Holocene-Emerged Notches and Tectonic Uplift Along the Jalisco Coast, Southwest Mexico
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Geomorphology 58 (2004) 291–304 www.elsevier.com/locate/geomorph Holocene-emerged notches and tectonic uplift along the Jalisco coast, Southwest Mexico M. Teresa Ramı´rez-Herreraa,*, Vladimir Kostoglodovb, Jaime Urrutia-Fucugauchib a Department of Geological Sciences, California State University Long Beach, 1250 Bellflower Boulevard, Long Beach, CA 90840, USA b Instituto de Geofı´sica, UNAM, Ciudad Universitaria, C.P. 04510, D.F., Mexico Received 24 July 2002; received in revised form 14 July 2003; accepted 16 July 2003 Abstract This paper presents the preliminary results from a study of Holocene-emerged shorelines, marine notches, and their tectonic implications along the Jalisco coast. The Pacific coast of Jalisco, SW Mexico, is an active tectonic margin. This coast has been the site of two of the largest earthquakes to occur in Mexico this century: the 1932 (Mw 8.2) Jalisco earthquake and the 1995 (Mw 8.0) Colima earthquake. Measurement and preliminary radiocarbon dating of emergent paleoshorelines along the Jalisco coast provide the first constraints upon the timing for tectonic uplift. Along this coastline, uplifted Holocene marine notches and wave-cut platforms occur at elevations ranging from ca. 1 to 4.5 m amsl. In situ intertidal organisms dated with radiocarbon, the first ever reported for the Jalisco area, provide preliminary results that record tectonic uplift during at least the past 1300 years BP at an average rate of about 3 mm/year. We propose a model in which coseismic subsidence produced by offshore earthquakes is rapidly recovered during the postseismic and interseismic periods. The long-term period is characterized by slow tectonic uplift of the Jalisco coast. We found no evidence of coastal interseismic and long-term subsidence along the Jalisco coast. D 2003 Elsevier B.V. All rights reserved. Keywords: Holocene; Tectonic uplift; Marine notches; Coastal tectonics; Earthquakes 1. Introduction crustal faulting within the Jalisco Block (Ramı´rez- Herrera and Urrutia-Fucugauchi, 1999; Ramı´rez-Her- The Jalisco coast, SW Mexico, has been the site of rera et al., 1999). Although there have been local three of the largest earthquakes to occur in Mexico studies of the tectonic deformation accompanying this century: the 1932 (Ms 8.1 and Ms 7.8) Jalisco large earthquakes along this active plate margin earthquakes and the 9 October 1995 (Mw 8.0) Colima (Cumming, 1933; Bodin and Klinger, 1986; Corona- earthquake. The structure and the morphology of this Esquivel et al., 1988; Hutton et al., 1997, 1998),an coast have been influenced by the subduction of the integrated approach to the study of emerged coastal Rivera Plate beneath the North America Plate and by features (ages and elevations) of this active margin has yet to be applied. * Corresponding author. Tel.: +1-562-985-8579; fax: +1-562- We identified in situ fossils of intertidal organisms 985-8638. and morphological indicators of relative sea-level E-mail address: [email protected] (M.T. Ramı´rez-Herrera). changes along the Jalisco coast. Intertidal organisms 0169-555X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2003.07.004 292 M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304 found in their growth position can be useful indica- 1996). Because the distribution of many marine tors of a rapid sea-level change. In situ fragile organisms is directly related to tide levels (or wave intertidal organisms, preserved above the supralittoral exposure), they are common sea-level indicators and, zone and above mean sea level, are possible only when preserved, may be used to identify former sea after a very rapid relative sea-level change (Pirazzoli, levels. Fossil intertidal organisms are very useful Fig. 1. Tectonic setting of the Jalisco coast. Boundaries of the Rivera Plate: Rivera fracture zone (Fz), the East Pacific Rise (EPR), the Tamayo Fracture Zone, and the Middle America Trench (MAT). Rivera–Cocos Plate boundary: El Gordo graben (EGG) (Bandy, 1992). Symbols: Unfilled arrows show direction of plate motion, and numbers indicate convergence rate in centimeters per year. Insert shows area of study and site locations. M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304 293 indications of former sea-level positions on the Plate subducts below the Jalisco Block of the North Jalisco coast. American Plate, with a shallow 10j dip angle down to We recognized a series of marine notches, wave- about a depth of 20 km in the seismogenic zone; the cut platforms, and remnants of marine terraces along dip gradually increases up to 46j at 65 km depth the Jalisco coast of Mexico along about 153 km, from (Pardo and Sua´rez, 1993; Hurtado et al., 1997). Barra de Navidad to Tomatlan that reflect relative sea- The Jalisco Block in SW Mexico is bounded inland level changes (Fig. 1). Emerged marine shorelines are on the NE by the Tepic–Zacoalco Rift (NW–SE) and the depositional and erosional remains of abandoned on the SW by the Colima Rift (NE–SW) (Fig. 1). shorelines (Lajoie, 1986). The number of emerged Rifting and volcanism are associated with movement shorelines, their spacing, and their character are a of the Jalisco Block away from the North America function of tectonic movement and regional and Plate toward the NW (Luhr et al., 1985; Allan, 1986). global sea-level change (eustatic change). The best- The rate of movement of the Jalisco Block relative to preserved records of sea level do not come from stable NorthAmericamaybeverylow(<5mm/year; coastlines but rather from coasts experiencing steady, DeMets and Stein, 1990; Bandy and Pardo, 1994). long-term uplift. Few studies on Holocene eustatic and relative sea-level changes along the southwestern 2.2. Morphology and sea-level history Pacific coasts of Mexico exist (Curray et al., 1969; Ortlieb, 1987). Moreover, no published studies are 2.2.1. Morphology available of either emerged shorelines in this region or The Jalisco coast stretches along 180 km and of previously reported radiocarbon dates for emerged subparallels the Middle American trench. The conti- coastal features in southern Mexico. This paper nental shelf here is relatively narrow, locally < 1 km presents the preliminary results from a study of but reaching up to 2 km. The distance from the trench Holocene-emerged shorelines, marine notches, and to the coast averages f 50 km, but increases to their tectonic implications along the Jalisco coast. about 120 km near Bahia de Banderas (Fig. 1). The We present the first geomorphic study and preliminary coastal landscape is characterized by cliffs and rocky radiocarbon data of emerged shorelines in this region. promontories (mostly shaped on granite, rhyolite, and We discuss field-based results and the possible mech- breccia), alternating with narrow beaches. Inland, anisms for coastal emergence and propose a model for sparse low plains with lagoons and estuaries, highly Holocene tectonic uplift of the Jalisco coast. dissected hills and high mountains (up to ca. 2000 m) characterized the landscape. Some of these highly dissected hills are apparently remnants of emerged 2. Setting of the Jalisco coast marine terraces that extend intermittently along the Jalisco coast. Although not yet surveyed with preci- 2.1. Tectonics sion nor dated, these terrace remnants show consis- tent elevations ca. 40, 60, f 80, and 100 m amsl The Jalisco coast is located along an active con- (topographic map estimates). Terrace remnants along vergent margin where the Rivera oceanic plate sub- this coast are highly dissected and weathered, expos- ducts beneath the North American Plate along the ing outcrops with a thick regolith layer on top of the Middle American trench (Fig. 1). The Rivera Plate, a terraces surfaces. Beach deposits were missing from young plate (sea floor age is 9 Ma (Klitgord and the terrace surfaces, most probably washed away by Mammerickx, 1982; Kostoglodov and Bandy, 1995)), streams and runoff. We speculate that the poor moves to the N–NE with respect to the Rivera–North preservation of these terraces is related to climatic America Plate boundary, at rates of 2.0 and 3.3 cm/ conditions (subtropical climate) and long time expo- year, increasing from north to south (DeMets et al., sure, and that the apparent age of the terraces might 1994). Other studies suggest higher convergence rates be pre-Holocene (Ramı´rez-Herrera et al., 1998a,b). up to 5.0 cm/year (Bandy, 1992; Kostoglodov and We suggest further study of these features that Bandy, 1995; Kostoglodov et al., 1997; Bandy et al., are very interesting but beyond the scope of this 1997, 1998). Recent studies show that the Rivera work. 294 M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304 Fig. 2. Historic seismicity and rupture zones along the Jalisco coast. Dates indicate main historical events and their magnitude. Shaded encircle areas show rupture zones for the 1932 and 1995 events (Pacheco et al., 1997). M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304 295 2.2.2. Sea level history southern Jalisco, and Colima coasts, according to Little is known about Holocene sea level along the Cumming (1933) report. southern Pacific coasts of Mexico. During the late The October 1995 earthquake (Mw 8.0) located Quaternary (stages 5a/c and 5e, 82/105 and 120 ka, near Manzanillo ruptured part of the Rivera–North respectively), two sea-level maxima have been de- America plate boundary (Melbourne et al., 1997; Ortiz scribed for NW Mexico; but evidence of only one et al., 1999). Coseismic subsidence of 80 F 14 mm (2) transgression, which occurred after the sea level at Manzanillo, Colima, was estimated from GPS maximum (stage 5e), is distinguished in the Baja surveys performed before and after the earthquake California Peninsula (Ortlieb, 1987).