Onshore/Offshore Geophysical Survey in Danco Coast. NW Antarctic Peninsula
Within an area of around 119 km2 at Danco Coast (64° 09' SL y 60° 57' WL) a geophysical onshore/offshore survey was carried out during the 2005-06, 2006-07 and 2008-09 Antarctic summers. The Upper Cretaceous to Tertiary volcanic rocks intruded by a granodioritic pluton which outcrop in coastal areas belongs to the Mesozoic magmatic arc. Leucocratic and melanocratic andesitic dykes hosted in granodiorite at Cierva Cove bear sulphide mineralization. The main structures consist of two fault systems of WNW-ESE and NE-SW orientation which delineate the morphology of onland areas and the shoreline. Our geophysical and geologic survey was directed to acquire new data to define the geometry, lateral relationships and tectonic development of the geologic units of the Mesozoic magmatic arc exposed in the western border of the Antarctic Peninsula (AP). The geologic survey included lithologic and structural sampling, a total of 531 rock cores were obtained for AMS and paleomagnetic studies. The bathymetric survey of Hughes bay (including Sierva and Santucci creeks) has been carried out with the aim of determining the geometric features and spatial distribution of the main submerged depressions and their onshore continuity. This information is very useful for geophysical surveys and for comparing the submerged and emerged morphostructure. The magnetic survey was carried out with two magnetometers of proton free-precession: a Scintrex ENVI GRAD (used as permanent base) and a Geometrics G856 magnetometer employed for sampling stations (which measured the intensity of the total magnetic field and worked synchronized with the base magnetometer in order to make the corresponding corrections). A total of 1670 registrations were acquired in the 5803 GPS fixed onland stations within an area of 2.55 km2. A Zodiac MK3 was employed for the offshore magnetic survey. 11672 stations constitute the magnetic grid in offshore areas, organized along broadly orthogonal sections with a total length of around 548 km. Magnetic susceptibility data (?) were also acquired on outcrops in order to introduce this parameter in the modeling of magnetic anomalies. A Fugro GMS-2 susceptibilimeter was employed at each of the 989 GPS fixed sites in onshore areas. Preliminary processing of acquired data allowed producing an onshore/offshore map of magnetic anomalies which shows maximums (up to 3000 nT) and minimums (-500 nT) organized in parallel NE-SW oriented belts. The correlation of onland geology with the anomaly magnetic map suggests the offshore continuity of outcropping volcanic rocks. The obtained map also indicates the probable extension of volcanic dykes with sulphide mineralization in the Cierva Cove northwards along Danco Coast. The information gained from magnetometric survey combined with ongoing paleomagnetic and petrological work will produce further insights on the geodynamic evolution of NW AP and its relation with other plates of old GDW protoPacific margin.
Horizontal-axis Rotation of Lenticular Blocks in Strike-slip Fault Zones: Paleomagnetic Evidence from the Colombian Andes and Implications
Oblique convergence of the Nazca plate against the South American plate results in a broad and long zone of right-lateral strike-slip faulting along the Cauca Valley of the Colombian Andes. Field studies there and elsewhere reveal that lenticular bodies are common along major fault zones. Paleomagnetic studies of mid- to Late-Cenozoic intrusives in three zones of slightly different trends show similar unusual dispersion of paleomagnetic vectors in vertical planes. The planes of dispersion strike parallel to the trends of adjacent strike-slip faults of the Romeral and associated faults. A model invoking torques on asperities on lenticular bodies is examined. It suggests that the location of an asperity on a lenticular block can lead to torques that could account for rotation of lenticular blocks in the fault zone about axes normal to the plane of faulting. The model also suggests that displacement of the lenticular block in the fault zone may result in a direction oblique to the fault-slip direction. Thus in a vertical fault, a lenticular block will rotate about horizontal axes and be displaced upwards, or downwards, as a result of the net torque associated with the asperities distributed over its surface. The implications for faults of other orientations are explored for paleomagnetists to consider in interpreting results from fault-dominated tectonic environments.
Magnetic Mineralogy of Neoproterozoic Cap Carbonates from Brazil: Magnetization, Re- magnetization and Environmental Significance
Neoproterozoic glacial deposits are ubiquitously capped by carbonates with unusual sedimentary structures and negative carbon isotopic signature. These cap carbonates correspond to the transgressive sequence deposited after ice thaw, and include a basal dolomitic unit covered by a limestone unit. They record the relative influence of several factors such as ocean mixing, atmosphere and ocean warming, gas exchange, and continental and plataformal weathering. Three carbonate units from the Amazon and Sao Francisco cratons were studied for their paleomagnetism and magnetic mineralogy. Cap dolostones present a primary signal recording several reversals. In contrast, the cap limestones show evidence of pervasive re-magnetization. Hysteresis, IRM-acquisition curves and MPMS data, as well as SEM observations and other magnetic tests (e.g., Lowrie and Cisowski tests) show that the cap dolostones typically contain hematite (detrital) as the main magnetic carrier, whereas cap limestones contain pyrrhotite and magnetite (usually as authigenic framboids). The mechanisms of acquisition and resetting of magnetization in these carbonates will be discussed as well as the implications for the paleoenvironmental models proposed for the end of the Neoproterozoic.
The Vallenar Discontinuity and the Maipo Orocline: Regional significance of clockwise vertical-axis rotations in the central Chilean Andes.
One of the most prominent tectonic features of the Andes is the Central Andean Rotation Pattern (CARP), which is closely related to the Bolivian Orocline and characterized by paleomagnetically determined clockwise rotations in northern Chile and counterclockwise rotations in southern Peru (Arriagada et al., 2008). Along the Chilean margin, between 29°S and 38°S, three prominent curvatures are observed. The Vallenar Discontinuity near ∼29°S corresponds to the southern limit of the Bolivian Orocline. North of 29°S the major structural elements (Paleozoic basement highs and thrusts) are NNE oriented while from 29°S down to 32°S the structures are mainly NS. The central Chilean margin presents also significant bends near Santiago (∼33°S, Maipo Orocline) and in the Arauco region (∼38°S). Near Santiago, the Maipo bend coincides with the subduction of the Juan Fernandez Ridge (JFR). During the last five years we have undertaken new paleomagnetic and structural studies along the forearc of northern and central Chile in order to understand the origin of the bends in the Chilean margin and the consequence of its indentation by the JFR. Clockwise rotations are, consistently large (30°S- 45°) north of the Vallenar discontinuity, but south of the Vallenar discontinuity, the segment between 29°S to 32°S was not subjected to significant clockwise rotation. South of ∼33°S, significant clockwise deflections up to §39°§ of the declinations are again observed. Rotations occur both in Mesozoic rocks of the Coastal Cordillera and Tertiary rocks of the Main Cordillera. Whereas most of the CARP rotations, involving bending of the Bolivian Orocline and clockwise rotations north of the Vallenar Discontinuity, occurred essentially during the Paleogene, the paleomagnetic rotations obtained in Tertiary formations of central Chile constrain the maximum possible age for the occurrence of rotations of the Maipo Orocline to the Miocene. Neogene shortening in the foreland belt induced only slight orogenic curvature near the Vallenar Discontinuity. In the southern Central Andes, the along-strike variation in the magnitude of Neogene shortening may have been large enough to produce the Maipo Orocline. From 22 to 10Ma, the location of the subduction of the JFR moved rapidly southward from Arica (§18°§S) to Santiago (§33°§S) (Yanez et al., 2002). During the past 10 My, the JFR has been subducting continuously along the same part of the continental margin inducing flat slab and tectonic shortening. We will discuss the origins of the bends observed along the forearc of northern and central Chile and examine the role of the subduction of the JFR in bending the margin and inducing the observed clockwise rotations south of Santiago. Arriagada C., P. Roperch, C. Mpodozis, P. R. Cobbold (2008), Paleogene building of the Bolivian Orocline: Tectonic restoration of the central Andes in 2-D map view, Tectonics, 27, TC6014, doi:10.1029/2008TC002269. Yanez, G., J. Cembrano, M. Pardo, C. Ranero, D. Selles, (2002) The Challenger-Juan Fernandez-Maipo major tectonic transition of the Nazca-Andean subduction system at 33-34°S: geodynamic evidence and implications, Journal of South American Earth Sciences, 15, 23-38,doi: 10.1016/S0895-9811(02)00004-4.
Paleointensity and Rock Magnetic Study of Lavas From the Galapagos Islands: Preliminary Results
Initial results of paleomagnetic studies of basaltic lava flows from three islands, Isabela, Santa Cruz and Bartra, Galapagos archipielago, focusing on the rock magnetic analyses and preliminary paleointensity determinations, are presented. Remanent magnetizations are well defined and stable to thermal and AF demagnetization. Magnetic susceptibility, remanent magnetization, thermomagnetic curves and coercivity and unblocking temperature spectra are used to analyze magnetic mineralogy and sample selection for paleointensities. Relatively complex mineralogy shown in k-T curves appear correlated to quality in the Thellier- Thellier experiments. Galapagos Islands have formed from intense volcanic activity in the past few million years above the Galapagos hot spot in the Nazca plate, south of Galapagos spreading center. The archipielago consists of 15 islands and numerous islets and seamounts built on a large platform. Active volcanism occurs towards the west and islands get older to the east due to east-southeast Nazca plate motion. Sampling in Isabela concentrated in east and south flanks of Sierra Negra volcano, a structurally complex large caldera that represents the largest Galapagos volcano. Sampling sites are distributed along the shore west of Port Villamil and the east caldera flank with historic activity. Records show eleven eruptions from 1813 to 2005, which translates into intervals roughly from 4-37 yr with eruptions in 1963, 1979, and 2005. Lavas reaching Isabela coast are Late Pleistocene-Holocene. Sampling in Santa Cruz was done along NS transect from Las Bachas beach to Darwin Station and Punta Raton, including the Gemelos craters. Sampling in Bartra is in the southwest part of Salinas beach.
Time Average Field and Secular Variations of Pleistocene to Recent Lava Flows From the Ruiz-Tolima Volcanic Complex (Colombia)
Thirty three Pleistocene to recent lava flows from the Ruiz-Tolima Volcanic Complex (Colombian Andes) have been sampled for time average field (TAF) and paleosecular variation studies. A total of 10 cores were drilled per flow (site) and stepwise AF demagnetization has been carried out. After principal component analysis and mean-site direction calculations, 29 sites (25 and 4 with normal and reverse polarity, respectively), with α95 < 5.5° were selected for further calculations. The overall mean direction among the sites (D = 1.8°, I = 6.3°, α95 = 5.6°) closely fits (at the 95% confidence level) the expected paleomagnetic direction (at the area of study) of a geomagnetic field composed primarily by a geocentric axial dipole with 5% axial quadrupole component (I = 5.72°), but also coincides with a simple GAD model. VGP scatter (13°) is similar to that expected from Model G (12.8°).
Paleomagnetism in the Americas: Success and Failure of Paleomagnetism in Mexico
The paleomagnetic method has been applied early to tectonic problems in Mexico, especially for unraveling the tectonic evolution of the Transmexican Volcanic Belt (TMVB) and of the Baja California Peninsula. Discordant paleomagnetic data were found in both regions, and were interpreted in terms of neo-tectonic crustal block rotations (TMVB) and significant northward displacement of Baja California since the Cretaceous. Paleomagnetic data obtained during the last decade in these regions do not show such systematic discordant directions and therefore seem to invalidate the earlier hypothesis of tectonic movements. Here we compare the older and more recent results to analyze how such differences may have occurred.
Combined Paleomagnetic And Petromagnetic Study Of Oligocene Dyke Swarms From Tecalitlan Area: Implications For Relative Motion Of Jalisco And Michoacan Blocks
Results of detailed paleomagnetic studies from Tecalitlan area are reported. Sexteen sites (about 150 oriented samples) were collected including one Ar-Ar dated ignimbrite. Rock-magnetic experiments permitted identification of the magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements vs temperature, in most cases yield reasonably reversible curves with Curie points close to that of magnetite. Judging from the ratios of hysteresis parameters, all samples appear to fall in the pseudo- single domain grain-size region, probably indicating a mixture of multi-domain and a significant amount of single-domain grains. Reliable paleomagnetic directions were obtained for 14 sites. The mean inclination is in reasonably good agreement with the expected value, as derived from reference poles for the stable North America. Declination is significantly different from that expected which may suggests a counterclockwise tectonic rotation of 19°.