Rocas Verdes Marginal Basin Research Articles

Paleomagnetic Constraints From South Georgia on the Tectonic Reconstruction of the Early Cretaceous Rocas Verdes Marginal Basin System of Southernmost South America

AbstractSouth Georgia exists as a microcontinent along the North Scotia Ridge ∼1,700 km east of Cape Horn. The tectonostratigraphic units of South Georgia have long been correlated with those of the Fuegian Andes of southernmost South America. Accordingly, South Georgia has been regarded as a continuation of the Late Jurassic–Early Cretaceous Rocas Verdes marginal basin system, formerly situated south of Burdwood Bank and east of Cape Horn. To test this, paleomagnetic analysis of samples from the Larsen Harbour Complex, Drygalski Fjord Complex, and Annenkov Island Formation of South Georgia showed that 21 sites yield a mean direction of D = 328.5°, I = −62.1° (a95 = 3.5°) and a paleomagnetic pole at 068.2°E, 67.2°N, A95 = 4.7°. The consistency of directions and strong polarity bias, plus indications of a negative differential tilt test, point to a secondary magnetization acquired in the Late Cretaceous. Comparison of predicted versus observed directions for South Georgia relative to stable South America indicate 27.2 ± 11.2° of counter‐clockwise rotation (and 10.5° ± 4.5° of northward tilting) since the acquisition of magnetization. These results are consistent with paleomagnetic studies from the Fuegian Andes and support a paleoposition of the South Georgia microcontinent south of Burdwood Bank as strongly indicated by the geologic evidence. Partitioning this rotation between oroclinal bending during the Rocas Verdes basin collapse in the Late Cretaceous and left‐lateral translation along the North Scotia Ridge is not possible on paleomagnetic grounds, but the co‐linearity of Andean structures between the restored microcontinent and Tierra del Fuego indicates the former.

U-Pb zircon constraints on the age of the Cretaceous Mata Amarilla Formation, Southern Patagonia, Argentina: its relationship with the evolution of the Austral Basin

Despite the abundant fossil content of the Mata Amarilla Formation (Southern Patagonia, Santa Cruz Province, Argentina), its age has always generated a considerable number of questions and debates. The chronological data provided by invertebrates, dinosaurs, fish, turtles, plesiosaurs and fossil flora are contradictory. In this work, twenty U-Pb spot analyses by laser ablation were carried out on the outer parts of the zoned zircon crystals from a tuff layer of the middle section of the Mata Amarilla Formation, yielding a U-Pb concordia age of 96.23±0.71 Ma, which corresponds to the middle Cenomanian. The deposition of the lower section of the Mata Amarilla Formation marks the onset of the foreland stage of the Austral Basin (also known as Magallanes Basin); this transition is characterized by the west-east shift of the depositional systems, which is consistent with the progradation of the Cretaceous fold-and-thrust belt. Thus, the onset of the foreland stage could have occurred between the upper Albian and lower Cenomanian, as the underlying Piedra Clavada Formation is lower Albian in age. On comparing the data obtained with information from the Ultima Esperanza Province in Chile, it can be suggested that the initiation of the closure of the Rocas Verdes Marginal Basin occurred simultaneously.

Provenance of eastern Magallanes foreland basin sediments: Heavy mineral analysis reveals Paleogene tectonic unroofing of the Fuegian Andes hinterland

Documenting variations in the composition of foreland basin detrital sediments is a useful tool for reconstructing the kinematic history of associated orogenic systems. Herein we report the results of our SEM-EDS heavy mineral analysis of the Late Cretaceous to early Miocene eastern Magallanes basin of southernmost Argentina. Our data indicate that Campanian to middle Eocene sediments had a mafic/ophiolitic provenance, which we interpret as being derived from the Patagonian–Fuegian magmatic arc and the mafic floor of the preceding Rocas Verdes marginal basin. Upper middle Eocene to lower Miocene heavy minerals, on the other hand, indicate a metamorphic/metasedimentary provenance, which we interpret as being derived from the Cordillera Darwin metamorphic complex. Together, these data indicate an abrupt shift in sediment provenance in middle to late Eocene time, thereby corroborating recent interpretations of the basin's detrital–zircon geochrononology, and providing further support for temporal and possibly genetic relationships between development of the Patagonian orocline, the opening of Drake Passage and the Oi-1 glaciation of Antarctica.

Mesozoic-Cenozoic stratigraphy of the Fuegian Andes, Argentina

The stratigraphy of the Argentinean Fuegian Andes reveals contrasting Mesozoic-Cenozoic tectonic regimes. The Lemaire Formation, submarine complex of bimodal volcanites, breccias, and sedimentary rocks; the Yahgan Formation, volcaniclastic apron of deep-marine andesite-rich turbidites and mudstones; and the Beauvoir Formation, slope mudstones, all evidence late Jurassic early Cretaceous extension and consequent origination of the Rocas Verdes Marginal Basin. The basal late Cretaceous ductile deformation, isoclinal folding, and metamorphism of these rocks indicate a compressional tectonic regime that resulted in the closure of the marginal basin. The Fuegian Andes were uplifted by the late Campanian; subsequent propagation of the compressional deformation and subsidence by tectonic loading along the northern orogenic margin originated the Austral and Malvinas foreland basins. The Turonian-lower Campanian “Estratos de Buen Suceso” represents the final stages of the closure of the marginal basin and/or the beginning of the foreland basins. During the late Campanian to mid Miocene, the foraminiferal assemblages allow precise dating of the main tectonic events and with the associated trace fossils and sedimentary facies, the general characterization of the depositional settings. The thrust and fold belt of the Austral/Malvinas basins includes three depocenters: 1) the Bahia Thetis (upper Campanian-Maastrichtian), Policarpo (upper Maastrichtian/Danian), and Tres Amigos (upper Paleocene) Formations, mostly turbidite settings with cosmopolitan agglutinated foraminifers; 2) the Rio Claro Group, upper Paleocene-lower Eocene; and 3) the La Despedida Group, representing the extended upper mid Eocene-upper Eocene transgression with endemic foraminifers. A fourth depocenter, the Oligocene-mid Miocene Cabo Domingo Group mostly deposited below the calcite compensation depth and characterized by widespread agglutinated foraminifers, represents the last and deepest foredeep, originated just north of the fold belt during the waning compressional phase. Reduced exposures of mid Eocene bryozoan limestones, the Rio Bueno Formation, and fluvial upper Eocene lower Oligocene, the Punta Cactus and Sloggett Formations, conform reduced exposures within the fold belt. The late Miocene-Pliocene, shallow marine “estratos de la Maria Luisa” and Irigoyen Formation, the latter with foraminifers suggesting an Atlantic-Pacific connection, record Cenozoic transcurrency and pull-apart basins.

Structural geology of the Fuegian Andes and Magallanes fold-and-thrust belt - Tierra del Fuego Island

A synthesis of the structural geology of the Tierra del Fuego Island, which integrates a new data set derived from field surveys and literature data of the last few years, is presented here. The main geological features of the region developed during the Mesozoic-Cenozoic Andean orogenic cycle that started in the Middle to Late Jurassic with a back-arc extension, crustal stretching and widespread volcanism, related to the break-up of Gondwanaland. An extensional fault system deriving from the mechanical and thermal subsidence led the evolution of the Rocas Verdes marginal basin, which hosts the upper Jurassic volcanoclastic rocks, the lower Cretaceous turbiditic sequences and few isolated elongated ophiolitic complexes. From the Late Cretaceous onward, the orogenic cycle of the Fuegian Andes continued with the shortening and inversion of the back-arc margin through horizontal contraction and crustal thickening. The uplift of the Cordillera, the emplacement of plutonic rocks, and the intercontinental polyphase deformation resulted from thick-skinned tectonics. The thrust system developed from its deeper roots, where the Palaeozoic basement was involved in compressive deformation, and propagated to the shallower stratigraphic levels of the northward verging Magallanes fold-and-thrust belt. The Magallanes foreland basin developed in front of the orogenic wedge that records at least four syntectonic angular unconformities from Late Cretaceous to Lower Miocene. During the Late Cretaceous Andean compression, three distinct phases of penetrative ductile deformation defined by low-greenschist facies assemblages took place, both in the basement and in the cover units. These deformations are related to a single metamorphic event with foliation development, as observed from microscopic analysis of the schist in the Ushuaia area. The first foliation S1 is preserved either as relic sericite microfolds between microlithons of the dominant S2, or as early refolded veins of recrystallized quartz. The S2 foliation is defined by oriented white mica. The crenulation of S2, which is related to D3 and occurs in most strained zones, becomes a pressure solution S3 spaced foliation, lined by opaque minerals. From the Palaeogene to the present, EW sinistral wrench tectonics affected the region as a component of the relative motion between South America and the Antarctic Peninsula. This strike-slip activity is well documented from the Carbajal valley to the Canal de Beagle region south of the Magallanes-Fagnano transform fault system. Restraining bends and overlapping step-over geometry characterize few sectors of the strike-slip faults with pop-ups, pressure ridges and uplifted slivers of crust. Releasing step-over along the transform fault system, both in on-shore and off-shore zones, formed several elongated pull-apart basins with many tens of km in length and a few km in width. The Lago Fagnano represents the main morphotectonic expression of this structural setting. A N-S geological cross-section through the Fuegian Andes synthesizes all the geological and geophysical data. The major stacks of internal thick skinned basement involved in the thrusting are high-grade Upper Palaeozoic to Lower Tertiary metamorphic rocks. The geometry of the thrust complex is an upright, south plunging monocline of moderately tilted sedimentary cover strata, as well as related thrusts, faults and chevron folds involving the Upper Jurassic and Cretaceous rocks. The orogenic shortening of the Fuegian Andes, including the Cordillera and the Magallanes fold-and-thrust belt, reaches few hundred kilometres with a left-lateral wrenching component of many tens of meters. The Tierra del Fuego Island is characterized by low seismicity (M<3.5) and shallow crustal earthquakes. The southern part presents strong morphological evidence of the Quaternary activity of the E-W left-lateral strike-slip faults, with a present deformation pattern (DGPS) with a horizontal slip component of about 6 mm/year. Moreover, the northern sector of the Island is affected by extensional tectonics related to the normal fault systems of the eastern arms of the Magallanes Strait.

A paleomagnetic analysis of the Patagonian orocline

The dramatic change in the structural trend of the Andean orogen in southernmost Patagonia has long been considered a possible example of a true orocline. Paleomagnetism has proved a powerful tool to determine whether a curved orogen acquired its shape as a secondary feature. Paleomagnetic data obtained so far in the southern Patagonian and Fuegian Andes are reviewed in order to test the origin of the Patagonian orocline. Systematic counterclockwise (ccw) rotations along the arc are suggested by widespread ccw deflections of paleomagnetic declinations, which may be compatible with some secondary curvature of the orogen. Larger values of declination anomalies in the outer arc are consistent with some oroclinal bending related to mid-Cretaceous closure of the Late Jurassic – Early Cretaceous Rocas Verdes marginal basin. However, a thorough paleomagnetic test of the origin of the Patagonian Orocline is hampered by several problems that affect most results. These include scarce and unevenly distributed studied localities, low quality paleomagnetic data, lack of paleohorizontal control and uncertainty in magnetization ages. The fact that most data obtained so far is still scarce and questionable precludes a reliable answer to whether the southern Patagonian Fuegian Andes are a true orocline

Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite [formula omitted] age constraints on tectonic evolution of Southwestern Gondwanaland

Zircon U-Pb and muscovite 40Ar 39Ar isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic “back stop” of that wedge. Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ± 1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile. Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 ± 1.0 Ma, but their upper intercept is not well known. Inasmuch as this age is similar to the 40Ar 39Ar age of secondary muscovite growing in extensional fractures of pulled-apart feldspar phenocrysts in a Beagle suite granitic pluton (plateau age is 68.1 ± 0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the highgrade metamorphic complex of Cordillera Darwin in Tierra del Fuego.

Orogenesis at the southern tip of the Americas: the structural evolution of the Cordillera Darwin metamorphic complex, southernmost Chile

New, detailed lithologic and structural data are presented from three separately mapped areas along the southern boundary of the Cordillera Darwin metamorphic complex of southernmost Chile. Cordillera Darwin is a unique uplift because it exposes the highest grade rocks in the Andes south of Peru and averages 1 km higher in elevation than adjacent areas. The structural data indicate that Cordillera Darwin experienced mid-Late Cretaceous trans-pressional deformation with a partitioned strike-slip component localized along the Beagle Channel that forms the southern boundary to the range. Foliation, lineation and fold axis trends indicate NE-SW-directed contraction and NW-SE strike-slip shearing (present directions) during progressive D1 D2 Andean deformation. D2 deformation is marked by outcrop-to 10 km-scale south-southwest-vergent folds. Late Cretaceous-Tertiary brittle-ductile and brittle left-lateral strike-slip faults and shear zones crosscut all D1 D2 structures. Although limited structural evidence for extensional tectonics was documented in this study, apparent normal offsets across both arms of the Beagle Channel and previously documented field evidence for extension from other areas in Cordillera Darwin suggest that transtensional displacements also may have occurred in southern Cordillera Darwin during the Late Cretaceous-Early Tertiary. Cordillera Darwin's position within the evolving Patagonian Orocline adjacent to an evolving Mesozoic-Cenozoic left-lateral transform boundary between the South American and Antarctic plates, and later the South American and Scotia plates, necessitates consideration of the possible effects of regional counterclockwise rotation on development of structures. Regional counterclockwise rotation of Cordillera Darwin may have controlled the temporal and spatial transition of deformational regimes within Cordillera Darwin. Exhumation of the metamorphic core of Cordillera Darwin during the Late Cretaceous-Early Tertiary is interpreted to have been due to rapid uplift and normal rates of erosion. Rapid uplift may have been due to migration of contraction to northern Cordillera Darwin and crustal relaxation in southern Cordillera Darwin as contractional structures rotated into parallelism with σ 1 and either ceased to be active or were reactivated as strike-slip or oblique-slip faults. Rapid isostatic rebound of Cordillera Darwin may have also been a response to aborted continental subduction of southern Cordillera Darwin and detachment of a subducted mafic root during mid-Cretaceous closure of the Rocas Verdes marginal basin.

Uplifted ophiolitic rocks on Isla Gordon, southernmost Chile: implications for the closure history of the Rocas Verdes marginal basin and the tectonic evolution of the Beagle Channel region

A succession of mafic rocks that includes gabbro, sheeted dikes and deformed pillow basalts has been mapped in detail on Isla Gordon, southernmost Chile and is identified as an upper ophiolitic complex representing the uplifted floor of the Late Jurassic-Early Cretaceous Rocas Verdes marginal basin. The complex was uplifted, deformed, and regionally metamorphosed prior to the intrusion of an undeformed 90 Ma granodiorite that cuts the complex. The complex appears para-autochthonous, is gently tilted to the northeast and is internally sheared by near-vertical foliation zones. No evidence for obduction was observed although the base of the complex is not exposed. The ophiolitic rocks have been regionally metamorphosed to mid-upper greenschist levels. Isla Gordon is bounded by the northwest and southwest arms of the Beagle Channel, two important structural boundaries in the southernmost Andes that are interpreted to have accommodated north-side-up and left-lateral displacements. Directly north of Isla Gordon is the Cordillera Darwin metamorphic complex that exposes the highest grade metamorphic rocks in the Andes south of Peru. On the north coast of Isla Gordon a volcaniclastic turbidite sequence that is interpreted to have been deposited above the mafic floor is metamorphosed to lower greenschist levels in strong metamorphic contrast to amphibolite-grade othogneisses exposed in Cordillera Darwin only 2 km away across the northwest arm of the Beagle Channel. The profound metamorphic break across the northwest arm of the Beagle Channel and the regional northeast tilt of the ophiolitic complex are consistent with the previously proposed hypothesis that Isla Gordon represents the upper plate to an extensional fault that accommodated tectonic unroofing of Cordillera Darwin. However, limited structural evidence for extension was identified in this study to support the model and further work is needed to determine the relative importance of contractional, extensional and strike-slip displacements during the closure of the Rocas Verdes marginal basin and uplift of Cordillera Darwin. The Isla Gordon ophiolitic complex is correlative with other regional occurrences of ophiolitic rocks including the previously studied Tortuga, Sarmiento and Larsen Harbour complexes. The existence of the Isla Gordon ophiolitic complex helps link the known occurrences of the marginal basin floor into a semi-continuous belt that sheds light on the original continuity of the basin.

Strike‐slip faults in the southernmost andes and the development of the Patagonian orocline

The Patagonian orocline is the 90° bend in the southernmost Andes between 50°S and 56°S. Paleomagnetic and structural data indicate that the orocline is, at least in part, the product of tectonic rotation. Recent field work in the Beagle Channel region of southernmost Chile provides evidence for widespread left‐lateral strike‐slip faulting in the internal zones of the mountain belt. Both arms of the Beagle Channel are interpreted to be left‐lateral strike‐slip faults based on detailed study of mesoscale strike‐slip faults (Riedel shears) observed in coastal outcrops. Although much of the evidence indicates Cenozoic brittle strike‐slip faulting, other fabric data, including vertical foliation zones containing horizontal quartz stretching lineations and ductile left‐lateral kinematic indicators, suggest that Mesozoic ductile strike‐slip or oblique‐slip shearing also occurred. The implication is that the mid‐Cretaceous Andean orogeny involved the transpressional inversion of the Rocas Verdes marginal basin and that transpression has been the dominant deformational regime in the region for the last 120 Ma. Regional left‐lateral strike‐slip faults are now recognized in all lithotectonic provinces of the southernmost Andes. A statistical study of regional lineament trends using aerial photographs and satellite imagery suggests that many unstudied lineaments are also strike‐slip faults. A new model is proposed that integrates the development of strike‐slip faulting and the structural evolution and uplift of the southernmost Andes with the rotational development of the orocline. The Patagonian orocline appears to be the product of broad interplate shearing accommodated by strike‐slip faulting, block rotation, and contraction and is probably continuing to evolve today.

Role of subduction-plate boundary forces during the initial stages of Gondwana break-up: evidence from the proto-Pacific margin of Antarctica

Abstract In the West Antarctic sector of Gondwana, early stages of break-up are associated with the large Antarctic-Karoo-Tasman basalt province. Formation of this within-plate province was synchronous with active margin tectonics and development of both a proto-Pacific margin magmatic suite along the Antarctic Peninsula and the extensive Tobífera volcanic suite associated with the Rocas Verdes marginal basin system of southern South America and South Georgia. Extension, concurrent with subduction and oceanward migration of the magmatic focus, resulted in a broad extensional province in a back-arc and intra-arc-setting. High geothermal gradients and basalt underplating caused crustal melting on the east coast of the Antarctic Peninsula and formation of bimodal basalt-rhyolite suites. Large-ion lithophile element enriched initial rifting magmas were succeeded, at least in part of the Rocas Verdes basin, by early drift magmas of transitional chemistry and then by entirely asthenospheric MORB magmas representing lithospheric rupture and sea-floor spreading. A plate interaction model is proposed for the initial stages of Gondwana break-up relating the broad zone of mantle melting to a reduction in subduction-plate boundary forces. The change from Gondwanide compression to lithospheric extension in the Jurassic may be linked to a change from shallow to steeply dipping subduction, and to a slowing of subduction rates caused by a change in plate boundary zone parameters. A possible reduction of compressive boundary stresses may have enabled unconfined, overthickened Permo-Triassic crust to extend because of gravitational instability, thus facilitating break-up. We suggest that break-up was not plume-related, but was due to variations in the regional stress field associated with changing plate-boundary forces. The continental crust was placed under tension with substantial lithospheric thinning and decompression melting of an enriched mantle source forming the broad linear zone of within-plate magmatism. The presence of a plume beneath the Karoo province may have thermally weakened the lithosphere and induced local rifting, contributing to, but not causing the eventual separation of East and West Gondwana.

The Patagonian Orocline: New paleomagnetic data from the Andean magmatic arc in Tierra del Fuego, Chile

The Hardy Formation is a 1300‐m‐thick succession of Upper Jurassic‐Lower Cretaceous volcaniclastic sedimentary rocks interbedded with lava flows on Hoste Island at the southernmost tip of South America (55.5°S, 291.8°E). The strata are gently folded and metamorphosed to the prehnite‐pumpellyite grade. A well‐defined characteristic direction of magnetization, carried by magnetite, was readily identified in 95 samples from seven sites. At a given site, the directions group slightly better without structural correction. However, the means of the seven sites cluster better without tilt correction at the 99% significance level, implying that the magnetization postdates the folding event. It is most likely that the magnetization was acquired during the mid‐ to Late Cretaceous Andean orogeny that involved the folding and emplacement of the Patagonian Batholith. The fact that all samples are normally magnetized supports this age assignment. The pole position of 42.9°N, 156.6°E, α95=3.3° implies that the sampling area has rotated counterclockwise relative to cratonic South America by 90.1±11.9° with no significant flattening of inclination (F=1.9 ± 3.7°). Geologic considerations indicate that the rotation involved the entire Andean magmatic arc in Tierra Del Fuego. The results support interpretation of the Hardy Formation as part of the Andean magmatic arc deposited on the Pacific side of the Late Jurassic‐Early Cretaceous Rocas Verdes marginal basin. Oroclinal bending of the arc in southernmost South America accompanied inversion of the marginal basin and the development of a Late Cretaceous‐Cenozoic left‐lateral transform system (South America‐Antarctica) that later developed into the North Scotia Ridge.

Petrochemistry and tectonic significance of Lower Cretaceous Barros Arana Formation basalts, southernmost Chilean Andes

The Lower Cretaceous Barros Arana Formation (Albian, hornblende KAr age of 104 Ma), in the Magallanes region of Chile, consists of a sequence of spilitized clinopyroxene- and amphibole-bearing mafic dikes and lavas, and volcaniclastic breccias, occurring within the sedimentary infill of the Rocas Verdes marginal basin and its eastward extension onto the Cretaceous continental platform. Although the original alkali and alkaline earth element concentrations of the basaltic lavas and dikes have been altered by spilitization, the presence of relict pargasitic amphibole phenocrysts, the absence of orthopyroxene, and high LREE contents and LREE/HREE ratios imply mildly alkaline affinities for these basalts. Their low TiO 2 and HFSE (Zr, Nb, Ta, and Hf) contents and high LREE/HFSE ratios suggest affinities with convergent plate boundary arc magmas. The Barros Arana basalts are interpreted as mafic members of the mildly alkaline shoshonitic rock suite of subduction-related arcs. They may have formed as subduction geometry began to undergo the changes (flattening) that ultimately led to the initiation of the closure, deformation, and uplift of the Rocas Verdes basin by the late or post-Albian. The low initial 87Sr/ 86Sr (0.7031) and high initial 143Nd/ 144Nd (0.51277) of the basalts indicate that a generally extensional tectonic regime east of the main calc-alkaline arc allowed eruption of these mafic shoshonites without interaction with continental crust (in contrast to the contemporaneous plutons of the Patagonian batholith).

Modified Gulf of California model for South Georgia,north Scotia Ridge, and implications for the Rocas Verdes back-arc basin, southern Andes

New field mapping and an integrated trace element and Nd isotopic study have been undertaken on the Jurassic-Cretaceous Larsen Harbour Complex on the island of South Georgia, recognized as a displaced part of the southern Andes ensialic, autochthonous Rocas Verdes marginal basin. Five basalt groups (Groups I-V) are identified from trace element data; initial Nd isotopic compositions suggest that they are derived from only two sources. Magmas produced during early stages of continental lithospheric attenuation (Groups I-III) were derived by varying degrees of partial melting and fractional crystallization from a large ion lithophile element (LILE)-enriched, low-{epsilon}{sub Nd} mantle source relative to normal (N)-type mid-ocean ridge basalt (MORB). Magmas produced during later stages of rifting (Groups IV and V) were, however, derived from a high-{epsilon}{sub Nd} asthenospheric mantle source similar to N-type MORB, unaffected by earlier LILE enrichment. Our data do not occur with previous suggestions that the Larsen Harbour Complex and, by implication, the Rocas Verdes formed in a supra-subduction-zone setting. On the basis of geologic and geochemical evidence, we favor basin formation along an oblique-slip margin akin to that of the Gulf of California.

Associated middle to late Jurassic volcanism and extension in southern South America

The extrusive products of a Middle to Late Jurassic volcanic event occur throughout a wide area of southern South America. These volcanic rocks are associated in time and space with a series of NNW-trending grabens. The extension that produced the grabens began perhaps in the latest Triassic and continued throughout most of the Jurassic. The Middle to Late Jurassic volcanic rocks represent the culminating event of this period of extension. The Jurassic volcanic rocks described here are dominantly rhyolites and basalts, but flows of intermediate composition are also present. Major element geochemistry on a suite of samples taken from a west-east transect near 44°S latitude shows that these rocks are not related directly to convergent arc volcanism along the margin of South America, but are the products of a separate tectonic/magmatic event that involved significant crustal anatexis. The extension and related volcanism directly preceded the opening of the Rocas Verdes marginal basin along the western margin of Chile and may have led to the initial separation of South America and South Africa. As such, the Middle-to-Late Jurassic extension and volcanism heralded the breakup of part of Gondwanaland.