Calipuy volcanics," "volcánico Calipuy" and "Calipuy G r o u p " in an even more general manner in northern and central Peru to include all the rocks above, and probably some below, the "post- lncaic" uncon-formity; their principal purpose has been to distinguish these rocks f rom largely volcanic strata of Mesozoic age (Casma Group) pres-ent in the vicinity of the Coastal batholith. In the northern part of central Peru, Cobbing (1973b) has divided his "Calipuy Group" into two formations, the Yantac and the overlying Calipuy. On the recent geologic map of Perú (Instituto de Geología y Minería del Perú, 1975) the term "Volcánico Calipuy" is used to include all but the youngest post-"Incaic" strata. Although the name Rio Blanco

has priority and stratigraphically is the mostly precise, the term "Cal ipuy" is in common use at present.

LOCALITIES A N D ROCKS DATED

Conglomerate and volcanic and volcaniclastic rocks of Eocene age overlie the post-"Incaic" unconformity developed across rocks of three tectonic zones. To the southwest (loc. 5, Fig. 1) Eocene vol-canic rocks lie on eroded rocks of the Coastal batholith. Northeast of the Coastal batholith in the Western Cordillera (Iocs. 1, 6, 7, 8, 1 0 , 1 1 , 1 2 ) , where the Casapalca Formation and older sedimentary rocks have been intensely deformed, the overlying Eocene rocks are moderately to strongly unconformable. The intensity of "Incaic" deformation decreases farther to the northeast on the Altiplano (Iocs. 2, 3, 4, 9), and there Eocene volcanic and volcaniclastic rocks typically overlie the Casapalca Formation disconformably or with slight unconformity. (Most of the folding in Mesozoic and Cenozoic rocks of the southwestern Altiplano was produced by one or more pulses of Neogene deformation; Far::ar and Noble, 1976.)

Three of the more important localities (1, 6, and 8, Fig. 1; Table 1) are in the Western Cordillera where volcanic and volcaniclastic strata overlie the Casapalca Formation and older units with strong angular unconformity (Harrison, 1951, 1953, 1956; Megard, 1978). The magnitude of the unconformity decreases to the north-east, and at locality 2 on the Altiplano volcanic sandstone with in-tercalated beds of conglomerate, freshwater limestone, gypsum, and tuff overlie the Casapalca Formation with apparent con-formity. To the south at locality 4 the Casapalca Formation has been removed by erosion, and volcanic breccia of intermediate composition disconformably overlies older strata (Megard, 1968).

Figure 1. Map of cen-tral Peru showing loca-tions of radiometrically dated specimens. Stipple pattern shows distribu-tion of volcanic and vol-caniclastic, associated sedimentary, and certain i n t r u s i v e r o c k s o f Cenozoic age (modified f r o m I n s t i t u t o d e Geología y Minería del Perú, 1975).

LA OROYA

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HVAHCARO

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AfACUCHO

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EARLY TERTIARY "INCAIC" TECTONISM, UPLIFT, VOLCANIC ACTIVITY, PERU 905

At locality 9, no r th of Huancavel ica , volcanic and volcaniclastic s trata (erroneously included in the Casapalca Format ion by N a r -vaez and Guevara , 1968) overlie red beds of the Casapa lca Forma-tion wi th slight unconformi ty . At each of these localities we were able to ob ta in volcanic rocks suitable for po tass ium-argon dat ing f rom within 200 m strat igraphically of the unconformi ty .

At C h o n t a Pass (loc. 10, Fig. 1) volcanic breccia of intermediate composi t ion is overlain by a b o u t 200 m of volcanic sands tone con-taining lenses of conglomerate , slide megabreccia conta in ing blocks of Mesozoic l imestone as much as 5 0 m long, f reshwater l imestone, and tuff . This unit in turn is overlain by a sequence of intermediate to silicic volcanic rock several ki lometres thick. The unit of volcanic sands tone a t C h o n t a Pass w a s incorrectly mapped as Casapalca Format ion by Salazar (1970) and is incorrectly shown by the In-st i tuto de Geología y Miner ía del Perú (1975) and by M é g a r d (1978); nor thwes t and southeas t of the pass it overlies the Casapa lca Format ion unconfo rmab ly . This unit and overlying units of daci te breccia and lava and rhyolite tuff can be traced for long distances nor thwes t and southeas t of C h o n t a Pass (Salazar, 1970 ; D. C. Noble , unpub . mapping) . Specimen 11 is f rom the upper pa r t of the thick unit of dacite tha t directly overlies the unit of volcanic sandstone , and specimens 10A and 10B are f rom the lower pa r t of the unit of rhyolite tuff tha t in turn overlies the dacite breccia (Ta-ble 1).

Specimen 5 is f rom an unal tered ash-flow sheet more than 100 m thick tha t overlies eroded granit ic rocks of the Coas ta l bathol i th in the foothills of the Western Cordil lera near Lima (Noble and others , 1978).

Specimen 7 is f rom a widely distr ibuted unit of distinctive quar tz -and sanidine-rich rhyolit ic ash-flow tuff . At locality 7 the base of this unit is abou t 1 ,000 m stratigraphically above the base of the sequence of volcanic and volcaniclastic strata tha t overlies the pos t - " Inca ic" unconformi ty , whereas at locality 8 the base of the tuff is only abou t 200 m above the unconformi ty . Specimen 3, f r om a 10-m-thick unit of water- la id rhyolitic tuff , may represent the eastern distal end of the same ash-flow sheet. Specimen 12 is f rom a unit of rhyolitic tuff tha t lies abou t 1 ,000 m strat igraphi-

cally above the hor izon f r o m which specimens 10A and 10B were obta ined .

A m a j o r p rob lem encountered dur ing the course of this study w a s the pervasive low-grade al terat ion and/or burial me tamor -phism tha t affects the older Cenozoic volcanic and volcaniclastic rocks of the Wes te rn Cordi l lera . Because of this we were unable to date rocks f rom a n u m b e r of per t inent localities, including the well-studied section in the vicinity of Casapalca (McKinstry and Noble , 1932). The only rocks completely unaffected by some al terat ion are f rom localities ei ther southwest or nor theas t of the central p a r t of the Western Cordi l lera . Nevertheless, we have been able to prepare concentra tes of essentially pure and unal tered phenocryst ic biotite or fe ldspar for all bu t three of the rocks dated using s tandard magnetic, heavy liquid, electrostatic, and hand-picking techniques.

Under the microscope, bioti te f rom specimen 2 shows some evi-dence of weather ing and resul tant expans ion , an observat ion tha t wou ld appear to be suppor ted by the low potass ium conten t of this separate . However , Kulp and Engles (1963) showed tha t loss of potass ium may be accompanied by the loss of approximate ly equivalent a m o u n t s of radiogenic a rgon . Moreove r , an age of a b o u t 4 0 m.y. is suppor ted by charophytes collected near the site sampled for radiometr ic da t ing (Megard , 1978). Nodosochara, Harrisichara, and Tectochara (n.s.) of middle Eocene age were col-lected f rom a locality strat igraphically slightly be low tha t f rom wh ich specimen 2 w a s ob ta ined , and s t ra ta slightly above specimen 2 yielded Tectochara sp. and Chara (Grambastichara) of late Eocene or Oligocene age. Localities s trat igraphical ly above speci-men 2, bu t below tha t f rom which specimen 3 w a s collected, yielded Sphaerochara sp. of Oligocene aspect, bu t of possible late Eocene age, and Stephanochara( ?) sp. of Oligocene aspect bu t wi th a late Eocene to early Miocene range.

Biotite f rom specimen 12 appears to be s o m e w h a t altered, a l though it has a normal potass ium content . T h e plagioclase of specimen 6 contains possibly as much as 5 % secondary albite. The radiometr ic age de te rmina t ions made on these separates , therefore, can be only min imum est imates of the ages of the host rocks.

TABLE 1. POTASSIUM-ARGON AGE DETERMINATIONS ON VOLCANIC ROCKS OVERLYING THE POST-"INCAIC" UNCONFORMITY, CENTRAL PERU

Specimen Field Location Mineral k 2 o Radiogenic Ar40 Radiogenic Ar Age no. lat (S) long (W) dated (wt %) (mol/g x 10-'") (%) (m.y.)

1 ABI-1B2 11°33.5' 76°18.0' Plagioclase, quartz 0.388 0.2246 24.2 39.8 ± 1.9 2 CASA-7 12°00.7' 75°28.6' Biotite 6.19 3.585 73.9 39.8 ± 1.0 3 CASA-10 12°01.2' 75°29.7' Sanidine 8.97 4.797 73.4 36.7 ± 1.0 4 CASA-W 12°24.0' 75°18.4' Plagioclase 0.417 0.1884 10.4 31.1 ± 6 5 FM73-129 12°08.1' 76°34.6' Biotite 8.57 5.150 74.7 41.3 ± 0 . 5

12°08.1' 76°34.6' Plagioclase, quartz 0.249 0.1456 12.7 40.2 ± 4.0 6 Y-3C 12°26.2' 75°40.5' Plagioclase, quartz 0.445 0.2275 17.2 35.2 ± 1.8 7 Y-1A 12°29.2' 75°37.8' Sanidine 9.66 4.891 84.7 34.9 ± 0 . 7 8 T-9 12°46.9' 75°31.3 ' Plagioclase 0.507 0.2806 23.8 38.0 ± 1.9 9 HU76-M1A 12°36.2' 74°57.0' Plagioclase 0.607 0.3474 44.4 39.2 ± 1.1

10A B-58 13-05.4' 75°03.2' Biotite 8.66 5.293 88.7 40.9 ± 0.9 10B B-58B 13°05.2' 75°03.3' Biotite 8.52 5.236 71.2 41.2 ± 1.9 11 LAVA-S 13°24.9' 74°54.8' Plagioclase 0.424 0.2551 21.2 41.4 ± 1.9 12 PILPICACHA 13°19.7' 74°59.3' Biotite 8.31 3.764 56.0 31.2 ± 0 . 9

Note: Constants: K40Xe = 0.572 x 10-'" yr"1; K40Xe' = 8.78 x 10"'3 y r 1 ; K40A/3 = 4.96 x 10~'0 yr"'; atomic abundance K4" = 1.167 x 10"4 mol/mol K. Potassium and argon analyses were made in the laboratories of the U.S. Geological Survey, Menlo Park, California, using standard flame photometer and isotope dilution methods (Dalrymple and Lanphere, 1969).

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9 0 6 NOBLE AND OTHERS

Plagioclase separates 1 and 11 bo th conta in a b o u t 0 . 5 % to 1 %

secondary albite.

R E S U L T S A N D DISCUSSION

The po tass ium-argon ages (Table 1) show tha t de fo rmat ion as

well as subsequent uplif t and erosion had t aken place in central

Peru by 4 0 to 41 m.y. ago — middle to upper Eocene according to

the t ime scale of Berggren and o thers (1978). This min imum age for

" I n c a i c " tectonism also is p robab ly appl icable to sou thern and

no r the rn Peru. C h a n o v e and o thers (1969) repor ted charophytes of

post—middle Eocene and pe rhaps middle Ol igocene age f rom near

the base of the Puno G r o u p , which pos tda tes this de fo rma t ion , in

southeas te rn Peru.

It is more difficult to set a lower limit on the time, of tectonism.

The rad iomet r ic ages of S tewar t and o thers (1974) and P. A. Wil-

son (Bussell and others , 1976) , plus addi t ional unpub l i shed age de-

te rmina t ions (E. J . Cobb ing , 1978 , oral c o m m u n . ) r s h o w tha t the

C o a s t a l b a t h o l i t h w a s emp laced mos t ly in C r e t a c e o u s a n d

Paleocene time, between a b o u t 105 and 60 m.y. ago. If, as appears

p robab le , the pos t - " Inca ic" erosion surface cuts the young granit ic

rocks of Paleocene age, then a late Paleocene and/or Eocene age is

indicated for the uplift and erosion of the Coas ta l ba tho l i th and the

folded and faulted s t ra ta to the east . A l though this w o u l d no t re-

qu i re tha t the compressive de fo rma t ion t ook place dur ing this in-

terval , w e believe nevertheless tha t it is likely t ha t d e f o r m a t i o n and

upl i f t were relatively rapid and closely related events t ha t did not

precede by more than a few million years the deposi t ion of the vol-

canic and volcaniclastic rocks t ha t mant le the e ros ion surface.

T h e presence of several hund red metres of volcanic rock beneath

coarse conglomera tes and slide megabreccias composed largely of

blocks of pre-Tert iary rock southeas t of C h o n t a Pass (D. C. Noble ,

u n p u b . mapping) and the presence elsewhere of cobbles of volcanic

rocks in the basal cong lomera te tha t overlies the regional erosion

sur face suggest tha t the volcanism had begun dur ing , and possibly

before the onse t of, uplif t and erosion. The ages of 3 6 . 7 ± 1.0 and

35 .2 ± 1.8 m.y. ob ta ined on sanidine f rom specimens 3 and 6

suggest tha t the pulse of igneous activity t ha t had begun by abou t

41 m.y. ago lasted a b o u t 6 m.y. into early Oligocene time. Rela-

tively small a m o u n t s of volcanic rock overlie these dated hor izons ,

and the intensity of volcanic activity p robab ly decreased markedly

shortly af ter the deposi t ion of the da ted units. This pulse of vol-

canic activity w a s a m a j o r event: volcanic rocks of middle to late

Eocene age are preserved no t only in the Cordi l lera bu t also a long

the coast of central and nor the rn Peru (Ruegg, 1957 ; Petersen,

1958) and in the sub-Andean region east of the high Andes (Koch

and Blissenbach, 1960, as revised by Mega rd , 1978) . T h e remain-

der of the Oligocene appears to have been character ized by rela-

tively little tectonic and igneous activity (Petersen, 1958 ; Nob le and

others , 1974) wi th p r o n o u n c e d volcanic activity resuming only in

latest Ol igocene or early Miocene t ime.

If upl i f t and volcanic activity fol lowed closely upon the pulse of

" Inca i c " compressive de fo rma t ion , all can reasonably be included

as aspects of an Eocene tectonic pulse. This pulse w o u l d have been

coeval wi th the change in direct ion of absolute mo t ion of the Pacific

l i thospheric p la te a b o u t 43 m.y. ago (Clague and Ja r r a rd , 1973 ;

N o b l e and others , 1974 , 1978 ; Dal rymple and Clague, 1976 ; Ja r -

ra rd and Clague, 1977) and pe rhaps o ther tectonic and igneous

events elsewhere in the circum-Pacific region (for example , Coney,

1971b, 1972 ; Noble , 1972) which may have been secondary conse-

quences of the collision o t India w i t h the Euras ian plate . Alterna-

tively, if compress ion preceded uplif t and volcanism by more than a

few million years, only uplif t of the Andes and associated volcanic

activity may be related to a possible Eocene global tectonic event.

A C K N O W L E D G M E N T S

This w o r k w a s suppor ted in p a r t by the Na t iona l Science Foun-

dat ion and the Cent re Na t iona l de la Recherche Scientifique

(France). Field and logistical suppo r t w a s prov ided by C o m p a ñ í a

de M i n a s Buenaventura , S. A., C o m p a ñ í a Mine ra Cas t rovi r reyna ,

S. A., and the Insti tut Français d 'E tudes Andines. J.-P. Soulas, F. M .

Arias, Alber to Samaniego, and D. A. Scherkenbach par t ic ipated in

var ious phases of the field w o r k . W ^ are especially indebted to

Alber to Benavides Q . and Ulrich Petersen for their con t inued sup-

por t . T h e results of this s tudy were presented a t the F o u r t h Peru-

vian Geological Congress , Lima (McKee and others , 1978) .

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Geological Society of America Bulletin

doi: 10.1130/0016-7606(1979)90<903:ETITUA>2.0.CO;2 1979;90, no. 10;903-907Geological Society of America Bulletin

 DONALD C. NOBLE, EDWIN H. McKEE and FRANÇOIS MÉGARD PeruEarly Tertiary ''Incaic'' tectonism, uplift, and volcanic activity, Andes of central  

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