Margin Features Research Articles

Cretaceous Flexural History of Northern Oman Mountain Foredeep, United Arab Emirates

An important feature of the middle Cretaceous Arabian continental margin was the regional upwarping in the foreland of the northern Oman Mountains. We interpret this upwarp as an early expression of a cratonward-migrating peripheral swell associated with the emplacement of the Semail Ophiolite onto the margin. Subsurface stratigraphic data indicate that the swell was expressed during the Cenomanian and attained its maximum relief during the Turonian. By the Santonian, further downflexure of the margin took place around an axis west of the study area, burying the then-inactive Turonian swell beneath the deposits of the westward-expanding foreland basin. We suggest that establishment of the later, more cratonward flexural regime took place abruptly rather than as a continuo sly migrating wave. The middle Cretaceous swell axis paralleled the present-day axis of the Oman Mountains. The upwarp was slightly asymmetric to the east, and loosely constrained estimates of its width and height are 150 km and 244 (93 mi and 800 ft), respectively. The dimensions suggest an anomalously low flexural rigidity (1022 N m) for the middle Cretaceous continental lithosphere. We attribute this low apparent rigidity to the combined effects of remnant thermal and mechanical processes of an earlier rifting event and compression of the margin during the Cretaceous emplacement of the Semail Ophiolite. The small dimensions of the middle Cretaceous swell suggest that other continental margins with similar geologic histories may also have narrow extinct swells buried beneath foreland deposits. Such swells may have significant impact on the entrapment of hydrocarbons, as we propose for the Sajaa-Margham trend.

Symmetric conjugation of continent-ocean boundary structures along the Norwegian and East Greenland Margins

A characteristic structural feature of the Norwegian continental margin that has been described from multichannel seismic data, as well as DSDP and ODP drilling, is a major buildup of layered extrusive basaltic rocks forming wedges that dip seaward along the entire margin. Models for their origin describe them as either products of a late-stage intra-continental rift phenomenon, with the volcanic rocks overlying attenuated continental crust, or as an integral component of the oceanic crust formed during the first few million years of seafloor spreading in the Norwegian-Greenland Sea. From studies of the Norwegian margin alone it was not possible to unequivocally distinguish between the two proposed models so an investigation of the conjugate portions of the East Greenland margin was undertaken which used two-ship multichannel seismic acquisition as its focus. We found that the wedges were not only present on the East Greenland margin, but that details of their along-strike variability are mirrored on both margins, together with their spatial relationship to the oldest magnetic lineation pattern. The wedges are relatively small in the northern part of the margin where they occur almost entirely landward of lineation #24. Moving south we see that the wedges broaden, encroach into the pattern of lineation #24, then develop into two separate wedges, the seaward of which is associated with lineation #23. Lineations #24 and #23 on both margins are distorted when associated with wedges. We believe that these data lend strength to a tectonic association of the wedges with nascent plate boundary processes. Their observation in many margins throughout the world suggest that a prolific volcanic phase may be commonly associated with the inception of a divergent plate boundary, and we believe that the most likely mechanism to generate the excess volcanism is the ‘convective partial melting’ scheme recently proposed by Mutter and Buck (1986) and Mutter et al. (in press).

Evolution of the Cape Fear Terrace: A complex interaction between the Gulf Stream and a paleo-shelf edge delta

A high-resolution seismic stratigraphic study of the Cape Fear Terrace (outer continental shelf off Cape Fear, North Carolina) combined with bio-lithostratigraphic data have yielded a complex chronostratigraphic framework of the Quaternary sequences that comprise this portion of the North American continental margin. The Cape Fear Terrace is one of six terraces/protuberances that form significant topographic irregularities along the South Carolina and North Carolina shelf-slope break. The Cape Fear Terrace is situated upon an anomalous, point-source, prograding, shelf margin feature which has protruded out onto the upper slope throughout much of the Quaternary. This upbuilding/outbuilding followed a period of active submarine erosion in which the ancestral Gulf Stream and/or associated meanders cut an erosional path beneath the present shelf margin during the early Pliocene. The protuberance was originally constructed during a relative lowstand of sea level as a shelf-edge delta. Severe modification of this deltaic lobe occurred during a relative highstand of sea level as Gulf Stream activities began to more effectively modify the margin. The anomalous thick accumulation of shelf-edge sediments acted as a barrier to flow inducing complex flow patterns of the Gulf Stream causing preferential erosion on its downstream side. Subsequent deposition may have resulted during more elevated highstands of sea level, as evidenced by the presence of active sand waves in the terrace protuberance region today. Once this shelf-edge bathymetric irregularity had been established, the Gulf Stream acted as a dynamic force inducing cellular flow structures within the shelf environment and enabling sediments to be transported seaward along the paleo-shoals complex.

Detachment faulting and the evolution of passive continental margins

Major detachment faults play a key role in the lithospheric extension process in the Basin and Range province and may also be important in other continental extension terranes. Such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift/subsidence patterns. Detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins. We therefore suggest the existence of upper-plate and lower-plate passive margins. These give rise to a complementary asymmetry of opposing margins after continental breakup. Transfer faults offset marginal features and allow margins to switch from upper-plate to lower-plate characteristics along strike.

Subglacial carbonate deposition: An isotopic study of a present-day case

Different types of calcite deposits may be formed in the subglacial environment. Patchy coatings on polished and striated bedrock pavements occur on outcrops recently exposed by retreating temperature glaciers; coated pebbles and thin limestone crusts in fissures are present in recent basal till exposures. Freezing of meltwater is the main process susceptible to explain most of these carbonate deposits. Partial freezing is accompanied by a rejection of ions from the growing ice and a concentration increase in the residual water, eventually until saturation is reached. Glacier sliding over a protuberance, by pressure-melting on the stoss side and refreezing on the lee side, is likely to be responsible for subglacial carbonate deposition. A fixed bedrock protuberance will give rise to fluted and furrowed coatings on its lee side. A mobile protuberance like a pebble at the ice-bedrock interface, because it is susceptible to rotate while in traction, will have a coating on most of its surface. A present-day case in the Swiss Alps is studied by a comparison of the isotopic composition of calcite deposits, ice and meltwater from the glacier. This study indicates that melting of the basal ice layer, not of glacier ice, is at the origin of the initial water giving rise to the precipitate. Since the basal ice layer is formed in the marginal zone of the glacier, subglacial carbonate deposition is a marginal feature in this case.

Surging of the southwestern part of the Laurentide Ice Sheet

The southwestern part of the Laurentidc Ice Sheet, in central North America, repeatedly surged during the last part of the Wisconsin Glaciation. Evidence includes the extreme lobation of the ice margin, the gentle slopes of lateral moraines and other marginal features, a radiocarbon chronology indicating extremely rapid marginal advance and retreat, and the abundance of supraglacial flow till. Rapid ice movement was caused by subglacial water and was probably limited to areas of slowly permeable substrate, which slowed the escape of the water.

Structural setting and genesis of ore bodies in carbonate rocks in the M.te Atzei-M.te Ega mining area (Sulcis, SW Sardinia)

At least seven different types of ore bodies, with an ore stock of Ba, F, Fe, Pb, Zn, Cu and Sb occur in Paleozoic rocks (Lower-Middle Cambrian and Ordovician) in an ≃15 km2 area north of the Narcao village (SW Sardinia, Italy). The ore bodies are related both to syngenetic accumulations, and later epigenetic recirculation of the primary ores. In the Lower Cambrian the area studied shows evidence of synsedimentary tectonic instability, probably related to carbonate shelf margin features. The Upper Cambrian is lacking and the transgressive Ordovician sediments cover large areas of the previously gently folded Cambrian rocks (Sardic phase of Caledonian orogenesis). The main Hercynian tectonic phases had a much stronger compressive folding that is mirrored in a sequence of tight folds and upthrusts. The geometric setting resulting from the tectonic frame of the Cambrian and Ordovician sediments must be taken into account to understand the distribution and the genesis of most of the ore bodies.

At the Margin of Discourse: Footnotes in the Fictional Text

The referential and marginal features of footnotes serve different functions in criticism and literature: scholarly footnotes shore up the text by enclosing it and limiting its claims; in fiction, footnotes extend textual authority by enlarging the fictional context. Both inner- and outer-directed, these two kinds of notations display a self-conscious anxiety about the critical and creative acts they annotate. Scholarly notes mask this ambivalence by claiming extratextual authority; literary notes highlight the ambivalence by consciously dividing the text against itself. This essay examines the ways footnotes in Tom Jones, Tristram Shandy, and Finnegans Wake parody the notational convention and draw attention to the faulted authority of its discourse by flouting scholarly claims to objectivity and neutrality, by calling into question the relations of author and reader on textual grounds, and by using self-reflexive narrative methods to illustrate the rhetorical double bind that keeps all language at the margin of discourse.

Radiocarbon dated raised beaches and glacial history of the northern coast of Spitsbergen, Svalbard

Raised beaches, glacial erratics, glacial striae and ice marginal features along the north coast of Spitsbergen have been studied. Two series of raised beaches occur in the area bordering the outer part of Woodfjorden. The lower marine limit has an altitude of approximately 40 m and is close to 11,000 years in age. The upper marine limit is at 80 m above sea level and predates 40,000 years B.P. Emergence between 11,000 and 10,000 years ago took place at a rate exceeding 2 m/century, and approximately one-half of the total emergence took place during this interval. Three main directions of glacier ice movement are recorded, and marginal features from the Late Weichselian maximum glaciation occur on both sides of Liefdefjorden. Sediment cores from six shallow lakes on Reinsdyrflya show extremely slow rates of sedimentation and low organic production. The maximum sediment thickness encountered was 143 cm (Lake F, at 35 m above sea level). No radiocarbon dates have been obtained from these lakes. The Younger Dryas (11,000 to 10,000 years B.P.) seems to be a period of deglaciation. The deglaciation on the northern coast of Spitsbergen predates the final deglaciation in a number of other Arctic regions. Mytilus edulis immigrated to the area about 9400 years ago or earlier, indicating a warm early Holocene climate on the north coast of Spitsbergen.

Radiocarbon dated raised beaches and glacial history of the northern coast of Spitsbergen, Svalbard

Raised beaches, glacial erratics, glacial striae and ice marginal features along the north coast of Spitsbergen have been studied. Two series of raised beaches occur in the area bordering the outer part of Woodfjorden. The lower marine limit has an altitude of approximately 40 m and is close to 11,000 years in age. The upper marine limit is at 80 m above sea level and predates 40,000 years B.P. Emergence between 11,000 and 10,000 years ago took place at a rate exceeding 2 m/century, and approximately one-half of the total emergence took place during this interval. Three main directions of glacier ice movement are recorded, and marginal features from the Late Weichselian maximum glaciation occur on both sides of Liefdefjorden. Sediment cores from six shallow lakes on Reinsdyrflya show extremely slow rates of sedimentation and low organic production. The maximum sediment thickness encountered was 143 cm (Lake F, at 35 m above sea level). No radiocarbon dates have been obtained from these lakes. The Younger Dryas (11,000 to 10,000 years B.P.) seems to be a period of deglaciation. The deglaciation on the northern coast of Spitsbergen predates the final deglaciation in a number of other Arctic regions. Mytilus edulis immigrated to the area about 9400 years ago or earlier, indicating a warm early Holocene climate on the north coast of Spitsbergen.

Geomorphological criteria for the reconstruction of late Pleistocene ice caps in Northern Eurasia

Abstract The positions of ice dispersal centers and of ice cap limits, both on land and on the sea bed, have been located fairly precisely by means of an analysis of marginal features produced by the last glaciation of Northern Eurasia, using in particular new data on the geomorphology of the beds of the Barents and Kara seas. The configuration of the various ice caps was then reconstructed on the basis of both glaciological and geomorphological data. Of particular interest is the conclusion that both the Barents and Kara seas were covered by a number of independent ice caps, contiguous to varying degrees and for verying periods, rather than by one massive ice cap, as had been earlier proposed. (The translation is by William Barr, University of Saskatchewan, Saskatoon.)

Status of Circum-Pacific Map Project--Southwest Quadrant, Mid-1982: ABSTRACT

The Geographic and Plate-Tectonic Maps of the Circum-Pacific Maps Project, Southwest Quadrant, have already been published. The Plate-Tectonic Map is a best guess for many areas, because much remains unknown about spreading centers, plate boundaries, and passive margin features. Compilation of the Geologic Map is practically complete, with modification of guidelines so that quadrant geology can be adequately summarized. The fundamental geologic elements within the Quadrant are fragments of Gondwanaland encircled by oceanic crust and plate boundaries; passive margin-style Mesozoic and Cainozoic deposits were added to the Gondwana Paleozoic and Precambrian terrain while interactions at the boundaries between the Pacific, Australia-India, and various Asian Plates were producing other kinds of deposits. Units hosting energy and mineral resources are also emphasized. The concurrent draft compilation for the Quadrant Tectonic Map directly complements the Geologic Map. The draft basically shows plate interiors and plate margins, each divided into basement and cover rock areas. Major orogenies are emphasized and detailed subdivision results in about 50 kinds of mostly strato-tectonic units. Drafts for the quadrant Energy Resources Map are in progress, but little progress has been made on the Mineral Resources Map. Most of the work on the Geodynamics Map is the responsibility of project headquarters. End_of_Article - Last_Page 964------------

Recording Marginal Glacial Deposits by Satellite Imagery

Based on imagery and experiences from areas influenced by the last Scandinavian ice sheet (Weichsel), the potential use of satellites (Landsat) for imaging large-scale glacial accumulations, especially marginal deposits, is discussed. The ability of satellite imagery to bring consistency in apparently disperse terrain elements and to visualize landscape systems because of its wide overview is pointed at.The testified advantage of multiband recording for getting a differentiated portrait of the landscape composition and material properties is valid also for this topic, but of still greater importance is the multidate approach, as a repeated recording will offer chances to obtain optimal (contrast-rich) imagery. The possibility of getting information on marginal features in less known and remote areas for detecting accumulation systems, or for making geomorphological comparison between marginal areas in different parts of a former glaciation, is illustrated with examples from the Verchojansk mountains and the Kola peninsula.

Geophysical review of the continental margins of eastern and western Canada

The evolution and geophysical features of the continental margins of eastern and western Canada are reviewed in light of recent plate-tectonic concepts. The two margins are very different in age, structure, and origin. The eastern margins were formed either by rifting or by transform motion during the latest separation of the continents around the Atlantic that occurred from Jurassic to Tertiary times. Studies of these margins centre around a reconstruction of plate motions, the inception of which occurred over 70 Ma ago, and on subsequent processes such as subsidence and sedimentation. The subsidence of the margin is explicable in terms of cooling of the lithosphere and sediment loading. Deep crustal features are inferred from seismic, gravity, and magnetic data. The recognition of the ocean–continent boundary at these margins involves consideration of edge effects, magnetic quiet zones and rifting mechanisms. The western Canadian margins are present active plate boundaries. Recent geophysical studies of these margins centre around the detailed definition of the present plate boundaries and relative plate motions, and those of the recent past (about the past 10 Ma), and involve spreading ridges, transform faults, and subduction zones. The plate convergence predicted by offshore geophysical data has a pronounced effect on the continental crust and upper mantle extending several hundred kilometres inland from the coast. In southwestern Canada patterns characteristic of subduction zones are seen in seismic structure, the gravitational and magnetic fields, heat flow, and deep electrical structure.

AGU Midwest Meeting Report

Approximately 150 Midwestern geophysicists attended the third annual AGU Midwest Meeting, which was held at Purdue University, September 26–28, 1977. The majority of the 75 papers presented were related to topical special sessions arranged by seven sections of the Union to be of particular, current importance to Midwestern geophysics. The interest shown in the meeting indicates not only the vigor of current geophysical research in the Midwest but also the desirability of continuing these meetings.The majority of the papers presented were in the area of solid‐earth geophysics, and these primarily were concerned with the paleogeodynamics and contemporary geodynamics of the Midwest. They reflect the growing interest in deciphering the complex evolutionary history of the continental interior by utilizing a wide range of methodologies and the concepts of plate tectonics which have generally emphasized oceanic and continental margin features. The papers in the session on Geology and Geophysics of the Precambrian Basement Rocks of the Midcontinent Region demonstrated that geophysical data can be used to clarify and amplify our understanding of the basement geology in poorly exposed regions. By extension of the data base to include buried portions of the basement, it soon may be possible to determine the major structural, lithologic, and chronologic provinces of the basement in the Midwest fairly accurately. The session dealing with Geodynamics of Plate Interiors demonstrated that the structures present in the midcontinent reflect a long and complex midplate tectonism which is not completely understood. The 15 papers of the sessions on the New Madrid Seismic Zone, the 38th Parallel Lineament, and the Seismicity of the Midwest illustrate the rapidly broadening interest in and knowledge of the tectonic framework of the seismically active New Madrid area and environs. It was emphasized that this area is an excellent example of reactivation of ancient structures during the Phanerozoic and locally into the present. Interpretation of surface, air, and satellite geophysical and geological data suggests the presence of an ancient easterly striking rift south of the 38th Parallel Lineament and a northeast‐trending graben lying along the axis of the New Madrid Seismic Zone. Microearthquakes together with historical seismicity are being used to delineate active faults of this area and estimate earthquake recurrence rates and the local stress state.

Stratiform sulfide mineralization at Oamites copper mine, South West Africa

A lithofacies change from terrestrial arenites to marine Oamites Formation sediments and possible volcanics is a major feature of the southern margin of the late Precambrian Damara eugeosyncline in the Oamites area. The MO sequence, which is the predominantly rudaceous middle section of the Oamites Formation, has a maximum thickness of 55 meters and hosts the 600-m-long Oamites Cu-Ag orebody near the major lithofacies change. Upper greenschist facies metamorphism and structural deformation have not obscured the main primary characteristics of this stratiform sulfide deposit.The weakly mineralized lower MO sequence, consisting mainly of conglomerate, was deposited during rapid marine transgression, possibly following a volcanic episode. The basal MO pink pebble rudites contain iron oxides and ilmenite, but in the overlying reduced white pebble conglomerates and white quartzites virtually complete sulfidization of iron in these detrital components has been effected. The upper MO sequence quartzites, conglomerates, and dolomites were deposited during the initial stages of regression and are often well mineralized. The reduced lagoonal facies grades into a pink oxidized facies at the eastern limit of the orebody, indicating proximity to the paleostrandline. Sulfide mineral zonation cuts across bed boundaries but tends to parallel oxidized lithofacies/reduced lithofacies boundaries. The Cu content of sulfides increases upward stratigraphically and toward the paleostrandline laterally. As is the case in many other stratiform deposits, Pb and Zn are concentrated above the Cu orebody in a stratigraphic sense and in a deeper water depositional environment laterally. Mineralogic investigation revealed a well-defined paragenetic sequence for the MO sulfides: Pyrite is first replaced by chalcopyrite which is, in turn, progressively replaced by bornite, neodigenite, and chalcocite.A basically syndepositional genetic model for the Oamites sulfide deposit is proposed: Base metals and Ag were carried by rivers into a lagoon where H 2 S in the anoxic zone caused their selective precipitation. Algal reefs developed near the strandline played an important role in the development of restrictive environments favoring concentration of Cu and Ag. Albitization of K-feldspar, which is characteristic of the orebody, suggests that the lagoonal waters were highly saline. It is concluded that the Oamites deposit is in many ways similar to the Zambian Copperbelt deposits, although the copper content at Oamites is relatively low.

Neotectonic Fold-Faulted Morphostructures in the Surroundings of the Town of Brno

A characteristic relief feature of the SE margin of the Česká vysočina (Bohemian Highlands) in the surroundings of the town of Brno are neotectonic forms designated by the authors as fold-faulted morphostructures. These morphostructures established by a detailed morphostructural analysis developed by Post Upper Miocene vertical movements. Together with horst and grabens they form a complicated monocline of the SE margin of the Bohemian Highlands. A part of the monocline is covered by Miocene deposits of the Carpathian Foredeep. In the territory investigated involving parts of the Českomoravská vrchovina (Highland), the Brněnská vrchovina (Highland) and the Drahanská vrchovina (Highland) five fold-faulted morphostructures were established (Encl. 1). The fold-faulted morphostructures have an irregular shape in their groundplan controlled by the course of faults. They are further characterized by inner faulting of various intensity (Figs. 1, 2). Their longer axes are as much as 18 km long and run in substance perpendicularly to the margin of the Bohemian Highlands. The summits of the fold-faulted morphostructures are situated in altitudes between 460 to 560 m. They do not occur in the centre of the fold-faulted morphostructures but rather in the interior of the Bohemian Highlands (Fig. 2). Along supposed longer axes of fold-faulted morphostructures deformed remnants of planation surface with monadnocks and outliers usually kept best preserved. Differences in heights between the uppermost and lowermost spots of fold-faulted morphostructures range between 200 and 300 m. A radial valley pattern is characteristic of fold-faulted morphostructures. But in details several smaller river basins have a rectangular valley pattern. Some river basins have a very atypical valley pattern, for instance with tributaries only from one side (a fan-like shape, Fig. 3). Numerous valleys exhibit a distinct height assymetry of slopes. The individual valleys are either rectilinear (valleys on fault and dislocation zones) or curved (on fissures developed due to tensile stress in the course of up-doming). Simultaneously with the development of fold-faulted morphostructures satellite blocks developed at their margins. The territory between neighbouring described fold-faulted morphostructures consists either of grabens of various widths (often with Miocene deposits) or deep incised valleys on dislocation zones. The morphostructural investigations carried out so far in the territory of the Outer Carpathian Depressions adjacent to the part of the Bohemian Highlands under study have shown that fold-faulted morphostructures developed even here. Since the surface of this territory is built predominantly of less resistant Miocene sediments the establishment of deformations is very difficult. More detailed investigations of these morphostructures bave not been carried out so far. The establishment of fold-faulted morphostructures is of considerable significance for understanding of the geomorphological development and neotectonic movements of the marginal parts of the Bohemian Highlands in Upper Tertiary and Quaternary. The results of research can be applied in the delimitation of geomorphological units of lower taxonomic levels and in mineral prospecting.

Erebus Glacier Tongue, Mcmurdo Sound, Antarctica

Examination of the past and present behaviour of the Erebus Glacier tongue over the last 60 years indicates that a major calving from the tongue appears to be imminent. Calculations of the regime of the tongue indicate that bottom melt rates may exceed 1 m a−1. By successive mapping of the ice tongue between the years 1947 and 1970, longitudinal strain-rates were determined using the change in distance between a set of 15 teeth, which are a prominent marginal feature of the tongue. Assuming a flow law for ice of the form where τ is the effective shear stress and is the effective shear strain-rate, values of the exponent n = 3 and B = 1 × 108 N m−2 are determined. These are in fair agreement with published values.

Erebus Glacier Tongue, Mcmurdo Sound, Antarctica

Examination of the past and present behaviour of the Erebus Glacier tongue over the last 60 years indicates that a major calving from the tongue appears to be imminent. Calculations of the regime of the tongue indicate that bottom melt rates may exceed 1 m a−1. By successive mapping of the ice tongue between the years 1947 and 1970, longitudinal strain-rates were determined using the change in distance between a set of 15 teeth, which are a prominent marginal feature of the tongue. Assuming a flow law for ice of the form where τ is the effective shear stress and is the effective shear strain-rate, values of the exponent n = 3 and B = 1 × 108 N m−2 are determined. These are in fair agreement with published values.

Contrasts in tectonic evolution of orogenic belts in the south-east pacific

Abstract The orogens of southern South America and Antarctic Peninsula, which are of similar age and apparently linked by the Scotia Arc, are shown to have evolved along markedly different patterns of geosynclinal and orogenic history. The Andes of Patagonia and Tierra del Fuego originated from a Late Mesozoic mio-eugeosynclinal couple and volcanic island arc created by subsidence, extension and rifting. The trend of this belt is unrelated to, and across the previously established Gondwanide framework. During the orogenic stage this marginal sea along the borders of an old continent, where apparently new oceanic crust had been formed, was welded to the continental block; the Andes thus constitute an accretionary belt. Orogenic polarity follows geosynclinal polarity; both geosynclinal and orogenic evolution are characterised by progressive outward migration towards the foreland, incorporating successive flysch and molasse basins in the fold belt. Alpino-type tectogenesis is accompanied by high-grade metamorphism and migmatisation. Recent calc-alkaline andesitic-dacitic volcanism in the Andes, their taphrogenic break-down along the Pacific and association with a deep-sea trench indicate a continuing orogenic regime. In the Antarctic Peninsula, no Late Mesozoic geosynclinal orogensis is recognised. I ts history is dominated by Gondwanide post-orogenic consolidation. Subsequent volcanism and localised, partly fault-bounded basins filled with terrestrial to shallowmarine sediments of limited thickness are the characteristic features of JurassicCretaceous time. Mid-Cretaceous large-scale intrusion of granodioritic magmas was accompanied and followed by regional unlift. This orogensis is superimposed upon, and parallel to the Gondwanide orogenic belt. Deformation is germanotype, except at the base of the Peninsula where tight folding occurred prior to the intrusions. Orogenic molasse sequences deposited in external foredeeps remained little disturbed tectonically. Cenozoic rifting and basaltic volcanism of subalkaline composition indicate that no orogenic regime is affecting this region. The Andean orogen of southernmost South America thus is a continental margin feature where accretion occurred, and which is unrelated to Gondwanide tectonics, whereas the Antarctic Peninsula is an intracontinental orogen of essentially Gondwanide inheritance. The contrasting tectonic setting and framework, sedimentary and orogenic history and structure of the two orogenic belts indicate that they have not been an originally continuous and straight mountain belt along the Pacific margin, prior to bending and disruption in the Scotia Arc region. The term “ Antarctandes” therefore is misleading and should not be used. The Scotia Arc appears as a composite feature of much greater complexity, which probably originated by juxtaposition of tectonic elements of entirely different origin and character, and with partial bending only.