ALBERT

Description: After a gap of nearly two decades since the Magsat mission in 1980, the dedicated low-orbit potential field mission CHAMP is now in the third of its seven year mission. Already, the new magnetic total intensity and vector data have yielded maps of the global crustal field of unprecedented accuracy and resolution. Here, we assess the value of these maps to infer deep crustal structure of regions overlain by younger cover. A GIS based modelling technique has been developed to model the various geological units of the continents starting from the geological map of the world. Depending upon the known rock types of the region, they are assigned a standard susceptibility value and using the global seismic crustal structure, a vertically integrated susceptibility (VIS) model is computed at each point of the region. Starting with this initial VIS model, the vertical field anomaly is computed at a satellite altitude of 400 km and compared with the corresponding CHAMP vertical field anomaly map. The first comparison is carried out against a model using the lateral extent of a cratonic region as given by published tectonic maps. In the subsequent modelling step, depending upon the extent of the observed anomaly pattern of that region, the surface geology is extended beneath the sediments until the recomputed map fits the observed magnetic anomaly map. Here, we focus on modelling results for the selected few provinces of the world where the initial model does not agree with the observed anomaly map. Similar modelling of CHAMP satellite magnetic anomalies can constrain the subsurface structure hidden by Phanerozoic cover in many parts of the world.

Description: A Databank was created using data from 25 local catalogues and 30 special studies of earthquakes in central, northern and northwestern Europe. Event types were discriminated, fake events and duplets eliminated, and different magnitudes and intensities converted to Mw. The conversions require the establishment of regression equations. The Catalogue contains tectonic events from the Databank within the area 44°N-72°N, 25°W-32°E and the time period 1300-1993 which have Mw magnitudes of 3.50 and larger. The area is covered by different polygons. Within each polygon only data from one or a small number of the local catalogues, supplemented by data from special studies, enter the Catalogue. If there are two or more such catalogues or studies providing a solution for an event, a priority algorithm selects one entry for the Catalogue. Then Mw is calculated from one of the magnitude types, or from macroseismic data, given by the selected entry according to another priority scheme. The origin time, location, Mw magnitude and reference are specified for each entry of the Catalogue. So is the epicentral intensity, I0, if provided by the original source. Following these criteria, a total of about 5,000 earthquakes constitute the Catalogue. Although originally derived for the purpose of seismic hazard calculation within GSHAP, the Catalogue provides a data base for many types of seismicity and seismic hazard studies.

Description: Polar motion data is available from the mid-19th century to the present. Based on time series with a variety of sampling intervals (monthly, 0.05-year, 5-day and daily), we have separated the low-frequency terms by low-pass filtering and the Chandler and annual terms by recursive band-pass filtering of the pole coordinates. Using a simple unweighted least-squares fit to the filtered low-frequency terms, the linear trends of the rotation pole were estimated. Assessing the estimates based on intercomparisons, the most reliable trend estimate was found. Using a Fast Fourier Transform, we have computed the prograde, retrograde and total amplitude spectra of the low-frequency part of polar motion in order to reveal the long-periodic signals. The characteristics and time evolution of the Chandler and annual wobbles are described by changes in their parameters (radii, directions and period lengths) over one century.

Description: This report describes the set-up, logistics and results of the CHICAGO (Chilean Coastal AeroGeophysical Observations) survey. It gives a short overview about the scientific intentions, detailed documentation of all technical aspects starting from the survey equipment via the aircraft installation to the GPS stations set-up and the experiences in flight. All processing results for the individual profiles are discussed in detail. Finally, the data is compared and combined with available recent marine gravity data and altimetry derived solutions.

Description: The LaCoste & Romberg gravity meter S124b and its associated system environment were installed and tested in conjunction with a strap-down gravity meter system (SAGS) of the Bayerische Akademie der Wissenschaften in Munich on a Cessna Grand Caravan of the DLR in Oberpfaffenhofen. This report describes the design and instrumentation of the aerogravimetry system, it documents the installation on the aircraft and it discusses some of the results of the test flights performed within AGFA (Airborne Gravity Flight Approach). Beyond the documentation of the system this report gives a short introduction to the basics of the instruments including a short theory of their operation and data processing. The intention is to give readers from disciplines other than aero-gravimetry and aero-altimetry a technical insight into how the system works and what it is capable of. This should help the reader to evaluate the systems usefulness in other geo-scientific projects. The experiences from the test flights are briefly summarized and an update of the current status and future plans for the individual instruments is given. The aerogravimetry system consists of two major instrument blocks: the gravimetry sensing system and the positioning system. The gravimetry sensors are the LaCoste & Romberg S124b and the SAGS-2.2 systems. The navigation block holds GPS receivers, an inertial navigation system and a laser altimeter. The aircraft used for the primary tests was a Cessna Grand Caravan of the DLR in Oberpfaffenhofen. It offers superb conditions for scientific installations and is widely used in geophysical exploration all over the world. The test flights were flown from Oberpfaffenhofen airport. One profile covers the Bavarian Alps to map short wavelength, topography induced gravity disturbances, and another flight crosses the Rhine Graben to map long wavelength structures of the deeper crust. The software for data processing for navigation, gravimetry and geoid calculations is briefly summarized.

Description: The Altiplano represents a key region of the Central Andes, where the interplay between faults and syn-tectonic sediments allow the reconstruction of the kinematic evolution of the Central Andean high plateau. This study aims, by the use of incrementally-balanced crosssections, interpretation of reflection-seismic profiles, 3D strain analysis, gravity data interpretation, isotopic-age dating, and surface observations, to reconstruct the geological and tectonic history of the Southern Altiplano at 21o S between the Eastern and the Western Cordillera. The Southern Altiplano is a complex intramontane basin with 6-8km Cenozoic fill. It can be structurally divided in three domains; the Eastern, Central, and Western Altiplano. 2D balanced cross-sections based on seismic-reflector analysis and field observations show that the Eastern Altiplano is the buried, thin-skinned deformation front of the western part of the Eastern Cordillera's bivergent thrust system. The 20-40° dipping, blind faults merge into a shallow, eastward-dipping detachment at 7-9km depth that continues into the Eastern Cordillera. The Central Altiplano forms a bivergent system with 30-90° dipping, basement-involving thrusts in the east, and fault-propagation folds in the west. The shallow, westward-dipping detachment lies at 9-10km depth and possibly continues into the Western Altiplano, which forms a separate bivergent thrust-system.The computer-aided (GeoSec and 2DMove), incremental restoration of the balanced crosssections of the Eastern and Central Altiplano, and preliminary line-length balancing of the Western Altiplano, yields 38km shortening due to folding and thrusting. 3D strain analysis of sandstone grain shapes reveals that an additional 7.7% of shortening was accumulated as ductile, micro-scale strain. This increases the total shortening in the entire cross-section of the Southern Altiplano at 21° S to 60km or 21%. In addition, I suggest that the contribution of outcrop-scale structures possibly accounts for another 20 km. 3D strain analysis further shows that the 7.7% of microscale strain were accompanied by 13% orogen-parallel extension. These shortening estimates more than double the published shortening values from the Altiplano. Crustal thickening and plateau uplift in the arc-backarc domain of the South American convergent margin took place during the Cenozoic. K-Ar and Ar-Ar age-dating on syn-tectonic sediments, together with seismic-sequence analysis, demonstrates that the Southern Altiplano structure formed during two independent compressional increments (Early Oligocene [〉27 Ma] and Middle/Late Miocene [17-8 Ma]), which were preceded by an Eocene/Oligocene extensional event that led to the formation of a half graben in the Central, and possibly a second in the Eastern Altiplano. Horizontal contraction of the Altiplano ended between 11-8 Ma, was indicated by the age of undeformed volcanic rocks. Detailed seismic analysis of single syn-tectonic basins combined with isotopic ages of syntectonic sediments, reveal a complex deformation history characterised by spatially and temporally irregular fault activation, which excludes the existence of large-scale eastward or westward propagating deformation during plateau formation. This diffuse pattern of deformation was characteristic for the entire plateau domain, i.e. from the western flank to the eastern edge of the Eastern Cordillera, during a first stage of plateau formation between 30 and 10 Ma. This possibly indicates that the plateau has remained flat since its formation and did not evolve from an initially doubly-vergent orogen. The syn-tectonic stratigraphic units of the Southern Altiplano domain overlie shallow marine, Late Cretaceous sediments that still form a sub-horizontal regional near sea level. This indicates that plateau surface-uplift in this part of the plateau was mainly achieved by sedimentary in fill of tectonically-controlled, internally-drained basins, and not by tectonic uplift. The tectonic evolution of the Southern Altiplano was largely accompanied by magmatic activity. An episode of strong volcanic activity affected the entire width of the Altiplano and adjacent parts of the Eastern Cordillera between 25-8 Ma. However, a causal relationship between magmatism and deformation could not be shown for the Southern Altiplano. Strong Oligocene/Miocene volcanic activity, together with the diffuse pattern of deformation, suggests that the formation of the Altiplano Plateau was initiated by magmatically-controlled thermal weakening of the crust, possibly as the result of the removal of the mantle lithosphere. At present, the Altiplano has a flat topography, high heat-flow, and is spatially related to a variety of geophysical anomalies that are interpreted as partial melting of the middle crust (20-40km depth). From this evidence, I propose that the process of plateau formation is still active.