KOYI, HEMIN A.1 and ANTONIO TEIXELL2
1Hans Ramberg Tectonic Laboratory,
Department of Earth Sciences, Uppsala University, Sweden
2Departament de Geologia Universitat Autonoma de Barcelona, Spain
Abstract: Where is the Footwall Flat? Answers from Sand Models
The ramp-flat kinematic model imposes a series of restrictions in interpreting and constructing cross-sections of deep, unexposed parts of thrust imbricate stacks. One of the principal geometric assumptions of the model is the necessity of a correspondence between hangingwall and footwall ramp and flat structure, the 'template constraints'. Since the ramp-flat model is essentially geometric, and natural strain states and kinematic effects of lithological and environmental variations are not considered, a straightforward application of the model may result in serious errors.
Evidence from sand models is added to emphasise the need for a reevaluation of the template constraints. During the analysis of sand models in terms of ramp-flat kinematics we were faced with hangingwall-footwall mismatches and departure from the ideal case. Therefore, in order to maintain compatibility in deformation between the hangingwall and footwall flats, the ramp-flat geometrical rules could not be obeyed.
In contrast to the template rules, in sand models, flats in hangingwall and footwall do not necessarily match; neither do they display the same length or amount of displacement. In many model imbricates, at a certain stratigraphic level, hangingwall flats do not have the correspondent footwall flat. Instead, footwall flat is represented by a wide zone, Of distributed deformation at deeper levels of the model. Even when a thrust may still be traced to the basal decollement of the system, it has a subplanar rather than a staircase-geometry, and shows much less offset at the deeper levels implying a decrease of slip with depth. There, the layers thicken by penetrative strain and folding. Hence, the potential space problems resulting from the ramp-flat mismatch are eliminated by the ductile deformation at deeper level.
It is emphasised here that the template constraints in-built in the rampflat kinematic model do not always account for the mechanics of internal deformation of thrust imbricates. Hence, the geometrical rules that follow cannot be applied indiscriminately in a self-similar way across nonexposed or poorly imaged parts of the crust without a consideration of the particular strain states.
AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas