Jorge Arzate

A method to predict the group fissuring and faulting caused by regional groundwater decline

Ground fissuring is a recurrent problem in many countries where water extraction surpasses the natural recharge of aquifers. Due to differential settlement, the soil layer undergoes deformation and cracks with serious consequences for civil infrastructure. Here, we propose an approximate analysis of the fissuring process that can be used to predict the location of cracks, which increasingly affect some middle- and large-sized cities in the world. For that purpose, the ground loss theory is applied to sediments overlying a sinusoidal-shaped graben. This analysis shows the existence of a tensile zone at the border of the graben with maximal values on its shoulder where tension cracks are more likely to appear. It also shows that soil deformation under differential settlement may evolve into ground faulting if water withdrawal continues. Finally, when a crack has completely developed, the tensile zone shifts towards the center of the graben, creating a new area for potential cracking and faulting.

Crustal‐scale transcurrent fault development in a weak‐layered crust from an integrated geophysical research: Carboneras Fault Zone, eastern Betic Cordillera, Spain

New magnetotelluric and receiver transfer function studies provide insights from the upper to the lower crust of the eastern Betic Cordillera, which is deformed by large folds, normal faults, and a major transcurrent left-lateral fault, the Carboneras Fault Zone (CFZ). Receiver function analysis determines a NNW dipping Moho reaching 20° that increases in depth, from 20 km south of the CFZ up to 34 km in the Sierra de Los Filabres. In addition, seismic discontinuities determined in the upper crust are interpreted as major contacts between metamorphic complexes that are detached and folded. The MT inversion model reveals a conductive zone, also representing a crustal seismic discontinuity, associated with the Alpujarride/Nevado-Filabride contact and fitting the N vergent geometry of the Sierra Alhamilla antiform. A small flexure at Moho coincides with the CFZ, as revealed by the Bouguer anomaly trend, in agreement with the receiver function results. Moreover, the Bahr strike and tipper angle at the stations placed closest to the CFZ clearly reveal the continuity of the CFZ at least down to approximately 15 km in depth, crossing all the detected crustal discontinuities up to the Moho. The lack of a clear Moho offset associated with the Carboneras Fault supports the idea that some large strike-slip faults tend to accommodate the deformation by a broadening fault zone at lower crustal levels. Its nucleation could occur at the base of a thin crust, where melting processes critically reduced the lithospheric strength during the late Miocene, to then propagate upward, reaching the topographic surface. Northward, the lithosphere comprised moderately larger strength, and the crustal discontinuities favored the development of larger folds with kilometric amplitude instead of strike-slip faults since the late Miocene.

Electrical image of the subducting Cocos plate from magnetotelluric observations

Magnetotelluric data acquired over the subducting Cocos plate in southern Mexico image the top of the oceanic plate for at least 150 km inland from the coast. Although fluid expulsion occurs in the accretionary prism, enough fluid appears to remain at the top of the subducting plate, because of sealing of pores and fractures on the underside of the continental plate, to produce the conductivity contrast necessary for electrical mapping. The results are supported by those of previous gravity and seismic refraction surveys that suggest the presence of more porous material on top of a denser, subducting oceanic crust. Earthquake epicenters also confirm the location of the top of the plate. Magnetotelluric data could, therefore, be used to map the Cocos plate along the entire Middle America Trench, where it is apparently broken into separate segments that subduct at different rates and dip at different angles.

Deep deformation pattern from electrical anisotropy in an arched orogen (Betic Cordillera, western Mediterranean)

Long-period magnetotelluric data acquired in the Iberian Massif and the Betic Cordillera arched orogen provide the first evidence of electrical anisotropy in the upper mantle of the Mediterranean region. Strike analysis at different periods reveals preferred structure orientation related to olivine elongation in the mantle, and points to a heterogeneous anisotropy pattern. At deep levels (periods ≥104 s), all the sites show a common north-south geoelectrical strike (∼N170°E), which may represent a low-intensity deformation, possibly related to “frozen” prealpine plate tectonics. For periods between 10 and 103 s, a north-south constant strike (∼N180°E) at the Betic Cordillera sites contrasts with the east-west strike (∼N85°E) in the Iberian Massif. An increase in the magnitude of the induction arrows from the Iberian Massif to the inner part of the Betic Cordillera probably reflects higher deformation toward the axis of the Eurasian-African plate boundary. The integration of electrical anisotropy data with seismic anisotropy allows us to discuss mantle deformation patterns produced by delamination and subduction, suggesting that the latter mechanism may be more suitable for the alpine evolution of the western Gibraltar Arc.

Reply to the comment by A. G. Jones et al. on “Deep resistivity cross section of the intraplate Atlas Mountains (NW Africa): New evidence of anomalous mantle and related Quaternary volcanism”

[1] Scientific discussion and different points of view are a basis of the advancement of knowledge. We acknowledge the comments of Jones et al. [2012] as an opportunity to publicly discuss the structure and origin of the Atlas Mountains. Moreover, we welcome the opportunity to compare our results with those recently published by the group responsible for the comment [Ledo et al., 2011], although it is not pertinent to comment in detail on a paper published in another journal. We also wish to remark that the paper of Ledo et al. [2011] was reviewed and published during the revision period of our contribution [Anahnah et al., 2011]; therefore, they are two different approaches and data sets, measured in different sites and by different instruments for the same region, lending readers the chance to compare different interpretations. The main differences on the data sets are: the profile of Anahnah et al. [2011] compared with the profile of Ledo et al. [2011] is 170 km longer, vertical magnetic data were obtained and lower frequencies were recorded. [2] We regret the style and way used by Jones et al. [2012]. We shall answer only those comments of Jones et al. [2012] related to objective issues. [3] One of the final conclusions of Jones et al. [2012] might serve as the starting point of our reply:

A volcanic centre in Mexico's Pacific continental shelf

Abstract On the continental platform of western Mexico, there is a young island of volcanic origin: Isla Isabel. It is the only volcanic manifestation in the region, contrasting with Islas Tres Marías, which are not volcanic. We aim at characterizing the source of the volcanic activity present in this particular location. Under Isla Isabel there is a bathymetric bulge that rises 60–80 m above the platform, and extends 20 km in the NW–SE direction and 17 km in the perpendicular direction. Isla Isabel is the only emerged portion of the bulge, extending only 1.8 km in the NW–SE direction. The island shows Plio-Pleistocene volcanic activity, including the formation of maars and the presence of mantle xenoliths. Using independent 2D modelling and 3D inversion methods for the gravity and magnetic fields, we analyse the nature of the bulge and its surroundings. A magnetotelluric station yields information about the electrical resistivity under the island, with penetration depths of approximately 20 km. The models are consistent with the presence of dense bodies of varying magnetizations that are interpreted as intrusive bodies. Results support the presence of an intrusion that locally has raised the ocean-floor topography. Volcanic activity projected from the bulge created Isla Isabel; the existence of additional, submerged volcanic centres in the area is most probable. We are inclined to identify the Isabel Bulge as a laccolith.