main idea: Although the Falkland islands are situated on the South American plate and are geographically close to South America, Paleomagnetic, stratigraphy, structural and paleocurrent data provide convincing evidence that the Falkland islands rotated from an original position off southeast Africa to their present position during break-up of Gondwana.
break-out model: the FIB rotation took place during or after 190Ma, and accomplished at ca 175Ma. This rotation may be related to a major magmatic pulse of a mantle plume beneath Africa and Antarctica.
This paper presented the conclusion surrounding the authors' detailed investigations of the tectonics of the South Atlantic. SEASAT altimeter data and underway marine geophysical data were used to focus the study to a series of fracture zones and associated magnetic anomaly patterns--more than 30 fracture zones can be traced for long distances, many entirely across the ocean floor. From these, a tectonic history of the area was reconstructed. Forty-five finite rotation poles in 2 million-year increments spanning the last 85 million years were found and studied, establishing the relative positions of the South American and African plates. From these, 12 were further studied to find corresponding stage poles to compare the plate motion through time.
According to the method used, the young end of the chron represents the magnetic anomaly. Moreover there is a marked topographic difference between a fracture zone where young crust abuts older crust compared to when young crust abuts younger crust. In the former, the newer crust has higher elevation; in the latter, the step is not as pronounced. The fracture zones were mapped from SEASAT altimeter data as profiles along the ship track. Where the SEASAT data was unclear, other geophysical methods like gravity, topography, and magnetic data were employed to map the fracture zones. Otherwise, these methods were used to complement the SEASAT data.
The paper shows that ridge spreading in the South Atlantic mirrors the pattern in the Central Atlantic, where changes in spreading occurred in the Cenozoic, shown by 's' curves in the fracture zones of the South Atlantic. In fact, these curves indicate that the tectonic history of the Early Cenozoic is marked by stage pole shifts. A comparison of fracture zones suggests that the Central Atlantic fracture zones south of 20 degrees N were generated by South Atlantic spreading 35-50 million years ago. Between the late Cretaceous and Early Cenozoic, spreading rates at 30 degrees S decreased from 75 mm/yr at chron C34 (90 million years ago) to 30 mm/yr at chron C27 (63 million years ago), then increasing around chron C20 to 50 mm/yr (45 million years ago). An inverse correlation between the slowest spreading (from chron C30 to chron C25--C70 to 60 million years ago) and the number and amplitude of fracture zone anomalies were found, possibly related to an increase in age offset caused by the fracture zone. The seafloor topography that formed before this time and hence topographically bound this interval show positive relief, as seen in the Eocene uplift of the Rio Grande Rise, and the rift propagation of the Walvis Ridge.
This research allows for a finely detailed study of this area. However,
large amounts of error could have been caused by ship uncertainty, then
amplified by the matrix rotations used when calculating poles. The paper
is generally clear in describing the kinematics of the South Atlantic
between the times indicated.
The paper introduces a large amount (14,000km) of `new' seismic data gathered over the relatively shallow (2km depth) elongate (1200km E-W, 300km N-S) Falkland/Malvinas Plateau (FP) - {Fig 1}, which affords the authors a new and detailed perspective on the geology and hence the tectonic evolution of the region. Their analysis is further supported by data from dredge hauling and DSDP wells.
The FP is a foundered complex of continental blocks differentially rifted during the opening of the South Atlantic in the middle Jurassic. The complex basement geometry is attributed to the triple-junction tectonics when the African, S. American and Antartic plates separated in the Late Jurassic - the FP is estimated to have elongated 400km in an E-W direction before the onset of drift - {see normal faulting in Fig 7}. Isopach mapping based on the seismic profiles - {Fig 3} - illustrates the great depth of the western central portion of this basin (filled with up to 7km of sediment). To the east, over the Maurice Ewing Bank, the basin is much shallower, overlain by only 0.5km of sediment in places. The FP is headed to the north by the Falkland Escarpment, several kilometers above the contiguous basin floor, where the basement actually outcrops (sampled by drege hauls). This northern margin is described as a continent/ocean transform boundary, and its thickness is attributed by the authors as due to thermally induced uplift caused by an oceanic ridge sliding along the transform - {Fig 17}. The authors further attempt to explain the clearly complex subsidence history of extreme depth in the west, with shallow basement in the east, in terms of delayed subsidence of the Maurice Ewing Bank due its extended proximity to the eastward moving mid-ocean ridge on its northern edge.
Consequent to this, up to 7km of sediments have overlain the basement in four widespread depositional sequences - {Fig16}. Lying unconformably over the Precambrian basement are Jurrasic sediment. From a shallow marine sediment of a few hundred metres at the centre of the plateau, this sequence thickens to a depth of over 2km of terrigenous sediment just south of the northern Escarpment - {Fig 5d} - likely sourced from the then adjacent African plate. The second and third depositional sequences are pelagic in nature, as is the fourth sequence, which is indicated by drift deposits likely due to deep ocean currents. Deposition was halted by the onset of the Antartic Circumpolar Current during the Oligocene.
The depth of faulting observed from the seismics indicate no major normal faulting occured after Early Cretaceous time, which they suggest shows extension occurs before drifting. The authors discuss the possiblilty of oceanic crust underlying the basin, due to the apparently very large E-W crustal extension.
The authors claim many of their arguments may be confirmed through drilling close to the Escarpment to examine the effects of the passing ridge, and to model gravitationaland magnetic anomalies to constrain the nature of the crust.
This paper (1988) predates some of the other papers read in the early part of the course. Thus, it does not benefit from insight gathered in particular from palaeomagnetic data as in the Storey (1999) paper on the break-out model for the Falklands within Gondwana. This paper concurs with that model in terms of the timing and geographical position of the FP breakout from modern Kwa-Zulu/Natal Province in South Africa during the middle and late Jurassic, though no mention and is made of the apparent rotation of the Falkland islands themselves that is documented in the Storey paper. Aside from this incongruity, this paper provides a thorough account of the formation and present geology of the Falkland Plateau.
This paper sets out to describe in detail the structure of the North Scotia Ridge - the boundary between the Scotia plate and the Suth American plate. This boundary lies roughly parallel to 55 deg S latitude, and stretches from Tierra del Fuego (at 65 deg W) to the Sandwich islands (around 30 deg. W). Earthquake studies have shown that some of the E-W relative motion between the Antarctic and South American plates is acommodated by strike-slip motion on the North Scotia ridge. The Scotia plate shallower (1 km depth) than the South American plate (2-3 km).
The authors present side-scan sonar data acquired with the GLORIA Mk II system, as well echo sounding and limited seismic reflection profiles. The GLORIA data consist 11 days worth of seafloor mapping data, covering 100,000 km^2. It provides a measure of the scattering properties of the seafloor with a resolution of 45m cross-track and o100-1000 m along track. The GLORIA system does not provide depth information, nor does it produce reliable data of the seafloor directly below the ship (due to specular reflection overwhelming the weaker backscatter). However, the backscatter properties of the seafloor provide information on the nature of the seafloor, e.g. a smooth area of undisturbed sedimentary deposity will appear dark in the sonograph.
The main scientific question adressed by this paper concerns the
history of motion of the North Scotia ridge boundary. Previous work
(Ludwig & Rabinowitz 1982) claimed that the presence of deformed
sedimentary deposits indicated that ongoing convergence. However,
this work shows that a) the exposed deformation front is probably at
least several million years old (sedimentation in the region is
inhibited by strong ocean currents), b) there are regions (e.g the FT)
where seismic reflection studies show the deformations to be buried
beneath a thick layer of smooth sediment, and c) previous earthquake
studies point to a more southerly location for the present locus of
slip.
This paper is mainly about the origin of two conjugate aseismic ridges in the South Atlantic--- the Islas Orcadas to the west end of the magnetic anomalies of South Atlantic ridge, and Meteor rise to the east end. >From Bathymetry map, we can see clearly these two rises at depths 1000-1500 meters shallower than surroundings regions. Geomagnetic data for these two rises are shown in Fig2A and Fig2B. A tectonic event closely related to these two rises is the ridge jump from Agulhas basin west to its present position at about Chron 25 time. We can clearly evidence for this ridge jump in geomagnetic anomaly map of Meteor rise in Fig2A. Chron 34 33 becomes younger in opposite direction as Chron 19-24.
From single-channel seismic data and Drilling 702 (in Islas Orcadas), 703, 704 (in Meteor Rise), two way travel time and age for sedimentary rock layers, two-way travel time from basement rock, were measured.
Seismic lines across Meteor rise are shown in Fig 4A, interpretations are shown in Fig 5B and Fig 6B: (1) Basement of the southern Meteor Rise is characterized by an elliptical depression, with its major axis oriented nearly N-S, bounded to the west by a basement ridge and to the east by a seamount province. Within the basement depression, the thickness of basement overburden reaches 1.4-s twt. (2) A distinct and uneven reflection event (reflector Z) appears 250 ms above the interpreted basement reflection in the eastern part of the elliptical basin and merges eastward with the base of the seamount province (Fig 6B). Its abrupt westward termination, roughness, and geometry (apron) strongly suggest an association with lava flow. Drilling at site 703 below the province, recovered highly altered porphyritic basalt, which has been interpreted as basement. The oldest sediments above the basalt are early middle Eocene (~50Ma). (3) several faults were interpreted from the seismic data, they might be some listric normal faults. Basement appears to faulted from the seamounts down into the basin on two profiles (M2 and M5).
Seismic lines across Islas Orcadas Rise are shown in Fig 7, interpretations of results are shown in Fig8B, (1) Basement on the Islas Orcadas Ridge is considerably smoother than on the Meteor Rise. (2) Maximum twt is about 0.8, smaller the Meteor Rise. The oldest sediments above basement rock are about 62Ma.
The authors then proposed three possible hypotheses that might account for the origin of the rises: (1) origin as rift shoulders created by effusive volcanism at a developing rift zone; (2) origin as pseudofaults on a propagating rift; (3) construction by midplate volcanism, perhaps a "hot spot".
From comparison of basement morphology of the two rises, (1) is not a good explanation. Because symmetrical constructional volcanic features are predicted from rift shoulder hypothesis. But fundamental differences existed for their basement structures. The Meteor Rise has rugged seamounts and a deep basin, the basement flooring the basin is suggestive of rotated fault blocks such as are produced at listric faults. The Islas Orcadas Rise is smooth (except for seamount in the east), and has experienced high-angle faulting. Since these two rises almost parallel to the isochrons, pseudofaults hypothesis was also denied. And the midplate volcanism is almost perpendicular to the possible Duncan's hot-spot track.
Final conclusion is that the basement of the two rises formed in the Late Cretaceous, just prior to the ridge jump, and subsequently split when seafloor spreading initiated at the new spreading axis at ~C25 (59 Ma). So, the new spreading center was right in the middle of these two rises and these two then drift apart with sea-floor spreading. The seamount province drilled at Site 703 is interpreted as a younger phase of volcanism based on the presence of a strong reflector within the basin associated with the seamount, the depth of the province, and its magnetic signature. This younger phase of volcanism is estimated to be contemporaneous with the initiation of seafloor spreading.