Downloaded from gsabulletin.gsapubs.org on May 4, 2011 Geological Society of America Bulletin

Structural Evolution of the , of

PETER J CONEY

Geological Society of America Bulletin 1971;82;1863-1884 doi: 10.1130/0016-7606(1971)82[1863:SEOTCH]2.0.CO;2

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Notes

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PETER J. CONEY Department oj'Geology, Middlebury College, Middlebury. Vermont 05753

Structural Evolution of the Cordillera Huayhuash, Andes of Peru

ABSTRACT 5,000 m high, extending in a northerly direc- An expedition to the Cordillera Huayhuash tion close to long. 75°55' W., between lat 10° in the north-central Andes of Peru has shown 4' and 10°25' S. Along its central portion the the range consists of complexly folded and range is dominated by six peaks more than 6,- thrust-faulted Cretaceous sedimentary rocks, 000 m high, culminating in Nevado Yerupaja and less deformed early to middle Tertiary(?) (6,634 m), the second highest mountain in andesitic volcanic rocks, all intruded by granitic Peru. The axial ridge of the Cordillera Huay- plutons thought to be as young as 9 m. y. B.P. huash forms the continental divide drained on A younger sequence of felsic ash flows has es- the west by tributaries of the Rio Pativilca, and caped folding and intrusion. Unlike the Cordill- by tributaries of the Rio Maranon on the east. era Blanca to the north (a batholith), the major Total relief in the region is about 4,000 m. 6,000 m summits of the Cordillera Huayhuash The Cordillera Huayhuash is most easily are carved from a complex synclinorium of Up- reached by 158 km of dirt road from Para- per Cretaceous Jumasha Formation carbonates. monga on the Pacific coast to Chiquian (3,400 The region has suffered two deformations. m), which is 35 km northwest of the range (Fig. The first deformation was the most severe, pro- 2). From the road-head at Chiquian, travel is on ducing tight flexural-slip and flexural-flow folds foot or horseback. Trail time to the foot of and east-directed thrust faults between Late glaciers is a two- to five-day effort. Cretaceous and middle(?) Tertiary. During No published systematic geologic study of deformation, detachment of the Cretaceous the Cordillera Huayhuash has appeared prior prism from some unexposed "basement" oc- to this report. In 1936, three Austrian geogra- curred at the horizon of basal Cretaceous Oyon phers explored the range, compiled a topo- Formation shale. Less severe middle(?) Terti- graphic map, and wrote a report (Kinzl and ary deformation warped andesitic volcanic others, 1942) including descriptions of orogra- rocks into broad gentle folds, and may have phy, glaciology, and cultural geography. Heim either reactivated or initiated thrust faults. (1948) commented on brief geologic observa- Field data and hypsometric (area altitude) tions in the range. Kinzl returned in 1954 and analysis suggest the middle to late Tertiary later published a book of photographs (Kinzl, Puna erosion surface is represented in the Cor- no date). Bodenlos and Ericksen (1955, p. 140- dillera Huayhuash by accordant flanking ridge- 153) published descriptions of several mines in tops and a landmass concentration below 5,000 and about Cerro Culebras on the northwest m. The 6,000 m axial ridge stood above this flank of the range. surface as a residual mass around which ash CRUSTAL CONSTITUTION flows were deposited. Latest Tertiary to Quat- ernary erogenic uplift, perhaps during the last General Statement 6 m.y., incised drainage and represents the Rocks of the Cretaceous, Tertiary, Quater- most recent deformation of the region. nary, and Holocene are exposed in the Cordill- era Huayhuash with a total thickness of about INTRODUCTION 4,500 m (Fig. 3). More than 80 percent of the The Cordillera Huayhuash (Fig. 1) stands in outcropping rocks are Cretaceous (Fig. 4) and splendid isolation on the crest of the north- are composed of 3,000 m of marine sediments. central Andes of Peru, 110 km northeast by Tertiary rocks include minor continental red east of the Pacific coast, and 200 km north of beds, more than 1,500 m of older intermediate Lima (Fig. 2). The range forms a serrate snow- volcanic rocks, and over 750 m of younger fel- and ice-clad ridge 40 km long and more than sic volcanic rocks. Cretaceous rocks, continen-

Geological Society of America Bulletin, v. 82, p. 1863-1884, 13 figs. July 1971 1863 1864 P. CONEY—EVOLUTION OF CORDILLERA HUAYHUASH, PERU

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Figure 1. Cordillera Huayhuash from above on snow-clad axial ridge are carved from folded Upper Chiquian. View southeasterly from 3,600 m. Rio Pati- Cretaceous limestones. Summits to right on Tsacra Spur vilca runs left to right in deep canyon (P). Quebrada are carved from Tertiary volcanics. Ridges and valley Llamac (L) is tributary to Rio Pativilca at 2,700 m. High- walls in foreground, and across Rio Pativilca, are carved est summit is Nevado Yerupaja (6,634 m). Snow line is from folded Lower Cretaceous sandstone and shale, at about 5,000 m. Nevado Yerupaja and summits to left

tal red beds, and the older volcanic rocks are p. 9), made up of basal Oyon Formation shale, intruded by scattered late Tertiary granitic plu- Chimu Formation orthoquartzite, Santa Forma- tons. Quaternary and Holocene rocks are gla- tion limestone, Carhuaz Formation sandstone cial deposits, valley alluvium, and talus. and shale, and the Farrat Formation orthoquart- Regional considerations suggest the Cretaceous zite. This group is followed above by Albian to sedimentary prism is floored by a Paleozoic Turonian carbonates with interspersed shale "basement" composed of metamorphic rocks made up of the Pariahuanca Formation lime- equivalent to the Excelsior Group (Jenks, stone, Chulec and Pariatambo Formations lime- 1956, p. 222; McLaughlin, 1924, p. 598; Har- stone and shale, and Jumasha Formation rison and Wilson, 1960, p. 34-35) of central limestone. The pre-Albian Goyllarisquisga Peru with the possibility of some Jurassic rocks Group thins just east of the range, loses lime- just below the Cretaceous (Steinmann, 1930, p. stone and shale, and changes facies into the 84-86; Bodenlos and Ericksen, 1955, p. 18- Goyllarisquisga Formation (Wilson, 1963, p. 21). 9-14). West of the range the early Tertiary coastal batholith obscures correlations, but Cretaceous Rocks Cretaceous detrital and volcanic rocks of The 3,000 m of Cretaceous sedimentary eugeosynclinal aspect crop out along the Pacific rocks in the Cordillera Huayhuash are the pre- coast (Wilson, 1%3, p. 6-8). The Cretaceous Albian Goyllarisquisga Group (Wilson, 1963, stratigraphy described by Benevides (1956) Downloaded from gsabulletin.gsapubs.org on May 4, 2011

Figure 2. Generalized location and geologic map of and others, 1956). the north-central Andes of Peru (geology after Belltdo Downloaded from gsabulletin.gsapubs.org on May 4, 2011

—4000

-3000

-2000

— 1000

METERS

Figure 3. Generalized composite geologic column for the Cordillera Huayhuash. Downloaded from gsabulletin.gsapubs.org on May 4, 2011 CRUSTAL CONSTITUTION 1867

and Wilson (1963) for the central Andes of dillera Huayhuash. Peru was found to extend into the Cordillera The Chimu Formation (Benevides, 1956, p. DownloadedHuayhuas from gsabulletin.gsapubs.orgh with only minor variations on May. 4, 2011 365) is prominent in the Cordillera Huayhuash Goyllarisquisga Group. Wilson (1963, p. with a thickness of at least 650 m. It is brought 12) described the Oyon Formation as more than to surface by all anticlines and upthrust belts 100 m of thin-bedded, dark, fine-grained sub- throughout the range (Fig. 5). The formation is graywacke shale interbedded with lenticular medium- to thick-bedded, occasionally mas- seams of coal exposed in the core of a thrusted sively bedded, very light gray to white, medi- anticline near Oyon, 30 km south of the Cor- um- to coarse-grained, occasionally conglo- dillera Huayhuash. In the Cordillera Huay- meratic, orthoquartzite interbedded with huash the Oyon Formation is restricted to the subordinate laminated beds of dark gray, silty, core of a complex anticline 1 m east of Car- carbonaceous shale and scattered seams of coal. huacocha, and possibly several other small The ratio of sandstone to shale beds is about highly sheared exposures in cores of anticlines 10:1. Sandstone units exhibit well-developed in the Chimu Formation in other parts of the diagonal and concave cross-bedding indicating range. Neither the base of the Oyon Formation current directions toward the southwest. nor any older rock was encountered in the Cor- The Santa Formation (Benevides, 1956, p.

Figure 5. Condor thrust fault. View south from (Kp) forms ridge east of overturned Carhuaz Formation 4,810 m on southwest ridge of Punta Huay. Fault (Kc). Somber terrain east of Pariahuanca ridge is in (dashed line) lies below cliffs of the Chimu Formation Pariatambo Formation (Km). (Kch). Vertical to overturned Pariahaunca Formation 1868 P. CONEY—EVOLUTION OF CORDILLERA HUAYHUASH, PERU

367; Wilson, 1963, p. 14) is present through of thin- to medium-bedded, light- to medium- out the range, flanking the Chimu Formation light gray, fine- to medium-grained cross-bed- on all anticlines, with a thickness of about 150 ded orthoquartzite with laminated, medium Downloaded fromm gsabulletin.gsapubs.org. The unit is medium-bedded on May, medium-ligh 4, 2011 t dark gray, silty shale and some light gray silt- to medium-dark gray, very fine-grained to mi- stone. Sandstone and shale beds are about crogranular fossiliferous limestone with minor equally numerous near the divide but the pro- laminated calcareous shale interbeds. The for- portion of sandstone increases eastward. Near mation thins and disappears eastward from the the top of the unit, 100 m of maroon silty shale range by facies change into the Goyllarisquisga is conspicuous. The formation extends several Formation (Wilson, 1963, p. 14). Benavides kilometers east of the mapped region, but is (1956, p. 368) dates the Santa Formation as reported absent at Lauricocha by Wilson Valanginian. (1963, p. 32), about 20 km east-southeast of The Carhuaz Formation (Benevides, 1956, p. the range, passing by facies change into the 368) forms subdued grass-covered valley walls Goyllarisquisga Formation. Benevides (1956, and serrate ridge-crests along flanks of folds on p. 369) dates the Carhuaz Formation as late both sides of the continental divide (Figs. 5, 6, Valanginian. 7, 8). About 800 m thick, the unit is composed Wilson (1963, p. 15) applied the name Far-

Figure 6. Nevado Tsacra Grande from the north. (Km), and Jumasha (Kj) Formations. Angular uncon- View from 4,925 m on ridge south of Jahuacocha. Tsacra formity (dashed line), Carhuaz beds, breccia, and volcan- volcanics (Tvt) and limestone breccia (Tr) overlie up- ics are all cut by hypabyssal quartz mon/onite dike turned Carhua/

rat Formation to a few tens of meters of coarse- on ridge-tops and quebrada walls (Figs. 5,7,8). grained quartz sandstone of late Aptian age About 150m thick, the unit is medium-bedded, above the Carhuaz Formation and below the medium gray to locally dark gray, very fine- Downloaded from gsabulletin.gsapubs.org on May 4, 2011 Pariahuanca Formation in the north-central grained fossiliferous limestone. Thick beds are Andes of Peru. Benevides (1956, p. 370) con- common in the middle part. Fossil hash and sidered the unit equivalent to the Goyllaris- abundant megafossils are common in many quisga Formation east of the continental divide. beds. Benevides (1956, p. 370) dated the As exposed in the Cordillera Huayhuash the Pariahuanca Formation as early Albian. The Farrat Formation is 60 m of thin- to medium- formation wedges out (Wilson, 1963, p. 15; bedded, medium gray, medium- to coarse- Benevides, 1956, p. 370) east of the range grained orthoquartzite interbedded with minor along a line closely following the facies change silty shale. The formation is not separately of the Goyllarisquisga Group to the Goyllaris- mapped on Figure 4 of this report and is in- quisga Formation. cluded in the Carhuaz Formation. Chulec and Pariatambo Formations. The Pariahuanca Formation. The Paria- Chulec and Pariatambo Formations were origi- huanca Formation (Benevides, 1956, p. 369) is nally defined by Mclaughlin (1924, p. 608) as distributed throughout the range in synclines members of the Machay Formation for thick

Figure ?. Nevado Yerupaja from the Llamac-Pacllon tion (Kc) forms flanks of fold. Jumasha Formation i Kj) ridge southwest of Pocpa. View south-southeasterly dips east on north face of Nevado Yerupaja. Flat glacial from 4,700 m. Culebras syncline, outlined by Paria- valley is Incahuain in Quebrada Pacllon. huanca Formation (Kp), crosses valley. Carhuaz Forma- 1870 P. CONEY—EVOLUTION OF CORDILLERA HUAYHUASH, PERU

limestone and shale exposed at Oroya, 180 km formation grades upward into platy, laminated, southeast of the Cordillera Huayhuash. Bene- dark gray limestone with less shale interbeds. Downloaded fromvide gsabulletin.gsapubs.orgs (1956, p. 373-376) raise ond May both 4, to 2011 forma- About 400 m of rocks belonging to the two tion rank, recognizing the basal Chulec formations were measured on the ridge south Formation limestone and sandy shale, and the of Calinca. Pariatambo Formation dark limestone and Jumasha Formation. Orographically, the shale. He placed both in the Middle Albian. Jumasha Formation (McLaughlin, 1924, p. The two formations are mapped as a single unit 609) is the most important lithologic unit in the on the geologic map of this report (Fig. 4), Cordillera Huayhuash, since it forms the axial because exposures generally prevent separa- ridge and all 6,000 m summits from Ishpac- tion. In the Cordillera Huayhuash the Chulec pampa south to Nevado Carnicero (Figs. 9, Formation ranges from 10 to 40 m of thin- 10). The formation is also prominent in the bedded, nodular, medium-light gray to yellow- Cordillera Raura southeast of the mapped re- ish-gray, fine-grained fossiliferous limestone in gion and in the Cordillera Huallanca north. beds 0.1 to 0.5 m thick, interbedded with dark Thicknesses of 400 to 600 m or more were gray calcerous shale in units up to 1 m thick. accounted for in the Cordillera Huayhuash, but Limestone beds are usually concretionary. The structural complication and difficulties of access

Figure 8. Cerro Culebras and Culebras syncline. (Km) in core. Jumasha Formation is ejected. Carhuaz View southeast from 3,700 m. Fold is outlined by Paria- Formation (Kc) forms flanks of fold. huanca Formation (Kp) with Pariatambo Formation CRUSTAL CONSTITUTION 1871

prevented detailed study. The formation is sha beds and dips 30° westward. The upper medium to massively bedded, very fine-grained mudstone and siltstone are locally dipping up to limestone and dolomite. Benevides (1956, p. 60° and the Chimu Formation appears to have Downloaded378 from) date gsabulletin.gsapubs.orgd the formation as Middl one AlbiaMay 4,n t2011o been thrust against them. In Quebrada Huacr- Turonian. ish a consolidated limestone breccia up to 30 m thick lies across steeply dipping beds of Car- Continental Red Beds huaz through Pariatambo Formations (Fig. 6). Continental red beds in the Cordillera Huay- The breccia is overlain by Tsacra volcanic huash are of scattered occurrence, are variable rocks, is intruded by quartz monzonite dikes, in thickness and structural setting, and show and dips up to 50° westward. Elsewhere, red several lithologic types. The largest exposure is beds are found beneath volcanics near Nevado east of Laguna Viconga in a narrow belt flank- Puscanturpa and west of Cuyocpunta. ing the western edge of the Cordillera Raura. The age of the red beds in the Cordillera A basal conglomerate 50m thick, composed of Huayhuash is not known, but is presumed Ter- elements of the Chimu Formation, is overlain tiary. The beds lie with angular unconformity by about 200 m of red mudstone and siltstone. over Cretaceous rocks, but are more nearly The conglomerate lies across overturned Juma- conformable with overlying Tsacra volcanic

Figure 9. Northern terminus of the Cordillera Huay- axial ridge with low hills and valleys below of the Paria- huash. View south from 4,600 m on southeast ridge of tambo Formation (Km). Tight chevron Yerupaja syn- Punta Huay. Jumasha Formation (Kj) in syncline forms cline (dashed line) over Punta Cumcush. 1872 P. CONEY—EVOLUTION OF CORDILLERA HUAYHUASH, PERU

rocks. They are intruded by quartz monzonite from them by angular unconformity, and pre- porphyry dikes. Continental red beds are exten- sumed early to middle Tertiary in age. There is sive in central Peru (Jenks, 1956, p. 228-230), no definitive evidence in the Cordillera Huay- Downloaded froman gsabulletin.gsapubs.orgd most are included in th one Casapalc May 4, a2011 Group huash to support correlation of red beds with (Petersen, 1958, p. 64-65; Mabire, 1961, p. either the Casapalca or Tacaza sequences, but 151-186; Szekeley, 1969, p. 558-559), which the strong angular unconformity between red lies generally disconformably on Cretaceous beds and the Cretaceous sequence, and close marine sediments, attains thicknesses of up to association with Tsacra volcanic rocks, suggests 3,000 m, and is presumed latest Cretaceous to the Tacaza correlation may be reasonable. early Tertiary in age. Hosmer (1959, p. 141) states the disconformity between red beds and Tertiary Volcanic Rocks Cretaceous sediments becomes progressively Tsacra Volcanics. The Tsacra volcanics more angular southwestward as the coastal have a wide distribution in the southwestern batholith is approached. Other red beds are at part of the Cordillera Huayhuash. They un- the base of and within an extensive sequence of derlie the summits along the Tsacra Spur (Fig. andesitic volcanics called the Tacaza volcanics 6) and Cerro Rosario, and are also found (Hosmer, 1959, p. 146). The Tacaza rocks are around Cuyocpunta, north of Nevados Puscan- younger than the Casapalca Group, separated turpa, and east of Laguna Viconga. The re-

Figure 10. Nevado Yerupaja from the south. View Upper Cretaceous (Kj). Nevado Yerupaja is apparently from 5,100 m on Cerro Magdalena. Yerupaja thrust formed from folded Jumasha Formation (see structure (dashed line) passes over col west of the peak bringing section B-B', Fig. 4). up Carhua/ Formation (Kc) against limestones of the Downloaded from gsabulletin.gsapubs.org on May 4, 2011 CRUSTAL CONSTITUTION 1873

gional extent of these rocks is considerable, and are gray to tan, aphanitic, very hard, and con- views from high elevations suggest they crop tain scattered phenocrysts of quartz. Higher out southwestward from the Cordillera Huay- units are tuffaceous with lithic, crystal, and huash for many kilometers. pumice fragments. Crystal fragments are em- The Tsacra volcanics are lithologically varia- bayed quartz, altered oligoclase, and scattered ble, but most outcrops show massively bedded, biotite. Limonite stain is prominent both in aphanitic-porphyritic textured, greenish-gray, groundmass and pumice fragments. red-purple andesitic flows with chalky pheno- The age of the Puscanturpa volcanics is not crysts of plagioclase. Locally, more felsic ma- known. They appear to overlie red beds and roon crystal tuff and breccia are found. Tsacra volcanics with angular unconformity. Microscopically, a few specimens have andesine They are not folded and dips were not noted and altered pyroxene, but most rocks show over about 10°. No contact metamorphism was strong propylitic alteration and all vestige of noted and they were not seen mineralized. ferromagnesium minerals is altered to chloride They were not seen to be intruded by quartz material. Pyrite is ubiquitous and hematite is monzonite dikes. Restricted distribution and common. difficulties of access prevent definitive conclu- At last 800 m of Tsacra volcanics are exposed sions, but they may be very young. Their hori- on the Tsacra Spur, and over 1,500 m are ex- zontal attitude and restriction to elevations in posed from the floor of Quebrada Seria to the excess of 4,800 m suggest they may have been top of Cerro Rosario. The basal contact undu- deposited on an extensive surface of erosion in lates through 800 m of relief cutting across ev- latest Tertiary prior to recent uplift and dissec- ery Cretaceous formation in the Cordillera tion. Huayhuash, with the exception of the Oyon Formation. On the ridge east of Quebrada Seria Tertiary Intrusive Rocks the volcanics are strongly altered, tourmali- Stock. A diorite stock with a cir- nized, and contact metamorphosed by felsic in- cular outcrop of about 1 sq km and an exposed trusive rocks which crop out just below. relief of about 600 m forms a prominent peak The age of the Tsacra volcanics is not known. in Quebrada Huacrish. The body is every- They lie in sharp angular unconformity over where in contact with steeply dipping Carhuaz strongly folded Cretaceous sediments. They Formation. Outcrops are somber-colored and have been contact metamorphosed by granodi- structureless, and hand specimens away from orite intrusions, intruded by quartz monzonite the aphanitic contact zone exhibit greenish-gray porphyry dikes, and mineralized. The massive phaneritic altered diorite. Microscopically, dis- flows and basal contact surface dip as much as tinct phenocrysts of altered andesine are seen in 40° in some places where the unit has been a matrix of feldspathic and chlorotic material. warped into broad folds. Altered ferromagnsium content makes up 30 Puscanturpa Volcanics. The Puscanturpa percent of the rock. The age of the stock is volcanics are distinctive felsic ash flows which unknown other than where it has intruded the extend in a narrow belt from Nevado Car- folded Carhuaz Formation, but its composition, nicero southward beyond the mapped region, alteration, and setting suggest it may have been including the prominent summits of Nevados a feeder for Tsacra volcanic flows. Puscanturpa for which they are named (Fig. Silicic Plutonic and Hypabyssal Rocks. 11). Outliers are found just east of Portachuelo Outcrops of quartz-bearing plutonic rocks are de Huayhuash, and west of Cuyocpunta. The found in Quebrada Seria, and around Sarapoco- outcrops are restricted to the continental divide cha. The composition of these rocks is mainly and the rock was never seen below 4,800 m in granodioritic, grading to quartz monzonite and the Cordillera Huayhuash. The total thickness quartz diorite. The plutons are clearly discord- is not known, but at Nevados Puscanturpa the ant to structural trends in Cretaceous rocks and base of the pile is close to 4,900 m and the rocks contact metamorphism is minor. Outcrops in appear to continue to the summit at 5,652 m. Quebrada Seria form much of the eastern wall This would indicate a minimum thickness of of the upper valley from the glacial headwall 750m. southward for about 1.5 km. The outcrops ex- The Puscanturpa volcanics form light tan to tend upward to near 5,000 m, but contact with yellow-brown cliffs and spires which exhibit overlying Tsacra volcanics which the body must massive beds up to 50 m thick and columnar intrude is obscured by talus, or inaccessible. jointing on a grand scale. Units near the base Most exposures are fine- to medium-grained, 1874 P. CONEY—EVOLUTION OF CORDILLERA HUAYHUASH, PERU

slightly prophyritic granodiorite grading to ridge north, and throughout the Sarapococha quartz diorite. Microscopically, anhedral quartz valley, indicating that soles of glaciers are biting Downloaded fromand gsabulletin.gsapubs.org orthoclase with scattered on biotit Maye an4, d2011 horn- into plutonic bodies at the base of much of the blende surround phenocrysts of andesine. On north-central part of the axial ridge of the Cor- the other side of the ridge, near Sarapococha, dillera Huayhuash. outcrops of granodiorite grading to quartz Hypabyssal quartz monzonite porphyry bo- monzonite are found in roches mountonnees dies include a stock-shaped mass at the south west and southwest of the toe of the glacier, and end of the Cordillera Huallanca near the base in larger exposures west of the glacier. Most of Punta Huay intruding the Pariahuanca and outcrops are gray but some are pinkish, due to Pariatambo Formations, and dikes and sills gen- potassium feldspar content. The rocks are medi- erally found a short distance west of the conti- um-grained with slight porphyritic develop- nental divide. The dikes and sills follow the ment of plagioclase. Both biotite and strike of Cretaceous sediments and jump in ir- hornblende are present. Granodiorite debris is regular fashion from one bedding plane to found in moraines at the head of Quebrada another. They are mostly 10 to 50 m wide and , in moraines of glaciers descending the some can be traced for as much as 2 km. They west side of Nevado Yerupaja and the axial appear as white bands in more somber rocks

Figure 11. Nevados Puscanturpa from the west. at base with massive near-horizontal beds and columnar View from 4,500 m at Cuyoc. Puscanturpa volcanics jointing. Field camp in foreground stands on recent lake (Tvp) form mountain from peak (5,652 m) to talus cones beds. Low hills beyond are glacial moraine. Downloaded from gsabulletin.gsapubs.org on May 4, 2011 1876 P. CONEY—EVOLUTION OF CORDILLERA HUAYHUASH, PERU

slightly asymmetrical, angular anticlines lies face. The fault appears to dip about 50" west along the eastern front. South of Quebrada Lla- along most of its length. The stratigraphic sepa- mac these folds are broken by westward dip- ration is up to 2,000 m or more at some points. ping thrusts. The thrusts lie in bedding planes South of Quebrada Llamac the Yerupaja of the Chimu Formation and do not appear to thrust breaks out of the core of the Paria anti- have disturbed Tsacra volcanic rocks on the cline, placing the Chimu and Santa Formations Tsacra Spur as they pass below them. over Carhuaz rocks to the east. At the pass west Jahuacocha Belt. Thejahuacocha belt lies of Nevado Rondoy the fault lies on a bedding northeast of the Pativilca belt and southwest of plane in the upper part of the Chimu Formation the Huayhuash belt extending from the north- and dips 70° southwest. Southward the fault is ern to the southern borders of the mapped re- obscured by the Yerupaja glacier. At the pass gion over a distance of 40 km. The belt is 10 between Nevado Yerupaja and Nevado , km wide at the north, narrowing to 7 km be- close to 5,800 m elevation, the upper part of tween Nevado Rasac and Punta Llanche, and is the Chimu Formation, with overlying Santa and convex northeastward with a curvature of Carhuaz beds, appear to be in discordant con- about 25°. The belt consists of a set of tight to tact with Upper Cretaceous limestones. The isoclinal, east-vergent, upright to slightly over- fault is probably the Yerupaja thrust, here dip- turned anticlines and synclines that bring rocks ping steeply westward (Fig. 10). Southward from the Chimu to Pariatambo Formations to along the eastern side of Sarapococha to the surface exposure. The eastern boundary of the ridge south of Cuyoc, the Carhuaz Formation, belt is a zone of east-directed overthrust fault- here in a sharply westward inclined anticline, ing. has been thrown against the Jumasha Formation The westernmost fold in the Jahuacocha belt, on the east. The break is nearly vertical and is the Culebras syncline (Fig. 4), is of particular considered to be the southward extension of interest, because for much of its length the fold the Yerupaja thrust. It is to be noted that folds is "strangled," displaying isoclinal geometry in are tighter and faults steeper in the narrow the Pariahuanca Formation with a core of southern part of the Jahuacocha belt compared sheared Pariatambo shale and limestone. In to the wider northern part. Quebrada Llamac the axis of the fold on top of Huayhuash Belt. Orographically the the Pariahuanca Formation is just above the Huayhuash belt is the most important structure river at 3,500 m. As the fold goes over the in the Cordillera Huayhuash, because the axial Llamac-Pacllon ridge (Fig. 8) the axial surface ridge, the continental divide, and the five 6,000 is vertical and both flanks in the Pariahuanca m summits which define the range are carved limestone parallel it to the top of the divide at from it (Figs. 9, 10). As a structure the belt 5,000 m. No trace of the massive Jumasha For- extends from the northern to the southern bor- mation remains in the core and one must con- der of the mapped region over a distance of 45 clude it has been detached and ejected. km and consists of a set of tight folds with the Two anticlines and an intervening syncline over-all aspect of a synclinorium. Structurally follow eastward of the Culebras syncline. The lower and topographically higher than flanking anticlines, particularly in the Chimu Formation, belts to east and west, the belt exposes mainly tend to be straight-flanked and angular, reflect- Jumasha rocks and forms a structural keel for ing massive brittle struts of quartzite interbed- the range. ded with ductile carbonaceous shale. The crests East of Ishpacpampa a syncline catches the in quartzite are commonly broken, and it is Jumasha Formation plunging southward. South from this zone that thrust faults break out along of this point, considered the northern terminus the eastern margin of the belt. of the Cordillera Huayhuash (Fig. 9), the fold From the western flank of the Cordillera has both flanks dipping 45° to 50° into a slightly Huallanca southward to Quebrada Rondoy, the eastward-inclined axial surface. Southward, eastern margin of the Jahuacocha belt is marked dips pass 60° and the Jumasha limestones are by the west-dipping Condor thrust (Figs. 4, 5). relayed southwestward in a series of right en The fault rises out of the core of an anticline in echelon folds. South of Matacancha to the the north, with the Chimu Formation on the southern border of the mapped region the prin- upper plate cutting across the eastern flank of cipal structure is the Yerupaja syncline, from the fold southward so that first Chimu, then which Nevados , Yerupaja Chico, successively Santa, Carhuaz, and eventually Yerupaja, Siula, and are carved. On the Pariahuanca Formations, are omitted at the sur- north face of Nevado Jirishanca the structure Downloaded from gsabulletin.gsapubs.org on May 4, 2011

STRUCTURAL GEOLOGY 1877 has the form of an upright, cuspate fold with debris, but it apparently has no effect on red both flanks dipping 55° to 70° into the axial beds as it passes beneath them. About where surface (Fig. 9)- The axial trace appears to pass the Raura thrust loses its action another thrust several hundred meters east of the summit and breaks out of the Viconga anticline south of continues southward along the east face of Quebrada Nupe, eventually migrating over the Nevado Yerupaja. South of Nevado Yerupaja eastern flank of the fold. North of Laguna the fold appears to nod westward and the east Viconga the fault apparently has disturbed red flank is nearly vertical. South of Nevado Sarapo beds placing Chimu quartzite against them. Far- the fold loses identity and breaks into isoclinal ther south, Tsacra volcanics appear to be also flexures. disturbed by the fault. Although the gross aspect of the Huayhuash Raura Belt. The Raura belt lies east of the belt is simple, details are complex. Numerous Carhuacocha belt and its western margin forms smaller folds flank major flexures and some of the eastern border of the mapped region. The the excessive thickness of Jumasha Formation part of the belt mapped is an eastward over- exposed is certainly due to considerable fault- turned syncline in the Jumasha Formation with ing along and across bedding planes. Little de- overturned dips of 70° west along the western tail could be worked out, because much of the flank.Dips along the eastern flank of the fold belt is confined to elevations over 6,000 m and are 50°, more westerly, and right side up. obscured by glacial ice and snow. Carhuacocha Belt. The Carhuacocha belt Structures in Tertiary Rocks lies east of the Huayhuash belt and enters the Tsacra Volcanic Trough. The Tsacra vol- mapped region just north of 10° 5' south. The canic pile and underlying red beds are warped belt is about 10 km wide and can be traced for into a south-plunging trough which extends 3 5 km to and beyond the southern boundary of from the Tsacra Spur southward beyond the the mapped region. The belt is characterized by mapped region. Along the eastern side of the south-plunging folds bounded on the east by trough, dips range from 15° to 40° or more the east-directed Raura thrust, and bounded on westward, while along the western side dips are the west in the northern part by the west- more gentle southeast and east. These dips directed Puka thrust. The southwestern part of place volcanics below 4,000 m in the floor of the belt is obscured by Puscanturpa volcanic Quebrada Seria from basal contacts near 5,000 rocks and glacial cover. m along the eastern margin of the trough. The The Puka thrust breaks out of the core and western part of the trough laps across the across the western flank of an anticline north of boundary between the Pativilca and Jahuacocha Punta Puka, placing the Chimu Formation belts, and frontal thrusts do not appear to have quartzite dipping 30° eastward over vertical disturbed the volcanics on the Tsacra Spur east beds of the Carhuaz Formation. The trace of the of Punta Llanche. fault has a trend about 20° from strike in the Puscanturpa Volcanic Sheet. Where ob- Carhuaz beds and eventually cuts out about 600 served in the Cordillera Huayhuash the Puscan- m of the formation. South of Punta Puka the turpa volcanics are nearly horizontal, but small upper plate plunges southward, taking Chimu warps and a regional southeasterly dip of sev- and Santa rocks below ground. At the head of eral degrees are evident. The base of the pile Quebrada Ninacocha the fault places upper along the west side is always close to 5,000 m, Carhuaz beds against the Jumasha Formation in while on the eastern side, near Portachuelo de the Yerupaja syncline, then dissipates in the Huayhuash, it is close to 4,800 m. What re- core of a slightly westward-inclined Jirishanca mains of the Puscanturpa volcanics has the as- anticline. pect of a sheet with a regional southeasterly dip The folds east of the Puka-Jirishanca axis of about 5°. plunge gently southward, gathering in succes- sively higher Cretaceous rocks from the Chimu Tectonic Analysis Formation in the north to Jumasha rocks in the The Cordillera Huayhuash has suffered two south. East of these folds is a structural complex "compressional" deformations. The first defor- bounded on the east by the Raura thrust, which mation was the most severe, involving rocks as places Chimu, Santa, and Carhuaz beds against young as the Middle Albian to Turonian Juma- westward-dipping overturned Jumasha rocks of sha Formation at the top of the Cretaceous the Raura belt to the east. South of Quebrada prism. Structures produced are tight folds and Nupe the Raura thrust is obscured by glacial thrust faults. Later deformation involved rocks Downloaded from gsabulletin.gsapubs.org on May 4, 2011 1878 P. CONEY—EVOLUTION OF CORDILLERA HUAYHUASH, PERU

as young as the Tsacra volcanics and was much ported by the fact that this "basement" is never less intense. Structures produced are mainly brought to surface exposure in the Cordillera broad warps or folds, and there was reactiva- Huayhuash. These relations suggest the Oyon tion or initiation of thrusting on some faults. Formation has served as a zone of local or re- The mechanism of folding in Cretaceous gional detachment (Dahlstrom, 1969), or that rocks during the first deformation was domi- the "basement" has deformed in a fashion quite nantly flexural-slip (Donath and Parker, 1964), distinct from overlying rocks (Compton, becoming flexural-flow where thick ductile 1966). The fact that this "basement" is never units such as the Oyon and Pariatambo Forma- exposed anywhere in the Cordillera Huay- tions were involved. Strong intraformation huash prevents further speculation. ductility contrast and bedding anisotropy Margins of structure belts are marked by (Donath and Parker, 1964), such as that ob- thrust faults of some magnitude. These faults tained by massive struts of quartzite and thin rise out of cores of anticlines placing one flank ductile shale in the Chimu Formation at the over and occasionally omitting the other flank. base of the Cretaceous prism, has resulted in With the exception of the Puka thrust they are sharply angular anticlines by slip between east-directed west-dipping faults with attitudes quartzite struts along shale horizons. Synclines of 50° to 85° as seen at the surface. The faults in the Chimu Formation tend to be more open often lie parallel to bedding planes in the upper (Fig. 4, section B-B'). Breaking at angular anti- plate, particularly where the Chimu Formation cline crests initiated thrust faulting out of anti- was involved. There is a marked steepening of clinal cores. Higher in the Cretaceous section, fault dips southward through the range coinci- near the top of the folded prism, synclines dent with a gentle southerly regional plunge, become sharp and cuspate (Fig. 9). The over-all tightening of folds, and narrowing of structural fold geometry is thus broadly concentric to an- belts. gular, the angularity occurring in anticlines As shown on structure sections (Fig. 4) the near the base of the prism and in synclines near several thrust faults are interpreted as steeply to the top. moderately dipping thrusts which rapidly Two stratigraphic units, the Oyon Formation become low-angle overthrusts within basal at the base of the Cretaceous prism, and the Cretaceous Oyon Formation carbonaceous Pariatambo Formation higher in the section, shale. This inference is based mainly on inter- have behaved in a ductile fashion, relative to pretation of the general tectonic style of the overlying formations. This has permitted devel- region as displayed in fold geometry which sug- opment of disharmonic fold geometry, in that gests local or regional detachment at the level fold form above these two units can be quite of Oyon shale, and on consideration of fault distinct from that found below. In the case of geometry when projected to depth. It can be the Pariatambo Formation the effect is local, in noted on section A-A' (Fig. 4) that the Condor that folds such as the Culebras syncline (Figs. 4, thrust is drawn passing rapidly to a low-angle 8) display isoclinal geometry in the Paria- overthrust under the folded eastern margin of huanca Formation with a sheared core of Paria- the Jahuacocha belt. The base of the Chimu tambo rocks. The massive Jumasha carbonates Formation is well controlled by the Santa For- have apparently become detached from under- mation exposed in Quebrada Asia on the upper lying rocks and ejected from the fold. Indeed, plate west of the fault trace. Dips in the Chimu much of the detached Jumasha rocks may have Formation remain near 50° west up to the fault glided eastward from the Jahuacocha belt and trace and any reasonable projection of the Santa piled up in the Huayhuash belt. The influence Formation on the upper plate and on the foot of the Oyon Formation has been more wide- wall indicate a dip separation on the Santa For- spread throughout the Cordillera Huayhuash mation of at least 2,000 m, and it could be and may have regional significance as well. This much more depending on how far the upper is suggested by the fact that in cores of anti- plate has ridden eastward. In an alternate inter- clines, as shown on structure sections (Fig. 4), pretation, if the Condor thrust is projected to it is difficult to include any considerable volume depth as a 50° west-dipping upthrust rooting in, of rock greater than that to be expected from and displacing the "basement" (Fig. 12), the Oyon shale. The geometric result is that fold dip separation on the base of the Chimu Forma- amplitude in the cretaceous prism is apparently tion is less than 1,000 m, which is less than considerably greater than undulations in the one-half that calculated on the Santa Formation. underlying "basement," an interpretation sup- Such disparate dip separation on two strati- Downloaded from gsabulletin.gsapubs.org on May 4, 2011 TECTONIC SIGNIFICANCE OF LAND-MASS DISTRIBUTION 1879

5000- -5000

4000- -40OO

3000-

7000-

Figure 12. Structure-section across Condor thrust pare with structure-section A-A', Fig. 4). showing alternative "deep seated rooting" of fault (corn- graphic horizons separated by only 650 m of particularly in early deformation (Coney, 1968, strata is difficult to explain without undue com- 1969). At Huallacocha Lakes in central Peru plication in fault geometry for which there is no the Pariatambo Formation served as a lubricat- surface evidence. Merging the fault surface ing horizon for detachment and gravity sliding with the level of detachment in the Oyon shale (Szekely, 1967, p. 1351). Northward from eliminates all displacement of the basement and Oroya, particularly from near Oyon northward seems more reasonable. into the Cordillera Huayhuash, the Oyon is the Some faults, such as the thrust which breaks detachment plane (Wilson, 1963, p. 13). It is from the core of the Viconga anticline (Fig. 4, significant that east of the line where the Goyl- section C-C'), are more difficult to interpret. larisquisga Group thins and loses Oyon car- Significantly, as well as having generally bonaceous shale, folding is more open and steeper dips than faults like the Condor thrust, evidence for imbricate thrusting and detach- they appear to have disturbed continental red ment of the Cretaceous prism is less obvious. beds and Tsacra volcanics. It seems possible Also, east of this same line the "basement" ap- that during post-Tsacra volcanics deformation pears to be involved in deformation along with these thrusts were steepened in dip by rotation Tertiary rocks and the Mesozoic sedimentary during deformation, and perhaps reactivated to prism (Harrison and Wilson, I960, see their cut post-Cretaceous rocks. It is also possible that Fig. 5). they may have initiated movement during later deformation, rooting deeper in the "base- TECTONIC SIGNIFICANCE OF ment," which was broadly folded as well. The LAND-MASS DISTRIBUTION wave length of these folds as expressed in the General Statement broad fold in the Tsacra volcanic pile in the southwestern part of the range (Fig. 4) is over When one attains a vantage point above 5,- twice that found in earlier folds in the Creta- 000 m in the Cordillera Huayhuash the striking ceous prism. element in the topography of the region is a Calculations based on structure sections general accordancy of ridge-tops and inter- throughout the range suggest that total shorten- stream divides, and a pronounced flattening of ing in the Cretaceous prism from folding and the horizon east and west. The axial ridge and thrust faulting amounts to about 30 percent in Tsacra Spur stand in isolation up to 1,600 m the north, increasing to as high as 40 percent or above this general level (Figs. 1,7), and deep more south of the deflection to southerly struc- gorges of flanking valleys are cut up to 1,000 m tural trends. These percentages represent up to below this level. The fact that the flat-lying base 15 km of total shortening of the original sedi- of the Puscanturpa volcanic sheet lies close to mentary prism now exposed in the mapped re- this general accordancy suggests these ash flows gion. may have been erupted upon a relatively sub- Evidence for detachment is often noted in dued erosion surface and around the flanks of the central and north-central Andes of Peru, residual monadnock massifs of an ancestral ax- Downloaded from gsabulletin.gsapubs.org on May 4, 2011

, o|< # * Downloaded from gsabulletin.gsapubs.org on May 4, 2011

REGIONAL ASPECTS 1881 ial ridge of the Cordillera Huayhuash. Recent nocks" above a land-mass undergoing vigorous dissection resulting in flanking valleys subse- dissection in flanking valleys. It is suggested quently cut into this maturing landscape to pro- that pronounced flattening near the inflection duce the deep gorges so characteristic of point of curves represents the effect of accord- Andean relief. ancy of ridge-tops, and the strong upward con- cavity above represents the residual massifs of Analysis the axial ridge. The pronounced convexity be- Hypsometric (area-altitude) analysis (Strah- low 4,500 m in the curve for the Cordillera ler, 1952, p. 1117-1141) has been carried out Blanca represents inequilibrium conditions of a on drainage basins in the Cordillera Huayhuash youthful incision of drainage. It is proposed and is presented in Figure 13, A, B, C. The that the accordancy of ridge-tops and subdued topographic data were taken from the topo- uplands represented by the flattening of hyp- graphic map of Kinzl and others (1942), using sometric curves is all that remains of an exten- the Pacllon, Queropalca, and Nupe drainage sive surface of subdued topography developed basins. Figure 13D is a composite curve for the across the region prior to recent erogenic uplift Cordillera Huayhuash, utilizing the entire and deep dissection of the Andean Cordillera. topographic map. This surface may represent the Puna erosion It is to be noted that the four curves for the surface (McLaughlin, 1924, p. 623) extensively Cordillera Huayhuash have low hypsometric preserved southeast of the Cordillera Huay- integrals and are characterized by land-mass huash in central Peru. concentration between 4,000 and 5,000 m, In the Cordillera Huayhuash the Puna ero- above which is a small mass of considerable sion surface appears to have beveled all rocks of relief. Since topographic control in the Cordil- the Cretaceous, leaving the synclinal structure lera Huayhuash extends down to only 4,000 m of the axial ridge as a monadnock in the Juma- above sea level, analysis of flanking valleys sha Formation. It would also appear that parts beyond the range is not possible. The topo- of the Tsacra volcanics were locally residual graphic map of the southern end of the Cordil- over the southwestern part of the range. The lera Blanca (Kinzl and Schneider, 1950) does fact that the base of the Puscanturpa volcanics permit analysis of lower elevations as it covers lies close to the proposed surface suggests these topography from the crest of residual massifs in rocks may have been poured out onto it over the down western-flanking beveled Cretaceous sediments, red beds, and valleys to within several hundred meters of sea some Tsacra volcanics, and around the base of level near the Pacific coast. Figure 13E is the residual massifs. The important point is that the result of hypsometric analysis of the drainage age of the Puscanturpa volcanics, which may be basin of Rio Forteleza, the headwaters of Rio very young, puts a lower limit on initiation of Santa, and the southernmost massifs of the Cor- dissection and uplift of the Andean Cordillera dillera Blanca (Fig. 2). The curve is character- in the region of the Cordillera Huayhuash. ized by considerable landmass up to about 4,500 m, above which is a small mass of high REGIONAL ASPECTS relief. It is to be noted that the upper part of the Pre-Cretaceous rocks are not exposed in the Cordillera Blanca curve strongly resembles the Cordillera Huayhuash, but facies associations in entire curves for the Cordillera Huayhuash, Cretaceous rocks (Benevides, 1956, p. 363; and suggests if similar control existed west of Wilson, 1963, p. 23-31) suggest the present the Cordillera Huayhuash a curve similar to site of the Cordillera Huayhuash stood near the that of the Cordillera Blanca would result. eastern margin of a transition zone from deeper The hypsometric curves prepared are "two- water continental margin conditions west of the cycle" in nature. The upper parts strongly range to miogeoclinal-continental shelf condi- resemble the "monadnock phase" of Strahler tions in the range itself. These conditions ap- (1952, /to Fig. 16; myFig. 13H), and the lower pear to have prevailed at least from Neocomian parts resemble the "inequilibrium phase" through Turonian time. (Strahler, 1952, his fig,. 14; myFig. 13F). In the In the Cordillera Negra, northwest of the Cordillera Huayhuash the axial ridge and Cordillera Huayhuash (Fig. 2), andesite flows Tsacra Spur appear to rise as residual "monad- and dacite tuffs are separated from steeply 4 folded Cretaceous rocks by an angular uncon- Figure 13. Hypsometric (area-altitude) curves for formity, but have been folded with dips up to the Cordillera Huayhuash. 30°. A still younger dacite tuff is undeformed Downloaded from gsabulletin.gsapubs.org on May 4, 2011 1882 P. CONEY—EVOLUTION OF CORDILLERA HUAYHUASH, PERU

(Bodenlos and Ericksen, 1957, p. 26-27). In the batholith of the Cordillera Blanca. Younger the southern Cordillera Blanca, andesitic hypabyssal dikes and sills would perhaps corre- Huanstan volcanics (Egeler and DeBooy, late with other smaller intrusives in the central 1956, p. 24-27) are gently folded above steeply Andes with cooling ages grouping around 6.9 folded Cretaceous rocks. The Huanstan volcan- m.y. ics are thermally metamorphosed by the bath- The Puscanturpa volcanics, here considered olith of the Cordillera Blanca. The Tsacra younger than the Tsacra volcanics and silicic volcanics of the Cordillera Huayhuash are pre- plutons, are correlated with younger, mainly sumed equivalent to older folded andesites in silicic, volcanics throughout the central Andes the Cordillera Negra, folded Huanstan volcan- of Peru, grouped as Sillipaca volcanics by Hos- ics in the Cordillera Blanca, and older, gener- mer (1959, p. 149-154; see also Jenks, 1946, p. ally folded, volcanics found throughout the 371). They would also correlate with generally Andes of Peru, termed the Tacaza volcanics by undeformed dacite tuffs in the Cordillera Hosmer (1959, p. 146-148; see also Jenks, Ericksen, (Bodenlos and Ericksen, 1957, p. 26- 1946, p. 368). 27) and the Cordillera Blanca (Egeler and The continental red beds of the Cordillera DeBooy, 1956, p. 27-28). If the Puscanturpa Huayhuash are more difficult to correlate. If volcanics were fed from silicic dikes, sills, and they are equivalent to the Casapalca Group of small stocks, such as those in the Cordillera central Peru, which is generally disconformable Huayhuash, they may be as young as 6 m.y. old. on Cretaceous marine sediments and intensely deformed along with them, this suggests an STRUCTURAL EVOLUTION early pre-Casapalca deformation in the Cordil- If the above correlations are correct, some- lera Huayhuash. An alternate interpretation time between Turonian and latest Cretaceous would be to assume the red beds of the Cordil- time, marine sedimentation ceased in the Cor- lera Huayhuash are distinctly younger than the dillera Huayhuash. Sometime between the Casapalca Group and equivalent to continental Turonian and middle(?) Tertiary the Cordillera red beds at the base of and within Tacaza vol- Huayhuash must have suffered major deforma- canics. In central Peru the Tacaza sequence is tion, and at this point it is not clear if it was pre-, separated from the Casapalca Group by an an- syn-, or post-Casapalca Group. Regionally it gular unconformity (Hosmer, 1959, p. 145). certainly was all of these. By middle(?) Tertiary Either correlation is possible, but if we assume there was sufficient erosion to remove close to the latter this suggests major deformation in the 50 percent of the folded Cretaceous prism, and Cordillera Huayhuash is post-Casapalca (latest much of the cover of Casapalca beds. Andesitic Cretaceous-earliest Tertiary) and pre-Tacaza volcanism and associated deposition of red beds volcanics, or early to middle Tertiary. followed, flooding the region with Tsacra vol- Plutonic and hypabyssal igneous rocks are canics. The region was again deformed, in- widespread in the central Andes of Peru (Jenks, truded by batholiths in late Miocene(P), and 1956, p. 233-235), the most important of still further eroded to a region of subdued relief which is the coastal Peruvian batholith. It in- of the Puna surface with residual massifs of an trudes Cretaceous marine sediments and has ancestral axial ridge of the Cordillera Huay- been presumed to be Late Cretaceous to very huash. Possibly as recently as 6 to 7 m.y. ago, early Tertiary in age (Petersen, 1958, p. 83; in Pliocene time, Puscanturpa volcanism, fed Hosmer, 1959, p. 173), apparently confirmed by silicic intrusives, flooded the region with ash by radiometric dates of 64 m.y. (Giletti and flows. In the last 6 m.y.(?), massive uplift has Day, 1968, p. 571). Smaller intrusives along carried the Andes to present heights, causing the continental divide and eastward have been deep dissection and eventually permitting onset considered distinctly younger than the coastal of Pleistocene glaciation. batholith (Petersen, 1958, p. 84-85; Jenks, 1956, p. 234-235; Hosmer, 1959, p. 181). Sev- ACKNOWLEDGMENTS eral of these intrusions have been dated giving I am most grateful to Victor Benevides- ages ranging from 9.1 m.y. for the batholith of Caceres for much assistance, including valuable the Cordillera Blanca to ages averaging about stratigraphic information. John Wilson kindly 6.9 m.y. for smaller stocks south of cordil- provided a summary of Cretaceous stratigraphy lera Cordillera Huayhaush and east of Lima in the central Andes of Peru prior to publica- (Giletti and Day, 1968). The silicic plutons of tion of his own report. Andres Bravo-Bresani, the Cordillera Huayhuash are correlated with Director of Mines in Peru, provided financial Downloaded from gsabulletin.gsapubs.org on May 4, 2011 REFERENCES CITED 1883 assistance. Dr. Issac Tafur, then director of the Giletti, B. J., and Day, H. W., 1968, Potassium- Comision Carta Geologica Nacional in Peru, argon ages of igneous intrusive rocks in Peru: provided transportation, aerial photographs, Nature, v. 220, p. 470-572. Harrison, J. V., and Wilson, J. J., I960, Geologiade use of office facilities, and a field assistant. Inter- la region comprendida entre Huacho y Vinchos: national Petroleum Company, Ltd., shipped Geol. Soc. Peru Bol., v. 35, p. 5-62. hand-specimens to the United States, and the Heim, A., 1948, Wunderland Peru: Bern, Verlag New Mexico Geological Society covered cost Hans Huber. of thin-sections and photography. Vincent C. Hosmer, H. L., 1959, Geology and structural deve- Kelley, Wolfgang Elston, Stuart Northrup, and lopment of the Andean system of Peru: Ph.D. Sherman Wengerd of the University of New thesis, Michigan Univ., Ann Arbor. Mexico provided much assistance and encour- Jenks, W. F., 1946, Preliminary note on geologic agement. Grants from Socony Mobile Co. and studies of the Pacific slope in southern Peru: Am. Jour. Sci., v. 244, p. 367-372. Pan American Petroleum defrayed costs. Luis 1956, Peru, mjenks, W. F., ed., Handbook of Vargas, of the Carta Geologica Nacional, Peru, South American geology: Geol. Soc. America served ably as field assistant, and Emilio An- Mem. 65, p. 219-247. geles, of Huaras, was a skillful packer and Kinzl, Hans, (no date), Cordillera Hauyhuash, Peru: guide. Countless native people throughout the Innsbruck, Verlag Tiroler Graphik. Cordillera Huayhuash showed kindness and Kinzl, Hans, and Schneider, E., 1950, Cordillera hospitality. Lastly, I am deeply grateful to my Blanca (Peru), with topographic map, scale 1:- wife who accompanied the expedition and 200,000: Innsbruck, Universitats-Verlag Wag- made a major contribution to its success. ner. Kinzl, Hans, Schneider, E., and Ebster, F., 1942, Die REFERENCES CITED karte der Kordillere von Huayhuash (Peru): Bellido, B. E., Narvaez, L. S., and Simons, F. S., Zeitschr. Ges. Erdkd., heft 1/2, p. 1-35. 1956, Mapa geologica del Peru: Geol. Soc. Mabire, B., 1961, Laseriede "capas rojas" Cretaceo- Peru, scale 1:2,000,000. Terceiarias en los Andes centrales del Peru: Benevides-Caceres, Victor, 1955, Cretaceous system Geol. Soc. Peru Bol., v. 36, p. 131-186. in northern Peru: Am. Mus. Nat. Hist. Bull., v. McLaughlin, D. H., 1924, Geology and physiogra- 108, art. 4, p. 359-493. phy of the Peruvian Cordillera, departments of Bodenlos, A. J., and Ericksen, G. E., 1955, Lead-zinc Junin and Lima: Geol. Soc. 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