Campbell Craddock

Cenozoic volcanism in Antarctica: Jones Mountains and Peter I Island

Abstract Cenozoic volcanism is widespread along the Pacific coast of Antarctica; a relationship to the rifting of the West Antarctic Rift System is obvious, but the possible role for mantle plumes is not established. We compare here complete geochemical and strontium, neodymium, and lead isotopic data for volcanic rocks from one probable plume just off the coast of Ellsworth Land (Peter I Island) with data from rift-related volcanic rocks from the nearby Jones Mountains, Ellsworth Land. The alkali basalts from Peter I Island are similar in most respects to average oceanic island basalt, and we propose recognition of this island as the most southerly known oceanic plume/hotspot—only unusually high 207/204 Pb ratios set the Peter I Island plume apart from other oceanic plumes. The volcanics from the Jones Mountains share many trace element and isotopic characteristics with the Peter I volcanics, including isotopic arrays which have as one end member the low 87/86 Sr-high 206/204 Pb component which is characteristic of Cenozoic volcanism throughout the West Antarctic Rift System. Basalts from the Jones Mountains are distinctive from those of Peter I Island principally in their much lower 207/204 Pb ratios, and their low and non-OIB-like Ce/Pb ratios (8.5–17.6). To explain this difference, we propose that a minor component of subduction-impregnated subcontinental lithosphere has been incorporated into the rift-related Jones Mountains volcanics that is not present in the oceanic Peter I Island plume.

Kinematics of Tertiary structures in upper Paleozoic and Mesozoic strata on Midterhuken, west Spitsbergen

A belt of folded and faulted Carboniferous-Cretaceous platform cover strata lies parallel to and inland of much of Spitsbergen9s west coast. Uplifted older basement rocks along the coast have been thrust eastward into and over these younger platform cover strata. This deformation occurred in early Tertiary time and was related to dextral movement of the Barents Shelf past northeastern Greenland during the opening of the Norwegian-Greenland Sea. On Midterhuken, a peninsula in Bellsund, an uplifted, tilted, and faulted sequence of the cover strata forms a 3.3-km-thick, 12-km-wide margin on the eastern side of the basement crustal welt. A series of closely spaced detachment horizons within Triassic strata occurs at the boundary between a lower unit of mainly sandstones and an upper unit of mainly thin-bedded shales and siltstones. The lower unit is tilted 60°–70° to the northeast but unfolded, whereas the upper unit is tightly and disharmonically folded. Hanging-wall–down movements on these and other detachment surfaces are indicated by offset of a dike and by fold-and-fault patterns. These foreland-dipping faults might be southwest-dipping thrusts later rotated, but more likely these faults and associated folds represent detachment structures formed close to their present position in response to the uplift and tilting of these strata.

Structural geology of the Nenana River segment of the Denali fault system, central Alaska Range

The Denali fault system, one of the major tectonic elements of southern Alaska, forms an arc 2,100 km long across southern Alaska. In the central Alaska Range, the system consists of a northern Hines Creek strand and a southern McKinley strand, 30 km apart, which divide the area into northern, central, and southern terranes. There is evidence for at least two episodes of deformation in the northern terrane, four in the central, and two in the southern during Paleozoic and Mesozoic time. During each, the inferred axis of maximum compressive strain was subhorizontal and about north-south, but the direction shifted to north-northwest–south-southeast during a late Paleocene–Eocene folding episode. Tectonic stability during Oligocene–middle Miocene time was followed by differential uplift of crustal blocks during late Miocene–Pliocene time. The Hines Creek fault may preserve a record of the early history of the fault system. Strong contrasts between lower and middle Paleozoic rocks juxtaposed along the fault suggest large dextral strike-slip displacement, but major convergent movement cannot be ruled out. Movement throughout the Hines Creek fault ceased by middle Cretaceous time, but local dip-slip movements continued into the Cenozoic era. The McKinley fault is an active dextral strike-slip fault, with Cenozoic offset of probably at least 30 km and possibly much greater. Mean Holocene displacement rates are 1 to 2 cm/yr. These rates would produce a 30-km offset in 1.5 to 3.0 m.y., or a 400-km offset in 20 to 40 m.y. The Denali fault may be part of a transform fault system connecting the Juan de Fuca Ridge and the landward extension of the Aleutian subduction zone. However, it is more likely that the fault forms the northern boundary of a small lithospheric plate caught between the Pacific and American plates.

Tectonic evolution of the west Spitsbergen fold belt

Abstract The west Spitsbergen fold belt has a complex tectonic history which is recorded in a thick, nearly complete upper Proterozoic-Phanerozoic layered sequence. Work since 1977 near Bellsund allows recognition of the main deformational events in that segment of the fold belt. The strata are grouped informally into the metamorphosed Hecla Hoek (HH) sequence (Proterozoic) and the Van Keulenfjorden (VK) sequence (Carboniferous-Cretaceous), separated by a pronounced unconformity. The HH is divided into the Antoniabreen succession, the Chamberlindalen succession, and the Kapp Lyell tillite; the first two consist of diverse clastic and carbonate rocks, along with some volcanic rocks. The VK is a platform sequence of shallow marine and terrestrial sedimentary rocks. These layered rocks are intruded by a few Mesozoic dolerite sills and dikes. Subhorizontal Paleogene sedimentary rocks are preserved in a small graben. Both layered sequences have undergone strong deformation. Structures (mainly Caledonian age) in the HH can be grouped as phase 1 (small isoclinal folds, subhorizontal axial planar foliation, ridge-groove lineation in the foliation, and large recumbent folds), phase 2 (tight to isoclinal folds, axial planar foliation), and younger (weak folds and foliations, kink bands, crinkles). Structures (mainly Tertiary age) in the VK include faults (thrust, reverse, down-to-the-east bedding-plane normal, other normal); folds (symmetric, asymmetric, overturned, recumbent, isoclinal); and foliation (in some tightly folded Triassic shales). Six deformational events can be identified here, each described below in terms of age, intensity, and kinematic pattern: • D1 - Vendian or early Paleozoic; very strong; NNW-SSE shortening?, NNW transport? • D2 - Early Paleozoic (pre-Carboniferous); strong; NE-SW shortening, NE vergence. • D3 - Middle (?) Carboniferous; moderate; unclear (limited exposure). • D4 - Early Cretaceous (?); weak; extension, direction unclear. • D5 - Early (?) Tertiary; very strong locally; uplift in W, ENE-WSW shortening, ENE translation. • D6 - Middle (?) Tertiary; moderate; NE-SW extension.

RUBIDIUM-STRONTIUM AGES FROM ANTARCTICA

Rubidium-strontium ages from two localities in the Transantarctic Mountains indicate that granitic rocks formed about 490 m.y. ago. Discordant ages and cataclastic texture in the specimen from the Queen Maud Range suggest younger early Paleozoic metamorphism of that granite. A single determination on biotite from a quartz diorite gneiss on Thurston Island yields a minimum age of 280 m.y.

Reconsidering caledonian deformation in southwest Spitsbergen

Geologic thinking about the Svalbard Caledonides has long been shaped by the hypothesis that large-scale strike-slip plate motions assembled the archipelago in mid-Paleozoic time. However, the first detailed studies of upper Proterozoic meta-sedimentary rocks along one of the postulated strike-slip zones, in northern Wedel Jarlsberg Land, southwest Spitsbergen, have produced no evidence for major Caledonian transcurrent shear in the region. An angular unconformity divides the greenschist-facies rocks into two sequences and chronicles a period of intense deformation in late (?) Proterozoic time. Rocks above the unconformity (conglomerates, dolomites, greenschists, and Vendian diamictites) provide a detailed record of early to mid-Paleozoic deformation. The widely distributed conglomerates allow definition of regional strain patterns and construction of a possible deformation path for the study area. Deformed conglomerate clasts range from oblate and parallel with bedding, to prolate and aligned parallel to the axes of northwest trending folds. Microstructural evidence indicates that net orogen-parallel elongation of clasts resulted from modification of an early subhorizontal flattening fabric by later progressive coaxial flattening in a vertical plane. This is consistent with macroscopic evidence for two distinct, though perhaps regionally diachronous, phases of deformation: a period of ductile thrusting and recumbent folding followed by upright to northeast vergent folding, in a progressive trend toward more brittle behavior. The structural fabric of the rocks may record their incorporation into an accretionary prism, in a process closely linked with blueschist emplacement in central west Spitsbergen. The rocks provide no evidence for vorticity about a vertical axis, as might be expected near a major strike-slip zone. Stratigraphic correlation of Proterozoic rocks on both sides of the postulated fault zone also argues against large-scale strike-slip displacement during Caledonian deformation in southwest Spitsbergen. The results of this study and others call for critical reexamination of the prevailing tectonic model for Caledonian Svalbard.

The Denali fault system and the tectonic development of Southern Alaska

Abstract The Denali fault system forms an arc, convex to the north, across southern Alaska. In the central Alaska Range, the system consists of a northern Hines Creek strand and a southern McKinley strand, up to 30 km apart. The Hines Creek fault may preserve a record of the early history of the fault system. Strong contrasts between juxtaposed lower Paleozoic rocks appear to require large dextral strike-slip or a combination of dipslip and strike-slip displacements along this fault. Thus the fault system may mark a reactivated suture zone between continental rocks to the north and a late Paleozoic island arc to the south, as suggested by Richter and Jones (1973). Major movements on the Hines Creek fault ceased by the Late Cretaceous, but local dip-slip movements continued into the Cenozoic. The McKinley fault is an active dextral strike-slip fault with a mean Holocene displacement rate of 1–2 cm/y. Post-Late Cretaceous dextral offset on this fault is probably at least 30 km and possibly as great as 400 km. Patterns of early Tertiary folding and reverse faulting indicate that the McKinley fault was active at that time and suggest that this fault developed shortly after strike-slip activity ceased on the Hines Creek fault. Oligocene — middle Miocene tectonic stability and late Miocene—Pliocene uplift of crustal blocks may reflect periods of quiescence and activity, on the McKinley fault. The two strands of the Denali fault divide the central Alaska Range into northern, central, and southern terranes. During the Paleozoic—Mesozoic there is evidence for at least two episodes of compressive deformation in the northern terrane, four in the central terrane, and two in the southern. During each, the axis of maximum compressive strain was subhorizontal and about north—south. This pattern suggests a Paleozoic and Mesozoic setting dominated by plate convergence, despite the possible large pre-Late Cretaceous lateral movement on the Hines Creek fault. The Cenozoic pattern of faulting and folding appears compatible with a plate tectonic model of (1) rapid northward movement of the Pacific plate relative to Alaska during the early Tertiary; (2) slow northwestward movement of the Pacific plate during the Oligicene and (3) rapid northwestward movement of the Pacific plate from the end of the Oligocene to the present.

Stratigraphy and structure of the Kapp Lyell diamictites (upper Proterozoic), Spitsbergen

Massive upper Proterozoic diamictites of the Kapp Lyell Group, south of Bellsund on the west coast of Spitsbergen, constitute a sequence of distinctive units that can be differentiated on the basis of megaclast compositions. Within the group, ten lithostratigraphic units are distinguished from modal estimates on the outcrop of the abundances of the three most common types of megaclasts—quartzite, dolomite, and limestone. The total thickness of the diamictite sequence is ∼3,000 m. The clasts found in the Kapp Lyell Group are almost entirely local in origin and can be correlated with older rocks to the east that underlie the diamictites. The diamictites were affected by two major periods of deformation. The first was either latest Precambrian (Jarlsbergian) or early Paleozoic (Caledonian) in age. This first episode of deformation (D 1 ) is expressed in the diamictites primarily as a flattening of megaclasts. The degree of deformation due to D 1 varies throughout the diamictite sequence. Part of this variation is due to differences in clast and matrix compositions and proportions, but some of it appears to be tectonic, suggesting that strain mechanisms were not uniform throughout the diamictite mass. Larger structures associated with D 1 are not observed in the field owing to the massive nature of the unit and the strong overprint of the later period of deformation (D 2 ). The second phase of deformation (D 2 ) is probably late Caledonian or Variscan in age, perhaps as young as early Carboniferous. Folds plunging gently to the west-northwest with an axial-planar foliation dipping southwest characterize this event. This axial-planar foliation occurs as an evenly spaced (0.5 to 2.0 cm) crenulation cleavage. The deformational intensity of D 2 also varies throughout the diamictites. It is particularly strong, however, near the basal contact of the diamictites with the underlying phyllites, suggesting that this contact may be tectonic rather than depositional, or possibly both. A northeast-southwest principal horizontal compression is inferred from the foliation for the D 2 event.