Geological studies in CordilleraDarwin, Tierra del Fuego,

southern Chile: R/VHero cruises77-4 and 78-2

ERIC P. NELSON, I. W. D. DALZIEL, andW. IAN RIDLEY

Lamont-Doherty Geological Observatory of Columbia UniversityPalicades, New York 10964

Cruise 77-4 of &v Hero, in July 1977, marked the be-ginning of the first detailed geological study of the Cor-dillera Darwin massif in the Chilean Andes in almost 40years—that is, since the study undertaken during theFinnish expedition of 1928-29 (Kranck, 1932).

This new study was continued during iJv Hero cruise78-2 (May and June 1978). We collected detailed fielddata, mainly from excellent coastal exposures alongnearly all the channels surrounding and penetratingCordillera Darwin. Given the extent of the massif icecover, these exposures constitute a large proportion ofthe exposed rock in the cordillera (see accompanyingmap). In addition, we mapped the rocks and structuresof Cordillera Darwin on a scale of 1:100,000, con-structed numerous cross sections of the massif, and col-lected material for laboratory analysis.

One of the underlying reasons for our studies was thatthis region is of major importance in understanding theevolution of the southernmost Andes and, in turn, theScotia Arc as a whole, in that the region exposes me-dium-grade (and, locally, high-grade) metamorphic rocksand extensive areas of pre-Middle or Upper Jurassicbasement. In the southern Andes not only are these fea-tures unique but so are the intensity and complexity ofthe late Mesozoic "early Andean" strains displayedthere. Geotectonically, Cordillera Darwin affords an un-usual opportunity to study the deformation and meta-morphism of terrain transitional between a marginalbasin and the adjacent continental margin along a west-ern Pacific type of convergent plate margin.

Prior to 1k/v Hero cruise 77-4, there were four majorgeological questions associated with Cordillera Darwin.

444"N

- LPATAG^ONIA

OEL

Coradlero

Cap. Horn

UNOEFORMED GRANITICROCKS? f7'00 Ep M Spto METAVOLCANiC COVER

J METASEDIMENTA PT COVER

LE110 SASEIDENT COMPLEX

+ ROCKS

PEAK S

OVER 2000 METERSWITHIN ICE CAP

Geology of Cordillera Darwin.

As a result of our work, we are now in a position tosupply answers. The questions and our answers are sum-marized below.

I. Do the so-called Central Schists of Cordillera Dar-win mapped by Kranck (1932) represent deformed andmetamorphosed upper Mesozoic cover rocks or do theyactually represent older pre-Middle or Upper Jurassicbasement?

The basement/cover unconformity previously recog-nized in Cordillera Darwin and elsewhere in the south-ern Andes was traced all along the northern margin ofCordillera Darwin. Lithologic correlation also permits itsrecognition in the southern part of the range, althoughthere the contact is everywhere tectonized and the basalbreccia of the cover sequence was not observed. Ournearly continuous mapping of the contact around Cor-dillera Darwin demonstrates that most of the CentralSchists therefore represent reactivated pre-Middle orpre-Upper Jurassic basement rocks (see map).

2. What is the lithologv of the basement rocks andwhat information can this provide concerning the earlyhistory of the southernmost Andes?

The basement complex consists of greenschist to am-phibolite grade metamorphic rocks of variable litholo-gies, including pelitic and silicic phyllite and schist, me-taquartzite and metachert, silicic metatuffs, andgreenstones. Lithologic similarity between the basementand the metavolcanic and metasedimentary cover indi-cates comparable paleogeographic settings along the tec-tonically active Pacific margin of southern South Amer-ica, both before and after the hiatus represented by theunconformity.

3. What is the basic geologic structure of CordilleraDarwin and how did it evolve?

Both basement and cover have been affected by atleast two and possibly three phases of strong "early An-dean" (i.e., mid-Cretaceous) deformation that gener-ally obliterates any pre-Andean fabric within the base-ment. The reactivated basement rocks are exposed inthe core of a horst-like structural culmination betweennorthward-verging Andean folds in the north andsouthward verging Andean folds in the south, indicatingthat major crustal shortening has occurred in the area.

4. What are the field relations and significance of thegranitoid rocks so conspicuous in the moraines aroundthe cordillera and previously referred to in the literatureas the Cordillera Darwin granites?

The granitoid rocks of Cordillera Darwin fall into twocategories. First, there are tonalite and quartz monzoniteplutons cutting the basement rocks that were themselvescut by a swarm of mafic sheets. These were deformedand metamorphosed together with the mafic sheets dur-ing the Andean orogeny to form a granitic gneiss-am-phibolite complex. Second, there are posttectonic gran-itic intrusives that are most common in the central andsouthern parts of the cordillera and also form the high -est peaks (see map).

In addition to providing bases for the above answers,our fieldwork also enabled us to make three other sig-nificant observations regarding Cordillera Darwin. First,the volcanic rocks of the cover in the south contain asignificantly higher proportion of intermediate to maficmaterial than those in the north. Second, most of thebasement complex and much of the lower cover Se-

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quence have been affected by low-grade (greenschist)recrystallization associated with the early Andean strains.The highest grade metamorphic rocks (upper amphib-olite facies), and local migmatites, are found in closeproximity to the early (pre-Andean) granitic rocks.Third, local growth of porphyroblastic minerals post-dates early Andean (D 1 and D2) structures, whereas me-tamorphism associated with the early granitic rocks musthave predated the D 1 deformation. Thus, either thehigh-grade metamorphic history in Cordillera Darwinwas long-lived or else there was a significant break inthe thermal history.

Our work was begun in July 1977 (iJv Hero cruise 77-4) by R. Forsythe, E. Nelson, and T. Wilson (Lamont-Doherty Geological Observatory), together with F. Hervéand E. Valenzuela (University of Chile) and M. Suárez

(Institute of Geological Investigations of Chile) (Nelsonet al., 1977). It was continued during cruise 78-2 by theauthors of this paper in conjunction with A. G. Milnes(the Swiss Federal Institute, Zurich), C. Mpodozis (Uni-versity of Chile), R. Guzman (Institute of Geological In-vestigations of Chile), and L. Oviedo (University of Con-cepcion).

ReferencesKranck, E. H. 1932, Geological investigations in the cordillera

of Tierra del Fuego. Acta Geographica Helsingfors, volume 4,number 2.

Nelson, E., R. Forsythe, F. Hervé, M. Suárez, E. Valenzuela,and T. Wilson. 1977. Observaciones Estructurales en LaCordillera Darwin, Provincias Antarctica y de Tierra delFuego: Crucero 77-4 del &v HERO. Notas Cientijicas, Go-municaciones, 21: 32-35.

Preliminary palynologicalinvestigation of Upper Paleozoic

and Mesozoic rocks in theAntarctic Peninsula area

ROSEMARY A. ASKIN

Institute of Polar StudiesThe Ohio State UniversityColumbus, Ohio 43210

During the 1979-80 austral summer it is planned tocollect rock samples suitable for palynologic study in thenorthern Antarctic Peninsula and South Shetland Is-lands in an attempt to obtain palynomorph assemblagesfor biostrati graphic and paelogeographic interpretation.Prior to this fieldwork a reconnaissance study has beenmade on some samples collected by David H. Elliot andother members of previous Institute of Polar Studiesexpeditions to the region, as well as on samples fromLivingston Island collected by Margaret A. Winslow ofthe Lamont-Doherty Geological Observatory, ColumbiaUniversity.

The samples processed are generally devoid of paly-nomorphs, although some contain carbonized woodyfragments and other plant debris. Many of these samplesare from the deformed Trinity Peninsula Formation andequivalents, and presumably are too highly metamor-phosed for palynomorph preservation, judging from theblack, opaque nature of the dispersed plant material.

One sample from the Gibbon Bay Shale of CoronationIsland, South Orkney Islands (Daiziel et al., 1977)yielded common, although poorly preserved, palyno-morphs. These include trilete spores and bisaccate pol-len, but unfortunately no age diagnostic species havebeen identified.

Four samples from Patella Island, northern AntarcticPeninsula (Elliot et al., 1978) also contain poorly pre-served palynomorphs. These are from a shale unit be-lieved to be equivalent to the plant-bearing Upper Jur-assic shales that crop out on Mount Flora at Hope Bay(Elliot, pers. comm.). Most of the palynomorphs are un-identifiable; those with recognizable morphological fea-tures include an assortment of trilete spores and bisac-cate pollen.

Many samples, including those from different strati-graphic units, have yet to be processed, and it is hopedthat a more systematic collection of suitable material spe-cifically for palynological study will increase the pros-pects for recovery of identifiable palynomorphs.

This study has been supported by National ScienceFoundation grant DPP 76-83030.

ReferencesDalziel, I. W. D., D. H. Elliot, J. W. Thomson, M. R. A. Thom-

son, N. A. Wells, and W. J. Zinsmeister. 1977. Geologic stud-ies in the South Orkney Islands: it/v Hero Cruise 77-1, Jan-uary 1977. Antarctic Journal of the United States, 12(4): 98-101.

Elliot, D. H., D. R. Watts, R. B. Alley, and T. M. (;racanin.1978. Geologic studies in the northern Antarctic Peninsula,iJv Hero cruise 78-1B, February 1978. Antarctic Journal ofthe United States. 13(4): 12-13.

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