As a result of complex extensional tectonics in northeast China and southern portions of Mongolia, some workers have interpreted the Cretaceous Maanshan Uplift and associated Chifeng basins as metamorphic core complex. Previous work has relied solely upon kinematic indicators to determine the structural origin of these basins. To fully understand the creation of these basins, the sedimentiological record was analyzed in this study. The early Cretaceous sedimentary fill of these basins was analyzed to determine if it is synextensional in nature, and if so what manner of extension was in progress during that deposition. The Chifeng basins are filled with four distinct facies associations and are floored by Late Jurassic and Early Cretaceous volcanics of intermediate and felsic composition. The facies associations observed are interpreted as lacustrine/fluvial deposits, alluvial fan conglomerates, distal fan deposits with fluvial deposits, and hanging wall derived fluvial deposits. These facies are composed of sediment shed from the footwall and hanging wall of the master faults and volcanic deposits. Paleocurrent indicators, primarily in the form of imbrication, along with clast count data show provenance directly off the structure separating the two basins and from the eastern margin of the southeast basin. The distribution of facies, as well as paleocurrent data, provenance data, and structural geometry, implies that these basins are of half-graben origin and that no sedimentological evidence exists of an Early Cretaceous metamorphic core complex near Chifeng.

An integrated field, petrographic , structural , and geophysical analysis of the Panther Creek half -graben (PCHG), in east -central Idaho, was undertaken to determine its tectonic origin. Two competing theories as to the origin of the PCHG exist. One suggests that it formed as part of the TransChallis fault system (TCFS) , a system of northeast-trending normal faults, eruptive centers, and graben that formed in Middle Eocene time. The graben of this system are bounded by northeast-striking normal faults, and show evidence of syn-tectonic deposition of basin-filling volcanic strata. The other theory suggests that the PCHG may have evolved as a result of a post-Challis volcanic extension event (Late Eocene to Early Oligocene). Graben of this event are bounded by NNW-striking normal faults, and Challis volcanic strata are uniformly tilted; this evidence suggests post-volcanic basin formation. This study demonstrates that formation of the PCHG was mostly synchronous with volcanism and that there was very little post-volcanic deformation. Other conclusions of this research are 1) Rates of slip on basin-bounding faults were high, and slip was episodic. Slip occurred on both NE and NNWstriking faults, but more occurred on the NE-striking basin-bounding fault. This is consistent with the dominant NE strike of faults within the TCFS. 2) Most of the older volcanic units in the basin dip more steeply to the SE than overlying younger units , this is evidence for synvolcanic deposition . 3) There is significant hanging wall deformation in the form of folds and normal faults, particularly in the northern and southern portions of the basin . 4) Paleocurrents determined from provenance studies and imbricate fabric in exposed conglomerate beds indicate a flow direction largely to the northwest. This paleocurrent flow crosses the strike of the syn-extensional basin-filling units . 5) The composition of the clasts in the post-volcanic basin -fill deposits suggests that the thick volcanic units preserved in the basin never blanketed highlands NE, E, and SE of the basin and that the footwall of the basinbounding fault system was the main source of sediment in the basin. 6) The magnetic and bouguer gravity surveys indicate a sizable intrusion (most likely related to the nearby Casto pluton) beneath the west-central portions of the basin. The intrusion(s) may be responsible for east-plunging folds in the tuff of Castle Rock. 7) Work initiated during this study to determine the 40Ar/39Ar incremental-release mineral ages and the polarity of the tuff of Castle Rock and the tuff of Challis Creek will help determine the relationship between these two units and will precisely date basin development. However, it is clear that the basin was nearly fully formed by the end of Challis volcanism.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A bacia sedimentar do Tacutu, localizada no Estado de Roraima, consiste em um segmento distensivo implantado no Mesozóico (Jurássico Superior - Cretáceo Inferior), hegráben encaixado no Escudo das Guianas, que se alonga na direção nordeste-sudeste, com cerca de 300 km de comprimento, variando entre 30 a 50 km de largura, estendendo-se da República da Guiana, à capital do Estado de Roraima, Boa Vista. Para realização dessa pesquisa foram levados em consideração os dados já existentes a cerca da área em estudo e diversas pesquisas em campo para entendimento do seu processo evolutivo. Dessa forma, buscamos fazer uma abordagem sobre a compartimentação geomorfológica do gráben do Tacutu, utilizando técnicas do aplicativo Argis 9.1, que favoreceu a construção de um modelo digital do terreno / The sedimentary basin of the Tacutu is located in the state of Roraima, Brazil. It is an implanted distensive segment in the Mesozóic (upper Jurassic - lower Cretaceo). It is an half-graben incased in the Shield of the Guianas, prolongated in the northeast direction - southwestern, with about 300 km of length varying 30 the 50 width km, extending itself of the Republic of Guyana to the capital of the state de Roraima, Boa Vista. We consideration the data already concerning the area in study and diverse research to understand the evolution of the process. The approach about the geomorphological compartimentation of graben of Tacutu using techniques of the Geografic Data Sistem by means of aplicatory ArcGis 9.1 that favored the construction of a digital model of the land