Mariano Cerca

The role of crustal heterogeneity in controlling vertical coupling during Laramide shortening and the development of the Caribbean-North America transform boundary in southern Mexico: insights from analogue models

Abstract 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.

Experimental modeling of rifting at craton margins

Lithospheric-scale centrifuge models are used to investigate the process of continental rifting at the margins of cratonic areas. Models reproduce extension between a resistant cratonic lithosphere and an adjacent, weaker mobile belt and investigate the influence of the strength contrast between craton and belt and the presence or absence of an intervening weak zone (such as a suture) on the extensional deformation. Model results suggest that regardless of craton and belt strength contrast, the presence of the weak zone strongly localizes deformation, leading to the development of narrow, deep rift valleys corresponding at depth to marked lithospheric thinning. Depending on the pre-rift rheology (in particular depending on the presence of a significant decrease of the brittle-ductile transition depth in the belt domain), the resulting basin can be largely asymmetric, with a major border fault system on the craton side. When the weak zone is absent, deformation is typically more distributed and lithospheric thinning more homogeneous. In these conditions the strength contrast between craton and belt strongly controls deformation: when the contrast is minimal, no major faults form at the craton-belt boundary, and a roughly symmetric deformation affects a wide region inside the strong mobile belt after the initial stages of extension. Conversely, for high strength contrasts, more asymmetric deformation is localized on a major fault system at the craton margin at the beginning of extension; with progressive extension, minor faulting propagates inside the weak belt, widening the deformed zone. Comparison with different natural examples suggests that these results may be important and have relevance for the development of continental rifts at the margins of cratonic areas.

Three-dimensional displacement fields measured in a deforming granular-media surface by combined fringe projection and speckle photography

D displacement fields on a diffuse surface are measured by a combination of two optical methods, fringe projection and speckle photography. The use of a single camera for recording information from the two methods implies that no calibration procedures are necessary, as is the case in stereoscopy-based techniques. Out-of-plane displacements are measured by fringe projection whereas speckle photography yields the 2D in-plane component. By using this technique, we analyze in detail the morphological spatial-temporal evolution of an analogue model of the Earths crust while subjected to compression forces. We discuss the experimental results and their relevance to the micromechanics of a surface of dry, non-cohesive and dilatant granular media. The results show that the combination of fringe projection and speckle photography is well suited for this type of study and allows the characterization of strain at the grain scale.

Correlation of near-surface stratigraphy and physical properties of clayey sediments from Chalco Basin, Mexico, using Ground Penetrating Radar

Detailed measurements of water content, liquid and plastic limits, electric conductivity, grain-size distribution, specific gravity, and compressibility were performed on the upper 7 m of the lacustrine sequence from the Chalco Basin, Valley of Mexico. Eight stratigraphic units consisting of alternating layers of clay, silt, sand, and gravel of volcanic origin are described for this sequence. The analysis of contrasts in the physical properties permitted to identify potential reflectors of radar waves: (i) change in the electrical conductivity at 0.4 m depth; (ii) increment in the clay and water content at 0.8 m depth; (iii) bimodal behavior of the water content at 1.3 m depth; (iv) increment in the sand content and decrease in water content at 2.6 m depth; and (v) the presence of a pyroclastic unit at 3.7 m depth. Radargrams with frequencies of 900 and 300 MHz were collected on a grid of profiles covering the study area. Correlation of radargrams with the reference section permitted the spatial interpolation of variations in the physical properties and the near-surface stratigraphy. Contrary to the expected in these clayey sediments, electric contrast enhanced by variations in water content and grain size permitted the recording of the near-surface sedimentary structures. Distinctive radar signatures were identified between reflectors. Furthermore, lateral discontinuities of the reflectors and their vertical displacements permitted the identification of deformational features within the sequence. D 2003 Elsevier B.V. All rights reserved.

Delineating the near-surface geometry of the fracture system affecting the Valley of Querétaro, Mexico: Correlation of GPR signatures and physical properties of sediments

We present a combined Ground Penetrating Radar (GPIR) and geotechnical investigation of a fault-fracture system that affects the Valley of Queretaro. The main fracture, named Falla Central (FC), is aligned with north-south striking regional faults suggesting that the buried fault scarps influences the geometry and propagation of fractures. Although the origin of fracl uring is closely related to geological factors, in the urbanized area of Queretaro the mechanical and hydraulic equilibrium in the <subsoil is also broken by anthropogenic activities, such as overexploitation of groundwater and overloading of high compressible grounds. In order to delineate the geometry of fracl uring we collected several vertical GPR profiles perpendicular to the main pattern of the fracture, with two different prospecting frequencies, 900 and 300 MHz. Processing of radar profiles was no needed because of high contrasting properties of the prospected sub-soil; we only used in some cases background removal, topographic correction or AGC amplitude corrtection so as to enhance stratigraphic related records. Perturbations in the radar signature help to identify changes in direction, width, and vertical displacement of fractures. Notably, in the radargrams from the northern part of the FC pertiirbations are concentrated near the fracture and vertical displlacement is higher, whereas in the southern part, the perturbations are distributed in a wider area in both sides of the FC. GPR profiles collected in areas with no vertical displacement along the trace of the FC recorded discontinuities suggesting that fraciures propagate from depth to surface. Detailed measurements of specific gravity, grain-size, plasticity, water cont(ent, and electrical conductivity were performed in samples collected in two shallow trenches in order to observe physical changes in the sedimentary sequence related to electrical contrasts. The near-surface stratigraphy consists of partially saturated fluvio-lacustrine granular and pyroclastic deposits. Dark clay with high plasticity and medium to firm consistency, grading to medium plastic silt with desiccation cracks characterizes the first meters. The analysis of the GPR profiles supported with the stratigraphic reco.rd permitted to identify variations in deformation of layers that can be explained in terms of differences of thickness and compressibility. Our results suggest that the evolution of the fracture is influenced by the conditions of deposition of the geological materials and by the anthropogenic activity. The analysis of clayey sediments suggests that the fracture is also controlled by the history of loads of the materials and the stratigraphic boundary between coarse and clayey sediments. Thus, the morphology of the near surface fracturing depends upon the spatial relation between regional fracturing and the depositional conditions of sediments. We propose that one of the main factors of propagation and orientation of the FC trace in the surface is the differential deformation because of the variation of vertical compaction of sediments. GPR profiles permitted to correlate vertical and lateral variations of the geological properties for a better understanding and forecasting of the distribution and behavior of fractures.

Holocene paleo-earthquakes recorded at the transfer zone of two major faults: The Pastores and Venta de Bravo faults (Trans-Mexican Volcanic Belt)

We present evidence of five late Holocene earthquake ruptures observed at two paleoseismological trenches in the Laguna Bani sag pond (Trans-Mexican Volcanic Belt, central Mexico). The trenches exposed two fault branches of the western termination of the Pastores fault, one of the major fault systems within the central Trans-Mexican Volcanic Belt. The site was studied by combining geomorphological and structural approaches, volcanic mapping, ground-penetrating radar, and paleoseismological analysis. The study revealed that coseismic surface rupture was noncharacteristic, and that the exposed fault branches had not always moved simultaneously. The fault tip has ruptured at least 5 times within the past 4 k.y., and the rupture events followed and preceded the deposition of an ignimbrite. The close temporal relationship of the seismic rupture with the volcanic activity of the area could be the result of volcanism triggered by faulting and its associated seismicity. The relatively high recurrence of seismic events (1.1–2.6 k.y.) and the noncharacteristic fault behavior observed at this tip of the Pastores fault suggest that the fault might have been active as a primary fault rupturing along segments of variable length or depth, and/or that the fault ruptured eventually as a secondary fault. The secondary ruptures would likely be related to earthquakes produced at major neighboring faults such as the Acambay fault, which moved during the 1912 Acambay earthquake, or the Venta de Bravo fault. A relatively large slip rate estimated for this fault branch (0.23–0.37 mm/yr) leads us to contemplate the possible connection at depth between the Pastores and the Venta de Bravo faults, increasing the maximum expected magnitude for central Mexico.

Analogue models of an Early Cenozoic transpressive regime in southern Mexico: implications on the evolution of the Xolapa complex and the North American-Caribbean Plate boundary

Abstract We present analogue models that illustrate the tectonic evolution of the continental margin of southwestern Mexico and the Early Cenozoic deformation of the Xolapa complex. Together with geological data they suggest that oblique convergence caused distributed deformation and mountain building near the present-day margin of southern Mexico in a general left-lateral transpressional regime. A similar deformation is also observed north of the Xolapa complex in Maastrichtian to Paleocene sedimentary and volcanic rock units. Since post-Oligocene exhumation of middle crust does not significantly affect Late Eocene to Oligocene volcanic rocks, we infer that the evolution of the transform margin led to the formation of discrete boundaries that eventually decoupled exhumed mid-lower crust from the onshore upper-crust sequences since the Late Eocene.

Structural evidence of enhanced active subsidence at the bottom of a maar: Rincón de Parangueo, México

Abstract Rincón de Parangueo is a Quaternary maar that has been recently desiccated. The crater was partially occupied by a soda lake, and near the shoreline microbialites have formed. Evaporites (mainly trona and halite) precipitated as the water level dropped. Active subsidence of the lake floor (c. 24 m since 1980) produced countless structures close to the lakeshore, where deformation is extensional. Closer to the depocentre, in the western half of the basin, gliding/spreading produced folds and mud-injection domes. The most remarkable structure throughout the basin is a monocline that forms a ring-like, nearly continuous scarp, approximately 15 m high, which in the eastern half of the basin was produced as a fault-propagation fold developed above the buried diatreme–country rock boundary. A more diffuse (wider) monocline, locally associated with compressive structures, occurs in the western half of the basin. These structures are interpreted as having developed above a gently inclined, irregular lake sediment–country rock (andesite) interphase. The monocline was modified by high-angle extensional faults/fractures with large heaves/apertures. In the eastern half of the basin, there is a second (outer) scarp, approximately 13 m high, formed by a high-angle, listric, normal fault. Rollover antiforms occur in the hanging wall of this structure. Rincón is an example of centripetal gravitational gliding/spreading.