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Geomorphology 58 (2004) 291–304

Holocene-emerged notches and tectonic uplift along the Jalisco

coast, Southwest Mexico

M. Teresa Ramırez-Herreraa,*, Vladimir Kostoglodovb, Jaime Urrutia-Fucugauchib

aDepartment of Geological Sciences, California State University Long Beach, 1250 Bellflower Boulevard, Long Beach, CA 90840, USAb Instituto de Geofısica, 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 (Ramırez-

The Jalisco coast, SW Mexico, has been the site of

three of the largest earthquakes to occur in Mexico

this century: the 1932 (Ms 8.1 and Ms 7.8) Jalisco

earthquakes and the 9 October 1995 (Mw 8.0) Colima

earthquake. The structure and the morphology of this

coast have been influenced by the subduction of the

Rivera Plate beneath the North America Plate and by

0169-555X/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.geomorph.2003.07.004

* Corresponding author. Tel.: +1-562-985-8579; fax: +1-562-

985-8638.

E-mail address: ramirezt@csulb.edu (M.T. Ramırez-Herrera).

Herrera and Urrutia-Fucugauchi, 1999; Ramırez-Her-

rera et al., 1999). Although there have been local

studies of the tectonic deformation accompanying

large earthquakes along this active plate margin

(Cumming, 1933; Bodin and Klinger, 1986; Corona-

Esquivel et al., 1988; Hutton et al., 1997, 1998), an

integrated approach to the study of emerged coastal

features (ages and elevations) of this active margin has

yet to be applied.

We identified in situ fossils of intertidal organisms

and morphological indicators of relative sea-level

changes along the Jalisco coast. Intertidal organisms

M.T. Ramırez-Herrera et al. / Geomorphology 58 (2004) 291–304292

found in their growth position can be useful indica-

tors of a rapid sea-level change. In situ fragile

intertidal organisms, preserved above the supralittoral

zone and above mean sea level, are possible only

after a very rapid relative sea-level change (Pirazzoli,

Fig. 1. Tectonic setting of the Jalisco coast. Boundaries of the Rivera Plate

Fracture Zone, and the Middle America Trench (MAT). Rivera–Cocos

Unfilled arrows show direction of plate motion, and numbers indicate con

site locations.

1996). Because the distribution of many marine

organisms is directly related to tide levels (or wave

exposure), they are common sea-level indicators and,

when preserved, may be used to identify former sea

levels. Fossil intertidal organisms are very useful

: Rivera fracture zone (Fz), the East Pacific Rise (EPR), the Tamayo

Plate boundary: El Gordo graben (EGG) (Bandy, 1992). Symbols:

vergence rate in centimeters per year. Insert shows area of study and

M.T. Ramırez-Herrera et al. / Geomorphology 58 (2004) 291–304 293

indications of former sea-level positions on the

Jalisco coast.

We recognized a series of marine notches, wave-

cut platforms, and remnants of marine terraces along

the Jalisco coast of Mexico along about 153 km, from

Barra de Navidad to Tomatlan that reflect relative sea-

level changes (Fig. 1). Emerged marine shorelines are

the depositional and erosional remains of abandoned

shorelines (Lajoie, 1986). The number of emerged

shorelines, their spacing, and their character are a

function of tectonic movement and regional and

global sea-level change (eustatic change). The best-

preserved records of sea level do not come from stable

coastlines but rather from coasts experiencing steady,

long-term uplift. Few studies on Holocene eustatic

and relative sea-level changes along the southwestern

Pacific coasts of Mexico exist (Curray et al., 1969;

Ortlieb, 1987). Moreover, no published studies are

available of either emerged shorelines in this region or

of previously reported radiocarbon dates for emerged

coastal features in southern Mexico. This paper

presents the preliminary results from a study of

Holocene-emerged shorelines, marine notches, and

their tectonic implications along the Jalisco coast.

We present the first geomorphic study and preliminary

radiocarbon data of emerged shorelines in this region.

We discuss field-based results and the possible mech-

anisms for coastal emergence and propose a model for

Holocene tectonic uplift of the Jalisco coast.

2. Setting of the Jalisco coast

2.1. Tectonics

The Jalisco coast is located along an active con-

vergent margin where the Rivera oceanic plate sub-

ducts beneath the North American Plate along the

Middle American trench (Fig. 1). The Rivera Plate, a

young plate (sea floor age is 9 Ma (Klitgord and

Mammerickx, 1982; Kostoglodov and Bandy, 1995)),

moves to the N–NE with respect to the Rivera–North

America Plate boundary, at rates of 2.0 and 3.3 cm/

year, increasing from north to south (DeMets et al.,

1994). Other studies suggest higher convergence rates

up to 5.0 cm/year (Bandy, 1992; Kostoglodov and

Bandy, 1995; Kostoglodov et al., 1997; Bandy et al.,

1997, 1998). Recent studies show that the Rivera

Plate subducts below the Jalisco Block of the North

American Plate, with a shallow 10j dip angle down to

about a depth of 20 km in the seismogenic zone; the

dip gradually increases up to 46j at 65 km depth

(Pardo and Suarez, 1993; Hurtado et al., 1997).

The Jalisco Block in SW Mexico is bounded inland

on the NE by the Tepic–Zacoalco Rift (NW–SE) and

on the SW by the Colima Rift (NE–SW) (Fig. 1).

Rifting and volcanism are associated with movement

of the Jalisco Block away from the North America

Plate toward the NW (Luhr et al., 1985; Allan, 1986).

The rate of movement of the Jalisco Block relative to

North America may be very low ( < 5 mm/year;

DeMets and Stein, 1990; Bandy and Pardo, 1994).

2.2. Morphology and sea-level history

2.2.1. Morphology

The Jalisco coast stretches along 180 km and

subparallels the Middle American trench. The conti-

nental shelf here is relatively narrow, locally < 1 km

but reaching up to 2 km. The distance from the trench

to the coast averages f 50 km, but increases to

about 120 km near Bahia de Banderas (Fig. 1). The

coastal landscape is characterized by cliffs and rocky

promontories (mostly shaped on granite, rhyolite, and

breccia), alternating with narrow beaches. Inland,

sparse low plains with lagoons and estuaries, highly

dissected hills and high mountains (up to ca. 2000 m)

characterized the landscape. Some of these highly

dissected hills are apparently remnants of emerged

marine terraces that extend intermittently along the

Jalisco coast. Although not yet surveyed with preci-

sion nor dated, these terrace remnants show consis-

tent elevations ca. 40, 60, f 80, and 100 m amsl

(topographic map estimates). Terrace remnants along

this coast are highly dissected and weathered, expos-

ing outcrops with a thick regolith layer on top of the

terraces surfaces. Beach deposits were missing from

the terrace surfaces, most probably washed away by

streams and runoff. We speculate that the poor

preservation of these terraces is related to climatic

conditions (subtropical climate) and long time expo-

sure, and that the apparent age of the terraces might

be pre-Holocene (Ramırez-Herrera et al., 1998a,b).

We suggest further study of these features that

are very interesting but beyond the scope of this

work.

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. Ramırez-Herrera et al. / Geomorphology 58 (2004) 291–304294

eomorphology 58 (2004) 291–304 295

2.2.2. Sea level history

Little is known about Holocene sea level along the

southern Pacific coasts of Mexico. During the late

Quaternary (stages 5a/c and 5e, 82/105 and 120 ka,

respectively), two sea-level maxima have been de-

scribed for NW Mexico; but evidence of only one

transgression, which occurred after the sea level

maximum (stage 5e), is distinguished in the Baja

California Peninsula (Ortlieb, 1987). A study in the

coast of Nayarit and Sinaloa (Curray et al., 1969),

south of the Gulf of California, suggests the following

chronology of events during the late Holocene. At ca.

18,000 years BP, sea level was at ca. � 125 m.

Between 18,000–7000 years BP, a rapid sea level

rise took place to ca. � 10 m. The middle Holocene

(7000–3600 years BP) was characterized by slower

rise of sea level to ca. � 2 m. Between 4750 and 3600

years BP occurred the stabilization of shorelines near

Nayarit and Sinaloa. Between 3600 and 1500 years

BP, relative sea level may have been near present

level, or could have risen rapidly to present level. By

1500 years BP, sea level reached its present level

(Curray et al., 1969). Globally, sea level has been

nearly stable during the past 6000 years. With most

deglaciation complete by about 6 ka, subsequent sea-

level changes are interpreted to be the consequence of

seismotectonic effects, isostatic adjustments, and cli-

matic changes (Pirazzoli, 1996).

2.3. Historic earthquakes and coastal movement

Large thrust earthquakes along the Mexican sub-

duction zone occur on a shallow N–E (10–15j)dipping plate boundary because of sudden slip along

the seismogenic zone between the Rivera and Cocos

Plates and the overriding North America Plate (Singh

et al., 1985). Along the Jalisco coast, three Ms>7.5

earthquakes have occurred during the past century: the

3 and 18 June 1932 Jalisco earthquakes (Ms 8.1 and

7.8, respectively) and the most recent 9 October 1995

Colima–Jalisco (Mw 8.0) earthquake (Fig. 2). Earth-

quakes of the nineteenth century have less reliable

location and magnitude estimates (Kostoglodov and

Ponce, 1994).

The 1932 earthquakes broke the Rivera–North

American Plate boundary (Singh et al., 1985) and

produced coastal subsidence estimated at 40–75 cm

between Barra de Navidad and south of Tecoman,

M.T. Ramırez-Herrera et al. / G

southern Jalisco, and Colima coasts, according to

Cumming (1933) report.

The October 1995 earthquake (Mw 8.0) located

near Manzanillo ruptured part of the Rivera–North

America plate boundary (Melbourne et al., 1997; Ortiz

et al., 1999). Coseismic subsidence of 80F 14 mm (2)

at Manzanillo, Colima, was estimated from GPS

surveys performed before and after the earthquake

(Melbourne et al., 1997). Pressure-gauge records indi-

cated subsidence of 44 cm in Barra de Navidad and 11

cm in Manzanillo, Colima (Kostoglodov et al., 1997).

Historical data are insufficient to determine earth-

quake recurrence periods for this region (Nishenko

and Singh, 1987). Singh et al. (1985) proposed an

earthquake recurrence period of 77 years for the

Jalisco coast based on historical earthquake records.

Although large earthquakes occurred in the region in

1806 (M 7.5), 1818 (M 7.7), and 1900 (M 7.6, 7.1)

(Singh et al., 1981), the rupture zones of these earth-

quakes have not been established (Pacheco et al.,

1997). The study of past earthquakes, inferred from

the geologic and geomorphic record, would improve

our understanding on the time–space behavior of

earthquakes along the Jalisco coast.

3. Holocene marine notches

On tectonically active coasts, marine notches are

one of the most precise indicators of rates and

patterns of uplift (Rust and Kershaw, 2000). On

coasts with a moderate tidal range, the most common

notch profiles have recumbent V-shaped or U-shaped

morphologies. In sheltered sites, the depth of marine

notches increase gradually above the notch floor to a

most convex part of the notch. The retreat point or

vertex, the most indented part of a notch, is located

near mean sea level (Fig. 3). From that point, the

depth decreases gradually upward and becomes neg-

ligible at the top of the notch near the highest tide

level. In exposed sites, the elevation of marine

notches amsl increases because wave action regularly

exceeds high-tide level in open and exposed coast-

lines (Pirazzoli, 1996).

Marine notches are best developed in the littoral

zone where the intersections of rock, air, and sea are

frequent and regular. Marine-notch development is

mainly related to bioerosion, however, marine notches

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.

M.T. Ramırez-Herrera et al. / Geomorphology 58 (2004) 291–304296

can be formed by abrasion and by differential erosion.

Zones of rapid marine-notch development correlate

well with zones of major eroding organisms. The

maximum density of grazing organisms is near mean

sea level. Rates of erosion of 0.2–5.0 mm/year have

been reported in the literature, the most frequent

values being 1.0–1.5 mm/year in the tropics (Fair-

bridge, 1968; Pirazzoli, 1996).

3.1. Field methods

We measured the relative elevations of marine

notches above mean sea level between the Barra de

Navidad and Cuitzmala sites with an Abney level

and stadia above water levels, considering tide level

at the time and date of observations. We estimated a

measurement error of ca. F 20 cm. One of the

problems in measuring elevations of marine notches

is related to the wave run-up, particularly in exposed

sites. In sheltered shores, such as Barra de Navidad,

the effect of wave run-up is significantly reduced

because this coast is protected from the open ocean

winds. In order to effectively diminish the wave run-

up effect when measuring notch elevations in ex-

posed sites, we performed measurements at low tide

and during the morning when winds were calm and

the wave run-up effect decreases on this coast. Some

notches located seaward were not accessible due to

the wave action and elevations could only be esti-

mated to the nearest level. Unfortunately, the nearest

tide gauge is located 40 km south of the study area

on a less-sheltered shore in the Manzanillo Bay, the

data for which was available from the archives of the

Institute of Geophysics at the National University of

Mexico (UNAM). Calculated mean tide level at

Manzanillo is 0.5 m, and 0.8 m at the Puerto Vallarta

coast.

We considered the influence of El Nino Southern

Oscillation (ENSO) on the sea level of the Pacific

coast because one of our field seasons (1998) oc-

curred during a strong El Nino event. We estimated

the ENSO influence on sea level during the 1997–

1998 field season using the image of the Pacific

ocean taken by the TOPEX/Poseidon satellite that

showed the sea surface along the SW Mexican coast

14–32 cm above normal. We used these corrections

to determine mean sea level at the time and date of

survey.

Table 1

Elevations and ages of emerged notches along the Jalisco coast

Location Averaged

elevations

(m)a

Elevation

amsl (m)

Radiocarbon

age

1 Barra de

Navidad

notchb

0.46 to 0.6b 0.6 to 0.9 Post 0 year BP

(ca. 1959 AD)d

2 Cuitzmala

notch

4.5 to 4.7 4.5 to 4.7 1262F 51

years BPc

Cuitzmala

sea caves

f 4.5 f 4.5 No datable

material

Cuitzmala

boulders

f 1.5 to 2.0 1.5 to 2.0 No datable

material

(8x 6� 6 m)

notches

3 Tecuan

notches

f 3.0 to 3.5e f 3.0 to 3.5 No datable

material

4 Benito Juarez

and Arroyo

Seco notches

0.8 to 1.0 0.8 to 1.0 No datable

material

5 Punta Farallon

notch

f 4.0 f 4.0 No datable

material

a Elevation data corrected for tide level at time of measurement;

Measurement error estimates F 0.2 m. Mean tide range for

locations near Manzanillo ca. F 0.5 m, and F 0.8 m for locations

near to Puerto Vallarta.b El Nino Correction (1998) + 0.14 to 0.32 m.c AA-22193.d 109% of modern (Beta-119880).e Above mean high tide level. amsl = above mean sea level.

eomorphology 58 (2004) 291–304 297

3.2. Marine notches along the Jalisco coast

We identified a suite of emerged notches along the

Jalisco coast between Punta Farallon and Barra

de Navidad (19j28VN, 105j04VW and 19j11VN,

104j40VW) (Fig. 1).

3.2.1. Barra de Navidad notch

A well-developed, slightly emerged notch cut into

the base of granite cliffs is located in the inner shore

of a lagoon protected from the open wave action by a

bar near Barra de Navidad (Fig. 1). At Barra de

Navidad, local tidal range is at ca. 0.5 m. The Barra

de Navidad notch extends 250 m along an almost

vertical cliff (4.6 m high). The retreat point of the

emerged notch reaches an elevation between 0.6 and

0.9 m amsl (Table 1). The notch profile at Barra de

Navidad shows a well-preserved floor. It exposes an

abrasion platform cut into granite. Even during high-

est high tides, the notch and abrasion platform are

still exposed above sea level (Fig. 3). On sheltered

shores, the retreat point (the most indented part of a

notch) is usually situated near mean sea level (Piraz-

zoli, 1996).

3.2.2. Cuitzmala notches

A series of notches cut into volcanic breccia

(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

this area vary from SW to SE during the year. The

shore here is moderately exposed to the NW, where

waves regularly exceed high-tide level and may in-

crease the elevation of the marine notches (Pirazzoli,

1996). Marine notches here are cut on near vertical

cliff faces (6–8 m high), and their retreat points reach

elevations ca. 4.5 to 4.7 m amsl (Fig. 4). We identified

other indicators of emergent shorelines in this area,

such as sea caves and a sea arch cut in volcanic

bedrock at elevations ca. 4.5 m. Several breccia

boulders (about 8� 6� 6 m) along the shore showed

an indented surface at ca. 1.5 to 2.0 m amsl that is

most probably a marine notch (Table 1).

3.2.3. El Tecuan notches

We recognized other notches along the Jalisco

coast. At a highly exposed site at El Tecuan, a

spectacular abrasion notch that is cut into breccia

M.T. Ramırez-Herrera et al. / G

shows double erosional benches and honeycombs on

the cliff profile. Although this notch reaches an

elevation of ca. 3.0 to 3.5 m above mean high tide

level, it is highly exposed to wave action; and local

accounts suggest that the highest tides reach the notch

and that breaking waves occasionally cover the ero-

sional benches and abrasion platform (Table 1).

3.2.4. Benito Juarez notches

At Benito Juarez, emerged notches are cut into

breccia at 1.0 m amsl. Tidal range at this locality is ca.

0.5 m, indicating that this notch has emerged. About

500 m seaward from the notch, several island stacks

also show notches and abrasion platforms emerged

above mean sea level (we were unable to measure the

island notches due to strong wave action). Northwest

of Benito Juarez, at the mouth of the Arroyo Seco

River, an abrasion platform and notches are cut into

andesites about 1 m amsl (Table 1).

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. Ramırez-Herrera et al. / Geomorphology 58 (2004) 291–304298

eomorphology 58 (2004) 291–304 299

3.2.5. Punta Farallon notch

At Punta Farallon, a well-developed notch with a

wave-cut platform at its base is cut into conglomerate

along a 20-m-high cliff face. We also observed a

modern notch at the foot of a paleowave-cut platform

at ca. 1.0 m above mean tide level. The coast at Punta

Farallon is very exposed, and the height of the

emerged marine notch (ca. 4.0 m) probably differed

from mean sea level at the time it was cut (Table 1).

Unfortunately, the majority of these notches and

wave-cut platforms are barren of organic material

for radiocarbon dating. Advanced dating techniques,

such as the use of cosmogenic isotope dating, should

be applied here in order to determine exposure ages of

these features.

3.3. Radiocarbon dating of Jalisco marine notches

Fossil intertidal barnacles, oyster shells, and red

algae (Balanus identified by Dr. A. Garcıa Cubas and

M.T. Ramırez-Herrera et al. / G

Fig. 5. Elevations of the retreat points of the notches along the Jalisco coa

segments. Radiocarbon dates for fossil organisms found on Cuitzmala (C)

(Post 0 year BP, or ca. AD 1959). Symbols: F = Punta Farallon, C =Cuit

masl =meters above sea level. Graphic indicates average regional: MSL=

highest high water level (for site location, see Fig. 1).

Dr. M. Reguero, UNAM; and red algae identified by

Dr. S. Tudhope from the University of Edinburgh)

were collected at the base of the Barra de Navidad

notch and inside the Cuitzmala notch at elevations

corresponding to former low and mean tide level,

respectively. All barnacles, oyster shells, and algae

were clearly located above mean sea level at emerged

notches (Fig. 5).

The AMS radiocarbon age for the red algae sample

collected at Cuitzmala is 1262F 51 14C years BP (cal.

AD 660–940) (AA-22193), suggesting that the emer-

gence of the Cuitzmala coast occurred at about that

time.

Shell samples from Barra de Navidad were ana-

lyzed for 14C activity using standard methods. The

13C/12C corrected age (Beta Analytic, Beta-119880)

obtained from the oyster shells sample at Barra de

Navidad is 109% of modern (Post 0 year BP, ca. AD

1959). Because barnacles collected from the same site

were encrusted on top of oyster shells, we assumed

st and sea level. Elevation uncertainty limits are marked by vertical

are 1262F 51 years BP, Barra de Navidad (BN) 109% of modern

zmala, BJ =Benito Juarez, T =El Tecuan, BN=Barra de Navidad;

mean sea level, MHWL=mean high water level, MHHWL=mean

M.T. Ramırez-Herrera et al. / Geomorphology 58 (2004) 291–304300

that the barnacles and shells were the same age.

However, the vertical zonation of barnacles indicates

that their living conditions can extend above the

maximum tidal range, and in most cases indicate

super-zero elevations. This helps explain the anoma-

lous radiocarbon date at Barra de Navidad.

4. Mechanisms of coastal emergence

4.1. Holocene tectonic uplift

The Cuitzmala radiocarbon age and marine notch

elevation indicate a relative sea-level fall on this

segment of the Jalisco coast during the last 1300

years BP. The global history of relative sea-level

change indicates that sea level has been at its present

level since 6 ka (Pirazzoli, 1996). Along the northern

coast of Jalisco, sea level was near or at present

height by 3600–1500 years BP (Curray et al., 1969),

and no major isostatic adjustments and climate

changes have been reported for the area for that

period of time. Two mechanisms might be involved

in the emergence of the Jalisco marine notches

intermittently exposed along 40 km of coast: (i)

rapid uplift produced by large-magnitude, late-Holo-

cene earthquakes generated on the active plate

boundary, and/or (ii) long-term uplift of the coast

produced by the permanent deformation of the over-

riding plate because of the subduction of the Rivera

Plate (Fig. 2). We discuss these two mechanisms

below.

First, rapid uplift movements might be coseismic

because the preservation of in situ sublittoral and

midlittoral shells, barnacles, and algae in the supra-

littoral zone would be enhanced by a very rapid,

relative sea-level change (Pirazzoli, 1996). Intertidal

organisms, red algae, were found in their growth

position and well preserved on the Cuitzmala notch.

The preservation of these organisms may be due to

rapid uplift of the coast associated with shallow, large-

magnitude earthquakes located close to the coast.

Similar coastal features produced by coseismic uplifts

have been observed in other parts of the world along

active subducting plate margins: for example, New

Zealand (Berryman et al., 1989; Ota et al., 1991),

Japan (Kawana and Pirazzoli, 1985), and Alaska

(Plafker and Rubin, 1967).

Alternatively, long-term deformation not directly

associated with earthquakes might also be a mech-

anism for coastal uplift in Jalisco. Elastic strain

deformation can be accumulated during the inter-

seismic period causing upward flexing of much of

the accretionary prism and adjacent local coastal

areas. However, this strain would be released dur-

ing the subsequent earthquake, producing abrupt

coseismic submergence on coastal areas. This sim-

ple model of interseimic and coseismic deformation

has been used to demonstrate the elastic behavior

of the overriding lithospheric plate on the Casca-

dian subduction zone where the Juan de Fuca

oceanic Plate underthrusts and where paleoseismic

records report coseismic subsidence (Atwater,

1992).

Rapid uplift might partially explain coastal emer-

gence evident by the Cuitzmala notches. Radiocarbon

dating of the ca. 4.5-m elevated Cuitzmala marine

notch indicates the time of uplift, inferred to be rapid

because of the presence of well-preserved marine

organisms found in their growth position. Uplift might

have occurred during a large-magnitude earthquake at

1262F 51 years BP (cal. AD 660–940). Other

emerged notches, exposed in the study region at

different elevations above mean sea level (ca. 1,

1.5–2, and 4 m) might have a similar history of

sudden uplifts associated with earthquakes. Unfortu-

nately, most of these features were barren of datable

material, and no ages could be used to determine

uplift rates. The absence of intertidal organisms on

these notches raises the question regarding whether a

rapid or a slow uplift raised these features above mean

sea level.

On the other hand, the elevation of the Cuitzmala

notch at 4.5 m amsl could be explained by a large-

magnitude shallow earthquake or a combination of a

series of shallow earthquakes located near the coast

and/or long-term tectonic uplift. None of these

hypotheses can be proven at this point because

historical records of earthquakes in this area do

not go back far enough to prove or disprove these

hypotheses.

The Barra de Navidad notch, elevated at ca. 0.6–

0.9 m, shows an anomalous (109% of modern)

radiocarbon age. According to this date, this shoreline

could have emerged a few decades before or after AD

1959. This date could as well correspond to an

M.T. Ramırez-Herrera et al. / Geomorphology 58 (2004) 291–304 301

earthquake that occurred in the 19th century. The

historical record indicates the occurrence of other

larger earthquakes in the 19th century (1806, 1818,

1837, 1875, 1900), nevertheless, the effects of these

earthquakes on the coast have not been documented.

However, the vertical zonation of barnacles indicates

that their living conditions can extend above the

maximum tidal range. This might be a probable

reason for the anomalous modern radiocarbon age of

the Barra de Navidad raised notch.

The spatial-height distribution of the raised notches

along the Jalisco coast illustrated in Fig. 5 suggests an

anomalous non-uniform spatial arrangement that

might be caused by differential uplift along the coast.

This non-uniform distribution could be related with

the presence of faults that make up the blocky

structure of the Jalisco megablock (Lugo-Hubp and

Ortiz-Perez, 1980; Ferrari and Rosas-Elguera, 2000;

Ferrari et al., 2000).

4.2. Historic interseismic and coseismic observations

Observations of the historic earthquakes of 1932

and 1995 showed that coseismic subsidence was

produced along the Jalisco coast (Cumming, 1933;

Melbourne et al., 1997). After the 1932 earthquake,

Cumming’s (1933) report indicates local coastal sub-

sidence of about 40–75 cm near Manzanillo, south of

Jalisco, produced by the1932 earthquakes. The arrival

of a tsunami was also reported by locals just after the

earthquake (Cumming, 1933).

In October 1995, initial coseismic coastal subsi-

dence (up to 20 cm at Chamela) was measured using

geodetic-GPS and tide gauge data (Hutton et al.,

1997). Measurements made barely 2 months after the

earthquake demonstrate a period of rapid uplift, with

some sites recovering almost the entire coseismic

drop (Hutton et al., 1997). Thus, a detailed GPS

survey before, during, and after the 1995 (Mw= 8)

Colima earthquake showed postseismic motion rang-

ing from 18% to 84% of the coseismic slip with

postseismic displacements decaying rapidly (Hutton

et al., 1998). GPS results indicate that almost 3 years

after the earthquake, the coastal coseismic subsi-

dence reported had been nearly completely recovered

during the postseismic and interseismic periods (Hut-

ton et al., 1997, 1998; O. Sanchez, UNAM, personal

communication).

5. Discussion

Studies at other plate boundaries around the Pacific

Rim showed that Holocene- and Pleistocene-emerged

shorelines recorded both eustatic and coseismic rela-

tive sea-level changes (Plafker, 1969; Matsuda et al.,

1978; Ota et al., 1991 Berryman, 1993). For example,

the Papua New Guinea coastline, which has one of the

most detailed records of emergent shorelines, shows

that meter-scale uplifts represent great earthquakes

(Ota and Chappell, 1996). Large coseismic uplifts

can be identified from the geomorphic record; how-

ever, identifying less than 1-m uplifts from the geo-

morphic record is not possible because the effects of

an instantaneous movement cannot be distinguish

clearly from those events occurring over decades or

centuries.

We believe that long-term slow uplift might also

have contributed to the total net emergence of former

Holocene shorelines on the Jalisco coast, although

distinguishing the amount of coseismic uplift from

that of the slow uplift produced during long-term

periods is not possible. The mechanism that explains

uplift produced during interseismic cycles and during

the long-term period on the coastal area, as discussed

above, can be related to the shortening of the accre-

tionary prism and a flexural uplift of the overriding

plate. Although the accretionary prism on this portion

of the margin is relatively thin, then the deformation

occurring on the overriding plate would be small also;

the possibility of an increase of buoyancy of the

subducting material might lead to an uplift of the

plate margin in the long term (Buiter et al., 2001).

Moreover, emerged shorelines clearly experienced

overall uplift; and we found no evidence of a pro-

longed interseismic and of long-term subsidence.

Finally, although coseismic centimeter-scale coast-

al subsidence was reported for the 1932 and 1995

earthquakes (Cumming, 1933; Hutton et al., 1997,

1998; Kostoglodov et al., 1997), the rapid postseismic

and interseismic recovery indicates a general trend to

tectonic uplift during long-term periods. Apparently,

interseismic and long-term uplift has been an impor-

tant, if not the most important factor, in the emergence

of the Jalisco coast. We propose a model for the

Jalisco coast in which (based on geomorphic, histor-

ical, and GPS observations) the coast subsides during

large earthquakes located offshore. This coseismic

M.T. Ramırez-Herrera et al. / Geomorphology 58 (2004) 291–304302

subsidence is rapidly recovered during a short period

of about 1–5 years. Then, during interseismic and

long-term periods, the coast is uplifted slowly by

upward flexing produced by the elastic deformation

of the overriding plate. This amount of long-term

uplift, although slow, does not seem to be completely

lost during coseismic subsidence produced by subse-

quent earthquake events. The possibility of large-

magnitude earthquakes producing tectonic uplift is

not excluded from this model. If earthquakes are

shallow and close to the coast, they are capable of

producing coastal uplift. Large coseismic uplift has

occurred in adjacent coastal segments of the subduc-

tion zone. The 1985 earthquake produced up to 1.10

m of coastal uplift in the adjacent Michoacan coastal

segment (Bodin and Klinger, 1986). However, we

found no further evidence of sudden uplift apart from

in situ intertidal organisms present on the Cuitzmala

notch and the history of cosesismic uplifts that

occurred on the adjacent coast of Michoacan. There-

fore, at this point, coseismic uplift cannot be proven.

However, while it is unfortunate that we have only two

radiocarbon dates, these dates for the Cuitzmala and

Barra de Navidad notches are consistent with their

elevations, showing a younger age for the lower Barra

de Navidad notch and an older age for the higher

Cuitzmala paleoshoreline. The record of emerged

coastal notches suggests a cumulative long-term slow

uplift rate estimated at 3 mm/year.

We have raised questions and suggested possible

explanations for Holocene uplift of the Jalisco coast in

SW Mexico. However, further work should be per-

formed on the absolute dating of emerged shorelines

to determine uplift rates and perhaps establish earth-

quake recurrence intervals for the Jalisco coast. The

lack of material suitable for radiocarbon dating on

most of these notches warrant further work along with

the introduction of other dating techniques on abraded

landforms. Cosmogenic isotopes might prove to be a

valuable tool in providing exposure ages for abrasion

platforms and marine terraces (Hancock et al., 1999;

Perg et al., 2001).

6. Conclusions

Intertidal notch elevations and their radiocarbon

age, first ever reported for this sector of the Mexican

active plate margin, show a relative sea-level ‘‘fall’’

in Jalisco during the late Holocene (at least during

the last 1300 years BP). Emerged marine notches,

ranging from 1 to 4.5 m in elevation, record coastal

emergence since at least 1262F 51 years BP. We

suggest that Holocene coastal uplift, registered by

emerged marine notches, represents (i) slow tectonic

uplift during the long-term period, and/or (ii) prob-

ably rapid tectonic uplift caused by great shallow

earthquakes that might have occurred close to the

Jalisco coast.

If the history of relative sea-level change in Jalisco

is similar to histories on other northern Mexican

coasts and globally, most of the relative sea-level fall

that we measured would be caused by tectonic uplift

of the Jalisco coast.

Although offshore earthquakes may produce local

centimeter-scale coastal subsidence along the Jalisco

coast, complete postseismic and interseismic recovery

appears to follow (Hutton et al., 1997, 1998) leading

to general coastal uplift. Slow long-term uplift is

considered to be the most important factor to the total

net vertical motion, uplift, along the Jalisco coast. We

have found no evidence of coastal interseismic and

long-term subsidence.

Judging by the morphologic evidence and the

radiocarbon date of the 4.5-m elevation Holocene

notches, we propose a Holocene long-term trend for

tectonic uplift at estimated rates f 3 mm/year along

the Jalisco coast. Further work on the precise dating of

emerged notches and terraces is necessary to deter-

mine uplift rates in this area.

Acknowledgements

This research was supported by California State

University-Long Beach, UNAM, and CONACYT.

Mike Summerfield and Alan Nelson provided val-

uable comments on an earlier version of this manu-

script. Thanks to Steven Lefton for the helpful

comments on this manuscript that helped us to

improve the final version. We thank J. Zamorano, R.

Reyna, R. Gutierrez, E. Leon, and M. Ortiz for their

assistance in the field. O. Sanchez provided tide

ranges. The University of Arizona Radiocarbon

Laboratory and Beta Analytic produced radiocarbon

dates. Thanks to R.A. Mastron, N. Pinter, and P.G.

M.T. Ramırez-Herrera et al. / Geomorphology 58 (2004) 291–304 303

Silva for their valuable comments and reviews of this

manuscript.

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