| Author||J.F. Savage|
|Title||Tectonic analysis of Lechada and Curavacas synclines, Yuso basin, León, NW Spain|
|Journal||Leidse Geologische Mededelingen|
|Abstract||The structures along the southern boundary of the flysch-filled Yuso Basin bear witness to continuous epeirogenic activity during the Upper Carboniferous. The Carda\u0148o Line foems the boundary proper but other structures, somewhat similar to this fundamental feature e.g. the Pe\u0148a Prieta line, trend away into the basin subdividing it, demonstrating their control over deposition. Sedimentary structures indicate the instability of the deposits near the edge of this basin.|
The present structures have grown out of initial subbasins most probably by a simple continuation of a similar type of epeirogenic movements. Recrystallization of the shales into slates accompanied some of this deformation, the other rocks though recrystallised show little sign of an oriented fabric. The slaty cleavage mainly developed as a concentric cleavage with a flatlying attitude dips on average 15° less than the bedding. Although some flap folds have such flatlying axial planes that it could be axial plane cleavage to them. The slaty cleavage deformation increased the dimensions of the bedding planes it seems as much, or even more, than demanded by the growing structures.
The west plunge corresponding to the slope of the basin reveals a spectrum of the structural developments at different levels. The Curavacas syncline, only moderately asymmetric in the extreme east, becomes rapidly steeper to the west. The north limb becomes overturned against the Pefias Matas fault causing complicated cascade folds in the Lechada Formation.
The Pefias Matas fault controls the box-form of the Lechada syncline to the west until it is lost in the Cardano Line near Barniedo. The syncline is then more symmetrical but cross folding here and near Portilla divert the trend to NW-SE.
Westwards very complex cascade and flap folds develop on either flank of the syncline folded inwards towards the core. In the extreme west the southern fold is dominant even involving Devonian rocks, one slab of which has been let-down by recent erosion to form a slip sheet.
The slaty cleavage is generally cut by a fracture cleavage found in almost all slates often parallel to the axial planes of minor folds. These range from the Portilla structure to minute puckers. Overall shortening must have been brought about during this deformation but due to the usual inclination this does not need to be more than 10 %: all could have been brought about by block tilting.
The fracture cleavage deformation has had its orientation perhaps more directly controlled by the fundamental lines. Hence, it cuts across the trend of the major structure especially where it swings NW. Here very steeply plunging folds with axial faults have developed. Some of these like the Portilla fold show younger rocks faulted upon older—typical for drape folds and suggesting further underlying control.
The evolution of the Lechada and Curavacas synclines is a single line of descent from the movements forming the basin in which the sediments were deposited through all the stages of deformation. Vertical and tilting movements are all that are required to induce such structures.
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