Subsidence and Strike-slip Tectonism of the Upper Continental Slope off

Manzanillo, Mexico

William L. Bandy1, Francois Michaud2, Jacques Bourgois3, Thierry Calmus4, Jérôme

Dyment5, Carlos Mortera-Gutiérrez1, Jose Ortega-Ramirez1, Bernard Pontoise6, Jean-

Yves Royer5, Bertrand Sichler7, Marc Sosson8, Mario Rebolledo-Vieyra1, Florence

Bigot-Cormier8, Oscar Diaz1, Angel D. Hurtado-Artunduaga1, Guillermo Pardo1 and

Corrine Trouillard-Perrot9

1Instituto de Geofisica, Universidad Nacional Autonoma de México, México D.F.,

México. 2Géosciences Azur, Université Pierre-et-Marie-Curie, Villefranche-sur-Mer,

France. 3Centre National de la Recherche Scientifique, Université Pierre-et-Marie-Curie, Paris,

France. 4Instituto de Geologia, Universidad Nacional Autonoma de México, Hermosillo, Sonora,

Mexíco. 5CNRS, Plouzané, France. 6 IRD-LGTE, Université Pierre-et-Marie-Curie, Paris, France. 7IFREMER, Plouzané, France. 8Géosciences Azur, Université de Nice-Sophia Antipolis, France. 9 Université de Bretagne Occidentale, Plouzané, France.

ABSTRACT

The direction of convergence between the Rivera and North American plates

becomes progressively more oblique (in a counter-clockwise sense as measured relative

to the trench-normal direction) northwestward along the Jalisco Subduction zone. By

analogy to other subduction zones, the forces resulting from this distribution of

convergence directions are expected to produce a NW moving, fore arc sliver and a NW-

SE stretching of the fore arc area. Also, a series of roughly arc parallel strike slip faults

may form in the fore arc area, both onshore and offshore, as is observed in the Aleutian

Arc.

1

In the Jalisco subduction zone, the Jalisco Block has been proposed to represent

such a fore arc sliver. However, this proposal has encountered one major problem.

Namely, right-lateral strike slip faulting within the fore arc sliver, and between the fore

arc sliver and the North American plate, should be observed. However, evidence for the

expected right-lateral strike slip faulting is sparse. Some evidence for right-lateral strike-

slip faulting along the Jalisco Block-North American plate boundary (the Tepic-Zacoalco

rift system) has been reported, although some disagreement exists. Right-lateral strike-

slip faulting has also been reported within the interior of the Jalisco Block and in the

southern Colima rift, which forms the SE boundary of the Jalisco block.

Multi-channel seismic reflection data was collected in the offshore area of the

Jalisco Subduction zone off Manzanillo in April 2002 during the FAMEX campaign of

the N/O L’Atalante. This three-fold data (25 meter CDP spacing) was acquired

employing 300 in3 GI guns tuned in harmonic mode and a hydrophone streamer with 6

hydrophone groups (48 hydrophones per group) spaced 50 meters apart. These data

provide additional evidence for recent strike-slip motion within the fore arc region of the

Jalisco Subduction zone. This faulting offsets right-laterally a prominent horst block

within the southern Colima rift, from which we conclude that the sense of motion along

the faulting is dextral. These data also provide additional evidence for recent subsidence

within the area offshore of Manzanillo, Mexico as has been proposed.

INTRODUCTION

The northern end of the Middle America trench (Figure 1), where the Rivera plate

subducts beneath the North American plate, is commonly termed the Jalisco subduction

zone (e.g., Bandy et al., 1999). Plate motion studies (e.g. Bandy, 1992; Lonsdale, 1995;

Kostoglodov and Bandy, 1995; DeMets and Wilson, 1997; Bandy et al., 1997, 2000;

DeMets and Traylen, 2000) indicate that the direction of convergence between the Rivera

and North American plate becomes progressively more oblique (in a counter-clockwise

sense as measured relative to the trench-normal direction) northwestward along the

Jalisco subduction zone. By analogy to other subduction zones, the forces resulting from

this distribution of convergence directions is expected to both produce a NW moving,

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forearc sliver and a NW-SE stretching of the forearc area (Fitch, 1972; Kimura, 1986;

Jarrard, 1986; Avé Lallemant and Guth, 1990; Pinet and Cobbold, 1992; Beck, 1991;

McCaffrey, 1991, 1992; Avé Lallemant, 1996). In the Jalisco subduction zone, the

Jalisco block has been proposed to be such a forearc sliver (Serpa et al., 1989), that is

moving slowly (Bandy and Pardo, 1994) to the NW, relative to the North American plate

(Luhr et al., 1985), and which is undergoing NW-SE oriented extension (Bandy, 1992;

Maillol and Bandy, 1994; Kostoglodov and Bandy, 1995; Maillol et al., 1997; Bandy et

al., 2001).

The proposal of forearc slivering at the Jalisco subduction zone has encountered

one major problem. Namely, the evidence for the expected right-lateral strike slip

faulting within the Jalisco block, and between the Jalisco block and the North American

plate, is sparse. Some right-lateral strike-slip faulting has been reported (Nieto-Obregon

et al., 1985; Allan et al., 1991; Michaud et al., 1991) along the Jalisco block-North

American boundary (the Tepic-Zacoalco rift), although some disagreement exists

(Johnson and Harrison, 1989; Ferrari et al., 1994). Right-lateral strike-slip faulting has

also been reported within the interior of the Jalisco block (Lange and Carmichael, 1990;

Righter et al., 1995; Carmichael et al., 1996; Maillol et al., 1997), although uncertainty

exists as to the ages of these faults (Maillol et al., 1997; Righter and Rosas-Elguera,

2001). Lastly, right-lateral strike slip faulting has been reported in the Southern Colima

rift (Serpa et al., 1992; Ortíz, 1993: Bougois et al., 1988), which forms the SE boundary

of the Jalisco block.

In this study, we present a newly collected, multichannel, seismic reflection

profile in the offshore area of the Jalisco block near Manzanillo that provides further

evidence for recent strike-slip deformation within the forearc region of the Jalisco

subduction zone. This faulting offsets right-laterally a prominent horst block within the

Southern Colima rift, from which we conclude that the sense of motion along the faulting

is dextral. These data also provide additional evidence for recent subsidence within the

area offshore of Manzanillo, Mexico (Mercier de Lépinay et al, 1997; Ramírez-Herrera

and Urrutia-Fucugauchi, 1999).

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TECTONIC SETTING

The study area (Figure 1) lies within the Southern Colima rift of southwest

Mexico, which is part of a prominent triple rift system comprised by the Tepic-Zacoalco,

Colima and Chapala/Citala rifts (e.g. Allan, 1986; Barrier et al., 1990; Johnson and

Harrison, 1990; Allan et al., 1991; Garduño-Monroy and Tibaldi, 1991; Michaud et al.,

1994). The Southern Colima rift extends from the La Cumbre fault zone, located just

south of the city of Colima, to the Middle America trench (Figure 2). The La Cumbre

fault zone is a topographically high region within the Colima rift. It consists of folded

and faulted Cretaceous limestones, marls and volcanoclastics (Michaud et al., 1989),

which are overprinted by north-northwest oriented high-angle faults that are interpreted

as having formed within a major dextral transpressive fault system (Serpa et al., 1992).

This fault system is a major structural boundary within the Colima rift: north of this fault

system the major structural trends are oriented north-south, whereas to the south, they are

orientated NE-SW. This shift in structural trends is quite abrupt, occurring over a

distance of less than 10 km. Earthquake studies (Pacheco et al., 1999, 2003; Suárez et al.,

1994) also indicate a change in the present day extensional stress axis from east-west in

the northern Colima rift to NW-SE in the southern Colima rift. Within the La Cumbre

fault zone, the structural trends are oriented NW-SE. The age of the strike slip faulting is

unknown, although the epicentres of some small earthquakes have been located along

these faults (Jiménez et al., 1996 as reported in Garduño et al., 1998).

Onshore, the Southern Colima rift is comprised of two major, NE-SW oriented

grabens separated by a highly eroded horst, the Manzanillo horst of Pacheco et al. (2003).

Gravity data (Bandy et al., 1993) from the offshore and coastal area in conjunction with

bathymetric data indicate that this morphology extends offshore to the Middle America

Trench.

The southeastern graben, herein called the Tecoman graben, is the larger of the

two grabens. It is marked by a broad coastal alluvial plain, on which lies the city of

Tecoman. Although no subsurface data is available, the graben may contain up to 9 km

of sediments (Bandy et al., 1993; Urrutia-Fucugauchi et al., 1999) in the coastal area.

Seismic reflection data collected within the offshore area of the Tecoman graben

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(Michaud et al., 1990; Khutorskoy et al., 1994) indicate that the graben contains a thick

sediment infill that is disrupted by large offset normal faults in its NW part. These

sediments may correspond, at least in the lower part of the sediment section, to the upper

Miocene to lower Pliocene siltstones observed during submersible dives on the lower

inner trench slope within the graben (Mercier de Lépinay et al., 1997). The basement

underlying the sediments was not clearly imaged in the seismic data; however, it most

likely consists of pre-Eocene (approximately 56 Ma) plutonic rocks (granodiorites and

gabbros) that outcrop both on the flanks of the graben and near the trench axis.

Of particular importance to the present study is the presence of a deep, fault

bounded trough, herein termed the Tecoman trough, which forms the northwest half of

the offshore part of the Tecoman graben. This trough does not extend onshore, but

instead terminates abruptly against the onshore part of the Manzanillo horst (Bourgois et

al., 1988). In other words, the offshore part of the Manzanillo horst is offset right-

laterally relative to its onshore counterpart. Of further interest is the presence of three

submarine canyons within the Tecoman graben. Two of these canyons coincide with the

two major rivers flowing within the Tecoman graben onshore; namely the Rio Armeria

and the Rio Coahuayana that flow along the NW and SE margin of the graben,

respectively. These submarine canyons are, respectively, the Armeria and Coahuayana

canyons (Bandy, 1992). Offshore, the Armeria canyon is located within the Tecoman

trough, along its southeast margin. Thus, the northwest margin of the onshore Tecoman

graben coincides with the southeast margin of the offshore Tecoman trough. Seismic

reflection data (Michaud et al., 1990; Khutorskoy et al., 1994) indicate little or no

sediment infill of these two canyons, suggesting that canyons are presently linked to the

onshore rivers and that erosion is occurring within these canyons (i.e. these canyons are

presently active). In contrast, the third canyon, the Cuyutlan canyon (Bandy, 1992),

which lies within the Tecoman trough adjacent to the Manzanillo horst, is a broad

sediment filled canyon as is not presently associated with any major river. The surface

deposits within the canyon do not show any sign of recent erosion, suggesting that the

canyon is presently inactive (i.e. the canyon has somehow been cut off from the river

system which was responsible for its formation).

5

The northwestern graben comprising the southern Colima rift, herein called the

Manzanillo graben, contains the Bahia de Manzanillo. The Sierra Perote bounds this

graben to the NW. Onshore, the graben contains little sediment infill, except along the

coast near Manzanillo and just SE of Barra de Navidad. Offshore, gravity modeling

(Bandy et al., 1993) suggests the presence of up to 8 km of sediment infill. Ross and

Shor (1965) report on a seismic reflection profile located offshore along the NW margin

of the Manzanillo graben (unfortunately the profile was not shown in their paper). They

describe the shelf sediments observed on the record as being folded and faulted, perhaps

reflecting the offshore continuation of the faulting noted in the adjacent onshore area by

Garduño et al. (1998).

The Manzanillo horst is oriented NE-SW. It terminates against the NW-SE

oriented Sierra Manantlan to the northeast and the Middle America Trench to the

southwest. The city of Manzanillo lies on its northwest flank. The surface of this horst is

comprised of Cretaceous granites with minor exposures of Cretaceous limestone. These

granites have been observed in submersible studies to extend to the trench axis (Mercier

de Lépinay et al., 1997). The geology of the offshore part of the Manzanillo horst is

poorly understood. An attempt to core the seaward slope of the horst (Ross and Shor,

1965) resulted in a bent core nose indicating that the surface of the horst is comprised, at

least in part of ‘rock’, probably either lithified sediments or perhaps surface exposures of

the granitic rocks observed to comprise the horst onshore.

Several structural features cut across the Manzanillo horst. In the onshore region,

Garduño et al. (1998) observed on Landsat images a series of NE-SW oriented lineations

that cut across the horst and the Manzanillo graben, which they termed the Tamazula

fault zone (Figure 3). Also, Ortíz et al. (1993) reported the presence of a WNW-ESE

oriented, 17 km long, right-lateral strike slip fault on the SE flank of the horst near the

town of Armería. In the offshore area, the horst is cut by a NW-SE oriented, sediment

filled (Ross and Shor, 1965), bathymetric depression, across which the horst is offset in a

right lateral sense.

The area of the Manzanillo horst and Manzanillo graben has received a lot of

recent attention due to the recent seismic activity in this area. A local seismic network

(RESCO) installed near the area of the southern Colima rift in 1985 has recorded

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numerous small events, the majority of which have occurred in the area of the Manzanillo

horst and the Manzanillo graben (Castellanos and Jiménez, 1995; Garduño et al., 1998;

Nuñez-Cornú et al., 2002). Further, a large, Mw=5.3, normal event (Figure 3) recently

occurred (6 March 2000) in the center of the Manzanillo horst (Zobin et al., 2000;

Pacheco et al., 2003). This event conclusively demonstrates that NW-SE oriented

extension is occurring in the southern Colima rift. Lastly, the hypocenter of the great

October 9, 1995 Jalisco earthquake (Mw 8) lies under the southeast flank of the

Manzanillo horst in the offshore area (Courboulex et al., 1997; Melbourne et al., 1997;

Pacheco et al., 1997; Zobin, 1997; Escobedo et al., 1998). Although the main event was

a subduction related thrust event, it activated normal faults along the Rio Marabasco,

which flows along the northwest margin of the Manzanillo graben, and on the northwest

flank of the Manzanillo horst near Manzanillo (Garduño et al., 1998).

DATA and METHODS

The 2-D seismic reflection data used in this study were collected in April 2002

during the FAMEX campaign of the N/O L’Atalante. The profile location is illustrated in

Figure 3. Three-fold data (25 meter CDP spacing) was acquired employing 300 in3 GI

guns tuned in harmonic mode and a hydrophone streamer with 6 hydrophone groups (48

hydrophones per group) spaced 50 meters apart. The data was sampled at 4 msec and

recorded in SEG-Y format.

The data was processed using the following processing sequence:

1. Geometry assignment

2. Spherical divergence correction

3. 10-70 Hz Bandpass filter with a high and low rolloff rate of 18 dB/octave

4. NMO correction

5. Stack

6. Gazdag phase-shift migration.

Instantaneous phase displays were also made for selected portions of the migrated data.

7

PROFILE INTERPRETATION

The seismic reflection profile (Figure 4) illustrates that the near-surface

sedimentation on the continental shelf off Manzanillo, Mexico, produces seismic

sequences typical of continental shelf/slope areas throughout the world. Specifically, the

sequences located on the landward side of the profile display seismic characteristics

typical of nearshore/inner neritic marine environments; whereas, the seismic reflection

characteristics on the seaward end of the profile are typical of those formed by deposition

in outer Neritic to upper bathyal marine environments. However, in the Manzanillo slope

area, this progression is disrupted in the central part of the upper slope, from CMP 90 to

CMP 540, by a 22 km wide zone of tectonism that appears to be related to strike-slip

faulting. In the following, the undisrupted zones in the upper continental slope to either

side of the tectonized zone will be described first, followed by a description of the

tectonized zone.

Landward Upper Continental Slope Sequences

The undisrupted area of the upper continental slope landward of the tectonized

zone imaged by the seismic reflection profile lies between CMP 5 and CMP 90 (Figure

5). Three seismic sequences are observed on the profile. The lower sequence (sequence

3) is bounded above by an erosional unconformity (Horizon C). The internal reflectors of

this sequence exhibit low amplitudes, a low vertical spatial frequency, and are

discontinuous. Horizon C is interpreted as an erosional unconformity based on the

truncated reflectors underlying this horizon between CMP’s 5 and 30, and based on two

apparent channel cut-and-fill sequence located below this horizon; the first located

between CMP’s 27 to 44 and the second between CMP’s 76 to 88. An abrupt change in

the dip of Horizon C occurs at CMP 22; between CMP 5 and CMP 22 the slope is 9°,

whereas the slope is 13° between CMP 22 and CMP 90 (in this paper, all depth

calculations employ a velocity of 1.5 and 2.0 km/sec for the water and sediment units,

respectively). The lithology of this sequence is unknown. As the onshore part of the

Manzanillo graben contains little sediment, and since this part of the seismic profile is

8

located close to the shore, sequence 3 may correlate with the Cretaceous granites that

outcrop onshore. However, the presence of internal reflections suggests a sedimentary

composition for this sequence.

Sequence 2 is bounded by horizons B and C. This sequence exhibits a seaward

thickening, wedge geometry. The internal reflectors exhibit variable amplitudes, low

continuity and a chaotic appearance typical of a deltaic/coastal, high-energy, depositional

environment. Between CMP 5 and CMP 35, the internal reflectors exhibit toplap against

Horizon B, indicating that Horizon B was a surface of non-deposition/erosion in the area

of the toplap. Horizon B has a fairly constant slope of 8° between CMP 5 and CMP 90.

Sequence 1 is bounded by Horizon A (the seafloor reflection) and Horizon B.

Like sequence 2, sequence 1 also exhibits a seaward thickening wedge geometry.

However, unlike sequence 1, the internal reflectors are divergent, and exhibit high

amplitude, high continuity, and high spatial frequencies typical of a shallow-water, low-

energy, depositional environment (inner neritic marine environment). The internal

reflectors at the base of this sequence diverge seaward, having a slope somewhat less

than that of Horizon B. In contrast, the uppermost reflections are parallel, having a slope

of about 1°. The lower internal reflectors of sequence 1 onlap Horizon B; the onlap

progresses landward from the base of the sequence upwards. The first observed

occurrence of onlap of the internal reflectors onto Horizon B occurs at CMP 68, at a

depth of 1140 ms TWTT (two-way travel time). The upper reflections within sequence 1

are truncated at the seafloor, indicating recent erosion at the seafloor. The seafloor dips

6° between CMP 5 and 40, and 3° between CMP 40 and 87. The reflection

characteristics indicate that the sediments of sequence 2 were deposited in a deeper-

water, lower energy environment than those of sequence 1.

No clear evidence for recent faulting is observed on the profile in this area.

However, an older fault system (FS1) is observed in the lower section between CMP’s 42

and 80. This faulting appears to be part of a positive flower structure, with the main

strike slip fault in the underlying basement being located near CMP 48. The faults

terminate at or below Horizon B were they are associated with an increased thickness of

the sediment unit just below Horizon B (this is clearly observed at CMP 57). Thus, the

last period of faulting within FS1 occurred at the time of formation of Horizon B. It may

9

be argued that the abrupt change in dip of the reflectors at CMP 46 is indicative of recent

movement along FS1; however, the reflectors show no breaks in continuity from which

one can clearly conclude that FS1 is presently active.

Seaward Upper Continental Slope Sequences

The undisrupted part of the upper continental slope SW of the tectonized zone lies

seaward of CMP 545 where the seafloor reflector is smooth and continuous. This area

consists of two main sequences (Figure 6). The upper sequence (OCSS-1) consists of

low amplitude, parallel, semi-continuous internal reflectors that are typical of an outer

neritic/upper bathyal marine environment. The continuity of these reflectors is disrupted

in several areas by large channel cut and fill sequences that are embedded within the

sequence, for example at CMP 600 and at CMP 650. Sequence 0CSS-1 overlies a

sequence (OCSS-2) of high amplitude discontinuous internal reflectors. The horizon

separating these two sequences is deeply eroded, as is clearly observed between CMP’s

545 and 570.

Although the seafloor indicates no recent faulting within this area, several normal

faults are observed to disrupt sequence OCSS-2 and perhaps the lower part of sequence

OCSS-1 (i.e., at CMP 580 and between CMP 680 to CMP 700).

Tectonized zone

The zone of recent tectonism extends from CMP 90 to CMP 542 and is marked by

abrupt offsets and undulations of the seafloor reflector. This zone is comprised of two

major morphotectonic units, namely a highly disrupted sedimentary basin to the NE and

an anticlinal zone to the SW.

Landward upper continental slope. The sedimentary sequences of the upper

continental slope extend into the tectonized zone, between CMP 90 and CMP 190 (Figure

7). The sediment section above Horizon C thins seaward and faulting becomes more

prevalent as one approaches the mid-slope basin. The internal reflectors of the upper part

10

of sequence 3 terminate against the seafloor reflector indicating that the thinning of the

sediment section is primarily due to recent erosion. Horizon B can be traced with

confidence only to CMP 108, whereas, Horizon C extends seaward to CMP 182 (and

perhaps to CMP 195), at which point the mid-slope basin begins.

Three fault systems are observed to disrupt the sediment section in the area

landward of the mid-slope basin. The first fault system (FS2) is centered on CMP 96

where a reversal in dip of Horizon C occurs. Horizon B also exhibits a reversal in dip

within FS1, although at a location slightly more seaward. The fault and reflection

geometry is characteristic of a negative-flower structure produced by motion along a

deep-seated strike slip fault. Breaks in reflector continuity within FS2 occur up to the

middle of sequence 3 above which the reflections are bent, without breaks in continuity.

Thus, this fault system was active more recently than FS1, however it is not clear if this

system is presently active.

The second fault system (FS3) is centered on CMP 122 where an offset in

Horizon C is observed. Breaks in the continuity of all reflectors lying between Horizon C

and the seafloor occur across the seaward most fault of this system, clearly indicating that

this fault is presently active. Although it is tempting to interpret this fault as a normal

fault, the breaks in reflector continuity indicate a vertical to seaward convex fault plane.

Further, a close inspection of Horizon C (Figure 8) clearly shows the ‘pop-up’ geometry,

and both normal and reverse offsets, of Horizon C that are characteristic of a positive

flower structure related to strike slip motion within the underlying basement. Thus, this

fault plane and bedding geometry is best interpreted as being part of a strike slip fault

system. The internal reflections within the upper part of Sequence 3 (1050 to 1150 ms)

between CMP 114 and 126 are dome shaped, and two reflectors are observed to onlap the

“domes” (the onlap occurs at two different reflectors). We interpret this geometric

pattern as indicating contemporaneous tectonic deformation and sedimentation at the time

of formation of these horizons.

It is conceivable that systems FS2 and FS3 are both linked to a single strike slip

fault within the underlying basement. If so, then the upwarping of Horizon C between

FS2 and FS3 suggests that a positive flower structure is associated with the combined

11

system. Unfortunately, employment of a larger sound source is needed to resolve this

possibility.

Third fault system, FS4, is centered on CMP 157 where another offset in Horizon

C occurs. Like in FS3, Horizon C again exhibits a ‘pop-up’ geometry characteristic of a

positive flower structure associated with a strike slip fault within the underlying

basement. It is unclear whether this fault system is presently active. Undulations in the

seafloor occur above FS4, however, these undulations may be produced by erosion

instead of faulting.

Mid-Slope Basin. The mid-slope basin (Figure 9) is 6.5 km wide, beginning at CMP

183, where Horizon C becomes highly disrupted, and extending to the prominent

subsurface anticline located at CMP 310. The seismic data images about 1 km (1 sec

TWTT) of the sediment section within the basin. The sediment section is most likely

thicker than 1 km as no clear basement reflection is observed.

The basin consists of four major seismic sequences that are best delineated on the

instantaneous phase display (Figure 10). The first sequence, B1, is the seaward extension

sequences 1 and 2. This sequence is underlain by Horizon C, which is traceable with

some degree of confidence to CMP 200, and perhaps to CMP 210. Horizon B has been

tentatively identified between CMP 182 and 197 based on the change from low amplitude

discontinuous reflections below the horizon to high amplitude, more continuous

reflectors above the horizon. Such a change is similar to that noted in the upper slope to

the NE. The internal reflections of sequence B1 terminate against the lower boundary of

sequence B2, which we interpret as indicating that the boundary between B1 and B2 is an

erosional unconformity. This is also best observed on the instantaneous phase display.

Sequence B2 is located in the basin between the seafloor and Horizon B2. The

sequence geometry is fill-shaped, and the internal reflections exhibit, for the most part,

low amplitudes and variable continuity. The internal reflectors onlap sequence B1 to the

NE and the buried anticline to the SW. A local erosional unconformity, horizon B1-1

separates sequence B2 into an upper and lower part. The lower part is best observed on

the instantaneous phase display. It exhibits a landward diverging, wedge geometry, and

is interpreted as a slope fill sequence. The upper part exhibits characteristics typical of a

12

complex channel cut and fill sequence. Thus, sedimentation appears to have first

occurred in a slowly subsiding basin, the maximum subsidence occurring on the

landward side. This was followed by the formation of a channel cut and fill system,

probably part of the present-day Manzanillo submarine canyon system.

Sequence B3, forms the basement in the NE half of the mid slope basin, and is

bounded above by Horizon B4. The sequence consists of steeply seaward dipping, high

amplitude, continuous to discontinuous internal reflectors. The nature of the rocks

forming this sequence is unknown but most likely consists of layered sediments.

Sequence B4 consists of high amplitude, discontinuous, landward dipping

reflectors. These reflectors are highly disrupted and downlap against Horizon B4. Like

sequence B3, the nature of the rocks forming sequence B4 is unknown but most likely

consists of layered sediments.

The exact nature of Horizon B4 is uncertain. One interpretation is that it is an

unconformity formed by tilting of the sediment layers of sequence B3 followed by

deposition of the sediments of sequence B4. This interpretation is analogous to that of

seismic reflection data located within the northern Elat Basin, a pull-apart basin located

within the Dead Sea Rift (Ben-Avraham et al., 1979). There, steeply dipping reflectors

are found on the west side of the basin that are downnlapped by the sediments to the east.

Alternatively, Horizon B may be a fault surface across which two distinct geologic units

have been juxtaposed by strike slip movement along the fault.

The area of the mid-slope basin is highly disrupted by faulting. These faults

exhibit the flower structure geometry indicative of a stike-slip fault system. On the NE

margin, Horizon C is down-dropped across a series of faults. There, the faulting appears

to be associated with a strike slip system within which several of the individual faults

offset the seafloor reflector, indicating that this system is still active. On the SW margin

of the basin, the internal reflectors of sequence B3 are highly disrupted and the buried

anticline appears to have been formed by strike-slip faulting similar to anticlines formed

in strike-slip regimens throughout the world. However, unlike the faulting on the

landward margin of the basin, here none of the faults clearly cut the seafloor reflector.

Instead, they terminate at or below horizon B2. Thus, most of the fault activity on the

SW margin of the basin ceased at the time of formation of Horizon B2. Unfortunately,

13

the age of Horizon B2 is unknown, but it might be determinable in the future by seafloor

dredging in the anticlinal area to the SW.

The observed fault pattern in conjunction with the similarity of seismic sequences

3 and 4 to those observed in the Elat basin, lead us to conclude that the mid slope basin

was most likely formed as a pull-apart basin located within a regional strike-slip fault

system.

Anticlinal Zone of the Manzanillo Horst. The seafloor in the NE half of the

offshore part of the Manzanillo horst, between CMP 310 and CMP 545, exhibits a

hummocky geometry and is underlain by three anticlines (Figures 11a and 11b), the

landward most of which forms the seaward margin of the midslope basin. The locations

of the two anticlines to the NE are clearly delineated by the geometry of Horizon B2,

which extends from the mid-slope basin to CMP 382 where it is truncated by the seafloor

reflector. In this zone, the sequence above Horizon B2 are undisrupted, onlap both

anticlines and exhibit small channel cut and fill sequences. The internal reflectors below

Horizon B2 are highly disrupted by faulting. These faults terminate at of below Horizon

B2, indicating that the strike slip faulting in this area of the first two anticlines is

presently inactive. However, it might be argued that the undulations in the seafloor are

the result continued, but minor, tectonic activity.

The third anticline is centered at CMP 415 and is marked by a doming of the

seafloor. Horizon B2 appears to have been eroded in this area suggesting that at some

time in the past, this represented the highest part of the Manzanillo horst (at least along

this profile). The character of the seafloor reflector changes abrubtly at the NE flank of

the anticline. Specifically, at CMP 395 the fluctuations in the seafloor are of higher

frequency than over the other two anticlines and the seafloor reflector is offset in several

places. This seafloor character continues to CMP 542, at which point the seafloor

reflector again become smooth and continuous. We interpret this as evidence of recent

fault activity in this area.

Seaward Continental Slope. The subsurface anticlinal zone terminates abruptly at CMP

435. At this point the previously described sequences of the seaward upper continental

slope sequences appear; namely, a thick sediment section overlying an erosional surface

14

(Figure 12). However, between CMP’s 435 and 542, the seafloor reflector exhibits high

frequency undulations and its continuity is broken in several places. These breaks in

continuity and undulations of the seafloor reflections appear to be the results of recent

strike slip faulting. A particularly well-developed flower structure is present at CMP

520, which produces a doming of the seafloor. Again we interpret this to indicate the

presence of recently active strike-slip faulting.

DISCUSSION

Subsidence and an Age Estimate of Horizon B

Large-scale subsidence of the lower inner trench slope of the southern Jalisco

subduction zone during the Neogene has been documented by observations and sampling

made within the Tecoman Graben (and Manzanillo Trough) during deep-sea dives with

the submersible Nautile (Mercier de Lepinay et al., 1997). Specifically, an erosional

unconformity between Paleocene plutonic rocks and upper Miocene-lower Pliocene

marine sediments (deposited in upper middle bathyal environment now) was observed to

outcrop between depths of 2823 m and 3950m. Also, Ramírez-Herrera and Urrutia-

Fucugauchi (1999) found morphologic evidence for a possible recent subsidence along

the coast of the state of Colima (i.e. within the southern Colima rift).

Subsidence of the continental slope region is also evidenced on the seismic

reflection profile. First, Horizon B2 is eroded over the anticline located at CMP 415:

however, it is not eroded over the other two anticlines located to the NE; which now lie at

shallower depths. These observations can be explained by subsidence and trenchward

tilting of the continental slope.

The second line of evidence comes from the characteristics of the seismic

sequences observed NE of the mid-slope basin. The chronostratigraphic chart (Figure

13) constructed from the seismic sequences observed in the upper continental slope. On

the landward side, following the deposition of sequence 2, a period of

nondeposition/erosion occurred. This was followed by the landward progressing

deposition of sequence 1. Presently, the sediments of sequence 1 are being eroded at the

15

seafloor. This pattern is a classic example (Vail et al., 1977; Weimer, 1984) of a relative

fall and subsequent rise of sea level. The observed toplap against Horizon B could have

been formed by sediment bypass during a relative stillstand of sea level prior to the

relative fall of sea level. Alternatively, it may have been produced by subarial erosion

during the relative fall of sea level, the fall in sea level occurring at the time of formation

of Horizon B. During the lowstand of sea level the deposition of a lowstand wedge

occurred (the lower part of sequence 1). Lastly, the relative rise in sea level produced the

transgressive deposits (upper part of sequence 1) which extend landward of the older

shoreline (i.e., the area of the toplap against Horizon B).

As is common in seismic sequence analyses, relative sea level changes can be

produced either by eustatic fluctuations in sea level or by tectonic subsidence/uplift (e.g.

Vail et al., 1977; Haq et al., 1987). In the area off Manzanillo, even though the

depositional pattern is a classic example of a large eustatic fall and rise of sea level,

tectonic subsidence of the area must have occurred. Specifically, in this area a shallow-

water, high-energy environment, or perhaps a subarial environment, must have extended

to at least CMP 35 as evidenced by the seaward extent of the toplap against Horizon B.

The depth to Horizon B at CMP 35 is presently about 700 meters below mean sea level

(assuming a 2 km/sec sediment velocity for the section overlying Horizon B). Thus, if

the relative rise of sea level was soley due to a eustatic rise of sea level, then its

magnitude was at least 700 meters. The eustatic curves of Haq et al. (1987) show that the

magnitude of the eustatic rises in sea level during the last 10 m.y. have been less than

about 150 meters. Thus, at least 550 meters of subsidence must have occurred in the

shelf area off Manzanillo since the time of formation of Horizon B. We conclude from

this that it seems likely that the seismic sequence pattern noted on the profile represents a

major fall in global sea level at the time of formation of Horizon B, superimposed on a

long term, ongoing, tectonic subsidence of the continental self/slope in this area.

The age of Horizon B is unknown. However, the average subsidence rate of 0.35

mm/yr determined by Mercier de Lepinay et al. (1997) may be used to estimate the age of

Horizon B. Using this rate and assuming 550 meters of subsidence yields an age for

Horizon B of 1.57 Ma. This age corresponds well with the 100 m eustatic fall in sea level

that occurred at 1.6 Ma (Haq et al., 1987). This age is also consistent with the

16

sedimentation rate data (Ross, 1971) collected in the area seaward of the profile (90 to

146 cm/1000 yrs within the trench axis and about 13 cm/1000 yrs for the lower

continental slope at a water depth of 3860 m). Specifically, 330 meters of sediments

overlie Horizon B at CDP 85. As this area lies seaward of the first onlap onto Horizon B,

we expect that no major hiatus in sedimentation should exist (Figure 13). Assuming that

these sediments compact to 30% of their original volume (Ross, 1971), then about 1 km

of uncompacted sediments have been deposited. If the age of Horizon B is 1.6 m.y., then

the minimum average sedimentation rate would be 63 cm/1000 yrs. Thus, we propose an

age estimate for Horizon B of 1.6 Ma based on the subsidence and sedimentation rates

calculated in the general vicinity of the profile as well as the global sea level curves.

Strike Slip Faulting and Tectonism of the continental Shelf/Slope

The seismic reflection profile of this study imaged a 22.5 km wide shear-zone

within the upper slope area off Manzanillo Mexico. If our age estimate for Horizon B is

correct, then this fault system has been active since at least 1.6 Ma as is indicated by fault

terminations within FS1 (CMP 60). The disruption of the seafloor at the NE margin of

the mid-slope basin and between CMP’s 400 to 530, SW of the mid-slope basin, suggest

that this fault system is presently active.

Trench-parallel, strike-slip fault systems are commonly observed in the forearc

region of subduction zones along which the relative motion between the converging

plates is not in a direction normal to the strike of the trench. Such an association has

been observed at various subduction zones worldwide: for example, the Sunda and

Philipine arcs (Fitch, 1972), the Aleutian Arc (McCaffrey, 1992; Avé Lallemant, 1996)

and the Kuril Arc (Kimura, 1986) (also see Jarrard (1986) for more examples). These

strike-slip systems can be quite complex as in the case of the Aleutian arc, where a series

of arc parallel, anastomosing strike slip faults (or shear zones) extend across the entire

fore arc region and into the back arc flank of the volcanic islands (Avé Lallemant, 1996).

Also, extensional basins and troughs are commonly observed in the forearc regions.

These basins and troughs have been proposed to form in several ways. These include: (1)

forming at the trailing edge of the forearc sliver (Kimura, 1986; Beck, 1991), (2) forming

17

in areas where the trench exhibits a change in strike (e.g. Huchon and Le Pichon, 1984;

Avé Lallemant, 1996) (3) forming between rotated blocks comprising the forearc area

(e.g., Geist, et al., 1988), (4) forming at offsets of the main faults (e.g., Kimura, 1986)

and (5) forming as a result of a progressive increase in the obliquity of subduction along a

subduction zone (e.g., Avé Lallemant, and Guth, 1990; McCaffrey, 1991, 1992).

In the onshore part of forearc region of the Jalsico subduction zone, strike-slip

faults and extensional basins and troughs are observed. Allan et al. (1991) presents a

review of earlier geologic studies which reported evidence of strike-slip faulting within

the Tepic-Zacoalco graben. Kostoglodov and Bandy (1995) proposed that the presently

active (Núñez-Cornú et al., 2002), trench-perpendicular oriented, Puerto Vallarta graben

and its offshore extension, the Bahia de Banderas, were formed as a result of a

progressive increase in the obliquity of subduction along the Jalisco subduction zone.

Alternatively, Alvarez (2002) proposed that the Bahia de Banderas formed at the edge of

the Jalisco Block. Maillol et al. (1997) proposed, based on Landsat images, a broad

right-lateral shear zone may exist in the NE part of the Jalisco Block, and that the major

grabens and extensional structures of this area are related to this shearing. Ortiz et al.

(1993) describe a WNW-ESE oriented, 17 kilometer long right-lateral strike-slip fault

which cuts the NW flank of the Manzanillo horst and offsets the Rio Armeria. Lastly, the

La Cumbre Fault zone contains right-lateral, strike-slip faults (Serpa, et al., 1992).

In the offshore part of the forearc region, strike-slip faulting has been proposed

based on morphologic evidence. Bourgois et al. (1988) first noted the right lateral offset

of the Tecoman Trough (which they termed the Manzanillo Graben) relative to the

onshore part of the Tecoman graben. In their Figure 3 they indicate a strike slip fault

located between the graben and trough; however, this fault was shown not to extend

northwestward of the Tecoman Trough. The results of the present study, along with the

observed right-lateral offset between the offshore and onshore parts of the Manzanillo

horst, suggest that the strike-slip faulting extends further to the NW than previously

thought, although more data is needed to constrain the exact nature and location of this

fault system. In the following, this fault system is referred to as the Bourgois fault

system.

18

The Tecoman trough exhibits additional interesting and important characteristics.

First, it is the only area of the offshore Tecoman graben that presents any clear evidence

of recent extension. Second, the two canyons located within the trough exhibit distinctly

different characteristics. The Cuyutlan canyon on its NW margin is broad, is filled by

sediments and is presently not connected with any major onshore river. In contrast, the

Armeria canyon on the SE margin is narrow, is sediment free and is connected to the Rio

Armeria onshore. Third, the southeast margin of this trough overlies the boundary

between the subducting Rivera and Cocos plates as presented in the plate boundary

models of Bandy (1992) and Bandy et al. (1995, 2000).

These characteristics, the NW extension of the Bourgois fault system and the

seafloor and coastal morphology of the area can be simply explained in terms of a forearc

sliver in the offshore/coastal area of the Jalisco subduction zone, the sliver moving

towards the NW relative to the rest of the Jalisco Block.

Figure 14 illustrates what we propose to be the gross strike-slip fault pattern of

this area. The dashed lines on this figure mark the positions of possible faults as inferred

from topographic/bathymetric lineations. The dotted lines illustrate proposed extensions

of previously proposed faults. This inferred fault pattern suggests the presence of a series

of anastomosing faults similar to that observed in the Aleution forearc. Although the

exact locations and geometries of the faults comprising the Bourgois fault system is still

unclear, we conclude from the seismic reflection data that this system contains recently

active faults, and we propose that it is a complex shear zone separating a narrow forearc

sliver to the SW from the rest of the Jalisco block. This fault system appears to extend

NW of the location of the seismic line. Ross and Shor (1965) found that the basin/ridge

morphology noted off Manzanillo continues to the NW until the Bahia de Banderas. The

sediments within the basins exhibit a high degree of folding and faulting. However, this

morphology is absent on their profile H located in the area of the NW boundary of the

Manzanillo graben. Regardless, they suggest that these features may belong to the same

fault. Thus, although the nature of the fault was not given, it may well be the extension of

the Bourgois fault system.

The Tecoman Trough lies at the trailing edge of this forearc sliver, and thus we

propose that it formed as a result of a NW movement of the sliver. This proposal

19

explains the right-lateral offset of the Manzanillo Horst. It also provides a plausible

explanation for the trench perpendicular orientation of the Tecoman Trough, for why it

lies over the boundary of the subducting Rivera and Cocos plates provides (i.e., in the

plate motion model of Bandy (1992), the Cocos plate exhibits trench normal subduction

whereas the Rivera plate does not) and why it is the only area of the offshore part of the

Tecoman Graben to be undergoing recent extension. This proposal also provides an

explanation for the different character of the Cuyutlan Canyon relative to the Armeria

and Coahuayana canyons. Specifically, in our proposal the Cuyutlan Canyon would have

been previously aligned with and formed by the Rio Armeria. It has since moved to the

NW relative to the Rio Armeria and a new canyon, the Armeria Canyon, formed at its

previous location. Such offsets in drainage patterns are commonly observed along strike

slip faults (e.g., Moody and Hill, 1956).

This shear system appears to be more extensive than that indicated on the seismic

reflection data. Specifically, it may include the strike slip fault described by Ortiz et al.

(1993). Also, the extremely straight coastline along the Laguna Cuyutlan, which lies SE

of Manzanillo and in front of the Manzanillo Horst, may also mark the location of

another strike slip fault of the Bourgois fault system. Further, the coastline to the NW of

Bahia Manzanillo is also fairly straight and is aligned parallel to the coastline SE of

Bahia Manzanillo.

The mid-slope basin imaged on the seismic reflection profile may have formed in

several ways within a strike slip system. With the presently available data, the exact

mechanism of formation is not determinable. However, in our model we tentatively

show it as being formed between bifurcating, diverging dextral faults similar to that

presented by Christie-Blick and Biddle (1985).

Although the structural and tectonic setting presented herein provides a plausible

explanation for many of the morphologic and structural features observed in the offshore

and coastal area of the southern Jalisco subduction zone, most of its details remain poorly

constrained. Clearly, a more detailed seismic reflection survey is needed. Also, seafloor

sampling in the area were horizon B2 is truncated at the seafloor could provide an age for

this horizon. This age is important as horizon B2 appears to mark a time of intense

20

tectonism in this area.

21

References:

Allan, J.F., 1986, Geology of the northern Colima and Zacoalco grabens, southwest

Mexico: Late Cenozoic rifting in the Mexican Volcanic Belt, Geol. Soc. Am. Bull, 97,

473-485,

Allan, J.F., S.A. Nelson, J.F. Luhr, I.S.E. Carmichael, M. Wopat and P.J. Wallace, 1991.

Pliocene-Recent rifting in SW Mexico and associated volcanism: an exotic terrane in the

making. In: The Gulf and Peninsular Province of the Californias, J.P. Dauphin and B.

Simoneit (Eds.). Am. Assoc. Petrol. Geol. Mem, 47, pp. 425-445.

Alvarez, R., 2002. Banderas rift zone: A plausible NW limit of the Jalisco Block,

Geophys. Res. Lett., 29, article 55.

Avé Lallemant, H.G., 1996. Displacement partitioning and arc-parallel extension in the

Aleution volcanic island arc. Tectonophys., 256, 279-293

Avé Lallemant, H.G. and L.R. Guth, 1990. Role of extensional tectonics in exhumation

of eclogites and blueschists in an oblique subduction setting, northwest Venezuela.

Geology, 18, 950-953.

Bandy, W.L., 1992, Geological and geophysical investigation of the Rivera-Cocos plate

boundary: Implications for plate fragmentation, Ph.D. dissertation, Texas A&M Univ.,

College Station, 195p.

Bandy, W. and M. Pardo, 1994. Statistical examination of the existence and relative

motion of the Jalisco and southern Mexico blocks. Tectonics, 13, 755-768.

Bandy, W.L., C.A. Mortera-Gutierrez and J. Urrutia-Fucugauchi, 1993, Gravity field of

the southern Colima graben, Mexico, Geofís. Int., 32, 561-567.

22

Bandy, W., C. Mortera-Gutierrez, J. Urrutia-Fucugauchi, and T.W.C. Hilde, 1995, The

subducted Rivera-Cocos plate boundary: Where is it, what is it, and what is its

relationship to the Colima rift?, Geophys. Res. Letts., 22, 3075-3078.

Bandy, W., V. Kostoglodov, S.K. Singh, M. Pardo, J. Pacheco and J. Urrutia-Fucugauchi,

1997. Implications of the October 1995 Colima-Jalisco Mexico earthquakes on the

Rivera-North American Euler vector, Geophys. Res. Lett., 24, 485-488.

Bandy, W., V. Kostoglodov, A. Hurtado-Díaz and M. Mena, 1999. Structure of the

southern Jalisco subduction zone, Mexico, as inferred from gravity and seismicity.

Geofís. Int., 38, 127-136.

Bandy, W.L., T.W.C. Hilde, and C.Y. Yan, 2000, The Rivera-Cocos plate boundary:

Implications for Rivera-Cocos relative motion and plate fragmentation, in Delgado-

Granados, H., Aguirre-Díaz, G., and Stock, J.M., eds., Cenozoic Tectonics and

Volcanism of Mexico: Boulder, Colorado, Geological Society of America Special Paper

334, p. 1-28.

Bandy, W.L., J. Urrutia-Fucugauchi, F.W. McDowell and O. Morton-Bermea, 2001. K-

Ar ages of four mafic lavas from the Central Jalisco Volcanic lineament: Supporting

evidence for a NW migration of volcanism within the Jalisco block, western Mexico,

Geofís. Int., 40, 259-269.

Barrier, E.,J. Bourgois and F. Michaud, 1990, The active Jalisco triple junction rift

system, C.R. Acad. Sci. Paris, 310, Serie II, 1513-1520.

Beck, M.E., Jr., 1991. Coastwise transport reconsidered: lateral displacements in

oblique subduction zones, and tectonic consequences. Phys. Earth Planet. Inter., 68, 1-8.

Ben-Avraham, Z., G. Almagor and Z. Garfunkel, 1979. Sediments and structure of the

Gulf of Elat (Aqaba)-northern Red Sea. Sedimentary Geology, 23, 239-267.

23

Bourgois, J., V. Renard, J. Aubouin, W. Bandy, E. Barrier, T. Calmus, J.-L. Carfantan, J.

Guerrero, J. Mammerickx, B. Mercier de Lepinay, F. Michaud, and M. Sosson, 1988,

Active fragmentation of the North American Plate: offshore boundary of the Jalisco

Block off Manzanillo, Comptes Rendus Académie des Siences Paris, 307, 1121-1130.

Carmichael, I.S.E., R.A. Lange and J.F. Luhr, 1996. Quaternary minettes ande associated

volcanic rocks of Mascota, western Mexico: A consequence of plate extension above a

subduction modified mantle wedge. Contrib. Mineral Petrol., 124, 302-333.

Castellanos, G. and Z. Jiménez, 1995. Sismología de Colima: Universidad de Colima,

Universidad Nacional Autónoma de México, Secretaria de Educación Pública, SESIC-

DES., 101 p.

Christie-Blick, N. and K.T. Biddle, 1985. Deformation and basin formation along strike-

slip faults, In: Strike-slip Deformation, Basin Formation and Sedimentation (edited by

Biddle, K.T. and Christie-Blick, N., Spec. Publs. Soc. Econ. Palaeont. Miner., Tulsa, 37,

1-34.

Courboulex, F., S.K. Singh, J.F. Pacheco and C. J. Ammon, 1997. The 1995 Cokima-

Jalisco, Mexico, earthquake (Mw 8): A study of the rupture process, Geophys. Res. Lett.,

24, 1019-1022.

DeMets, C. and D.S. Wilson, 1997. Relative motions of the Pacific, Rivera, North

American, and Cocos plates since 0.78 Ma. J. Geophys. Res., 102, 2789-2806.

DeMets, C. and S. Traylen, 2000, Motion of the Rivera plate since 10 Ma relative to the

Pacific and North American plates and the mantle, Tectonophysics, 318, 119-159.

24

Escobedo, D., J.F. Pacheco and G. Suárez, 1998. Teleseismic body-wave analysis of the

9 October, 1995 (Mw = 8.0), Colima-Jalisco, Mexico earthquake, and its largest

foreshock and aftershock, Geophys. Res. Lett., 25, 547-550.

Ferrari, L., G. Pasquaré, S., Venegas, D. Castillo, and F. Romero, 1994, Regional

tectonics of western Mexico and its implications for the northern boundary of the Jalisco

block, Geofís. Int., 33, 139-151.

Fitch, T.J., 1972. Plate convergence, transcurrent faults, and internal deformation

adjacent to southeast Asia and the western Pacific. J. Geophys. Res., 77, 4432-4460.

Garduño-Monroy, V.H. and Tibaldi, A., 1991, Kinematic evolution of the continental

active triple junction of the western Mexican Volcanic Belt, C.R. Acad. Sci. Paris, 312,

Serie II, 135-142.

Garduño-Monroy, V.H., R. Sauccedo-Girón, Z. Jiménez, J.C. Gavilanes-Ruiz, A. Cortés-

Cortés and R.M. Uribe-Cifuentes, 1998. La falla Tamazula, Limite suroriental del

Bloque Jalisco, y sus relaciones con el complejo volcanico de Colima, Mexico, Revista

Mexicana de Ciencias Geológicas, 15, 132-144.

Geist, E.L., J.R. Childs and D.W. Scholl, 1988. The origin of summit basins of the

Aleutian Ridge: Implications for block rotation of an arc massif, Tectonics, 7, 327-341.

Haq, B.H., J. Hardenbol and P.R. Vail, 1987. Chronology of fluctuating sea levels since

the Triassic. Science, 235, 1156-1167, 1987.

Huchon, P. an X. Le Pichon, 1984. Sunda Strait and central Sumatra fault, Geology, 12,

668-672.

Jarrard, R.D., 1986. Terrain motion by strike-slip faulting of forearc slivers, Geology,

14, 780-783.

25

Jiménez, Z., G. Reyes and A Ramírez, 1996, Actividad sismica observada en la frontera

sur-oriental del bloque de Jalisco de septiembre de 1995 a agosto de 1996 (abstract),

GEOS Boletín Informativo de la Unión Geofísica Mwxicana, 16, 210.

Johnson, C.A. and C.G.A. Harrison, 1989. Tectonics and volcanism in central Mexico: a

landsat thematic mapper perspective. Remote Sensing of the Environment, 28, 273-286.

Johnson, C.A. and C.G.A. Harrison, 1990, Neotectonics in central Mexico, Physics of the

Earth and Planetary Interiors, 64, 187-210.

Khutorskoy, M.D., L.A. Delgado-Argote, R. Fernández, V.I. Kononov, and B.G. Polyak,

1994, Tectonics of the offshore Manzanillo and Tecpan basins, Mexican Pacific, from

heat flow, bathymetric and seismic data, Geofís. Int., 33, 161-185.

Kimura, G., 1986. Oblique subduction and collision: Forearc tectonics of the Kuril arc,

Geology, 14, 404-407.

Kostoglodov, V. and W. Bandy, 1995. Seismotectonic constraints on the convergence

rate between the Rivera and North American plates, J. Geophys. Res., 100, 17,977-

17,989.

Lange, R.A. and I.S.E. Carmichael, 1990. Hydrous basaltic andesites associated with

minette and related lavas in western Mexico. J. Petrology, 31, 1225-1259.

Lonsdale, P., 1995. Segmentation and disruption of the East Pacific Rise in the mouth of

the Gulf of California. Marine Geophys. Res., 17, 323-359.

Luhr, J.F., S.A. Nelson, J.F. Allan and I.S.E. Carmichael, 1985, Active rifting in

southwest Mexico: Manifestations of an incipient eastward spreading ridge jump,

Geology, 13, 54-57.

26

Maillol, J.M. and W.L. Bandy, 1994. Paleomagnetism of the Talpa de Allende and

Mascota grabens, western Mexico: A preliminary report. Geofís. Int., 33, 153-160.

Maillol, J.M., W.L. Bandy and J. Ortega-Ramírez, 1997. Paleomagnetism of Plio-

Quaternary basalts in the Jalisco block, western Mexico. Geofís. Int., 36, 21-35.

McCaffrey, R., 1991. Slip vectors and stretching of the Sumatran forearc, Geology, 19,

881-884.

McCaffrey, R., 1992. Oblique plate convergence, slip vectors, and forearc deformation,

J. Geofhys. Res., 97, 8905-8915.

Melbourne, T., I. Carmichael, C. DeMets, K. Hudnut, O. Sanchez, J. Stock, G. Suarez

and F. Webb, 1997. The geodetic signature of the M8.0 Oct. 9,1995, Jalisco subduction

earthquake, Geophys. Res. Lett., 24, 715-718.

Mercier de Lépinay, B., F. Michaud, T. Calmus, J. Bourgois, G. Poupeau, P. Saint-Marc,

and The NAUTIMATE team, 1997, Large Neogene subsidence event along the Middle

America Trench off Mexico (18-19°N): Evidence from submersible observations,

Geology, 25, 387-390.

Michaud, F., J. Bourgois, E. Barrier and E. Fourcade, 1989, The cretaceous Tecoman

series (western Mexico) related to the western cordilleran geologic history, C.R. Acad.

Sci. Paris, 309, Serie II, 587-593.

Michaud, F., J. Bourgois and J. Aubouin, 1990. Fragmentation continentale en cours

dans la zone d’affrontement entre le continent Nord-Américain et la ride d’accrétion

océanique de Pacifique Est (Mexique). Oceanologica Acta, 10, 87-96.

27

Michaud, F., O. Quintero, E. Barrier and J. Bourgois, 1991, The northern boundary of the

Jalisco block (western Mexico): Location and evolution from 13 Ma to present, C.R.

Acad. Sci. Paris, 312, Serie II, 1359-1365.

Michaud, F., J. Bougois, J.-F. Parrot, H. Taud and M. Kasser, 1994. Tectonic

development of the Jalisco triple junction (western Mexico) from SPOT-DEM data

processing, , C.R. Acad. Sci. Paris, 318, Serie II, 1117-1124.

Moody, J.D. and M.J. Hill, 1956. Wrench-Fault Tectonics, Geol. Soc. Am. Bull., 67,

1207-1246.

Nieto-Obregon, J., L. Delgado-Argote and P.E. Damon, 1985. Geochronologic,

petrologic, and structural data related to large morphotectonic features between the Sierra

Madre Occidental and the Mexican volcanic belt. Geofís. Int., 24, 623-663.

Núñez-Cornú, F.J., R.L. Marta, F.A. Nava, G. Reyes-Davila and C. Suarez-Plascencia,

2002. Characteristics of seismicity in the coast and north of Jalisco Block, Mexico,

Physics of the Earth and Planetary Interiors, 132 (1-3), 141-155.

Otíz, M.A., J.J. Zamorano, and R. Bonifaz, 1993. Reconocimiento morfotectonico de

una falla reciente de tipo transcurrente en Colima, Mexico. Geofís. Int., 32, 569-574.

Pacheco, J., W. Bandy, G.A. Reyes-Dávila, F.J. Nuñez-Cornú, C.A. Ramírez-Vázquez

and J.R. Barrón, 2003. The Colima, Mexico, earthquake (Mw 5.3) of 7 March 2000:

Seismic activity along the southern Colima rift, Bull. Seismol. Soc. Am., 93, 1458-1476.

Pacheco, J.F., C.A. Mortera-Gutiérrez, H. Delgado, S.K. Singh, R.W. Valenzuela, N.M.

Shapiro, M.A. Santoyo, A. Hurtado, R. Barrón and E. Gutiérrez-Moguel, 1999. Tectonic

significance of an earthquake sequence in the Zacoalco half-graben, Jalisco, México, J.

South American Earth Sciences, 12, 557-565.

28

Pacheco, J, S.K. Singh, J. Domínguez, A. Hurtado, L. Quintanar, Z. Jiménez, J.

Yamamoto, C. Gutiérrez, M. Santoyo, W. Bandy, M. Guzmán and V. Kostoglodov, 1997.

The October 9, 1995 Colima-Jalisco, Mexico earthquake (Mw 8): An aftershock study

and a comparison of this earthquake with those of 1932, Geophys. Res. Lett., 24, 22223-

2226.

Pinet, N. and P.R. Cobbold, 1992. Experimental insights into the partitioning of motion

within zones of oblique subduction, Tectonophysics, 206, 371-388.

Ramírez-Herrera, M.-T. And J. Urrutia-Fucugauchi, 1999, Morphotectonic zones along

the coast of the Pacific continental margin, southern Mexico, Geomorphology, 28, 237-

250.

Righter, K. and J. Rosas-Elguera, 2001. Alkaline lavas in the volcanic front of the

western Mexican volcanic belt: Geology and Petrology of the Ayutla and Tapalpa

volcanic fields. J. Petrology, 12, 2333-2361.

Righter, K., I.S.E. Carmichael, T.A. Becker and R.P. Renne, 1995. Pliocene to

Quaternary volcanism and tectonism at the intersection of the Mexican Volcanic Belt and

the Gulf of California. Geol. Soc. Am Bull, 107, 612-626.

Ross, D.A., 1971. Sediments of the northern Middle America trench, Geol. Soc. Am.

Bull., 82, 303-322.

Ross, D.A. and G.G. Shor, Jr., 1965. Reflection profiles across the Middle America

Trench, J. Geophys. Res., 70, 5551-5572.

Serpa, L., C. Katz and C. Skidmore, 1989. The southeastern boundary of the Jalisco

block in west-central Mexico (abstract). Eos Trans. AGU, 70, 1319.

29

Serpa L., S. Smith, C. Katz, C. Skidmore, R. Sloan and T. Pavlis, 1992, A geophysical

investigation of the southern Jalisco block in the State of Colima, Mexico, Geofis. Int. 31,

475-492.

Suárez, G., V. García-Acosta and R. Gaulon, 1994. Active crustal deformation in the

Jalsico block, Mexico: evidence for a great historical earthquake in the 16th century,

Tectonophysics, 234, 117’127.

Urrutia-Fucugauchi, J., J.H. Flores-Ruiz, W.L. Bandy and C.A. Mortera-Gutiérrez, 1999.

Crustal structure of the Colima rift, western Mexico: gravity models revisited. Geofís.

Int., 38, 205-216.

Vail, P.R., R.M. Mitchum, Jr., and S. Thompson, III, 1977. Seismic stratigraphy and

global sea-level changes, part 3, in Seismic Stratigraphy- applications to hydrocarbon

exploration, Charles E, Payton (ed.), AAPG Mem. 26, pp.63-82, The American

Association of Petroleum Geologists, Tulsa, Oklahoma

Weimer, R.J., 1984. Relation of unconformities, tectonics, and aes-level changes,

Cretaceous of western interior, U.S.A., in Interregional Unconformities and

Hydrocarbon Accumulation, John S. Schlee (ed.), Am. Ass. Petr. Geol.,Memoir 36, pp.7-

35, The American Association of Petroleum Geologists, Tulsa, Oklahoma.

Zobin, V.M., 1997, The rupture history of the Mw 8.0 Jalisco, Mexico, earthquake of

1995 October 9, Geophys. J. Int., 130, 220-228.

Zobin, V.M., G.A. Reyes-Dávila, L.M. Pérez-Santa Ana, C.A. Ramírez-Vázquez and J.F.

Ventura-Ramírez, 2000. Estudio Macros´smico del temblor de Colima (Mw 5.3) del 6 de

Marzo del 2000. GEOS, 20, 414-417.

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Figure Captions

Figure 1. Study area location map.

Figure 2. Topogragphic/Bathymetric map illustrating the main morphotectonic features

of the southern Colima rift and immediate surroundings. Abbreviations are: AC =

Armeria Submarine Canyon; CC = Cuyutlan Submarine Canyon; CoC = Coahuayana

Submarine Canyon; LCFZ = La Cumbre Fault Zone; MH = Manzanillo Horst; SM =

Sierra Manantlan; EGG = El Gordo Graben; Man = Manzanillo. Filled squares mark

locations of the main cities of the area.

Figure 3. Shaded relief map. Illustrated are (1) the epicenters of the 6 March 2000 (solid

square) and the 9 October 1995 (solid circle) earthquakes, (2) the Tamazula Fault Zone

(TFZ) and the faults presented by Ortiz et al. (1993), (OF), and by Bourgois et al. (1988),

(BF), and (3) the location of the seismic reflection profile presented in this study

(numbers adjacent to the plus symbol are the CMP numbers).

Figure 4. Complete seismic reflection profile illustrating the tectonized zone located in

the continental slope region off Manzanillo, Mexico. Every tenth trace is displayed. See

figure 3 for profile location.

Figure 5. NE part of the seismic reflection profile illustrating the undisrupted sediments

of the continental slope landward of the tectonized zone. Arrows mark reflector

terminations.

Figure 6. SW part of the seismic reflection profile illustrating the tectonically

undisturbed area seaward of the tectonized zone. OCSS=Outer Continental Slope

Sequence.

Figure 7. Seismic reflection section of the NE margin of the tectonized zone.

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Figure 8. Close-up view of the 'pop-up' structure associated with fault system FS3.

Figure 9. Seismic reflection section of the mid-slope basin.

Figure 10. Instantaneous phase display illustrating the seismic sequences within the mid-

slope basin.

Figure 11. Seismic reflection sections of a) the NE half or the anticlinal zone and b) the

SW half of the anticlinal zone.

Figure 12. Seismic reflection section of the SW margin of the tectonized zone. Note that

the tectonism extends beyond the anticlinal zone and disrupts the reflector of sequences

OCSS-1 and OCSS-2 indicating that the tectonism in this area is fairly recent. Also note

the particularly well developed flower structure centered on CMP 520.

Figure 13. Chronostratigraphic chart constructed from the seismic sequences observed in

the upper continental slope between CMP 5 and CMP 80.

Figure 14. Map illustrating our preliminary estimate of the fault pattern in the area off

Manzanillo. In this proposal the faulting is due to a NW movement of a small forearc

sliver (white hatched area). The Tecoman Trough is located at the SE margin of this

forearc sliver.

32

Figure 1.

33

Figure 2.

34

Figure 3.

35

Figure 4.

36

Figure 5.

37

Figure 6.

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Figure 7.

39

Figure 8.

40

Figure 9.

41

Figure 10.

42

Figure 11a.

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Figure 11b.

44

Figure 12.

45

Figure 13.

46

Figure 14.

47