Marta E. Ghidella

Revised tectonic implications for the magnetic anomalies of the western Weddell Sea

Abstract Ten years after the USAC ( U.S. – A rgentina– C hile) Project, which was the most comprehensive aeromagnetic effort in the Antarctic Peninsula and surrounding ocean basins, questions remain regarding the kinematics of the early opening history of the Weddell Sea. Key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the Weddell Sea and merging the USAC data set with the other magnetic data sets in the region. For this purpose we reprocessed the USAC data set using a continuation between arbitrary surfaces and equivalent magnetic sources. The equivalent sources are located at a smooth crustal surface derived from the existing bathymetry/topography and depths estimated by magnetic inversions. The most critical area processed was the transition between the high altitude survey over the Antarctic Peninsula and the low altitude survey over the Weddell Sea that required downward continuation to equalize the distance to the magnetic source. This procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion. The enhancement of the Mesozoic sequence permits refining the interpretation of the seafloor-spreading anomalies. In particular, the change in shape and wavelength of an elongated positive in the central Weddell Sea suggests that it was formed during the Cretaceous Normal Polarity Interval. The older lineations in the southwestern Weddell Sea are tentatively attributed to susceptibility contrasts modeled as fracture zones. Numerical experimentation to adjust synthetic isochrons to seafloor-spreading lineations and flow lines to fracture zones yields stage poles for the opening of the Weddell Sea since 160 Ma to anomaly 34 time. The corresponding reconstructions look reasonable within the known constraints for the motions of the Antarctic and South America plates. However, closure is not attained between 160 and 118 Ma if independent published East Antarctica–Africa–South America rotations are considered. The lack of closure may be overcome by considering relative motion between the Antarctic Peninsula and East Antarctica until 118 Ma time, an important component of convergence.

Continental stretching preceding the opening of the Drake Passage: Evidence from Tierra del Fuego

Age estimates for the onset of a seaway through the Drake Passage range from middle Eocene to early Miocene, complicating interpretations of the relation between ocean circulation and Cenozoic global cooling. Here we present evidence for the presence of a latest Paleocene–early Eocene extensional basin (i.e., lateral rift) in Tierra del Fuego. An accurately dated postrift unconformity indicates that extensional faulting ended in the studied area ca. 49 Ma, in concurrence with a previously reported eightfold increase in South America–Antarctica separation rate, and the proposed onset of oceanic basins in the incipient Drake Passage. The coincidence of these facts indicates progressive strain concentration on the zone of future crustal separation (i.e., Drake Passage) after abandonment of lateral rifts ca. 49 Ma. Although the described extensional depocenters did not allow the exchange of water between the Pacifi c and Atlantic Oceans, they represent a direct indication of continental lithospheric stretching preceding the recently proposed Eocene opening of embryonic basins in the West Scotia Sea.

Bathymetry, depth to magnetic basement, and sediment thickness estimates from aerogeophysical data over the western Weddell Basin

We estimated the bathymetry and sediment thickness of a remote and difficult to access portion of the Antarctic continental margin using aerogeophysical surveying techniques. The U.S., Argentina, Chile aerogeophysical survey collected magnetic and gravity data over the basins surrounding the Antarctic Peninsula. Thirty-seven of these flight lines were used to estimate bathymetry and depth to magnetic basement for the western Weddell Basin. A wavenumber technique was applied to individual magnetic anomaly profiles in an automated fashion to obtain estimates of the depth to magnetic basement. The bathymetric estimates were obtained by admittance inversions of the gravity field. The results were then gridded at a 40-km interval for the region 64°W, 44°W, 73°S, and 62°S. Bathymetric estimates and depth to magnetic basement estimates were differenced at each grid point to obtain a regional estimate of the thickness of nonmagnetic overburden (assumed to be sediment). Subsequent spot measurements of topography in the estimated region of the continental margin generally agree to about 52 m. The estimated magnetic basement deepens from the Antarctic Peninsula margin eastward to a maximum of 10–12 km near 54°W. We also postulate the existence of two moderately large basins flanking the eastward continuation of the Jason Peninsula. Farther east, the basement steps upward, with a correspondent thinning of the sedimentary layer. Along the east coast of the peninsula, results agree well with seismic studies on James Ross Island and magnetotelluric studies on Marambio Island and the Larsen nunatak, as well as the British Antarctic Survey basement estimates from aeromagnetic data. This study further demonstrates the utility of combined application of airborne and satellite geophysical techniques in the study of structure and tectonic evolution of continental margins and marine basins.

Exploring the Recovery Lakes region and interior Dronning Maud Land, East Antarctica, with airborne gravity, magnetic and radar measurements

Abstract Long-range airborne geophysical measurements were carried out in the ICEGRAV campaigns, covering hitherto unexplored parts of interior East Antarctica and part of the Antarctic Peninsula. The airborne surveys provided a regional coverage of gravity, magnetic and ice-penetrating radar measurements for major Dronning Maud Land ice stream systems, from the grounding lines up to the Recovery Lakes drainage basin, and filled in major data voids in Antarctic data compilations, such as AntGP for gravity data, ADMAP for magnetic data and BEDMAP2 for ice thickness data and the sub-ice topography. We present the first maps of gravity, magnetic and ice thickness data and bedrock topography for the region and show examples of bedrock topography and basal reflectivity patterns. The 2013 Recovery Lakes campaign was carried out with a British Antarctic Survey Twin Otter aircraft operating from the Halley and Belgrano II stations, as well as a remote field camp located at the Recovery subglacial Lake B site. Gravity measurements were the primary driver for the survey, with two airborne gravimeters (Lacoste and Romberg and Chekan-AM) providing measurements at an accuracy level of around 2 mGal r.m.s., supplementing GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) satellite data and confirming an excellent sub-milligal agreement between satellite and airborne data at longer wavelengths.

New Magnetic Anomaly Map of the Antarctic

The second generation Antarctic magnetic anomaly compilation for the region south of 60°S includes some 3.5 million line‐km of aeromagnetic and marine magnetic data that more than doubles the initial maps near‐surface database. For the new compilation, the magnetic data sets were corrected for the International Geomagnetic Reference Field, diurnal effects, and high‐frequency errors and leveled, gridded, and stitched together. The new magnetic data further constrain the crustal architecture and geological evolution of the Antarctic Peninsula and the West Antarctic Rift System in West Antarctica, as well as Dronning Maud Land, the Gamburtsev Subglacial Mountains, the Prince Charles Mountains, Princess Elizabeth Land, and Wilkes Land in East Antarctica and the circumjacent oceanic margins. Overall, the magnetic anomaly compilation helps unify disparate regional geologic and geophysical studies by providing new constraints on major tectonic and magmatic processes that affected the Antarctic from Precambrian to Cenozoic times.

Esquema de tres interfaces con compensación isostática local en el margen continental Argentino

In the present paper, we investigate the Airy type isostatic compensation scheme for three interfaces: water-sediments, sediments-crust and crust-mantle, using forward modelling to calculate the isostatic anomaly with different approximations of Parker’s expression in the Argentine continental margin. The isostatic anomaly is estimated as the residue between the observed free air anomaly and the calculated free air anomaly through the sum of the contributions from the tree interfaces. After that, we perform the inversion of the isostatic anomaly in order to find the deflection of the Moho necessary to compensate it (or minimize it) using one term of Parker’s expression in the inversion, as an experiment to test the numerical methods. The Moho here considered is obtained from knowing the depth of the basement and crustal thickness. Another way to estimate the crust-mantle interface (Moho) is by the inversion with the iterative method of Parker-Oldenburg (Parker, 1973; Oldenburg, 1974), which allows inverting the Bouguer anomaly with more than one term. With the Moho thus obtained, a free-air gravity anomaly can be calculated, which reproduces the observed anomaly quite well. Even considering that the experiment constitutes a schematic model just to test the numerical methods involved, from the comparison with the only available digitized refraction profile in the study area, we find that the large wavelength of the inverted Moho is reproduced fairly well by the modeled Moho for both methods. However, the iterative method yields smoother surfaces, possibly more realistic surfaces given the roughness that the other estimates present.

Gravimetric Time Series Recording at the Argentine Antarctic Stations Belgrano II and San Martín for the Improvement of Ocean Tide Models

As a joint project, the Instituto Antartico Argentino (IAA) and TU Dresden (TUD) carried out gravimetric time series recordings at the Argentine Antarctic stations Belgrano II and San Martin. At both stations gravimetric data were recorded for about 1 year, using LaCoste&Romberg gravity meters. At Belgrano II the observations were carried out from February to November 2007, and at San Martin from February 2008 to January 2009. The set-up of the gravimetric stations and the instrumentation utilized are shown. We discuss and compare the data gained at the two stations as well as the analyses in order to solve for the tidal constituents. Generally, tidal gravimetry has the potential to provide independent data to validate and to improve ocean tide models at the Antarctic seas, especially since in-situ data to be used for the establishment of the models are sparse and satellite altimetry has limitations in high latitudes and over ice-covered regions. We discuss the feasibility of the obtained data to pursue this goal of an improvement of ocean tide models. Finally, an outlook for further investigations is given.