Structural Geology and Tectonics 

of the Orville Coast Region, 

Southern Antarctic Peninsula, 

Antarctica

Work done in cooperation with the 

National Science Foundation

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1498
COVER View to the northeast of chevron folds in the Latady Formation along a north-trending 

ridge in the northern Hauberg Mountains, 34 km east of Janke Nunatak. Pronounced southerly 

vergence is visible. Length of ridge is about 5 km.
Structural Geology and Tectonics 

of the Orville Coast Region, 

Southern Antarctic Peninsula, 

Antarctica

By KARL S. KELLOGG and PETER D. ROWLEY

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1498

Work done in cooperation with the 

National Science Foundation

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1989
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, industry, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Kellogg, Karl 8.

Structural geology and tectonics of the Orville Coast region, southern Antarctic Peninsula, 

Antarctica

(U.S. Geological Survey professional paper ; 1498)

Bibliography: p.

Supt. of Docs, no.: I 19.16:1498

1. Geology, Structural. 2. Geology Antarctic regions Orville Coast Region. I. Rowley, Peter 

D. II. National Science Foundation (U.S.) III. Title. IV. Series. 

QE601.K367 1989 551.8'0998'9 88-607945

For sale by the Books and Open-File Reports Section, U.S. Geological Survey 

Federal Center, Box 25425, Denver, CO 80225
CONTENTS

Abstract ........................................... 1

Introduction ....................................... 1

Acknowledgments ............................... 3

Geologic setting .................................... 3

Structural geology .................................. 6

Structures of the Latady and Mount Poster Formations 6

Folds ...................................... 6

Axial-plane cleavage .......................... 8

Thrust faults ................................ 8

Joints ...................................... 8

Quartz-filled fractures in sedimentary rocks ...... 9

Structures of the Lassiter Coast Intrusive Suite ..... 12

Joints ...................................... 12

Dikes ...................................... 13

Page

Structural geology Continued

High-angle faults ................................ 13

Paleomagnetic evidence for relative rotation about vertical axes

in the Sweeney and Hauberg Mountains .......... 14

Discussion ......................................... 18

The Latady basin as a back-arc rift ................ 18

Opening of the Weddell Sea ....................... 19

Origin of the Palmer Land deformational event ...... 19

Lassiter Coast Intrusive Suite ..................... 21

Southern tectonic boundary of the Orville Coast ..... 21

Tertiary extension: Uplift, block faulting, and alkalic

volcanism ................................. 22

Conclusions ........................................ 22

References cited .................................... 23

ILLUSTRATIONS

FIGURE 1. Index map of part of West Antarctica showing major geographic features discussed in text ...................... 2

2. Sketch map of Antarctica showing proposed continental crustal blocks of West Antarctica ....................... 3

3. Geologic map of the Orville Coast and eastern Ellsworth Land ............................................... 4

4. Chart showing geologic and tectonic events for the Orville Coast region ....................................... 5

5-8. Photographs of folds in the Latady Formation:

5. Prominent anticline and syncline on west face of Mt. Dewe, eastern Hauberg Mountains .................... 6

6. Chevron folds along a ridge 28 km east of Janke Nunatak, northwestern Hauberg Mountains ................ 6

7. Tight folds on northern part of ridge that contains Mt. Dewe, eastern Hauberg Mountains .................. 6

8. Tight recumbent syncline cut by high-angle apparent normal faults on flank of Mt. Leek, northeastern Hauberg Mountains ........................................................................................ 6

9. Contoured equal-area projections of poles to bedding (S0) in the Latady and Mount Poster Formations ............. 7

10. Photograph of fine-grained sandstone and shale of the Latady Formation on west face of Mt. McKibben, McCaw Ridge

area ............................................................................................. 8

11. Equal-area projections of fold axes in the Latady Formation ................................................. 9

12. Photograph of steeply plunging fold in the Latady Formation at contact with a granodiorite pluton that underlies the eastern

Thomas Mountains ............................................................................... 9

13-19. Contoured equal-area projections of:

13. Poles to axial-plane cleavage (S:) in the Latady and Mount Poster Formations ............................ 10

14. Poles to fracture cleavage in massive rhyodacite porphyry of the Mount Poster Formation .................. 10

15. Poles to joint surfaces in folded layered rocks of the Latady Formation .................................. 11

16. Poles to joint surfaces in massive rhyodacite porphyry and layered volcanic rocks of the Mount Poster Formation 11

17. Poles to quartz-filled fractures in sedimentary rocks of the Latady Formation ............................. 12

18. Poles to joint surfaces in stocks .................................................................... 12

19. Poles to dike orientations .......................................................................... 13

20. Rose diagram of strike directions of all observed high-angle faults cutting the Latady Formation in the Orville Coast and

eastern Ellsworth Land ............................................................................. 13

21. Paleomagnetic poles for 111-to 103-Maplutons from eastern Ellsworth Land, Sweeney and Hauberg Mountains, Lassiter

Coast, and Shetland Islands ......................................................................... 16

22. Demagnetization diagrams for four selected samples from the Behrendt Mountains ............................. 17

23. Schematic cross section showing major tectonic elements in Late Jurassic and middle Cretaceous time ............ 18

24. Diagram of hypothetical plate configuration of the Antarctic Peninsula area during the Late Cretaceous and early Tertiary 20

in
IV CONTENTS

TABLES

Page 

TABLE 1. Paleomagnetic and geochronologic data from mid-Cretaceous rocks from the Behrendt and Merrick Mountains, eastern

Ellsworth Land .................................................................................... 15

2. Paleomagnetic and geochronologic data from mid-Cretaceous rocks from the Antarctic Peninsula .................. 15
STRUCTURAL GEOLOGY AND TECTONICS OF THE ORVILLE COAST REGION, 

SOUTHERN ANTARCTIC PENINSULA, ANTARCTICA

By KARL S. KELLOGG and PETER D. ROWLEY

ABSTRACT

The oldest rocks in the Orville Coast of the southern Antarctic Peninsula and in adjacent parts of eastern Ellsworth Land belong to a Middle and Late Jurassic calc-alkalic magmatic arc that developed along 

the axial part of the peninsula. The main exposed product of the 

magmatism, which developed in response to eastward subduction 

beneath the Pacific margin of the peninsula, is mostly andesitic to 

dacitic volcanic rocks of the Mount Poster Formation. These rocks 

interfinger southeastward with Middle and Upper Jurassic sedimentary rocks of the Latady Formation, deposited in a back-arc basin. 

Deposition was concurrent with early stages of opening of the Weddell 

Sea, during which time the Antarctic Peninsula was moving right 

laterally with respect to East Antarctica along the Weddell fault, a 

proposed transform east of the peninsula that is continuous with the 

Transantarctic rift system.

The Latady and Mount Poster Formations were strongly folded 

during Late Jurassic and (or) Early Cretaceous time during the Palmer 

Land deformational event. Fold axes are parallel to the prominent bend 

or orocline in the southern Antarctic Peninsula, and vergence is to 

the south or southeast. Axial-plane cleavage is well developed in finer 

grained rocks, as is a set of extension joints oriented perpendicular 

to fold axes. Thrust faults with apparently small displacement dip 

to the north or northwest in the Hauberg and Wilkins Mountains. 

These features suggest that deformation is thin skinned, similar to 

fold-and-thrust belts elsewhere, and was perhaps caused by gravitational spreading from the thickened magmatic arc. A collisional origin 

for the Palmer Land deformational event, however, cannot be discounted. For example, the deformational event may be related to the 

Rangitata orogeny recognized in New Zealand, which is thought to 

have resulted from the suturing of two tectonostratigraphic terranes, 

or it may have resulted from the collision of the Antarctic Peninsula 

magmatic arc with the Gondwana craton, which now lies to the south 

of the Orville Coast.

Renewed arc magmatism, resulting in emplacement of gabbroic to 

granitic plutons of the Lassiter Coast Intrusive Suite, began before 

115 Ma, sometime after folding ceased, and continued for at least 15 

m.y. Dikes of similar composition to the plutonic rocks and anastomosing, randomly oriented, hydrothermal quartz veins were intruded 

during late stages of plutonism; copper mineralization associated with 

the late hydrothermal activity took place in the Sky-Hi Nunataks and 

Merrick Mountains. Dikes are generally oriented parallel to the 

extension-joint direction in sedimentary rocks in the Orville Coast but 

are oblique to that direction in eastern Ellsworth Land. The dikes may 

be related to northwest-directed shear produced by early stages of 

movement along the Ellsworth fault system, an extinct transform fault

Manuscript approved for publication October 28, 1988.

that is thought to have formed a major crustal discontinuity between 

the peninsula and ice-covered regions to the southwest.

Small-displacement, predominantly right-lateral strike-slip faults, 

oriented about N. 70° W., are probably satellitic to the right-lateral 

Ellsworth fault system. Drag along the Ellsworth fault system may 

account for clockwise-rotated paleomagnetic directions from Lower 

Cretaceous intrusive rocks in the Sweeney and Hauberg Mountains 

of the Orville Coast; the directions are rotated about 50° compared 

to those from similar-age rocks elsewhere on the Antarctic Peninsula. 

The Ellsworth fault system may be a continuation of the Tharp fracture zone in the southeast Pacific Ocean, which was active between 

about 85 and 50 Ma. The southern peninsula underwent deep erosion 

and peneplanation in the Late Cretaceous and early Tertiary, followed by gradual arching and rifting. Continuation of extension into the 

late Tertiary is suggested by occurrences of alkalic basalt of apparent 

late Miocene and Pliocene age.

INTRODUCTION

The Orville Coast is a region of isolated mountain 

ranges and nunataks along the southeastern coast of 

Palmer Land, Antarctic Peninsula, adjacent to the 

Ronne Ice Shelf (fig. 1). The rocks of the Orville Coast 

region define the southernmost limit in the Antarctic 

Peninsula of a Mesozoic and Cenozoic orogenic belt that 

is continuous with the Andean orogenic belt of South 

America. The orogenic belt is characterized by 

widespread volcanism, calc-alkalic plutonism, and deformation. This igneous and tectonic belt bends west 

through the Orville Coast and eastern Ellsworth Land 

and continues along the Pacific coast of Ellsworth Land 

(Rowley and others, 1985, in press) and Marie Byrd 

Land (LeMasurier and Wade, 1976). Plate tectonic 

reconstructions indicate that New Zealand forms part 

of a small continental-crustal plate (Weissel and others, 

1977) that was attached to Marie Byrd Land prior to 

80 Ma. Therefore many Mesozoic structures of New 

Zealand may have been part of the same orogenic belt 

recognized in the Antarctic Peninsula (Cooper and 

others, 1982).

The change in trend of the orogenic belt around the 

geographic bend of the southern Antarctic Peninsula 

is referred to as an orocline in this report. The original

1
STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA

60° W

EXPLANATION

i r- Ice shelf

FIGURE 1. Index map of part of West Antarctica showing major geographic features discussed in text.

definition of orocline incorporated the idea that such 

curvature is due to horizontal bending of the crust or 

"deformation in plan" (Carey, 1958). No such genetic 

interpretation is implied by our use of the term orocline, 

although, as we shall discuss, deformation in plan locally may have occurred.

Most rocks and geologic structures of the southern 

Antarctic Peninsula reflect subduction beneath the west 

side of the peninsula during Mesozoic and perhaps late 

Paleozoic time (Weissel and others, 1977; Barker, 1982; 

Storey and Garrett, 1985). An axial, calc-alkalic 

magmatic-arc sequence, as old as Early Jurassic (Thomson, 1982; Rowley and others, 1983), and a Middle and 

Upper Jurassic sedimentary back-arc basin sequence 

(Suarez, 1976; Laudon and others, 1983; Thomson, 1983) 

are well represented in the area. On the Pacific side of 

the peninsula, a late Mesozoic accretionary wedge

(LeMay Group; Tranter, 1986, in press) and fore-arc 

basin (Fossil Bluff Formation; Taylor and others, 1979) 

are well exposed on Alexander Island. Whereas 

subduction-related features in Palmer Land are almost 

entirely of Mesozoic age, similar features in Graham 

Land are as young as Tertiary (Thomson and 

Pankhurst, 1983). This northward younging of 

magmatic events is related to an incremental northward 

cessation of subduction as transform-bounded segments 

of the Aluk-West Antarctic plate intersected the trench 

along the west side of the peninsula (Barker, 1982). Subduction ceased beneath the southern peninsula about 

50 Ma (Barker, 1982); however, subduction is still occurring adjacent to and north of the Shetland Islands 

(Weissel and others, 1977).

The Antarctic Peninsula is one of several crustal 

blocks that have been proposed to make up West
GEOLOGIC SETTING

Antarctica (Dalziel and Elliot, 1982; Dalziel and others, 

1987) (fig. 2), although the blocks probably are 

separated by small extensional basins along which there 

has been a minimum of transcurrent movement (Storey, 

in press). Orville Coast and eastern Ellsworth Land are 

centrally located within the proposed cluster of crustal 

blocks, which underlines the importance of understanding the geology of this region.

The geology of eastern Ellsworth Land was discussed 

by Laudon and others (1970) and Laudon (1972). Rowley 

(1978) and Rowley and others (1983) summarized the 

geologic history of the Orville Coast and eastern 

Ellsworth Land, including a general discussion of some 

structural features. Stanek (1987) reported observations 

made on the geology of the eastern Orville Coast. Thomson and others (1978) and Laudon and others (1983) 

discussed the sedimentary rocks of the Latady Formation, and Thomson (1983) described fossils from the 

Latady Formation. The bearing of paleomagnetic data 

from Cretaceous intrusive rocks on possible oroclinal 

bending of the southern peninsula has been given by 

Kellogg (1980).

ACKNOWLEDGMENTS

We are grateful to all members of the Orville Coast 

expedition (1979-1980), J.M. Boyles, P.E. Carrara, T.S. 

Laudon, M.R.A. Thomson, and W.R. Vennum, all of 

whom collected structural data. The input of A.B. Ford, 

Warren Hamilton, R.L. Reynolds, and D.L. Schmidt 

during numerous and stimulating discussions greatly 

improved this report, as did thorough reviews by A.B. 

Ford and D.J. Lidke.

GEOLOGIC SETTING

The geology of the Orville Coast and vicinity is shown 

on figure 3 and the geologic and tectonic events of the 

region are summarized on figure 4; a discussion of many 

of the regional tectonic features shown on figure 4 are 

found in later sections of the report.

The oldest known rocks in the Orville Coast belong 

to locally fossiliferous, fine- to medium-grained, largely volcanogenic siltstone and shale and subordinate 

sandstone of the Latady Formation, first described 

from its type locality in the southern Lassiter Coast by 

Williams and others (1972). In northerly exposures, 

especially in the Sweeney Mountains, interbedded, 

mostly dacitic lava flows and ash-flow tuffs are included 

in the Latady Formation where sedimentary rocks 

predominate. Similar and probably correlative sedimentary rocks in northern and central Palmer Land are

90°W

West 

Antarctica

0°E

180 C

FIGURE 2. Sketch map of Antarctica showing continental crustal 

blocks (shaded) of West Antarctica, as proposed by Dalziel and 

others (1987). AP, Antarctic Peninsula crustal block; TI, Thurston 

Island-Eights Coast crustal block; MBL, Marie Byrd Land crustal 

block; EWM, Ellsworth and Whitmore Mountains crustal block.

called the Mount Hill Formation (Singleton, 1980). The 

Latady Formation is Middle and Late Jurassic in the 

Behrendt Mountains (fig. 3) but is known only to be 

Late Jurassic elsewhere (Thomson, 1983).

The Latady Formation is at least several thousand 

meters thick; neither the top nor bottom has been 

observed. Depositional facies vary from north to south 

from lagoonal, fluvial, and lacustrine near the volcanic 

arc in the Sweeney Mountains, to marine deltaic in the 

Hauberg and Wilkins Mountains, and to marine shelf 

at Cape Zumberge (Thomson and others, 1978; Laudon 

and others, 1983; Thomson, 1983).

Toward the axis of the Antarctic Peninsula, rocks of 

the Latady Formation interfinger with and grade into 

calc-alkalic basaltic to rhyolitic lava flows, ash-flow tuff, 

and subordinate volcaniclastic sedimentary rocks correlated with the Mount Poster Formation of the 

Lassiter Coast (Rowley and others, 1982); the Mount 

Poster Formation is considered part of the Antarctic 

Peninsula Volcanic Group of Thomson (1982). These interbedded sedimentary and volcanic rocks, best exposed 

in the Sweeney Mountains, are interpreted as the interfingering contact between sedimentary rocks of a mostly marine back-arc basin (Suarez, 1976) in the southeast 

(Latady Formation) and volcanic rocks of the Mount 

Poster Formation deposited subaerially in the northwest (Rowley and others, 1983). The Mount Poster Formation has not been directly dated, but the inferred 

interfingering relationship with the Latady Formation 

suggests a Middle to Late Jurassic age.
74° W 72° W EXPLANATION

Alkalic basalt (Late Miocene and Pliocene) 

Lassiter Coast intrusive suite (Middle Cretaceous)Mount Poster Formation (Middle and Upper Jurassic)

Massive rhyodacite porphyry Layered volcanic rocks 

Latady Formation (Middle and Upper Jurassic)

Contact Dotted where concealed 

Thrust fault Sawteeth on upper plate

Fold axis

Horizontal, or direction of plunge unknown

Vertical fracture cleavage In massive 

rhyodacite porphyry

66° W

76° S

8

>

a»

M 

O

FIGURE 3. Geologic map of the Orville Coast and eastern Ellsworth Land. Six areas subdivided for purpose of structural analysis are: A, eastern Ellsworth Land; B, western 

Sweeney Mountains; C, eastern Sweeney Mountains; D, Hauberg Mountains; E, Wilkins Mountains; F, McCaw Ridge area. The southern Lassiter Coast (Scaife Mountains 

area) is not included in the structural analysis.
GEOLOGIC SETTING

MA 

 0

 50

-100

Slow arching and 

block faulting

Alkalic basalts T Glaciation

Peneplanation?

Movement on Ellsworth 

fault system

H Emplacement of Lassiter 

I Coast Intrusive Suite

Palmer Land deformational 

event (folding, thrusting)

150 : 

Mount Poster Formation 

magmatic arc (north)

 i

Latady Formation 

Back-arc basin (south)

200

L 250

Subduction of Aluk-West 

Antarctic Ridge

Subduction along Pacific 

margin of peninsula; 

time of origin unknown

Opening of Weddell 

Sea; movement along 

Weddell fault system

FIGURE 4. Geologic and tectonic events for the Orville Coast region. Dashed line represents uncertain placement and query indicates that

event may extend in age for unknown time.

Massive rhyodacite porphyry occupies much of the 

northern, inland portions of the area westward from the 

northern Sweeney Mountains; similar rocks have been 

mapped as far west as the English Coast (Rowley and 

others, in press). Although no contact has been observed 

between this massive unit and either the Latady Formation or the layered volcanic rocks of the Mount 

Poster Formation, it is correlated with the Mount 

Poster Formation. Rocks of the Mount Poster Formation mapped in eastern Ellsworth Land (area .A of fig. 3) 

are composed largely of unstratified rhyodacite, and except in the area of Olander and Horner Nunataks, the 

rock is porphyritic and contains partially resorbed beta 

quartz and plagioclase phenocrysts as large as 1 cm 

across. This unit probably represents thick lava flows, 

ash-flow tuff, and volcanic domes (Rowley and others, 

1983), or a strongly welded intracaldera tuff.

After deposition of the Mount Poster and Latady Formations, the layered rocks were strongly folded about 

axes that are parallel to the arcuate trend of the 

southern peninsula (fig. 3); the extent of folding in the 

unstratified rhyodacite is unknown. This period of Late 

Jurassic and (or) Early Cretaceous folding was named 

the Palmer Land deformational event by Kellogg and 

Rowley (in press). Folds and local thrusts of this event, 

including those in the Lassiter Coast and the English

Coast of eastern Ellsworth Land, have been described 

by Williams and others (1972), Kellogg and Rowley 

(1974), Kellogg (1979), Rowley and Williams (1982), and 

Rowley and others (1983, in press).

Following the Palmer Land deformational event, folded Jurassic rocks were intruded by calc-alkalic gabbroic 

to granitic stocks and batholiths and magmatic, finegrained dikes of similar composition, all of the middle 

Cretaceous Lassiter Coast Intrusive Suite (Vennum and 

Rowley, 1986). With the exception of a 122-Ma K-Ar 

(potassium-argon) age on hornblende from the Sky-Hi 

Nunataks, rocks of the intrusive suite have been dated 

within the study area by K-Ar or Rb-Sr (rubidiumstrontium) methods as about 113-100 Ma (Halpern, 

1967; Mehnert and others, 1975; Farrar and others, 

1982; Rowley and others, 1988; Pankhurst and Rowley, 

in press). Systematic geographic trends in the ages are 

not apparent in the southern peninsula, although mafic 

phases are generally older (Pankhurst and Rowley, in 

press).

The area of exposed rock of the Lassiter Coast Intrusive Suite is significantly less in the Orville Coast 

than farther northeast on the Lassiter Coast (Vennum 

and Rowley, 1986); this probably reflects more restricted plutonism and (or) less uplift southwestward 

along the axis of the arc.
STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA

Other than upper Tertiary and Quaternary glacial 

deposits, the only Cenozoic material in the region is 

olivine-phyric basaltic lava flows in the Merrick Mountains (Laudon and others, 1970; Vennum and Laudon, 

1988) and at Henry Nunataks. Halpern (1971) reported 

a K-Ar age of 6 Ma for the basaltic rocks of the Merrick 

Mountains.

STRUCTURAL GEOLOGY

We have divided the region into six areas (fig. 3) in 

order to show the changes in different structural 

elements through the oroclinal bend in the Orville Coast 

and eastern Ellsworth Land: A, eastern Ellsworth Land; 

.B, western Sweeney Mountains; C, eastern Sweeney 

Mountains; D, Hauberg Mountains; E, Wilkins Mountains; F, McCaw Ridge area.

STRUCTURES OF THE LATADY AND 

MOUNT POSTER FORMATIONS

FOLDS

The Latady Formation and at least the layered volcanic rocks of the Mount Poster Formation were everywhere folded during the Late Jurassic and (or) Early 

Cretaceous Palmer Land deformational event (Kellogg 

and Rowley, in press). Folds are open to isoclinal, and 

wavelengths of folds range generally from several tens 

of meters to a kilometer or more (figs. 5-8). Chevron 

geometry is common (figs. 6 and 8 and front cover). Fold 

axes are horizontal to gently plunging and are oriented 

parallel to the general trend of the coastline (fig. 3). 

Thus, the structural fabric defines an orocline, striking 

about N. 50° E. in the northeastern Orville Coast, eastwest in the central Hauberg Mountains, and N. 75 ° W. 

in eastern Ellsworth Land. Figure 9 shows contoured 

equal-area projections of poles to bedding (S^) for the six

35* «ESI

FIGURE 5. View to the east of prominent anticline and syncline in 

the Latady Formation on west face of Mt. Dewe, eastern Hauberg 

Mountains. Fold axes are nearly horizontal. Distance along ridge 

is about 2 km.

FIGURE 6. View to the east of chevron folds in the Latady Formation along a small ridge 28 km due east of Janke Nunatak, northwestern Hauberg Mountains. Prominent vergence is to the south. 

Distance along ridge is about 2 km. A flat fault, with no more 

than about 20 m displacement, offsets axial plane of prominent 

anticline in left side of photograph.

FIGURE 7. View to the east of tight folds in the Latady Formation 

on northern part of ridge that contains Mt. Dewe (fig. 5), eastern 

Hauberg Mountains. Vergence is to the south, although this is 

not apparent in the photograph. Distance along ridge is about 

4 km.

FIGURE 8. View to the northeast of tight recumbent syncline cut by 

high-angle apparent normal faults in the Latady Formation on 

flank of Mt. Leek, northeastern Hauberg Mountains. Distance 

along skyline is approximately 0.5 km.
STRUCTURAL GEOLOGY

areas of the Orville Coast and eastern Ellsworth Land. 

Note that the change in strike of fold axes is not 

continuously more northerly toward the northeast; in 

the McCaw Ridge area (fig. 9F) fold axes are more easterly than in the Wilkins Mountains (fig. QE) to the 

southwest.

Many of the folds are asymmetric (figs. 6, 7, and 10) 

and some have overturned limbs (figs. 8 and 10). Axial 

planes generally dip steeply to the north and northwest, indicating compression perpendicular to the peninsula and vergence toward the south and southwest. 

Asymmetric folding is not ubiquitous however; throughout most of the Hauberg Mountains the folds are nearly 

symmetric (figs. 5 and 9Z>), although southern vergence 

is locally common (figs. 6 and 7 and front cover).

Fold axes were determined from pi diagrams, in which 

the best great circle through a set of poles to bedding

is determined, and from bedding-cleavage (SQ-Sj) intersections. Mean directions determined by both methods 

are similar (fig. 11); mean fold axes are horizontal or 

nearly horizontal in all areas.

Local, disharmonic folds in the Latady Formation 

associated with forceful intrusion of Cretaceous plutons 

are rare in the Orville Coast, implying that most plutons 

were passively emplaced. Steeply plunging small folds 

were locally observed, however, in the Thomas Mountains of eastern Ellsworth Land (fig. 12). In contrast to 

the apparent scarcity of evidence for forceful intrusion 

in the Orville Coast region, disharmonic folding adjacent to plutons is common in the northern Lassiter 

Coast (Kellogg and Rowley, 1974). This may simply 

reflect a comparatively deeper level of erosion in the 

northern Lassiter Coast, such that effects of higher 

temperature, more-ductile deformation are exposed.

A, Eastern Ellsworth Land 

N

N=19 

B, Western Sweeney Mountains

N

N=94 

C, Eastern Sweeney Mountains

N

N=152

D, Hauberg Mountains E, Wilkins Mountains F, McCaw Ridge area

FIGURE 9. Contoured equal-area projections of poles to bedding (S0) in the Latady and Mount Poster Formations for the six areas shown 

on figure 3. Successive contours are at 2, 4, 6, and 8 percent of poles per 1 percent area. N is number of observations. Dashed lines 

are estimated best mean girdle for folds in each area. Mean fold axis, normal to this girdle, is also indicated.
STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA

FIGURE 10. View to the northeast of outcrop of fine-grained sandstone (massive, light-colored rock) and shale (cleaved, dark-colored 

rock) of the Latady Formation on west face of Mt. McKibben, 

McCaw Ridge area. Rocks are overturned, on southeast limb of a 

recumbent anticline; axial-plane cleavage (Sj) dips more gently than 

does bedding (S0). A quartz-filled fracture, as wide as 1 cm, is in 

lower part of photograph, "hor" shows horizontal line marked on 

outcrop. Width of outcrop at bottom of photograph is about 1 m.

AXIAL-PLANE CLEAVAGE

Well-developed, penetrative, axial-plane cleavage (S^ 

is ubiquitous in the finer grained sedimentary rocks and 

most layered volcanic rocks of the Latady and Mount 

Poster Formations (fig. 13). Axial-plane cleavage is more 

poorly developed in the less abundant, coarser grained 

rocks of the Latady Formation. The strike of Sj^ directions is generally parallel to the axis of the orocline, and 

the dip of Sj directions reflects the fold symmetry. Dips 

toward the northwest in the Wilkins Mountains and 

McCaw Ridge areas (figs. 10 and 13E-F) contrast with 

the more nearly vertical axial planes in much of the 

Hauberg and Sweeney Mountains (figs. 13B-D),

although north- to northwest-dipping axial planes are 

common in the Hauberg Mountains as well.

A nonpenetrative fracture cleavage, spaced typically 1-10 cm apart, occurs in the massive rhyodacite porphyry of the Sweeney Mountains (fig. 14) and is parallel 

to the Sj direction observed in nearby sedimentary and 

volcanic rocks. However, fracture cleavage was not 

observed in the massive rhyodacite porphyry of eastern 

Ellsworth Land (fig. 3).

THRUST FAULTS

Although spectacular folds are a prominent feature 

in the southern Antarctic Peninsula, thrust faults apparently are not common. A general lack of both 

regional marker beds and continuous, accessible exposures make the recognition of thrusts difficult and 

preclude the estimation of displacements. Nonetheless, 

thrusts with apparently small offsets, dipping about 

20°-40° N., were observed and mapped in the Hauberg 

and Wilkins Mountains (fig. 3). Repetition of fossil 

assemblages in the southern Behrendt Mountains also 

suggests either that thrusts exist there (Laudon and 

others, 1970) or that fossils have been reworked. Stanek 

(1987) reported that thrusts observed in the eastern Orville Coast and southern Lassiter Coast may be Tertiary 

in age, although we found no evidence that suggests 

that thrusting is younger than Early Cretaceous.

JOINTS

Joints in folded, layered rocks of the Latady Formation (fig. 15) are predominantly vertical and are perpendicular to both fold axes (fig. 11) and cleavage directions 

(fig. 13) in folded layered rocks. No other consistent 

joint direction occurs in either sedimentary or layered 

volcanic rocks. The joints formed perpendicular to the 

minimum principal stress direction (a3) and probably 

formed during a period of extension in the a3 direction. 

In many places, infillings of drusy quartz crystals, 

epidote, iron oxides, and (or) chlorite reflect that the 

joints were open for some period after formation. 

Hydrothermal fluids from devolitizing Early Cretaceous 

plutons are probably responsible for much of the 

mineralization along joint surfaces.

Joints in both the layered and massive volcanic rocks 

of the Mount Poster Formation show much poorer 

development of preferred orientation (fig. 16) than for 

joints in the Latady Formation (fig. 15). Alignment is 

best developed in the eastern Sweeney Mountains 

(fig. 16C), where joints strike roughly parallel to the 

extension-joint direction in nearby layered sedimentary
STRUCTURAL GEOLOGY

N

A, Eastern Ellsworth Land

N

B, Western Sweeney Mountains

N

C, Eastern Sweeney Mountains

N

D, Hauberg Mountains E, Wilkins Mountains F, McCaw Ridge area

FIGURE 11. Equal-area projections of fold axes in the Latady Formation for the six areas shown on figure 3. Solid circles represent fold 

axes calculated from bedding-cleavage (S^-S,) intersections; open circles are mean fold axes calculated from pi analysis across individual 

folds.

FIGURE 12. Steeply plunging fold in the Latady Formation at contact with a granodiorite pluton that underlies the eastern Thomas 

Mountains (Laudon and others, 1970). Photograph by T.S. Laudon.

rocks. Except for a slight tendency toward vertical 

orientations, the joint directions in volcanic rocks from 

eastern Ellsworth Land (fig. 16A) and the western 

Sweeney Mountains (fig. 16B) appear nearly random.

QUARTZ-FILLED FRACTURES IN SEDIMENTARY ROCKS

Quartz-filled fractures, as wide as several centimeters, 

cut sedimentary rocks of the Latady Formation in 

anastomosing patterns; quartz-filled fractures were also 

locally observed cutting volcanic rocks of the Mount 

Poster Formation but are not known in plutonic rocks. 

Quartz-filled fractures in sedimentary rocks make up 

more than 50 percent of some outcrops, although such 

zones are generally less than 10m across. The orientation of individual fractures is nearly random (fig. 17); 

it has a slight tendency towards being parallel to the 

predominant joint direction in the sedimentary rocks
10 STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA 

N N N

N=34

A, Eastern Ellsworth Land

N

B, Western Sweeney Mountains 

N

C, Eastern Sweeney Mountains 

N

N=135

D, Hauberg Mountains £, Wilkins Mountains F, McCaw Ridge area

FIGURE 13. Contoured equal-area projections of poles to axial-plane cleavage (S:) in the Latady and Mount Poster Formations for the six 

areas shown on figure 3. Successive contours are at 2, 4, 6, and 8 percent of poles per 1 percent area. N is number of observations.

IM=20

FIGURE 14. Contoured equal-area projections of poles to fracture cleavage in massive 

rhyodacite porphyry of the Mount Poster Formation for two of the six areas shown on 

figure 3. A, western Sweeney Mountains (areaB of fig. 3); B, eastern Sweeney Mountains 

(area C of fig. 3). Volcanic rocks of eastern Ellsworth Land are not commonly cleaved and 

so are not shown. Successive contours are at 2, 4, 6, and 8 percent of poles per 1 percent 

area N is number of observations.
N

STRUCTURAL GEOLOGY 

N

11

A, Eastern Ellsworth Land 

N

N=17

B, Western Sweeney Mountains 

N

N=60

C, Eastern Sweeney Mountains 

N

N=70

D, Hauberg Mountains E, Wilkins Mountains F, McCaw Ridge area

FIGURE 15. Contoured equal-area projections of poles to joint surfaces in folded layered rocks of the Latady Formation for the six areas 

shown on figure 3. Successive contours are at 2, 4, and 6 percent of poles per 1 percent area. N is number of observations.

N=85

FIGURE 16. Contoured equal-area projections of poles to joint surfaces in massive rhyodacite porphyry and layered volcanic rocks of the 

Mount Poster Formation for three of the six areas shown on figure 3. A, eastern Ellsworth Land; B, western Sweeney Mountains; C, 

eastern Sweeney Mountains. Successive contours are at 2 and 4 percent of poles per 1 percent area. N is number of observations.
12 STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA

(fig. 15). Quartz also locally infills slaty cleavage (S^ 

planes, and irregular quartz-filled fractures are more 

densely grouped in the hinge zone of some folds, indicating that at least some mineralization is synchronous 

with or postdates folding.

STRUCTURES OF THE LASSITER COAST 

INTRUSIVE SUITE

JOINTS

Joint directions in plutonic rocks of the Lassiter Coast 

Intrusive Suite (Vennum and Rowley, 1986) in the 

Orville Coast (fig. ISB), excluding data from eastern 

Ellsworth Land (area A of fig. 3), are almost randomly 

oriented, except for a slight preference towards being 

vertical. However, although joint directions in the 

107-Ma (Halpern, 1967; age corrected for new decay 

constants of Steiger and Jager, 1977) Merrick Mountains diorite of eastern Ellsworth Land (Vennum and 

Laudon, 1988) (fig. I8A) are scattered, they do suggest 

a preferred joint orientation of about N. 20° W.

In contrast to the general lack of preferred orientation of joints in plutonic rocks of the Orville Coast, joint 

orientations in rocks of the same plutonic suite in the 

Lassiter Coast are strongly oriented parallel to the 

regional extension-joint direction (Kellogg and Rowley, 

1974; unpub. data). This preferred orientation hi the 

Lassiter Coast area is particularly well defined by 

open joints that contain iron-oxide minerals (probably 

formerly pyrite), chlorite, and quartz. Joints of this 

orientation are probably the conduits through which

N=34

FIGURE 17. Contoured equal-area projection 

of poles to quartz-filled fractures in sedimentary rocks of the Latady Formation, Orville 

Coast (combined plot of data for areas B-F 

of fig. 3); no data were gathered for eastern 

Ellsworth Land (area A of fig. 3), although 

quartz-filled fractures are common there. 

Successive contours are at 2 and 4 percent 

of poles per 1 percent area. N is number of 

observations.

hydrothermal fluids ascended during formation of copper occurrences in the Copper Nunataks of the Lassiter 

Coast (Rowley and others, 1977) and the Sky-Hi 

Nunataks and Merrick Mountains of eastern Ellsworth 

Land (Rowley and others, 1988) (fig. 3).

The fact that joint orientations are better defined in 

plutonic rocks of the Lassiter Coast than in similar-age

N=35

FIGURE 18. Contoured equal-area projections of poles to joint surfaces in A, Merrick Mountains diorite stock, eastern Ellsworth Land (area A of fig. 3) and B, all stocks of the Orville Coast (combined plot of data for areas B-F of fig. 3). Successive contours are at 2 

and 4 percent of poles per 1 percent area. N is number of observations.
STRUCTURAL GEOLOGY 13

(Pankhurst and Rowley, in press) rocks of the Orville 

Coast suggests that cross-peninsular compression, 

culminating during the Palmer Land deformational 

event, lingered longer in the Lassiter Coast than in the 

Orville Coast.

DIKES

Basaltic to rhyolitic dikes of the Lassiter Coast Intrusive Suite cut both plutons and surrounding sedimentary and volcanic rocks throughout the Orville 

Coast and eastern Ellsworth Land. K-Ar ages from the 

Lassiter Coast (Mehnert and others, 1975) and crosscutting relationships indicate that the dikes are related to 

late stages of pluton emplacement, which occurred 

about 100 Ma. This age is supported by paleomagnetic 

data from the Lassiter Coast (Kellogg, 1980), where 

dikes similar in composition to those of the Orville Coast 

and eastern Ellsworth Land have almost identical 

paleomagnetic directions to those for the approximately 

100-Ma plutonic rocks of the Lassiter Coast. Undated 

aplite and subordinate pegmatite dikes, related to late 

plutonic activity, cut plutons with random orientations 

(data not shown).

Dike orientations in the Orville Coast and eastern 

Ellsworth Land (fig. 19) suggest an extension direction 

that is considerably different from that indicated from 

joints in sedimentary rocks from the same area (fig. 15). 

Most dikes, of which more than half (44 of 77 observations) are in eastern Ellsworth Land (area A of fig. 3),

N=77

FIGURE 19. Contoured equal-area projection 

of poles to dike orientations, Orville Coast 

and eastern Ellsworth Land (combined plot 

of data for areas A-F of fig. 3). More than 

half of dike orientations (44 of 77) are from 

eastern Ellsworth Land (area A of fig. 3). 

Successive contours are at 2 and 4 percent 

of poles per 1 percent area. N is number of 

observations.

are vertical and strike north-northwest, reflecting eastnortheast extension. Joint directions in sedimentary 

rocks from eastern Ellsworth Land suggest westnorthwest extension during folding (fig. I5A). This large 

difference in extension direction probably reflects a 

change in the principal stress trajectories between the 

time the Palmer Land deformational event culminated 

and the time the dikes intruded; that is, between 

sometime in the Early Cretaceous (140-120 Ma?) and 

about 100 Ma. This change in the stress trajectories 

may have been from a state of pure shear during crosspeninsular compression to one of right-lateral simple 

shear induced by early movement along a large, northwest-trending, right-lateral fault system (Ellsworth 

fault of Ford, 1972), which may bound the southern 

margin of the peninsula (see section entitled "Southern 

tectonic boundary of the Orville Coast"). In eastern 

Ellsworth Land, the fault system would have produced 

north- to north-northwest-oriented extension planes 

along which dikes intruded.

HIGH-ANGLE FAULTS

Numerous high-angle faults of small displacement 

were observed cutting the Latady Formation in the Orville Coast and eastern Ellsworth Land (figs. 8 and 20). 

In contrast, high-angle faults were rarely seen in the 

Lassiter Coast to the north, although high-angle faults

wFIGURE 20. Rose diagram of strike directions of all observed highangle faults cutting the Latady Formation in the Orville Coast and 

eastern Ellsworth Land. Of 69 observations, 15 are from Laudon 

and others (1970).
14 STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA

oriented parallel to the extension-joint direction and 

associated with hydrothermally altered and coppermineralized rocks were mapped in the Copper Nunataks 

of the Lassiter Coast (Rowley and others, 1977). Highangle faults cutting sedimentary rocks in the Orville 

Coast and eastern Ellsworth Land show as much as 

several tens of meters of apparent strike-slip movement 

where displacement could be measured. The predominant strike direction for faults throughout the Orville 

Coast and eastern Ellsworth Land is about N. 70 ° W. 

(Kellogg and Rowley, in press) (fig. 20), although a 

smaller group of fault orientations occurs at about right 

angles to the larger set. In the predominant set, 14 of 

69 faults show discernible offset in an apparent rightlateral sense; three others show apparent left-lateral offset. These relationships suggest a regional right-lateral 

shear regime along west-northwest trends.

PALEOMAGNETIC EVIDENCE FOR

RELATIVE ROTATION ABOUT

VERTICAL AXES IN THE SWEENEY

AND HAUBERG MOUNTAINS

The paleomagnetic declinations from middle Cretaceous plutons in the Orville Coast and eastern Ellsworth 

Land are rotated clockwise by an average of 50 ° relative 

to the paleomagnetic direction from coeval plutons in 

the Lassiter Coast (Kellogg, 1980). Rotation of the 

Orville Coast and eastern Ellsworth Land poles was 

thought to be due to drag along the right-lateral 

Ellsworth fault of Ford (1972) the Ellsworth fault 

system of this report which may bound the southern 

Antarctic Peninsula (see section entitled "Southern 

tectonic boundary of the Orville Coast").

The suggestion that paleomagnetic vectors were 

rotated by drag along the Ellsworth fault system 

(Kellogg, 1980) was challenged by Watts and others 

(1984). They contended that the difference in direction 

between middle Cretaceous plutonic rocks of the 

Lassiter Coast and plutonic rocks from the Orville Coast 

and eastern Ellsworth Land was due to apparent polar 

wander rather than to tectonic rotation. This contention was based on an overly literal interpretation of 

geochronologic data of Kellogg (1980); these data were 

used to infer that the studied rocks of the Orville Coast 

and eastern Ellsworth Land were about 8 m.y. older 

than other ("unrotated") intrusive rocks in the southern 

part of the peninsula. We feel, however, that the available geochronologic data do not support this conclusion.

Table 1 presents previously published and new paleomagnetic and geochronologic data for the Behrendt and 

Merrick Mountains; table 2 summarizes all of the

paleomagnetic and geochronologic data for middle 

Cretaceous rocks from the Antarctic Peninsula. These 

data point to two major conclusions. Firstly, uncertainties in K-Ar and Rb-Sr dates (Halpern, 1967; Farrar 

and others, 1982; Pankhurst and Rowley, in press) of 

plutons sampled for paleomagnetic study do not support the notion that ages are significantly different between the plutonic rocks of the Orville Coast and 

eastern Ellsworth Land and other middle Cretaceous 

plutons with "unrotated" declinations. Secondly, it 

appears that all plutons with clockwise-rotated paleomagnetic declinations are in the Sweeney and Hauberg 

Mountains; paleomagnetic directions from eastern 

Ellsworth Land are not significantly different from 

"unrotated" localities farther north on the peninsula 

(table 2). This implies, therefore, that oroclinal bending 

occurred only in the Orville Coast.

A late Early Cretaceous (110-95 Ma) paleomagnetic 

pole is well established from numerous localities on the 

Lassiter Coast (Kellogg and Reynolds, 1978) (table 2 and 

point LC on fig. 21). A 111- to 104-Ma pole is also determined for the Shetland Islands from the data of Watts 

and others (1984) (table 2 and point SI on fig. 21) and 

is in close agreement with the Lassiter Coast pole.

The only dated plutons from the Orville Coast are 

from the Sweeney Mountains. They yield a K-Ar age 

on biotite of 107±4 Ma from the granite of Mount 

Smart (Rowley and others, 1988), a Rb-Sr age of 

104±11 Ma from the Mount Edwards pluton (Pankhurst and Rowley, in press), and a Rb-Sr age of 112.6± 

1.2 Ma from the diorite of Hagerty Peak (Pankhurst and 

Rowley, in press). The combined mean paleomagnetic 

pole from 113- to 104-Ma plutons of the Sweeney 

Mountains (Kellogg, 1980; localities KelOl (Mount 

Smart), Kel54 and L31 (Mount Edwards), and Kel58 

(Hagerty Peak)) is lat 60° S., long 177° W. («95 (the 

semiangle of the cone of 95-percent confidence)=9°, calculated from site-mean directions). Although plutons 

and dikes of the Hauberg Mountains are undated, 

paleomagnetic results from these rocks are similar to 

those from the Sweeney Mountains; the mean pole from 

three localities in the Hauberg Mountains (Kellogg, 

1980; localities Kel27, C15, and Ro410) is lat 71° S., 

long 134 °W. (a95 =7°). Eleven dated and undated 

localities (72 samples) from the Sweeney and Hauberg 

Mountains give a mean pole of lat 65 ° S., long 165 ° W. 

(A95 (the semiangle of the cone of 95-percent confidence 

about the virtual geomagnetic pole) = 9 °), which is not 

significantly different from the pole determined from 

dated plutons. The combined pole from the 11 localities 

of the Sweeney and Hauberg Mountains is shown on 

figure 21 (point labeled SH).

Dated and undated paleomagnetic rocks from the 

Behrendt and Merrick Mountains of eastern Ellsworth
PALEOMAGNETIC EVIDENCE, SWEENEY AND HAUBERG MOUNTAINS 15

TABLE 1. Paleomagnetic and geochronologic data from mid-Cretaceous rocks from the Behrendt and Merrick Mountains, eastern Ellsworth

Land

[Three localities (Ke237, Ke239, Ke240) are new; two localities (Kel89, Kel90) are from Kellogg (1980). N, number of sites; (n), total number of samples; demagnetization field (in milliteslas) 

is that at which the paleomagnetic direction is chosen; D, mean declination in degrees, clockwise from true north; I, mean inclination in degrees, negative upward; a95, semiangle 

of cone of 95-percent confidence; R, magnitude of vector sum of the N unit vectors; dm and dp, semimajor and semiminor axes, respectively, of oval of 95-percent confidence. Leaders 

( ). not calculated]

Demagnetization Virtual 

Site number and rock type Location N(n> field n T R geomagnetic pole dm

Lat

Behrendt Mountains1 : 

1. Ke237 granodiorite, 

Mt. Caywood ........ 75.3 °S

2. Ke239 granodiorite, 

Mt. Suggs ........... 75.3°S

3. Kel 90 granodiorite, 

Mt. Trimpi .......... 75.4°S

4. Mean, Ke237, Ke239, 

Kel90 ...............

5. Ke240 gabbro, west 

of Mt. Caywood ...... 75.3 °S

Merrick Mountains: 

Kel89 diorite ........ 75.1 °S

Mean, Behrendt and Merrick

Long (mT)

72.5 °W 1(8) 30 

72.3 °W 1(14) 30 

72.8 °W 1(8) 20 

3(30) 20-30 

72.5 °W 1(9) 30

72.0 °W 1(12) 5 

5(51)

Lat

296 -78 16 7.46 69 °S 

92 -88 4 13.86 75 °S 

315 -79 8 7.85 73°S 

327 -88 5 28.93 81 °S 

265 -84 6 8.91 71 °S

221 -88 5 11.85 72 °S 

__ __ __ __ 75053

Long

6°E 30° 

89°W 8° 

63 °W 10° 

41 °W 10° 

63 °W 11°

63 °W 10°

47 °W3 

, Age 

dp (Ma)

29° 2100±2 

8° 2100±2 

9° 210o±2 

10° 2100±2 

11°

10° 2107±10 

 =100

 ^Numbers 1-5 refer to corresponding numbers used on figure 21 to plot virtual geomagnetic poles. 

2Rb-Sr ages (Halpern, 1967) corrected for new decay constants of Steiger and Jager (1977). 

A95 (Semiangle of cone of 95-percent confidence around mean virtual geomagnetic pole)=5°.

TABLE 2. Paleomagnetic and geochronologic data from mid-Cretaceous rocks from the Antarctic Peninsula

[N, number of sites; (n), total number of samples; D, mean declination in degrees, clockwise from true north; I, mean inclination in degrees, negative upward; A95, semiangle of cone 

of 95-percent confidence around mean virtual geomagnetic pole; (a95), semiangle of cone of 95-percent confidence around mean of site-mean directions. Leaders ( ), not calculated]

Locality1 Age (Ma) N(n) A95<a95>

Paleomagnetic pole

Lat Long

=115-100 8(52)

1157-100? 3(20)

5-108 11(72)

Sweeney and Hauberg Mountains: 

Sweeney Mountains, dated localities

(KelOl, Kel54, Kel58, L31)2 ...... 2-3113-104 4(24)

Sweeney Mountains, all localities (Dated

localities and undated localities Kel41,

Kel42, Kel45, Kel57)4 ...........

Hauberg Mountains, (Undated localities

Kel27, C15, Ro410)4 .............

Mean, Sweeney and Hauberg Mountains,

all localities (point SH, fig. 21) ....

Eastern Ellsworth Land, mean from table

1 (point EE, fig. 21) ............ . . =100 5(51)

Lassiter Coast:

All dated intrusives (Crowell pluton, 

M206, M307, M308, Ke39)6 .......

All intrusives (point LC, fig. 21 .....

Shetland Islands9, dated rocks only 

(point SI, fig. 21) ................. 111-104 4(29)

3,7109-96 5(28) 

8=110-95 17(96)

53

55

73

-69

-71

-81

(9°)

(6°) 

(7°) 

9°

5

12

-68

60 °S

61 °S

71 °S 

65 °S

75 °S

85 °S 

87 °S

79 °S

177°W

173°W 

134 °W 

165 °W

47 °W

37 °W 

131 °W

126 °E

 ^Numbers with letter prefix are site-locality numbers.

2Rowley and others (1988); K-Ar ages.

3Pankhurst and Rowley (in press).

4Kellogg (1980).

"Age estimated from four dated plutons.

6Kellogg and Reynolds (1978).

7Mehnert and others (1975); K-Ar age on biotite corrected for new decay constants of

Steiger and Jager (1977).

8 Age range estimated from five dated plutons. 

9Watts and others (1984).
16 STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA

GO90° E

FIGURE 21. Paleomagnetic poles, surrounded by circles of 95-percent confidence, for 111- to 103-Ma plutons from eastern Ellsworth Land 

(EE; table 2), Sweeney and Hauberg Mountains, Orville Coast (SH; table 2), Lassiter Coast (LC; table 2), and Shetland Islands (SI; table 

2). Numbers refer to site-mean poles determined from the Behrendt and Merrick Mountains, eastern Ellsworth Land (table 1).

Land define poles (tables 1 and 2 and point EE on 

fig. 21) that are significantly different from those of the 

Sweeney and Hauberg Mountains and are more consistent with the mean poles from the Lassiter Coast and 

Shetland Islands (table 2 and points LC and SI on 

fig. 21). The Behrendt Mountains pluton has a Rb-Sr 

age of 100±2 Ma, and the Merrick Mountains diorite 

has a Rb-Sr age of 107±10 Ma (Halpern, 1967; ages corrected for new decay constants of Steiger and Jager,

1977). Kellogg (1980) reported two virtual geomagnetic 

poles for localities in the Behrendt and Merrick Mountains plutons (table 1). Three previously unreported 

poles, of which two are from localities in the Behrendt 

Mountains pluton and another is from a locality in an 

undated gabbro in the Behrendt Mountains, are also 

presented here (table 1).

Demagnetization diagrams for samples from dioritic 

inclusions in the northern Behrendt Mountains pluton
PALEOMAGNETIC EVIDENCE, SWEENEY AND HAUBERG MOUNTAINS

N(up)

0.4--

17

NRM

E (horizontal)

B

W-

-0.1

0.2--

0.1--GJ |100 mT 

70 mT

,100 mT

0.1 0.0 

S (down)

(Amperes/meter)

E (horizontal)

N(up)

50 mT

30 mT 

100 mT

NRM

70 mT j£e 50 mT

E (horizontal)

D

6--

,NRM

10 mT

20 mT

30 mT ^NRM

S^1 J- E (horizontal)

(Amperes/meter) 

S (down)

(Amperes/meter) 

-0.2-1- 

S (down)

FIGURE 22. Demagnetization diagrams for four selected samples from the Behrendt Mountains. Diagrams A-C show results for 

samples Ke237A, Ke237I, and Ke239A, respectively, from the northern Behrendt Mountains pluton; diagram D shows results 

for sample Ke240C from an undated gabbro on west side of the northern Behrendt Mountains pluton. "Ke" and the next three 

numbers designate the site; the last letter designates the sample from that site Each diagram displays two views of paleomagnetic 

vector at each demagnetizing level: circles represent projection of paleomagnetic vector onto horizontal plane (N, S, E, and W 

axes); squares represent paleomagnetic vector on vertical plane containing paleomagnetic vector (up, down, and horizontal axes). 

NRM, natural remanent magnetization; mT, milliteslas.

(figs. 22A-Q demonstrate that the remanence remaining after demagnetization at inductions above 20 mT 

(milliteslas) is, in almost all cases, single component. One

sample (Ke237A, fig. 22^4), although showing convincing single-component magnetization, has an unusually 

shallow inclination of unknown origin. Another sample
18 STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA

(Ke237I, fig. 22.B) appears to have a small secondary 

component at high demagnetizing inductions that is 

probably due to the acquisition of a component of 

anhysteretic remanent magnetization, an artifact of the 

demagnetization process itself. Some scatter in the data 

for sample Ke239A (fig. 22C) may be caused by acquisition of viscous remanence during measurement; such 

behavior is typical of the majority of samples collected 

from more-felsic plutonic rocks, regardless of whether 

the samples were from inclusions or from coarser and 

more-felsic host rock. It should be noted, however, that 

within the Behrendt Mountains pluton the mean directions for each sampling locality at each demagnetization level do not show a significant or consistent change 

in direction. This indicates that viscous remanence acquired during demagnetization is randomly oriented.

Sample Ke240C, from an undated gabbro body on the 

west side of the Behrendt Mountains pluton, shows strong 

evidence for single-component magnetization (fig. 22D).

It is clear that the paleomagnetic pole from the 

Sweeney and Hauberg Mountains is significantly different from all other late Early Cretaceous paleomagnetic

poles from the Antarctic Peninsula at the 95-percent confidence level (fig. 21). Because of age equivalency of the 

rocks, the pole differences indicate that tectonic rotation 

has displaced the Sweeney-Hauberg Mountains pole from 

the others. Clockwise rotation of the Sweeney-Hauberg 

Mountains pole by about 50° -60° about a vertical axis 

since the middle Cretaceous is necessary to account for 

the pole differences. Although the rotated area includes 

the Sweeney and Hauberg Mountains, its total extent is 

unknown.

DISCUSSION 

THE LATADY BASIN AS A BACK-ARC RIFT

Most geologic features of the Orville Coast and eastern 

Ellsworth Land are related to the evolution of a magmatic arc between Middle Jurassic and Late Cretaceous 

time (fig. 23). If the predominantly marine Latady Formation occupies a back-arc basin behind the volcanic 

rocks of the Mount Poster Formation (Suarez, 1976), then

Back-arc basin

Accretionary wedge Magmatic arc

Trench

WEST 

KILOMETERS

Forearc basin 

Fossil Bluff Formation

Caldera

complex Mount Poster

> Formation Ocean

Present coastline of 

Orville Coast

Latady Formation

Subducting oceanic 

lithosphere

Dewatering of slab near 

100 km causes melting

Caldera complex

Lassiter Coast lntmsive uite Folded and thrusted KILOMETERS

Latady Format.on Ocean

Caldera 

Present erosion surface

, Plutons "--I- r- \ O,i#Ltet;y .^ ' -- '- 5 -"

Subducting oceanic 

lithosphere

100- -100

FIGURE 23. Schematic cross section, oriented approximately along line A-A' on figure 24, showing major tectonic elements in A, Late 

Jurassic time and B, middle Cretaceous time. Based in part on model for Sunda arc in the East Indies (Hamilton, 1987).
DISCUSSION 19

the Latady Formation, as well as the Mount Hill Formation to the north, may have been deposited in a basin 

formed by normal faulting related to back-arc spreading, a common process in developing magmatic arcs 

(Hamilton, 1985, 1987).

Evidence for a back-arc-rift origin for the Latady 

basin is far from unequivocal. Mesozoic rift structures 

have not been identified in the study area or in the 

Lassiter Coast to the north (Kellogg and Rowley, 1974; 

Rowley and Williams, 1982), although they have been 

inferred in the Black Coast in northern Palmer Land 

(Storey and others, 1987). Gabbroic plutons in northern 

Palmer Land and some ductile, north-trending, highangle faults in central Palmer Land may represent extensional development of a back-arc-rift basin in Middle Jurassic to earliest Cretaceous time (Meneilly and 

others, 1987). In the central Black Coast, additional sequences of locally sheared basalt flows (amygdaloidal 

basalt of Kamenev Nunatak) and large, sheared alkalic 

gabbro bodies that may be the source plutons of the 

basalts (Rowley and others, 1987; Storey and others, 

1987) also suggest an extensional environment.

The significance and age of the shearing in the Black 

Coast, however, is still uncertain. Kinematic indicators 

within the northerly striking, ductile, high-angle shear 

zones that cut the basalt and alkalic gabbro plutons in 

the central Black Coast reveal a complex deformational 

history (Storey and others, 1987); components of normal, reverse, dextral, and sinistral movement are all 

present. These sheared rocks may be as old as Late 

Jurassic, although McGibbon and Wever (in press) inferred a Late Cretaceous age for both the undated 

alkalic gabbros and the period of shearing.

OPENING OF THE WEDDELL SEA

The Weddell Sea began to form during the breakup 

of Gondwanaland, beginning with rifting in the Middle 

Jurassic (Ford and Kistler, 1980) and followed by seafloor spreading starting about 165 Ma (late Middle 

Jurassic; LaBrecque, 1986). Relative to modern coordinates, the rift axis was east-west, as indicated by linear 

magnetic anomalies. The present edge of the continental slope, south of which is unextended continental 

crust, is at about 74° S. (Kristoffersen and Haugland, 

1986).

The west boundary of the opening Weddell Sea is inferred to have been a north-striking, right-lateral transform fault (Barker and Griffiths, 1977), which we call 

the Weddell fault system (fig. 24). The Weddell fault 

system probably was initially part of a Mesozoic rift 

that extended the length of the Transantarctic Mountains, separating East and West Antarctica (Ford and

Kistler, 1980), and named the Transantarctic rift 

system by Schmidt and Rowley (1986). Eastern parts 

of the rift system in the Weddell Sea were identified by 

Kristoffersen and Hinz (in press). During opening of the 

Weddell Sea, West Antarctica may have moved about 

500-1,000 km right laterally along faults of the Transantarctic rift system (Schmidt and Rowley, 1986). Opening of the Weddell Sea probably ceased by the end of 

the Cretaceous or early Cenozoic time (LaBrecque and 

Barker, 1981; Barker and Lonsdale, in press).

The Latady Formation, as well as the presumed correlative Mount Hill Formation to the north, were 

deposited concurrently with early stages of opening of 

the Weddell Sea (fig. 4), although the east side of the 

Latady-Mount Hill basin was underlain by continental crust (fig. 24). In fact, if the model for right-lateral 

movement along the Weddell fault system is correct, 

then most of the Antarctic Peninsula was backed to the 

east by continental crust at least until latest Middle 

Jurassic time.

Modern analogues exist for the Weddell fault system 

and its relationship to an adjacent magmatic arc. For 

example, the Sumatra fault in Indonesia is a major leftlateral fault that runs the length of the island of 

Sumatra, which is part of the East Indian Sunda arc 

(Hamilton, 1987). The Sumatra fault is within or near 

the axis of the magmatic arc and is due to oblique subduction along the Sunda trench.

ORIGIN OF THE PALMER LAND 

DEFORMATIONAL EVENT

The origin is enigmatic for the Late Jurassic to Early 

Cretaceous period of deformation (the Palmer Land 

deformational event of Kellogg and Rowley, in press). 

Three origins are possible. The first two possibilities involve collision of the magmatic arc with another crustal 

block. The third possibility, which seems preferable to 

us, involves gravitational spreading and east- to 

southeast-directed thrusting off the thickened magmatic arc.

Firstly, the deformation may be related to the Early 

Cretaceous main-phase Rangitata orogeny of New 

Zealand (Bradshaw and others, 1981; Cooper and others, 

1982; Kellogg and Rowley, in press), during which time 

New Zealand, including the Campbell Plateau, were attached to the Marie Byrd Land coast (Cooper and 

others, 1982; Barker, 1982). In New Zealand, deformational, metamorphic, and plutonic events of the Rangitata orogeny have been related to the suturing of two 

tectonostratigraphic terranes the Tbrlesse terrane (containing a thick clastic sequence) and a terrane consisting 

of a continental crustal block with a magmatic-arc
20 STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA

T

EXPLANATION

Fault Dashed where approximately located; dotted 

where concealed; queried where doubtful. 

Opposed arrows show relative direction of 

motion across strike-slip faults

Active subduction zone Teeth on upper plate

Spreading center

- 2,000 m - Bathymetric contour

A A'Cross section line See figure 23

FIGURE 24. Hypothetical plate configuration of the Antarctic Peninsula area during the Late Cretaceous and early Tertiary, based in part 

on Kellogg (1980), Barker (1982), LaBrecque (1986), and Schmidt and Rowley (1986). The modern shoreline configuration (shaded) is 

shown for reference. AWAR, Aluk-West Antarctic ridge; EPS, Ellsworth fault system; EM L (along dotted line), Evans Ice Stream magnetic 

lineament (Garrett, in press); HN, Haag Nunataks.

margin (Bradshaw and others, 1981). The Rangitata 

orogeny has been related to an Early Cretaceous period 

of deformation and plutonism also noted in Marie Byrd

Land (Cooper and others, 1982). The Early Cretaceous 

orogenic belt in Marie Byrd Land is part of the same 

("Andean") orogenic belt as in the southern Antarctic
DISCUSSION 21

Peninsula, so it seems plausible that the effects of the 

Rangitata orogeny may have extended as far as the 

southern peninsula. Early Cretaceous deformation does, 

in fact, decrease northward along the peninsula, generally away from Marie Byrd Land; in the northern peninsula, there is little evidence for deformation of this age 

(Thomson and Pankhurst, 1983).

Secondly, the Palmer Land deformational event may 

have been due to collision of the Antarctic Peninsula 

magmatic arc with a block of continental lithosphere 

containing the Haag Nunataks (fig. 24) that now lies 

to the south of the Antarctic Peninsula (Rowley and 

others, 1983); the Haag Nunataks are underlain by 

Precambrian gneiss, clearly of continental affinity (Garrett and others, 1987). Although such a collision should 

thicken the crust, gravity and magnetic modeling indicate that the crust is not significantly thickened 

across the region to the south of the Orville Coast 

(Behrendt, 1964). There is a sharp magnetic lineament, 

however, aligned northwest along and parallel to the 

Evans Ice Stream (fig. 24). The magnetic lineament 

separates a southwestern area of sharp, shortwavelength anomalies, apparently underlain by dikeinjected Precambrian crust, from a northeastern area 

of subdued, longer wavelength anomalies (Garrett, in 

press), and could be an expression of a major crustal 

suture.

Thirdly (and favored by us), the folds and thrusts of 

the southern Antarctic Pensula may be part of a foldand-thrust belt, such as formed elsewhere in the world 

(for example, Boyer and Elliott, 1982), although observed thrusts are minor and rare in the Orville Coast. 

The style of folding suggests approximately 50 percent 

horizontal shortening of the Latady Formation. As 

geophysical evidence indicates that the crust under the 

region has not been thickened (Behrendt, 1964), it seems 

unlikely that the entire lithosphere has been similarly 

shortened. The only reasonable way to account for 

horizontal shortening of the Latady Formation is to 

have a regional decollement underlying the region. 

Thrust imbricates would overlie such a decollement, so 

their presence also seems likely.

Foreland folding and thrusting may be due to gravitational spreading of thrust sheets off crust thickened and 

isostatically elevated by arc magmatism during lengthy 

subduction (Hamilton, 1985). Spreading does not involve crustal compression or horizontal shortening of 

the middle or lower crust, as might occur during a collisional event, although compression would be expected 

in the gravitationally spreading upper crustal rocks. 

Spreading involves the development of a regional 

decollement, above which thrust splays imbricate rock 

sequences. Displacement along the decollement increases toward the axis of the arc, eventually disappearing into a zone of plastic deformation at middle crustal

levels. The few possible thrusts observed in the 

Hauberg and Wilkins Mountains would, therefore, be 

part of a system of larger, probably mostly hidden 

thrusts that may exist in the region.

LASSITER COAST INTRUSIVE SUITE

Compression across the Antarctic Peninsula waned 

after the Palmer Land deformational event but continued weakly until at least 100-95 Ma during emplacement of the Lassiter Coast Intrusive Suite. Plutons of 

the suite He in a belt parallel to the axis of the peninsula, southeast and south of the axis of inferred Middle to Late Jurassic arc volcanism (Vennum and 

Rowley, 1986). This more southeasterly position may 

be due to migration with time of the axis of magmatic 

activity toward the back-arc basin; this in turn is probably related to the Cretaceous episode of world-wide 

rapid sea-floor spreading and a flattening of the subducting slab beneath the Pacific side of the Antarctic 

Peninsula (Larson and Pitman, 1972). The plutons probably lie above the approximately 100-km-deep position 

of the subducting slab, where partial melting results 

from increased water produced by dehydration reactions (Hamilton, 1981).

SOUTHERN TECTONIC BOUNDARY OF THE 

ORVILLE COAST

Geologic and geophysical features in the mostly icecovered region south of the Orville Coast and north of 

the Ellsworth Mountains suggest that the Latady basin 

was bounded on its south side by extended crust of the 

Gondwana craton. The southern tectonic margin of the 

Antarctic Peninsula forms a 1-km-high, sub-ice, 

dissected escarpment, down to the south, approximately parallel to and about 10 km south of the Orville Coast 

shoreline (Bentley and Clough, 1972; Garrett and 

others, 1987). South of this escarpment, graben-like 

structures between the Orville Coast and the Ellsworth 

Mountains have been noted by numerous investigators 

(Masolov and others, 1981; Doake and others, 1983; 

Garrett and others, 1987). Storey (in press) used the 

term "chocolate-tablet boudinage" to describe such 

regions in West Antarctica that consist of extended and 

transcurrently faulted, crustally thinned zones surrounding thicker, undeformed crustal blocks.

The prominent escarpment along the Orville Coast 

shoreline probably represents a major rifted margin 

that may be the locus of at least some strike-slip movement associated with the Ellsworth fault system. Small, 

high-angle faults noted in the Orville Coast and eastern 

Ellsworth Land (fig. 20) cut both sedimentary and
22 STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA

plutonic rocks and are approximately parallel to and 

have the same sense of right-lateral movement as the 

Ellsworth fault system. For these reasons, Kellogg and 

Rowley (in press) suggested that these small faults may 

be satellitic to the Ellsworth fault system. Kellogg and 

Rowley (in press) also suggested that the Ellsworth fault 

system may be a right-lateral continuation of the Tharp 

fracture zone (fig. 24) (Barker, 1982), which formed about 

85 Ma during the initial rifting of the Campbell Plateau 

margin of the New Zealand block away from Marie Byrd 

Land; movement along the Tharp fracture zone ceased 

about 50 Ma when the Aluk-West Antarctic ridge intersected the subduction zone (Barker, 1982). Herron 

and Tucholke (1976) had also proposed that a rightlateral transform south of the Antarctic Peninsula was 

continuous with an unnamed transform fault that offsets the Aluk-West Antarctic ridge.

The major north-northwest-trending magnetic discontinuity along the west side of Evans Ice Stream also 

marks a 1-km-high sub-ice escarpment that may be a 

boundary between extended Precambrian crust to the 

southwest, similar to that found at Haag Nunataks 

(Garrett and others, 1987), and a thick sequence of 

sedimentary rocks of the Latady basin to the northeast. 

This prominent magnetic boundary is in contrast to the 

very subdued anomalies associated with the major topographic escarpment along the Orville Coast shorelina 

The magnetic and topographic boundary may be an expression of a prominent strand of the Ellsworth fault 

system (fig. 24).

Strike-slip movement along the Ellsworth fault system could not have been large. Paleomagnetic data 

indicate that the Antarctic Peninsula has remained essentially fixed relative to East Antarctica since middle 

Cretaceous time (Watts and other, 1984). Furthermore, 

the Antarctic Peninsula and the part of Ellsworth Land 

to the south have remained fixed, within the limits of 

uncertainty of the paleomagnetic data at the 95-percent 

confidence level, since the Middle Jurassic (Longshaw 

and Griffiths, 1983; Grunow and others, 1987).

Drag along the Ellsworth fault system may be responsible for clockwise rotations of paleomagnetic vectors by as much as about 60° in the Sweeney and 

Hauberg Mountains. The rotation mechanism may have 

been ductile (oroclinal bending), brittle (fault-block 

movements), or some combination of the two.

TERTIARY EXTENSION: UPLIFT, BLOCK 

FAULTING, AND ALKALIC VOLCANISM

Laudon (1972, p. 220) inferred that a lower Tertiary 

erosion surface in the southern Antarctic Peninsula and 

eastern Ellsworth Land was "arched, or tilted and

disrupted by block faulting in the late Tertiary." 

Although the higher elevations toward the axis of the 

peninsula (as much as 3,000 m above sea level) and the 

existence of deep, linear, ice-filled basins (Behrendt, 

1964) indicate that arching and block faulting have occurred in this area, the existence of an Upper Cretaceous 

or lower Tertiary erosion surface has yet to be 

demonstrated.

The trends in both the shape and structural fabric of 

McCaw Ridge and the Hauberg, Wilkins, and Sweeney 

Mountains are parallel to the coastline. Seismicreflection and gravity data define rough sub-ice topography, including valleys nearly 1,000 m below sea level 

(Behrendt, 1964). High-angle faults parallel to the structural fabric have not been seen, although the presence 

of upper Tertiary alkalic olivine basalt in the Merrick 

Mountains (dated at 6 Ma; Halpern, 1971) and Henry 

Nunataks (Rowley and others, in press) indicates that 

the southern Antarctic Peninsula marks the eastern 

limit of a zone of rift-related volcanism that extends 

across West Antarctica from Marie Byrd Land 

(LeMasurier and Wade, 1976).

CONCLUSIONS

The structural features of the Orville Coast and 

eastern Ellsworth Land are consistent with the following concepts:

1. The Latady Formation was deposited in a back-arc 

basin behind a Middle and Late Jurassic magmatic 

arc.

2. The Latady basin, at the time of its formation, was 

(and still is) backed to the south and east by 

crystalline crust that is as old as Precambrian.

3. A Late Jurassic to Early Cretaceous period of folding 

and thrust faulting (Palmer Land deformational 

event) may be related to gravitational spreading 

toward the back arc of an elevated crust thickened 

by a developing magmatic arc. Gravitational 

spreading involved the development of a regional 

decollement. Alternatively, deformation may have 

been due to collision of the Antarctic Peninsula 

magmatic arc with a continental block. Two 

possibilities for such an arc-continent collision are 

(a) the convergence of crustal blocks that might 

have caused the Rangitata orogeny of New 

Zealand at the time that New Zealand was still 

attached to Antarctica, or (b) the collision of 

the southern Antarctic Peninsula with a crustal 

block to the south of the present Orville Coast. 

Collision involved lithospheric shortening without necessarily the development of a regional 

decollement.
REFERENCES CITED 23

4. Stresses across the Antarctic Peninsula associated 

with the Palmer Land deformational event lingered 

until at least 100-95 Ma during the intrusion of 

plutons of the Lassiter Coast Intrusive Suite.

5. Copper mineralization, such as found in the Sky-Hi 

Nunataks, is probably related to hydrothermal 

fluids, generated during late stages of plutonism, 

that ascended fractures parallel to the extensionjoint direction, perpendicular to fold axes, in the 

Latady Formation.

6. The southern Antarctic Peninsula is bounded on the 

east by a north-trending, right-lateral transform 

fault, the Weddell fault system, which is part of 

the Transantarctic rift system of Schmidt and Rowley (1986). The fault was active during opening of 

the Weddell Sea basin, starting about 165 Ma.

7. Small-displacement, high-angle faults are approximately parallel to a right-lateral transform, the 

Ellsworth fault system, which lies along the south 

edge of the peninsula. The Ellsworth fault system 

may be a continuation of the Tharp fracture zone 

in the southeast Pacific, which ceased activity 

about 50 Ma when the Aluk-West Antarctic 

spreading center intersected the trench on the 

Pacific side of the peninsula.

8. The origin of the eastern Ellsworth Land orocline may 

be due, at least in part, to drag along the rightlateral Ellsworth fault system, an idea supported 

by about 60° clockwise rotation of paleomagnetic 

directions from the Sweeney and Hauberg Mountains. The rotation mechanism is unknown, but 

may have been by ductile bending, fault-block rotation, or a combination of the two.

9. The formation of an Upper Cretaceous or lower Tertiary erosion surface across the southern peninsula 

remains equivocal, although it appears that the 

region was arched and block faulted. The axis of 

the arch and the strike of major, sub-ice faults 

bounding tectonic blocks are both parallel to the 

shape of the peninsula.

10. The alkalic nature of upper Tertiary olivine-phyric 

basaltic lava flows in the Merrick Mountains and 

Henry Nunataks reflects development of the 

eastern end of a rift system that extends into 

Marie Byrd Land.

REFERENCES CITED

Barker, P.P., 1982, The Cenozoic subduction history of the Pacific 

margin of the Antarctic Peninsula Ridge crest-trench interactions: Journal of the Geological Society of London, v. 139, 

p. 787-801.

Barker, P.F., and Griffiths, D.H., 1977, Towards a more certain 

reconstruction of Gondwanaland: Philosophical Transactions of 

the Royal Society of London, ser. B, v. 279, p. 143-159.

Barker PR, and Lonsdale, M.J., in press, A multichannel seismic profile across the Weddell Sea margin of the Antarctic PeninsulaRegional tectonic implications, in Thomson, M.R.A., ed., Fifth International Symposium on Antarctic Earth Sciences, Cambridge, 

England, August 23-28, 1987.

Behrendt, J.C., 1964, The crustal geology of Ellsworth Land and 

southern Antarctic Peninsula from gravity and magnetic 

anomalies: Journal of Geophysical Research, v. 69, p. 2047-2063.

Bentley, C.R., and Clough, J.W., 1972, Seismic refraction shooting in 

Ellsworth Land and Dronning Maud Land, in Adie, R.J., ed., Antarctic geology and geophysics: Oslo, Norway, Universitetsforlaget, 

p. 169-172.

Boyer, S.E., and Elliott, David, 1982, Thrust systems: American 

Association of Petroleum Geologists Bulletin, v. 66, p. 1196-1230.

Bradshaw, J.D., Adams, C.J., and Andrews, P.B., 1981, Carboniferous 

to Cretaceous on the Pacific margin of Gondwana The Rangitata 

phase of New Zealand, in Cresswell, M.M., and Vella, P., eds., Gondwana Five: Fifth International Gondwana Symposium, Wellington, 

New Zealand, A.A. Balkema, p. 217-222.

Carey, S.W., 1958, A tectonic approach to continental drift, in Carey, 

S.W., ed., Continental drift A symposium: Hobart, Australia, 

University of Tasmania, p. 177-355.

Cooper, R.A., Landis, C.A., LeMasurier, WE., and Speden, I.G., 1982, 

Geological history and regional patterns in New Zealand and West 

Antarctica Their paleotectonic and paleogeographic significance, 

in Craddock, Campbell, ed., Antarctic geoscience: Madison, Wis., 

University of Wisconsin Press, p. 43-53.

Dalziel, I.W.D., and Elliot, D.H., 1982, West Antarctica Problem child 

of Gondwanaland: Tectonics, v. 1, p. 3-9.

Dalziel, I.W.D., Garrett, S.W., Grunow, A.M., Pankhurst, R.J., Storey, 

B.C., and Vennum, W.R., 1987, The Ellsworth-Whitmore crustal 

block Its role in the tectonic evolution of West Antarctica, in 

McKenzie, G.D., ed., Gondwana six Structure, tectonics, and 

geophysics: Washington, D.C., American Geophysical Union, 

Geophysical Monograph 40, p. 109-115.

Doake, C.S.M., Crabtree, R.D., and Dalziel, I.W.D., 1983, Subglacial morphology between Ellsworth Mountains and Antarctic PeninsulaNew data and tectonic significance, in Oliver, R.L., James, PR., 

and Jago, J.B., eds., Antarctic earth science: Australian Academy 

of Science and Cambridge University Press, p. 270-273.

Farrar, Edward, McBride, S.L., and Rowley, P.O., 1982, Ages and tectonic implications of Andean plutonism in the southern Antarctic Peninsula, in Craddock, Campbell, ed., Antarctic geoscience: 

Madison, Wis., University of Wisconsin Press, p. 349-356.

Ford, A.B., 1972, Fit of Gondwana continents Drift reconstructions 

from the Antarctic continental viewpoint: 24th International 

Geological Congress, Montreal, Canada, section 3, p. 113-121.

Ford, A.B., and Kistler, R.W., 1980, K-Ar age, composition, and origin 

of Mesozoic mafic rocks related to Ferrar Group, Pensacola Mountains, Antarctica: New Zealand Journal of Geology and Geophysics, 

v. 23, p. 371-390.

Garrett, S.W., in press, Aeromagnetic studies of crustal blocks in West 

Antarctica A review, in Thomson, M.R.A., ed., Fifth International 

Symposium on Antarctic Earth Sciences, Cambridge, England, 

August 23-28, 1987.

Garrett, S.W., Herrod, L.D.B., and Mantripp, D.R., 1987, Crustal structure of the area around Haag Nunataks, West Antarctica New 

aeromagnetic and bedrock elevation data, in McKenzie, G.D., ed., 

Gondwana Six Structure, tectonics, and geophysics: Washington, 

D.C., American Geophysical Union, Geophysical Monograph 40, 

p. 109-115.
24 STRUCTURAL GEOLOGY AND TECTONICS, ORVILLE COAST REGION, ANTARCTICA

Grunow, A.M., Dalziel, I.W.D., and Kent, D.V., 1987, EllsworthWhitmore Mountains crustal block, western Antarctica New 

paleomagnetic results and their tectonic significance, in McKenzie, G.D., ed, Gondwana Six Structure, tectonics, and 

geophysics: Washington, D.C., American Geophysical Union, 

Geophysical Monograph 40, p. 161-171.

Halpern, Martin, 1967, Rubidium-strontium age measurement of 

plutonic igneous rocks in eastern Ellsworth Land and northern 

Antarctic Peninsula, Antarctica: Journal of Geophysical Research, 

v. 72, p. 5133-5142.

___1971, Evidence for Gondwanaland from a review of West Antarctic radiometric ages, in Quam, L.O., ed., Research in the Antarctic: American Academy for the Advancement of Science, 

p. 717-730.

Hamilton, Warren, 1981, Crustal evolution by arc magmatism: 

Philosophical Transactions of the Royal Society of London, ser. 

A, v. 301, p. 279-291.

___1985, Subduction, magmatic arcs, and foreland deformation, 

in Howell, D.G., ed., Tectonostratigraphic terranes: Circumpacific 

Council for Energy and Mineral Resources, p. 259-262. 

.1987, Convergent-plate tectonics viewed from the Indonesian

region, in Sengor, A.M.C., ed., Tectonic evolution of the Tethyan 

domain: Amsterdam, The Netherlands, Reidel Press, p. 1-44.

Herron, E.M., and Tucholke, B.E., 1976, Sea-floor magnetic patterns 

and basement structures in the southeastern Pacific, in Hollister, 

C.D., Craddock, Campbell, and others, eds., Initial reports of the 

Deep Sea Drilling Project, v. 35: U.S. Government Printing Office, p. 263-278.

Kellogg, K.S., 1979, Structural geology of Orville Coast and eastern 

Ellsworth Land: Antarctic Journal of the United States, v. 14, 

no. 5, p. 19-21.

___1980, Paleomagnetic evidence for oroclinal bending of the 

southern Antarctic Peninsula: Geological Society of America 

Bulletin, v. 91, p. 414-420.

Kellogg, K.S., and Reynolds, R.L., 1978, Paleomagnetic results from 

the Lassiter Coast, Antarctica, and a test for oroclinal bending 

of the Antarctic Peninsula: Journal of Geophysical Research, 

v. 83, p. 2293-2299.

Kellogg, K.S., and Rowley, P.D., 1974, Structural geology of the Lassiter 

Coast: Antarctic Journal of the United States, v. 9, p. 224-225.

___in press, Tectonic evolution of southeastern Antarctic Peninsula, in Thomson, M.R.A., ed., Fifth International Symposium 

on Antarctic Earth Sciences, Cambridge, England, August 23-28, 

1987.

Kristoffersen, Yngve, and Haugland, Kristen, 1986, Geophysical 

evidence for the East Antarctic plate boundary in the Weddell 

Sea: Nature, v. 322, no. 6079, p. 538-541.

Kristoffersen, Yngve, and Hinz, Karl, in press, Complexities of the 

East-Antarctic Gondwana plate boundary in the Weddell Sea, in 

Thomson, M.R.A., ed., Fifth International Symposium on Antarctic Earth Sciences, Cambridge, England, August 23-28, 1987.

LaBrecque, J.L., 1986, South Atlantic Ocean and adjacent Antarctic 

continental margin: Ocean Margin Drilling Program, Regional 

Atlas Series, Atlas 13, Woods Hole, Mass., Marine Science International, 21 sheets.

LaBrecque, J.L., and Barker, P.F., 1981, The age of the Weddell Basin: 

Nature, v. 290, p. 489-492.

Larson, R.L., and Pitman, W.C., 1972, World-wide correlation of 

Mesozoic magnetic anomalies and its implications: Geological 

Society of America Bulletin, v. 83, p. 3645-3662.

Laudon, T.S., 1972, Stratigraphy of eastern Ellsworth Land, in Adie, 

R. J., ed., Antarctic geology and geophysics: Oslo, Norway, Universitetsforlaget, p. 215-223.

Laudon, T.S., Lackey, L.L., Quilty, P.G., and Otway, P.M., 1970, 

Geology of eastern Ellsworth Land, in Bushnell, N.C., and

Craddock, Campbell, eds., Geologic maps of Antarctica: American 

Geographic Society, Antarctic Map Folio Series, Folio 12, Plate 

III, scale 1:1,000,000.

Laudon, T.S., Thomson, M.R.A., Williams, P.L., Milliken, K.L., 

Rowley, P.O., and Boyles, J.M., 1983, The Jurassic Latady Formation, southern Antarctic Peninsula, in Oliver, R.L., James, P.R., 

and Jago, J.B., eds., Antarctic earth science: Australian Academy 

of Science and Cambridge University Press, p. 308-319.

LeMasurier, W.E., and Wade, F.A., 1976, Volcanic history in Marie 

Byrd Land Implications with regard to southern hemisphere tectonic reconstructions, in Gonzales-Ferran, Oscar, ed., Andean and 

Antarctic volcanology problems: Rome, Italy, International 

Association of Volcanology and Chemistry of the Earth's Interior, 

p. 398-424.

Longshaw, S.K., and Griffiths, D.H., 1983, A paleomagnetic study 

of Jurassic rocks from the Antarctic Peninsula and its implications: Journal of the Geological Society of London, v. 140, p. 

945-954.

Masolov, V.N., Kurinin, R.G., and Grikurov, G.E., 1981, Crustal structure and tectonic significance of Antarctic rift zones, in Cresswell, 

M.M., and Vella, P., eds., Gondwana Five: Fifth International 

Gondwana Symposium, Wellington, New Zealand, A.A. Balkema, 

p. 303-309.

McGibbon, K.J., and Wever, H.E., in press, Magnetic evidence for 

Late Cretaceous back-arc extension in the Black Coast, Palmer 

Land, in Thomson, M.R.A., ed., Fifth International Symposium 

on Antarctic Earth Sciences, Cambridge, England, August 23-28, 

1987.

Mehnert, H.H., Rowley, P.O., and Schmidt, D.L., 1975, K-Ar ages 

of plutonic rocks in the Lassiter Coast area, Antarctica: U.S. 

Geological Survey Journal of Research, v. 3, p. 233-236.

Meneilly, A.W., Harrison, S.M., Piercy, B.A., and Storey, B.C., 1987, 

Structural evolution of the magmatic arc in northern Palmer Land, 

Antarctic Peninsula, in McKenzie, G.D., ed., Gondwana SixStructure, tectonics, and geophysics: Washington, D.C., American 

Geophysical Union, Geophysical Monograph 40, p. 209-219.

Pankhurst, R.J., and Rowley, P.D., in press, Rb-Sr study of 

Cretaceous plutons from the Orville and Lassiter Coasts, Antarctic Peninsula, in Thomson, M.R.A., ed., Fifth International Symposium on Antarctic Earth Sciences, Cambridge, England, August 

23-28, 1987.

Rowley, P.D., 1978, Geologic studies in Orville Coast and eastern 

Ellsworth Land, Antarctic Peninsula: Antarctic Journal of the 

United States, v. 13, no. 4, p. 7-9.

Rowley, P.O., Farrar, Edward, Carrara, P.E., Vennum, W.R., and 

Kellogg, K.S., 1988, Porphyry-type copper deposits and K-Ar 

ages of plutonic rocks of the Orville Coast and eastern Ellsworth 

Land, Antarctica, in Rowley, P.D., and Vennum, W.R., eds., 

Studies of the geology and mineral resources of the southern 

Antarctic Peninsula and eastern Ellsworth Land, Antarctica: U.S. 

Geological Survey Professional Paper 1351-C, p. 35-49.

Rowley, P.O., Kellogg, K.S., and Vennum, W.R., 1985, Geologic 

studies in the English Coast, eastern Ellsworth Land: Antarctic 

Journal of the United States, v. 19, p. 34-36.

Rowley, P.O., Kellogg, K.S., Vennum, W.R., Laudon, T.S., Thomson, 

J.W., O'Neill, J.M., and Lidke, D.J., in press, Tectonic setting 

of the English Coast, eastern Ellsworth Land, Antarctica, in 

Thomson, M.R.A., ed., Fifth International Symposium on Antarctic Earth Sciences, Cambridge, England, August 23-28, 1987.

Rowley, P.O., Schmidt, D.L., and Williams, P.L., 1982, Mount Poster 

Formation, southern Antarctic Peninsula and eastern Ellsworth 

Land: Antarctic Journal of the United States, v. 17, p. 38-39.

Rowley, P.O., Storey, B.C., Ford, A.B., and Wever, H.E., 1987, 

Geology of the central Black Coast, Antarctic Peninsula: Antarctic 

Journal of the United States, v. 22, no. 5, p. 1-3.
REFERENCES CITED 25

Rowley, P.D, Vennum, W.R., Kellogg, K.S., Laudon, T.S., Carrara, RE., 

Boyles, J.M., and Thomson, M.R.A., 1983, Geology and plate tectonic setting of the Orville Coast and eastern Ellsworth Land, Antarctica, in Oliver, R.L., James, PR., and Jago, J.B., eds., Antarctic 

earth science: Australian Academy of Science and Cambridge 

University Press, p. 345-350.

Rowley, P.D., and Williams, PL., 1982, Geology of the northern Lassiter 

Coast and southern Black Coast, Antarctic Peninsula, in Craddock, Campbell, ed., Antarctic geoscience: Madison, Wis., University of Wisconsin Press, p. 339-348.

Rowley, P.D., Williams, PL., and Schmidt, D.L., 1977, Geology of an 

Upper Cretaceous copper deposit in the Andean province, Lassiter 

Coast, Antarctic Peninsula: U.S. Geological Survey Professional 

Paper 984, 36 p.

Schmidt, D.L., and Rowley, P.D., 1986, Continental rifting and 

transform faulting along the Jurassic Transantarctic rift, Antarctica: Tectonics, v. 5, p. 279-291.

Singleton, D.G., 1980, The geology of the central Black Coast, Palmer 

Land: British Antarctic Survey Scientific Reports, no. 102, 50 p.

Stanek, K.P., 1987, Beitrag zur geologic der sudichen Lassiter Coast 

und der ostlichen Orville Coast (Antarktische Halbinsel) [Contribution to the geology of the southern Lassiter Coast and eastern 

Orville Coast (Antarctic Peninsula)]: Freiberger Forschungshefte, 

v. C-412, p. 5-50.

Steiger, R.H., and Jager, E., 1977, Subcommission on geochronology 

Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, 

p. 359-362.

Storey, B.C., in press, West Antarctica crustal fragments within 

Gondwanaland New constraints, in Thomson, M.R.A., eds., Fifth 

International Symposium on Antarctic Earth Sciences, Cambridge, England, August 23-28, 1987.

Storey, B.C., and Garrett, S.W., 1985, Crustal growth of the Antarctic 

Peninsula by accretion, magmatism, and extension: Geological 

Magazine, v. 122, p. 5-14.

Storey, B.C., Wever, H.E., Rowley, P.D., and Ford, A.B., 1987, The 

geology of the central Black Coast, eastern Palmer Land: British 

Antarctic Survey Bulletin, no. 77, p. 145-155.

Suarez, Manuel, 1976, Plate tectonic models for southern Antarctic 

Peninsula and its relation to southern Andes: Geology, v. 4, 

p. 211-214.

Taylor, B.J., Thomson, M.R.A., and Willey, L.E., 1979, The geology of 

the Ablation Point-Keystone Cliffs area, Alexander Island: British 

Antarctic Survey Scientific Reports, no. 82, 65 p.

Thomson, M.R.A., 1982, Mesozoic paleogeography of West Antarctica, in Craddock, Campbell, ed., Antarctic geoscience: Madison, 

Wis., University of Wisconsin Press, p. 331-337.

___1983, Late Jurassic ammonites from the Orville Coast, Antarctica, in Oliver, R.L., James, P.R., and Jago, J.B., eds., Antarctic 

earth science: Australian Academy of Science and Cambridge 

University Press, p. 328-333.

Thomson, M.R.A., Laudon, T.S., and Boyles, J.M., 1978, 

Stratigraphical studies in Orville Coast and eastern Ellsworth Land: Antarctic Journal of the United States, v. 13, 

p. 9-10.

Thomson, M.R.A., and Pankhurst, R.J., 1983, Age of post-Gondwanian 

calc-alkaline volcanism in the Antarctic Peninsula, in Oliver, R.L., 

James, P.R., and Jago, J.B., eds., Antarctic earth science: 

Australian Academy of Science and Cambridge University Press, 

p. 328-333.

Tranter, T.H., 1986, The LeMay Group of central Alexander Island: 

British Antarctic Survey Bulletin, no. 71, p. 57-67.

___in press, Accretion and subduction processes along the Pacific 

margin of Gondwana, central Alexander Island, in Thomson, 

M.R.A., ed., Fifth International Symposium on Antarctic Earth 

Sciences, Cambridge, England, August 23-28, 1987.

Vennum, W.R., and Laudon, T.S., 1988, Igneous petrology of the Merrick Mountains, eastern Ellsworth Land, Antarctica, in Rowley, 

P.D., and Vennum, W.R., eds., Studies of the geology and mineral 

resources of the southern Antarctic Peninsula and eastern 

Ellsworth Land, Antarctica: U.S. Geological Survey Professional 

Paper 1351-B, p. 21-34.

Vennum, W.R., and Rowley, P.D., 1986, Reconnaissance geochemistry of the Lassiter Coast Intrusive Suite, southern Antarctic 

'^Peninsula: Geological Society of America Bulletin, v. 97, 

p. 1521-1533.

Watts, D.R., Watts, G.C., and Bramall, A.M., 1984, Cretaceous and 

early Tertiary paleomagnetic results for the Antarctic Peninsula: 

Tectonics, v. 3, p. 333-346.

Weissel, J.K., Hayes, D.E., and Herron, E.M., 1977, Plate tectonic 

synthesis The displacements between Antarctica, New Zealand, 

and Antarctica since the Late Cretaceous: Marine Geology, v. 25, 

p. 231-277.

Williams, P.L., Schmidt, D.L., Plummer, C.C., and Brown, L.E., 1972, 

Geology of the Lassiter Coast area, Antarctic PeninsulaPreliminary report, in Adie, R.J., ed., Antarctic geology and 

geophysics: Oslo, Norway, Universitetsforlaget, p. 143-148.

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