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Transactions on Engineering and Computing Sciences - Vol. 12, No. 3

Publication Date: June 25, 2024

DOI:10.14738/tecs.123.17139.

Alvarez, R., & Camacho, M. (2024). The N-S Extension of The Mexico Basin is Confirmed by Gravity Analysis. Transactions on

Engineering and Computing Sciences, 12(3). 149-174.

Services for Science and Education – United Kingdom

The N-S Extension of The Mexico Basin is Confirmed by Gravity

Analysis

R. Alvarez

ORCID: 0000-0002-2606-0904

Instituto de Investigaciones en Matemáticas Aplicadas y en Sistemas (IIMAS),

Universidad Nacional Autónoma de México, Coyoacán CDMX 04510, Mexico

M. Camacho

ORCID: 0000-0002-3871-6843

Posgrado en Ciencias de la Tierra, Universidad Nacional

Autónoma de México, Coyoacán CDMX 04510, Mexico

ABSTRACT

The Mexico Basin is in the eastern portion of the Trans-Mexican Volcanic Belt,

where it is repeatedly affected by strong ground motions. Notwithstanding, the

tectonic setting of this region is not adequately defined, and several efforts are

underway to improve its characterization. Here we use a high-resolution (220 m),

satellite-derived gravity data set to acquire the matching Bouguer anomaly, from

which 3D inversions are performed at varying resolutions, to obtain density

distributions to depths of up to 15 km. The gravity signature agrees with an N-S

extensional process previously recognized, based on geological arguments. Two

gravity saddle points are identified flanking a minimum in the S portion of the basin;

immediately southward is located Sierra del Chichináutzin, which we interpret as

resulting from the N-S extensional process. Two relevant, not previously reported

gravity anomalies are identified: the Ajusco Gravity Anomaly (AGA) and the Central

Gravity Anomaly (CGA), the former associated with Ajusco volcano, and the latter

with the central portion of Mexico City. The AGA contains two polygenetic

structures: Ajusco volcano and San Miguel volcano; given their proximity to the

Mexico and Toluca basins, a detailed exploration of their density distributions is

performed. A comparison is made with seismic results reported elsewhere,

between density and seismic velocity, finding good matches to depths up to 10 km

below sea level.

Keywords: Basin of Mexico, Geological extension, Satellite-derived gravity, Gravity saddle

point, Bouguer anomaly, Density distribution.

INTRODUCTION

Mexico Basin (MB), also known as Mexico City Basin (MCB), located in the eastern portion of

the Trans-Mexican Volcanic Belt (TMVB). Mexico City is the largest, most populated city in the

country, with a population of over 20 million, it is also a heavily industrialized area. The city

area is continuously affected by seismic events, some of which have induced strong damage to

it, particularly the 1985 earthquake (M 8.1) originating SW of the city, in a location where the

Cocos plate begins to subduct under the North American plate [1,2]. The more recent one,

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Transactions on Engineering and Computing Sciences (TECS) Vol 12, Issue 3, June - 2024

Services for Science and Education – United Kingdom

known as the 2017 Puebla earthquake, it struck at 13:14 local time on 19 September 2017 with

an estimated magnitude of 7.1 Mw. Despite the seismic and volcanic risks pending on the city,

the characteristics of its tectonic setting have not been adequately defined, particularly from

geophysical approaches. The present one attempts to describe such characteristics using

gravity field analyses, to depths of ~15 km. Figure 1 is a location map of Mexico City and

neighboring volcanic structures in the TMVB.

In previous research about the deep characteristics of the Mexico Basin [3-7]) the only gravity

data available at the time was used; it was a ground gravity survey [8] restricted in size to the

Mexico City area. In the present study, we use satellite-derived gravity data designated GGM+

[9], discussed ahead, which has much larger coverage and considerably more dense

distribution than the previous data set; this results in a more regional and more precise gravity

data set, which allows for the identification of extensional processes occurring in the area,

among other procedures.

We first analyze the Bouguer anomaly in the area from a general viewpoint to highlight its

characteristics, and then proceed to superpose geologic maps on it to identify cause-effect

relationships. In this frame, we can associate volcanic formations with tectonic processes. Then,

we proceed to perform 3D inversions of the gravity data in the Mexico City area and extract

density cross-sections of pertinent locations, which allows for an analysis of the first 15 km

below the surface.

Fig. 1: Location map of the study area in central Mexico. Cities (yellow letters): T, Toluca. M,

Mexico City, C, Cuernavaca. U, Puebla. Volcanic structures (white): N, Nevado de Toluca; H,

Sierra Chichinautzin; P, Popocatépetl; I, Iztaccihuatl; L, Tláloc; A, Malinche. Sierra de Las Cruces

trends NW, dividing Mexico (M) and Toluca (T) basins. DEM from GeomapApp [10];

http://www.geomapapp.org

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Alvarez, R., & Camacho, M. (2024). The N-S Extension of The Mexico Basin is Confirmed by Gravity Analysis. Transactions on Engineering and

Computing Sciences, 12(3). 149-174.

URL: http://dx.doi.org/10.14738/tecs.123.17139

GEOLOGY

Three tectonic events appear to have shaped the present Mexico Basin. The first one

corresponds to the deposition of limestones and terrigenous rocks in the associated volcano- tectonic depression; its development started in the Cretaceous [11]. After the Laramide

orogeny that occurred in the 80-35 Ma period, an E-W extension appeared to have increased

the size and shape of the basin, subsequently filled by volcanic materials of the initial Trans- Mexican Volcanic Belt [12]. The second tectonic incident gave rise to the Mixhuca fault [13],

inside the graben structure, and the formation of Sierra Las Cruces, which partially filled the

graben. About 5 Ma, a new fault system began, trending SW-NE, associated with the

Tenochtitlan mega shear [14]; these authors used the volcanic cones in the area, about 200

cones, to determine the local fracture distribution, identifying a NE-SW modern system of

lateral displacement. They determined the azimuth of two or more monogenetic cones as the

orientation of the main tectonic stress, parallel or subparallel to extensional shear fractures. In

Sierra del Chichináutzin, south of Mexico City, they analyzed the relations between 132 cones,

determining that orientation N55-75E tends to predominate over orientation E-W, and may be

genetically related to faults in Sierra Las Cruces, Sierra de Guadalupe, and El Tezoyo, which have

the same orientation.

Fig. 2: Complete Bouguer anomaly map of the Mexico City area, detrended with a second-degree

polynomial, from satellite-derived gravity data (GGM+); gravity values at ~220-m interval. The

approximate location of the Mexico City area is outlined by the continuous black line. The large,