TMVB - grabens

TA = Tzitzio Anticline, NT = Nevado de Toluca, DF = Distrito Federal, G = Guadalajara, IZ = Ixtapa Zihuantanejo, C = Nevado de Colima, and Ce = Ceboruco. Note the association of the Tzitzio Gap with the subduction trajectory of the OFZ. LA = Los Azufres caldera, A = Amealco caldera, H = Huichipan caldera, TA = Tzitzio Anticline, Zr = Zirahuato domes, NT = Volcan Nevado de Toluca, M = Morelia, PG = Penajamillo Graben, CG = Cuitzeo Graben, TQ = Taxco-Queretaro fault system, and AG = Amealco Graben

Paleoseismology of the southwestern Morelia–Acambay fault system, central Mexico -http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0016-71692009000300006

The Morelia–Acambay fault system (MAFS) is an E–W structure in the Trans–Mexican Volcanic Belt (TMVB), in the central part of the Chapala–Tula Fault zone (Johnson and Harrison, 1989). Previous studies report that the E–W fault system in the central part of the TMVB is seismically active. Recurrence periods on these faults are unknown. For the Acambay fault in the eastern part of the system, a recurrence interval of 3600 yr was proposed. There is a connection between this tectonic system and the seismicity, in the Araro zone and the El Chino fault at Los Azufres geothermal field. Historical records contain a large number of events that might be related to the activity of MAFS. In the eighteenth century, several earthquakes were reported in1786 and1787 that cause damage in Tlalpujahua, Araro and Zinapйcuaro. These earthquakes struck the gold and silver mines in Tlalpujahua. Earthquake repots in 1806, 1837, 1839, 1843, 1849, 1857, 1858, 1873, 1875, 1887, 1894 and 1895 for of Morelia, Tlalpujahua, Zinapйcuaro, Querendaro and Araro, included casualties. Most of the earthquake damage was concentrated along the E–W fault segments of MAFS. Important earthquakes with hypocenters located between the towns of Acambay and Maravatнo occurred in1912, 1979 and 1998). The town of Acambay was partly destroyed by the 1912 event on the Acambay fault , Maravatio, had swarm activity in 1979 and 1998 on the Venta de Bravo fault , followed by some activity up to Mg=4.
In this work we describe the western segment of MAFS, with special attention to the Morelia–Cuitzeo–Zinapйcuaro region. A review of the stratigraphy and structural geology of the region is first presented. Stratigraphic, paleontological and geochronological data provide new information on past earthquakes in the region, and morphology and length of the fault scarps, associated deposits, and paleoseismologic data provide new insights on the ongoing activity of the segment.
Tectonic setting
Three regional fault systems affect the central part of the TMVB; the older NW–SE to N–S trending faults of Basin and Range type; the NE–SW trending system; and the E–W MAFS faults, which are the subject of this study. The first two fault systems offset the MAFS. All three fault systems are exposed over the study area, particularly in the vicinity of Morelia, which is located at the intersection of these systems.
The NW–SE fault system
Regional extensional faulting in northern Mйxico trends NW–SE to NNE–SSW and may belong to the Basin and Range province. The activity of these faults began 30 Ma in the USA and northern and continues today. In the TMVB, these faults may be contemporaneous with the E–W extensional faults. South of the TMVB (Tzitzio region), faults with this orientation have experienced mostly lateral movement. The most important structure of this system is the Taxco–Queretaro fault zone. Other faults with similar orientation appear in the Maravatio–Ciudad Hidalgo depression, to the E and SE of Morelia and in the vicinity of Indaparapeo.
Near Morelia, a NW–SE fault cuts the Miocene andesite at Sierra de Mil Cumbres. These rocks are overlain by a 12 Ma. Morelia ignimbrites originated at the Atйcuaro caldera (M. Suter, 2001, personal communication, Garduсo et al., 1999b). Rнo Chiquito River follows the NW–SE oriented faults with a 40 m drop at its SW bank. Outcrops along an access road to the Guadalajara–Morelia highway south of Morelia reveal shear structures and semihorizontal and semivertical superimposed striate representing first right–lateral, and later, normal movements. Further east, in the vicinity of Indaparapeo and Atapaneo, NW–SE oriented faults form a small graben with surface drops of ~20 m. These faults cut lava flows, recent lacustrine sediments, and apparently, also E–W oriented faults. Another NW–SE fault with a 500–m surface drop exists between Los Azufres geothermal field and the Zinapйcuaro area and the Cuitzeo Lake. East of the study area, in the vicinity of Temascalcingo, a series of NW oriented faults form grabens and half–grabens that displace the base of the present soil (unpublished data). Thus the NW–SE oriented faults may still be active as concluded by Suter et al. (1995a). The Tzitzio–Valle de Santiago regional fault that generated the Tzitzio fold displaces the Querendaro–Indaparapeo lava flows dated at 700 ka. NW–SE faults generated 10 km long right–hand displacements in the Cuitzeo depression related to the Valle de Santiago volcanism as shown by morphological data. Similar structures have generated the west stepping geometry of the lacustrine depressions. Other regional structures include the Infiernillo–Penjamillo fault. In conclusion, that the NNW–SSE fault have been reactivated as right–lateral faults along the deformation tensor of the TMVB.
The NE–SW fault system
Regional NE–SW oriented faults were recognized in various locations of the central TMVB. Such faults might underlay Mйxico City. They might reach the Apan region. They may also be found near Nevado de Toluca volcano. In the western portion of the study area and in the Michoacan–Guanajuato volcanic field, faults with this orientation are associated to alignments of cinder cones. This NE–SW structural trend separates the Chapala half–graben from the Cuitzeo half–graben by offsetting the position of their axes. Aeromagnetic maps of the region (CRM, 1996, 1998) and the gravimetric data (M. Mena, 2002, personal communication) show NE–SW oriented alignments of magnetic and gravimetric anomalies in the western portion of the study region. The older NNW–SSE trends may have been reactivated with a normal–right–hand movement.
Locally, slip planes with NE–SW orientation are common throughout the MAFS (unpublished data). In the vicinity of Morelia, these faults have two generations of striae, the first one pitches 0–15° with left–lateral movement, and the second one has semivertical striae with normal movement. These faults may date from 15 Ma as left–lateral strike–slip faults, reactivated as normal faults in early Pliocene. A similar scenario has been inferred for the NE–SW oriented faults in the Apan region (Garcнa–Palomo et al., 2002) and in the Mйxico Basin. Israde (1995) showed that the NE–SW oriented faults were highly active between 3–5 Ma when of the Morelia and Cuitzeo lacustrine basins were formed in response to extensional faulting.
The MAFS fault system
MAFS is the most seismically active system of the TMVB. In its western part, around Morelia, MAFS consists of two main faults with several segments that define a tectonic boundary between two distinct geologic provinces: to the north, the Plio–Quaternary active volcanism and fluvio–lacustrine sedimentation of the TMVB, and to the south, Miocene volcanic rocks of Sierra de Mil Cumbres that host silicic calderas (e.g. Atecuaro, La Escalera). MAFS comprises segments with seismic historic activity. The origin of these faults goes back to Late Miocene – Early Pleistocene, as E–W trending faults along the southern borders of the Miocene basins of Charo, Maravatнo and Venta de Bravo (Israde et. al., 1992; Israde, 1995; Israde and Garduсo, 1999). It caused the structural and morphological reorganization of the central and western parts of the TMVB with formation of fluvio–lacustrine basins and 200–m high fault scarps. At Morelia and Cuitzeo, MAFS structures show hanging walls to the north (Israde–Alcantara and Garduсo–Monroy, 1999; Garduсo et al., 1997; Israde, 1995; Johnson and Harrison, 1990) along as the southern edge of the lacustrine basins (Israde, 1995)). These faults have progressively tilted lake–beds toward the modern Cuitzeo Lake, a rotational movement of the MAFS fault system associated with listric faults. As in Chapala lake, west of the study area, MAFS faults at the southern edge of the Cuitzeo, Tarнmbaro and Morelia depressions are more important than the corresponding northern bounding faults.

Regional easth earthquakes related to MAFS
The area has experienced four types of seismic activity: (1) deep focus, subduction earthquakes, (2) intraplate, intermediate–depth, normal slab earthquakes, (3) intraplate, shallow, normal earthquakes (Singh et al., 1996), and (4) silent, slip earthquakes (Garduno–Monroy et al., 2000).
Subduction earthquakes occur along the Pacific coast. Large magnitude events are felt over broad areas of southern and central Mexico. These are the most destructive earthquakes experienced in Mйxico, the 1985 earthquake that severely damaged Mexico City. The damage caused by these earthquakes is unrelated to MAFS.
Intraplate, intermediate earthquakes caused by normal faulting in the subducting slab are related to historical events. Two large earthquakes of this type may have occurred in 1845 and 1858 (Singh et al., 1996). The isoseismals of the 1858 earthquake by Figueroa (1974) assign a 7.0 of intensity and locate the epicenter in the Tarasco corridor nearby the site of Parнcutin volcano (19°39''N, 102°11''W). This author estimated the Mercalli intensity of this event at IX in of Morelia and Patzcuaro. Parts of Mйxico City, Morelia, Patzcuaro and Araro were damaged in this earthquake. In Araro, the cathedral was damaged. the 1845 and 1858 earthquakes, were caused by a similar mechanism. During both earthquakes, the Patzcuaro area behaved as a high–intensity island .The isoseismals tend to parallel the MAFS structures in the area.
Paleoseismological studies around Jaracuaro south of Patzcuaro, sugest that at least two important earthquakes may have occurred during the last 10,000 years. A debris avalanche occurred after 28,000 yr ago plus volcanic activity during the last 8,000 yr.
The 1845 and 1858 epicenters fall within the area of maximum seismic and volcanic activity during the Pleistocene–Recent period.
Earthquakes directly related to normal faulting on MAFS structures include the events of 1912, 1979 and 1998. The Acambay earthquake of 1912, magnitude 6.7 ruptured the Acambay–Tixmadeje fault and several other minor faults in the Acambay graben partly destroying the town of Acambay. The February 1979 earthquake had a magnitude of 5.3 with an epicenter on the Venta de Bravo fault. The June 1998, Maravatio swarm included nine events with magnitudes over 3 and epicenters at depths between 3–11 km on the Venta de Bravo fault. The focal mechanism of this earthquake consisted of N–S oriented extension with a small left–lateral component.

---

next