Contribution to the tectonics of the northern portion of the central sector of the trans-Mexican Volcanic Belt

Abstract

Gravity data from the central sector of the trans-Mexican Volcanic Belt indi-cate that the area bounded on the east by the Mezquital valley (99° W), on the west by the Queretaro fault zone (100.5° W), and to the north by the Sierra Madre Oriental fold and thrust belt (20.6° N) consists of an east-west-oriented basement depression. The eastern part of this depression is a single 50-70-km-wide (northsouth) basin. To the west, it narrows and consists of several smaller basins separated by tilted fault blocks that become progressively more pronounced westward. Presumably, the block faulting is related to an older (Cenozoic) extensional event because no surface deformation is associated with these faults. Contrary to previous studies, the gravity data indicate that the Pastores fault (which forms the southern boundary of the Acambay graben) terminates at 99.3° W and does not continue eastward to the Mezquital valley, as previously proposed. The fault pattern inferred from the gravity data, as well as earthquake data, is consistent both with a recent and an older phase of north-south-directed extensional tectonics of intraplate origin (intra-arc extension).

Regional Path Effects on Seismic Wave Propagation in Central Mexico

“…The diffracted pulses (apparent velocity for the whole source-station path is between 2.7 and 3.5 km/sec for CRXI) must travel the distance PLIG-CRXI with a group velocity between 1.5 and 3.5 km/ sec. These low values of group velocity are compatible with the decrease in density observed by Campos-Enríquez et al (2000) and with the observed low values of phase velocity along the PLIG-CRXI path.…”

“…However, if we are right in identifying it with the volcanic valley of Figure 7, the western limit must be the contact between the volcanic valley and its metamorphic basement, mapped by Mooser et al (1996) at 100Њ/W (around 20 km to the west of CRXI). To the north, both Mooser et al (1996) and Campos-Enríquez et al (2000) placed a limit to the volcanic valley at 20Њ/N, where these deposits become very thin. However, we do not have seismological observations in this region.…”

“…In addition to the study of Mooser et al (1996), the structure of the volcanic deposits in central Mexico was studied in Campos-Enríquez et al (2000) using gravity measurements. The measurements were done every 200 m on a net of closed traverses 5-8 km long.…”

“…Campos-Enríquez et al (2000) presented results for seven profiles, 100-200 km long, oriented north-south, one of them extending from 150 km north of Mexico City to 50 km to the south. Their results show a relatively low density structure extending from 19Њ N to 20Њ45Ј N, with a maximum depth of 2000 m and small thickness around 20Њ N. According to Campos-Enríquez et al (2000), this structure consists of undifferentiated OligoceneMiocene volcanic materials. The average density in this lowdensity structure is 2500 kg/m 3 , and it overlies a calcareous basement of Cretacic age and an average density of 2700 kg/m 3 .…”

Geology of the Tierras Blancas area in the southeastern part of the Acambay graben, central Mexico

“…1; Johnson and Harrison, 1990;Suter et al, 1995). These tectonic basins are the result of intra-arc extension within the Trans-Mexican Volcanic Belt that began in the late Miocene (Suter et al, 1995;Campos-Enriquez et al, 2000;Suter et al, 2001). The Acambay graben ( Fig.…”