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Vertical Axis Block Rotations in the Upper Crust, Horizontal and Vertical Partitioning and Implications for Vertical Coupling-Decoupling in the Lighosphere |
1 Centro de Geociencias, Universidad Nacional Autónoma de Mexico, Campus Juriquilla, Apartado Postal 1-742, Querétaro 76230, Mexico
2 Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, Sezione di Firenze, via G. La Pira 4, 50121 Firenze, Italy
3 Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via S. Maria 53, 56126 Pisa, Italy
4 Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, via G. Moruzzi 1, 56124 Pisa, Italy
(e-mail: mcerca{at}geociencias.unam.mx)
(e-mail: luca{at}geociencias.unam.mx)
Analogue models of polyphase deformation involving crustal differences in strength, thickness and density give insights into lateral and vertical strain propagation during Late Cretaceous shortening and Early Tertiary left-lateral shearing related to the early development of the North America-Caribbean plate boundary in southern Mexico. Analogue models reproduce a two-phase deformation characterized by a first stage of compression orthogonal to the plate boundary, simulating deformation induced by the Laramide orogeny, followed by a later stage of left-lateral transpression associated with the transfer of the Chortis block from the North American to the Caribbean plate during the early stage of development of the new plate boundary in Early Tertiary times. Based on detailed structural observations in the Guerrero-Morelos platform and the western part of the Mixteco terrane of southern Mexico, we document that a transpressive regime affected continental red bed sequences of Early Paleocene to Late Eocene, and rotated and refolded Laramide structures during this second phase. Our model ends before the transtensional regime that affected the region, which is marked by a volcanic episode of Late Eocene-Oligocene. This change in the deformation regime records the passage of the NW tip of the Chortis block (North America-Cocos-Caribbean triple junction), when subduction replaced transform faulting along the southern Mexico margin. The models focus on the structures formed around the flanks of a thicker/more rigid crustal block that simulates the rock assemblages of the Palaeozoic orogens of southern Mexico (Mixteco-Oaxaca-Juarez block, MOJB). The comparison of the mechanism of deformation of three different analogue models with the natural prototype explains most of the structures observed around the MOJB. Counterclockwise vertical-axis rotations of pre-existing structures in the western flank of the MOJB observed in the Guerrero-Morelos platform are consistent with the modelled structures. Vertical movements of the modelled MOJB induced by the transpressive regime can explain the Papalutla thrust and the basement upheaval and gravitational sliding of the cover in the Tentzo Ranges observed at the western and northern margins of the MOJB, respectively. The growth and propagation of thrusting controlled by the geometry of the block along the eastern margin also correlates with the Vista Hermosa fault. The propagation of strain to the north increases with higher contrast in strength of the thick block with respect to the adjacent modelled crust. Analogue modelling failed to reproduce all the structural details of southern Mexico and, specifically, the structures observed inside the MOJB. The latter, however, are controlled by pre-existing discontinuities, which are not simulated in the model. As a whole, the results demonstrate that crustal heterogeneity in a developing left-lateral plate boundary zone produces a stronger vertical coupling between ductile and brittle crust and a widening of the deformation zone along the margin of the North America plate in southern Mexico.
