TECTONIC IMPLICATIONS OF GEOMORPHOLOGIC AND STRUCTURAL CHARACTERISTICS OF THE MORELIA – ACAMBAY FAULT SYSTEM, CENTRAL TRANS-MEXICAN VOLCANIC BELT

The roughly 200 km long and 15 to 60 km wide Morelia – Acambay fault system (MAFS) cuts the central part of the Trans-Mexican Volcanic Belt (TMVB) along its length. The MAFS is one of the main factors controlling the topography, deformation and sedimentation of the region since the late Miocene. In spite of this, neither the reasons behind the formation of the MAFS (e.g. above the older suture zone or as a gravitational collapse structure, nor its kinematics (either extensional, strike-slip or some combination of the two) are agreed upon. Although a few studies estimated the slip rates of several faults of the MAFS, its space-time evolution and growth pattern remain unknown. The nature of the interaction of the system with the older structures, and the issue if the pre-MAFS faults are active in the present-day displacement field are unclear too.

This work is based on analysis of a digital elevation model (DEM), interpretation of aerial photographs and on fieldwork. Shaded images, morphometric maps, topographic profiles and long river profiles were obtained from the DEM in order to evaluate the morphostructural segmentation of the area and to identify active structures. The photointerpretation and fieldwork allowed the evaluation of the fault scarp morphology and hence the relative chronology of fault formation. The structural data collected during fieldwork consists of fault slip plane measurements, dip of layered deposits, and stratigraphic observations. The integration of the two approaches shows that: (1) The studied part of MAFS is superposed on up- and down-thrown morphotectonic blocks whose fault-bounded limits are transversal to the MAFS, and at least some of them are presently active. (2) Pre-volcanic basement lithology seems to control the changes of the deformation style along the MAFS. This changes are: listric faulting to the west of the Los Azufres caldera and planar faulting to the east of it, and also the apparent change in the growth pattern or deformation stage of the MAFS across the same boundary. (3) NNW – SSE oriented extension accompanied by NNE – SSW oriented contraction drive the motion of the MAFS since it appeared in the late Miocene. The former became more pronounced with time, what is probably the effect of the increasing tendency to gravitational collapse of the volcanic arc during its growth.