Holocene-Emerged Notches and Tectonic Uplift Along the Jalisco Coast, Southwest Mexico

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 consistent 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). A study in the (Melbourne et al., 1997). Pressure-gauge records indi- coast of Nayarit and Sinaloa (Curray et al., 1969), cated subsidence of 44 cm in Barra de Navidad and 11 south of the Gulf of California, suggests the following cm in Manzanillo, Colima (Kostoglodov et al., 1997). chronology of events during the late Holocene. At ca. Historical data are insufficient to determine earth- 18,000 years BP, sea level was at ca. À 125 m. quake recurrence periods for this region (Nishenko Between 18,000–7000 years BP, a rapid sea level and Singh, 1987). Singh et al. (1985) proposed an rise took place to ca. À 10 m. The middle Holocene earthquake recurrence period of 77 years for the (7000–3600 years BP) was characterized by slower Jalisco coast based on historical earthquake records. rise of sea level to ca. À 2 m. Between 4750 and 3600 Although large earthquakes occurred in the region in years BP occurred the stabilization of shorelines near 1806 (M 7.5), 1818 (M 7.7), and 1900 (M 7.6, 7.1) Nayarit and Sinaloa. Between 3600 and 1500 years (Singh et al., 1981), the rupture zones of these earth- BP, relative sea level may have been near present quakes have not been established (Pacheco et al., level, or could have risen rapidly to present level. By 1997). The study of past earthquakes, inferred from 1500 years BP, sea level reached its present level the geologic and geomorphic record, would improve (Curray et al., 1969). Globally, sea level has been our understanding on the time–space behavior of nearly stable during the past 6000 years. With most earthquakes along the Jalisco coast. deglaciation complete by about 6 ka, subsequent sea- level changes are interpreted to be the consequence of seismotectonic effects, isostatic adjustments, and cli- 3. Holocene marine notches matic changes (Pirazzoli, 1996). On tectonically active coasts, marine notches are 2.3. Historic earthquakes and coastal movement one of the most precise indicators of rates and patterns of uplift (Rust and Kershaw, 2000).On Large thrust earthquakes along the Mexican sub- coasts with a moderate tidal range, the most common duction zone occur on a shallow N–E (10–15j) notch profiles have recumbent V-shaped or U-shaped dipping plate boundary because of sudden slip along morphologies. In sheltered sites, the depth of marine the seismogenic zone between the Rivera and Cocos notches increase gradually above the notch floor to a Plates and the overriding North America Plate (Singh most convex part of the notch. The retreat point or et al., 1985). Along the Jalisco coast, three Ms>7.5 vertex, the most indented part of a notch, is located earthquakes have occurred during the past century: the near mean sea level (Fig. 3). From that point, the 3 and 18 June 1932 Jalisco earthquakes (Ms 8.1 and depth decreases gradually upward and becomes neg- 7.8, respectively) and the most recent 9 October 1995 ligible at the top of the notch near the highest tide Colima–Jalisco (Mw 8.0) earthquake (Fig. 2). Earth- level. In exposed sites, the elevation of marine quakes of the nineteenth century have less reliable notches amsl increases because wave action regularly location and magnitude estimates (Kostoglodov and exceeds high-tide level in open and exposed coast- Ponce, 1994). lines (Pirazzoli, 1996). The 1932 earthquakes broke the Rivera–North Marine notches are best developed in the littoral American Plate boundary (Singh et al., 1985) and zone where the intersections of rock, air, and sea are produced coastal subsidence estimated at 40–75 cm frequent and regular. Marine-notch development is between Barra de Navidad and south of Tecoman, mainly related to bioerosion, however, marine notches 296 M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304

Fig. 3. Emerged tidal notch and abrasion platform cut into granite by a former Holocene sea level near Barra de Navidad, southern Jalisco coast. Tidal notch profile: (A) top, (B) retreat point, (C) base. Ruler bars are 10 cm each. Retreat point (C) is located between 0.62 and 0.95 m above mean sea level. Local tide range is 0.536 m. Fossil intertidal barnacles were sampled from this site. can be formed by abrasion and by differential erosion. posed sites, we performed measurements at low tide Zones of rapid marine-notch development correlate and during the morning when winds were calm and well with zones of major eroding organisms. The the wave run-up effect decreases on this coast. Some maximum density of grazing organisms is near mean notches located seaward were not accessible due to sea level. Rates of erosion of 0.2–5.0 mm/year have the wave action and elevations could only be esti- been reported in the literature, the most frequent mated to the nearest level. Unfortunately, the nearest values being 1.0–1.5 mm/year in the tropics (Fair- tide gauge is located 40 km south of the study area bridge, 1968; Pirazzoli, 1996). on a less-sheltered shore in the Manzanillo Bay, the data for which was available from the archives of the 3.1. Field methods Institute of Geophysics at the National University of Mexico (UNAM). Calculated mean tide level at We measured the relative elevations of marine Manzanillo is 0.5 m, and 0.8 m at the Puerto Vallarta notches above mean sea level between the Barra de coast. Navidad and Cuitzmala sites with an Abney level We considered the influence of El Nin˜o Southern and stadia above water levels, considering tide level Oscillation (ENSO) on the sea level of the Pacific at the time and date of observations. We estimated a coast because one of our field seasons (1998) oc- measurement error of ca. F 20 cm. One of the curred during a strong El Nin˜o event. We estimated problems in measuring elevations of marine notches the ENSO influence on sea level during the 1997– is related to the wave run-up, particularly in exposed 1998 field season using the image of the Pacific sites. In sheltered shores, such as Barra de Navidad, ocean taken by the TOPEX/Poseidon satellite that the effect of wave run-up is significantly reduced showed the sea surface along the SW Mexican coast because this coast is protected from the open ocean 14–32 cm above normal. We used these corrections winds. In order to effectively diminish the wave run- to determine mean sea level at the time and date of up effect when measuring notch elevations in ex- survey. M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304 297

3.2. Marine notches along the Jalisco coast Table 1 Elevations and ages of emerged notches along the Jalisco coast We identified a suite of emerged notches along the Location Averaged Elevation Radiocarbon Jalisco coast between Punta Farallon and Barra elevations amsl (m) age (m)a de Navidad (19j28VN, 105j04VW and 19j11VN, b 104j40VW) (Fig. 1). 1 Barra de 0.46 to 0.6 0.6 to 0.9 Post 0 year BP Navidad (ca. 1959 AD)d notchb 3.2.1. Barra de Navidad notch 2 Cuitzmala 4.5 to 4.7 4.5 to 4.7 1262 F 51 A well-developed, slightly emerged notch cut into notch years BPc the base of granite cliffs is located in the inner shore Cuitzmala f 4.5 f 4.5 No datable of a lagoon protected from the open wave action by a sea caves material Cuitzmala f 1.5 to 2.0 1.5 to 2.0 No datable bar near Barra de Navidad (Fig. 1). At Barra de boulders material Navidad, local tidal range is at ca. 0.5 m. The Barra (8x 6 Â 6m) de Navidad notch extends 250 m along an almost notches vertical cliff (4.6 m high). The retreat point of the 3 Tecuan f 3.0 to 3.5e f 3.0 to 3.5 No datable emerged notch reaches an elevation between 0.6 and notches material 4 Benito Juarez 0.8 to 1.0 0.8 to 1.0 No datable 0.9 m amsl (Table 1). The notch profile at Barra de and Arroyo material Navidad shows a well-preserved floor. It exposes an Seco notches abrasion platform cut into granite. Even during high- 5 Punta Farallon f 4.0 f 4.0 No datable est high tides, the notch and abrasion platform are notch material still exposed above sea level (Fig. 3). On sheltered a Elevation data corrected for tide level at time of measurement; shores, the retreat point (the most indented part of a Measurement error estimates F 0.2 m. Mean tide range for F F notch) is usually situated near mean sea level (Piraz- locations near Manzanillo ca. 0.5 m, and 0.8 m for locations near to Puerto Vallarta. zoli, 1996). b El Nin˜o Correction (1998) + 0.14 to 0.32 m. c AA-22193. 3.2.2. Cuitzmala notches d 109% of modern (Beta-119880). e A series of notches cut into volcanic breccia Above mean high tide level. amsl = above mean sea level. (composed of porphyritic andesite in a lithic matrix) and andesitic lavas are exposed on an open shore near the mouth of the Cuitzmala River (Fig. 4). Winds in shows double erosional benches and honeycombs on this area vary from SW to SE during the year. The the cliff profile. Although this notch reaches an shore here is moderately exposed to the NW, where elevation of ca. 3.0 to 3.5 m above mean high tide waves regularly exceed high-tide level and may in- level, it is highly exposed to wave action; and local crease the elevation of the marine notches (Pirazzoli, accounts suggest that the highest tides reach the notch 1996). Marine notches here are cut on near vertical and that breaking waves occasionally cover the ero- cliff faces (6–8 m high), and their retreat points reach sional benches and abrasion platform (Table 1). elevations ca. 4.5 to 4.7 m amsl (Fig. 4). We identified other indicators of emergent shorelines in this area, 3.2.4. Benito Jua´rez notches such as sea caves and a sea arch cut in volcanic At Benito Jua´rez, emerged notches are cut into bedrock at elevations ca. 4.5 m. Several breccia breccia at 1.0 m amsl. Tidal range at this locality is ca. boulders (about 8 Â 6 Â 6 m) along the shore showed 0.5 m, indicating that this notch has emerged. About an indented surface at ca. 1.5 to 2.0 m amsl that is 500 m seaward from the notch, several island stacks most probably a marine notch (Table 1). also show notches and abrasion platforms emerged above mean sea level (we were unable to measure the 3.2.3. El Tecuan notches island notches due to strong wave action). Northwest We recognized other notches along the Jalisco of Benito Jua´rez, at the mouth of the Arroyo Seco coast. At a highly exposed site at El Tecuan, a River, an abrasion platform and notches are cut into spectacular abrasion notch that is cut into breccia andesites about 1 m amsl (Table 1). 298 M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304

Fig. 4. Emerged tidal notches cut into volcanic bedrock and conglomerates near Cuitzmala. Tidal notches are on almost vertical cliff faces and their retreat points reach elevations ca. 4.5 m above mean sea level. Photographs show the ca. 4.5 m uplifted notch seawards (A), and the same notch extended inland (B). Tidal range is between 0.5 and 0.8 m. Symbols as in Fig. 3. M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304 299

3.2.5. Punta Farallon notch Dr. M. Reguero, UNAM; and red algae identified by At Punta Farallon, a well-developed notch with a Dr. S. Tudhope from the University of Edinburgh) wave-cut platform at its base is cut into conglomerate were collected at the base of the Barra de Navidad along a 20-m-high cliff face. We also observed a notch and inside the Cuitzmala notch at elevations modern notch at the foot of a paleowave-cut platform corresponding to former low and mean tide level, at ca. 1.0 m above mean tide level. The coast at Punta respectively. All barnacles, oyster shells, and algae Farallon is very exposed, and the height of the were clearly located above mean sea level at emerged emerged marine notch (ca. 4.0 m) probably differed notches (Fig. 5). from mean sea level at the time it was cut (Table 1). The AMS radiocarbon age for the red algae sample Unfortunately, the majority of these notches and collected at Cuitzmala is 1262 F 51 14C years BP (cal. wave-cut platforms are barren of organic material AD 660–940) (AA-22193), suggesting that the emer- for radiocarbon dating. Advanced dating techniques, gence of the Cuitzmala coast occurred at about that such as the use of cosmogenic isotope dating, should time. be applied here in order to determine exposure ages of Shell samples from Barra de Navidad were ana- these features. lyzed for 14C activity using standard methods. The 13C/12C corrected age (Beta Analytic, Beta-119880) 3.3. Radiocarbon dating of Jalisco marine notches obtained from the oyster shells sample at Barra de Navidad is 109% of modern (Post 0 year BP, ca. AD Fossil intertidal barnacles, oyster shells, and red 1959). Because barnacles collected from the same site algae (Balanus identified by Dr. A. Garcı´a Cubas and were encrusted on top of oyster shells, we assumed

Fig. 5. Elevations of the retreat points of the notches along the Jalisco coast and sea level. Elevation uncertainty limits are marked by vertical segments. Radiocarbon dates for fossil organisms found on Cuitzmala (C) are 1262 F 51 years BP, Barra de Navidad (BN) 109% of modern (Post 0 year BP, or ca. AD 1959). Symbols: F = Punta Farallon, C = Cuitzmala, BJ = Benito Juarez, T = El Tecuan, BN = Barra de Navidad; masl = meters above sea level. Graphic indicates average regional: MSL = mean sea level, MHWL= mean high water level, MHHWL = mean highest high water level (for site location, see Fig. 1). 300 M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304 that the barnacles and shells were the same age. Alternatively, long-term deformation not directly However, the vertical zonation of barnacles indicates associated with earthquakes might also be a mech- that their living conditions can extend above the anism for coastal uplift in Jalisco. Elastic strain maximum tidal range, and in most cases indicate deformation can be accumulated during the inter- super-zero elevations. This helps explain the anoma- seismic period causing upward flexing of much of lous radiocarbon date at Barra de Navidad. the accretionary prism and adjacent local coastal areas. However, this strain would be released during the subsequent earthquake, producing abrupt 4. Mechanisms of coastal emergence coseismic submergence on coastal areas. This sim- ple model of interseimic and coseismic deformation 4.1. Holocene tectonic uplift has been used to demonstrate the elastic behavior of the overriding lithospheric plate on the Casca- The Cuitzmala radiocarbon age and marine notch dian subduction zone where the Juan de Fuca elevation indicate a relative sea-level fall on this oceanic Plate underthrusts and where paleoseismic segment of the Jalisco coast during the last 1300 records report coseismic subsidence (Atwater, years BP. The global history of relative sea-level 1992). change indicates that sea level has been at its present Rapid uplift might partially explain coastal emer- level since 6 ka (Pirazzoli, 1996). Along the northern gence evident by the Cuitzmala notches. Radiocarbon coast of Jalisco, sea level was near or at present dating of the ca. 4.5-m elevated Cuitzmala marine height by 3600–1500 years BP (Curray et al., 1969), notch indicates the time of uplift, inferred to be rapid and no major isostatic adjustments and climate because of the presence of well-preserved marine changes have been reported for the area for that organisms found in their growth position. Uplift might period of time. Two mechanisms might be involved have occurred during a large-magnitude earthquake at in the emergence of the Jalisco marine notches 1262 F 51 years BP (cal. AD 660–940). Other intermittently exposed along 40 km of coast: (i) emerged notches, exposed in the study region at rapid uplift produced by large-magnitude, late-Holo- different elevations above mean sea level (ca. 1, cene earthquakes generated on the active plate 1.5–2, and 4 m) might have a similar history of boundary, and/or (ii) long-term uplift of the coast sudden uplifts associated with earthquakes. Unfortu- produced by the permanent deformation of the over- nately, most of these features were barren of datable riding plate because of the subduction of the Rivera material, and no ages could be used to determine Plate (Fig. 2). We discuss these two mechanisms uplift rates. The absence of intertidal organisms on below. these notches raises the question regarding whether a First, rapid uplift movements might be coseismic rapid or a slow uplift raised these features above mean because the preservation of in situ sublittoral and sea level. midlittoral shells, barnacles, and algae in the supra- On the other hand, the elevation of the Cuitzmala littoral zone would be enhanced by a very rapid, notch at 4.5 m amsl could be explained by a large- relative sea-level change (Pirazzoli, 1996). Intertidal magnitude shallow earthquake or a combination of a organisms, red algae, were found in their growth series of shallow earthquakes located near the coast position and well preserved on the Cuitzmala notch. and/or long-term tectonic uplift. None of these The preservation of these organisms may be due to hypotheses can be proven at this point because rapid uplift of the coast associated with shallow, large- historical records of earthquakes in this area do magnitude earthquakes located close to the coast. not go back far enough to prove or disprove these Similar coastal features produced by coseismic uplifts hypotheses. have been observed in other parts of the world along The Barra de Navidad notch, elevated at ca. 0.6– active subducting plate margins: for example, New 0.9 m, shows an anomalous (109% of modern) Zealand (Berryman et al., 1989; Ota et al., 1991), radiocarbon age. According to this date, this shoreline Japan (Kawana and Pirazzoli, 1985),andAlaska could have emerged a few decades before or after AD (Plafker and Rubin, 1967). 1959. This date could as well correspond to an M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304 301 earthquake that occurred in the 19th century. The 5. Discussion historical record indicates the occurrence of other larger earthquakes in the 19th century (1806, 1818, Studies at other plate boundaries around the Pacific 1837, 1875, 1900), nevertheless, the effects of these Rim showed that Holocene- and Pleistocene-emerged earthquakes on the coast have not been documented. shorelines recorded both eustatic and coseismic rela- However, the vertical zonation of barnacles indicates tive sea-level changes (Plafker, 1969; Matsuda et al., that their living conditions can extend above the 1978; Ota et al., 1991 Berryman, 1993). For example, maximum tidal range. This might be a probable the Papua New Guinea coastline, which has one of the reason for the anomalous modern radiocarbon age of most detailed records of emergent shorelines, shows the Barra de Navidad raised notch. that meter-scale uplifts represent great earthquakes The spatial-height distribution of the raised notches (Ota and Chappell, 1996). Large coseismic uplifts along the Jalisco coast illustrated in Fig. 5 suggests an can be identified from the geomorphic record; how- anomalous non-uniform spatial arrangement that ever, identifying less than 1-m uplifts from the geo- might be caused by differential uplift along the coast. morphic record is not possible because the effects of This non-uniform distribution could be related with an instantaneous movement cannot be distinguish the presence of faults that make up the blocky clearly from those events occurring over decades or structure of the Jalisco megablock (Lugo-Hubp and centuries. Ortiz-Perez, 1980; Ferrari and Rosas-Elguera, 2000; We believe that long-term slow uplift might also Ferrari et al., 2000). have contributed to the total net emergence of former Holocene shorelines on the Jalisco coast, although 4.2. Historic interseismic and coseismic observations distinguishing the amount of coseismic uplift from that of the slow uplift produced during long-term Observations of the historic earthquakes of 1932 periods is not possible. The mechanism that explains and 1995 showed that coseismic subsidence was uplift produced during interseismic cycles and during produced along the Jalisco coast (Cumming, 1933; the long-term period on the coastal area, as discussed Melbourne et al., 1997). After the 1932 earthquake, above, can be related to the shortening of the accre- Cumming’s (1933) report indicates local coastal sub- tionary prism and a flexural uplift of the overriding sidence of about 40–75 cm near Manzanillo, south of plate. Although the accretionary prism on this portion Jalisco, produced by the1932 earthquakes. The arrival of the margin is relatively thin, then the deformation of a tsunami was also reported by locals just after the occurring on the overriding plate would be small also; earthquake (Cumming, 1933). the possibility of an increase of buoyancy of the In October 1995, initial coseismic coastal subsi- subducting material might lead to an uplift of the dence (up to 20 cm at Chamela) was measured using plate margin in the long term (Buiter et al., 2001). geodetic-GPS and tide gauge data (Hutton et al., Moreover, emerged shorelines clearly experienced 1997). Measurements made barely 2 months after the overall uplift; and we found no evidence of a pro- earthquake demonstrate a period of rapid uplift, with longed interseismic and of long-term subsidence. some sites recovering almost the entire coseismic Finally, although coseismic centimeter-scale coast- drop (Hutton et al., 1997). Thus, a detailed GPS al subsidence was reported for the 1932 and 1995 survey before, during, and after the 1995 (Mw = 8) earthquakes (Cumming, 1933; Hutton et al., 1997, Colima earthquake showed postseismic motion rang- 1998; Kostoglodov et al., 1997), the rapid postseismic ing from 18% to 84% of the coseismic slip with and interseismic recovery indicates a general trend to postseismic displacements decaying rapidly (Hutton tectonic uplift during long-term periods. Apparently, et al., 1998). GPS results indicate that almost 3 years interseismic and long-term uplift has been an impor- after the earthquake, the coastal coseismic subsi- tant, if not the most important factor, in the emergence dence reported had been nearly completely recovered of the Jalisco coast. We propose a model for the during the postseismic and interseismic periods (Hut- Jalisco coast in which (based on geomorphic, histor- ton et al., 1997, 1998; O. Sanchez, UNAM, personal ical, and GPS observations) the coast subsides during communication). large earthquakes located offshore. This coseismic 302 M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304 subsidence is rapidly recovered during a short period active plate margin, show a relative sea-level ‘‘fall’’ of about 1–5 years. Then, during interseismic and in Jalisco during the late Holocene (at least during long-term periods, the coast is uplifted slowly by the last 1300 years BP). Emerged marine notches, upward flexing produced by the elastic deformation ranging from 1 to 4.5 m in elevation, record coastal of the overriding plate. This amount of long-term emergence since at least 1262 F 51 years BP. We uplift, although slow, does not seem to be completely suggest that Holocene coastal uplift, registered by lost during coseismic subsidence produced by subse- emerged marine notches, represents (i) slow tectonic quent earthquake events. The possibility of large- uplift during the long-term period, and/or (ii) prob- magnitude earthquakes producing tectonic uplift is ably rapid tectonic uplift caused by great shallow not excluded from this model. If earthquakes are earthquakes that might have occurred close to the shallow and close to the coast, they are capable of Jalisco coast. producing coastal uplift. Large coseismic uplift has If the history of relative sea-level change in Jalisco occurred in adjacent coastal segments of the subduc- is similar to histories on other northern Mexican tion zone. The 1985 earthquake produced up to 1.10 coasts and globally, most of the relative sea-level fall m of coastal uplift in the adjacent Michoacan coastal that we measured would be caused by tectonic uplift segment (Bodin and Klinger, 1986). However, we of the Jalisco coast. found no further evidence of sudden uplift apart from Although offshore earthquakes may produce local in situ intertidal organisms present on the Cuitzmala centimeter-scale coastal subsidence along the Jalisco notch and the history of cosesismic uplifts that coast, complete postseismic and interseismic recovery occurred on the adjacent coast of Michoacan. There- appears to follow (Hutton et al., 1997, 1998) leading fore, at this point, coseismic uplift cannot be proven. to general coastal uplift. Slow long-term uplift is However, while it is unfortunate that we have only two considered to be the most important factor to the total radiocarbon dates, these dates for the Cuitzmala and net vertical motion, uplift, along the Jalisco coast. We Barra de Navidad notches are consistent with their have found no evidence of coastal interseismic and elevations, showing a younger age for the lower Barra long-term subsidence. de Navidad notch and an older age for the higher Judging by the morphologic evidence and the Cuitzmala paleoshoreline. The record of emerged radiocarbon date of the 4.5-m elevation Holocene coastal notches suggests a cumulative long-term slow notches, we propose a Holocene long-term trend for uplift rate estimated at 3 mm/year. tectonic uplift at estimated rates f 3 mm/year along We have raised questions and suggested possible the Jalisco coast. Further work on the precise dating of explanations for Holocene uplift of the Jalisco coast in emerged notches and terraces is necessary to deter- SW Mexico. However, further work should be per- mine uplift rates in this area. formed on the absolute dating of emerged shorelines to determine uplift rates and perhaps establish earthquake recurrence intervals for the Jalisco coast. The Acknowledgements lack of material suitable for radiocarbon dating on most of these notches warrant further work along with This research was supported by California State the introduction of other dating techniques on abraded University-Long Beach, UNAM, and CONACYT. landforms. Cosmogenic isotopes might prove to be a Mike Summerfield and Alan Nelson provided val- valuable tool in providing exposure ages for abrasion uable comments on an earlier version of this manu- platforms and marine terraces (Hancock et al., 1999; script. Thanks to Steven Lefton for the helpful Perg et al., 2001). comments on this manuscript that helped us to improve the final version. We thank J. Zamorano, R. Reyna, R. Gutierrez, E. Leo´n, and M. Ortiz for their 6. Conclusions assistance in the field. O. Sanchez provided tide ranges. The University of Arizona Radiocarbon Intertidal notch elevations and their radiocarbon Laboratory and Beta Analytic produced radiocarbon age, first ever reported for this sector of the Mexican dates. Thanks to R.A. Mastron, N. Pinter, and P.G. M.T. Ramı´rez-Herrera et al. / Geomorphology 58 (2004) 291–304 303

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