Page 172 - Caldera Volcanism Analysis, Modelling and Response

The Sierra Madre Occidental, Mexico 147

ignimbrites, and (3) an upper suite of Miocene basalts and basaltic-andesites that
occur dispersed throughout the SMO. Because the main features of the SMO are
the large-volume ignimbrites of the second package, the SMO has been restricted
to the Mid-Tertiary ignimbrites occurring between the Latitudes 211 to 311 N.
However, Mid-Tertiary ignimbrites similar to those of the SMO continue as
discontinuous outcrops farther to the South of the Mexican Volcanic Belt, as well as
to the North of the U.S.–Mexico border, extending as far as Parallel 181 Lat. N to
the South, and up to Latitude 351 N, to the North, including the Mogollon-Datil
volcanic field (Figure 1). The SMO changes its width from North to South; it is
about 600 km wide in its northern sector, including the states of Sonora,
Chihuahua, and the westernmost portion of Texas, and it narrows to about 250 km
wide in the central sector, including the states of Sinaloa and Durango. It is about
550 km wide at the southern sector, including the states of Nayarit, Zacatecas,
Jalisco, Aguascalientes, Guanajuato, San Luis Potosı ´, and Quere ´taro. In order to
visualise the size of this volcanic province, the area that it covers is about the same
size as Italy (Figure 1).
3
A conservative estimated volume of the SMO ignimbrites is about 400,000 km ,
assuming an average thickness of 1 km and the actual surface distribution of the
mid-Tertiary ignimbrites between Latitudes 211 and 311 N(Table 1 —in Aguirre-
Dı ´az and Labarthe-Herna ´ndez, 2003). The estimate of Aguirre-Dı ´az and Labarthe-
Herna ´ndez (2003) does not consider the erosion of the ignimbrites, nor the
extension caused by the Basin and Range (up to 22% according to Henry and
Aranda-Go ´mez, 2000), and the consequent separation of Baja California from
mainland Mexico. Based upon its dimensions and volume, the SMO has been
catalogued as the largest ignimbrite province in the world (Swanson and McDowell,
1984; Aguirre-Dı ´az and Labarthe-Herna ´ndez, 2003; Swanson et al., 2006).
There have been several studies on the SMO province. These include classic
regional geology studies (McDowell and Clabaugh, 1979; Gastil et al., 1979; Mauger,
1981; Labarthe-Herna ´ndez et al., 1982; Swanson and McDowell, 1984; Henry and

Table 1 Volume of mid-Tertiary ignimbrites in Mexico.
3
2
Province Area (km ) Thickness (km) Volume (km )
Sierra Madre Occidental main 392,775 1 392,775
Eastern Chihuahua 66,583 1 66,583
Sonora 7,982 1 7,982
South of Mexican volcanic belt 35,287 1 35,287
Baja California 36,421 1 36,421
Inferred beneath the Mexican volcanic belt 32,136 1 32,136
Total 586,727 586,727
Total of San Juan Mountains a 20,000
Source: modified from Aguirre-Dı ´az and Labarthe-Herna ´ndez (2003), with permission from Geological Society of
America.
Note: volume is approximated and related to 17 calderas.
a
Data from Lipman et al. (1970).