Geo07VIMexicoExtension.pdf

Westward migration of extension in the

northern Gulf of California, Mexico

Manuel Aragón-Arreola* and Arturo Martín-Barajas*

División de Ciencias de la Tierra, Centro de Investigación Cientíﬁ ca y Educación Superior de Ensenada,

Km 107 Carretera Tijuana-Ensenada, Ensenada, Baja California 22860, México

ABSTRACT

Interpretation of industry seismic lines indicates that the eastern margin of the northern

Gulf of California contains the inactive Adair-Tepoca and Upper Tiburón basins. The western

margin is active and includes the Wagner, Consag, Upper Delfín, and Lower Delfín basins.

These basin systems are separated by a wide basement high across which the upper strata in

the inactive basins correspond to the middle and lower strata in the active basins, recording

the westward migration of strain and subsidence during late Pliocene time. Our results illus-

trate the formation of an abandoned rift margin along the eastern Gulf of California.

Keywords: Gulf of California, oblique rifting, rift kinematics, seismic reﬂ ection, rift basins.

INTRODUCTION

The Gulf of California is a well-developed transtensional plate

margin linked to the rupture of Baja California from southwestern

North America (Fig. 1). In middle Miocene time, extensional deforma-

tion affected the Gulf Extensional Province (Stock and Hodges, 1989;

Gans, 1997; Axen and Fletcher, 1998), but sometime during the middle

to late Miocene, the strain localized into the highly oblique Gulf of Cali-

fornia rift, which is now dominated by large transform faults linked by

pull-apart basins (Lonsdale, 1989; Stock and Hodges, 1989; Oskin et al.,

2001). However, little is known about how strain evolved during the his-

tory of focused rifting. Based on processing and interpretation of seismic

proﬁ les, we compiled a structural map of the northern Gulf of California

(the northern Gulf), documenting structures related to the initial stage of

focused rifting. We show that the crustal strain and subsidence migrated

westward to form the modern rift conﬁ guration. Finally, we propose that

the eastern Gulf forms an abandoned rift margin.

GEOLOGIC FRAMEWORK

Evolution of the Gulf of California is related to transfer of the

Baja California from the North America to the Paciﬁ c plate. This proc-

ess started ~12 m.y. ago and imposed a northwestward motion on Baja

California that triggered the localization of most of the Paciﬁ c–North

America plate motion into the Gulf Extensional Province (Stock and

Hodges, 1989). The timing of onset of focused extension is controversial;

it has been suggested that oblique extension was preceded by a period of

nearly orthogonal extension known as the proto-Gulf stage, which was

accompanied by the ﬁ rst marine incursions (12–6 Ma; Stock and Hodges,

1989; Henry and Aranda-Gomez, 2000; Umhoefer et al., 2002). Around

the northern Gulf, the early marine sedimentation occurred during latest

middle Miocene time (Delgado-Argote et al., 2000; Gastil et al., 1999;

Helenes and Carreño, 1999; McDougall et al., 1999; Helenes et al.,

2005), but it is unclear if deposition was related to oblique or orthogonal

extension. Nevertheless, the marine environment became established by

6.5–6.3 Ma, synchronous with the full localization of oblique strain into

the Gulf (Oskin et al., 2001; Oskin and Stock, 2003).

Previous works documented that the eastern margin of the Gulf con-

tains several inactive basins, including the Upper Tiburón, Lower Tiburón,

and Yaqui basins; it is interpreted that these basins were formed as

Figure 1. A: Regional tectonic framework of western North America

and northwestern Mexico (after Stock and Hodges, 1989; Lons-

dale, 1989; Fenby and Gastil, 1991). Arrow shows relative motion of

Paciﬁ c–North America plates. B: Layout of processed seismic lines.

C: Structural map of northern Gulf of California. Eastern margin

contains inactive basins and faults, while western margin includes

active basins of modern rift. Pattern of active depocenters and shal-

low fault arrays is from Persaud et al. (2003). Faults outside data

coverage are from Fenby and Gastil (1991). PA—Paciﬁ c plate; GC—

Gulf of California; GEP—Gulf Extensional Province; B&R—Basin

and Range Province; SMO—Sierra Madre Occidental; CP—Colorado

Plateau: ITI—Isla Tiburón; IAG—Isla Ángel de la Guarda.

*E-mails: maragon@cicese.mx; amartin@cicese.mx.

Geology , June 2007; v. 35; no. 6; p. 571–574; doi: 10.1130/G23360A.1; 3 ﬁ gures; Data Repository item 2007130.

571

GEOLOGY, June 2007

incipient spreading centers (Phillips, 1964; Fenby and Gastil, 1991; Lons-

dale, 1989) abandoned ~3 m.y. ago (Stock, 2000). However, these studies

lacked the data to resolve the structural pattern of the inactive basins, such

that the database used here may help to clarify these conclusions.

The crustal structure displays contrasts across the northern Gulf.

Toward the east, the crust is continental and thicker than in the west, where

it is likely formed by a mix of continental, igneous, and sedimentary rocks

(Phillips, 1964; Persaud et al., 2003; González-Fernández et al., 2005).

Moreover, the shoulders of the northern Gulf differ in the amount and age

of extension and volcanism; in central Sonora, the extension produced

basin-and-range–type deformation and the exhumation of metamorphic

core complexes during late Oligocene–middle Miocene time, coeval with

arc-related volcanism (Nourse et al., 1994; Gans, 1997; Martín-Barajas,

2000). In contrast, extension in Baja California was accrued in high- to

moderate-angle normal faults, and some detachment faults, and started

in late-middle Miocene time after cessation of arc volcanism (Axen,

1995; Lee et al., 1996). Moreover, discrete postsubduction volcanism has

occurred along the coastal plain of Baja California (Sawlan, 1991; Martín-

Barajas, 2000). Thus, extensional strain is older and of larger magnitude in

the eastern than in the western margin of the northern Gulf.

folds and are cut by multiple ~N-S–striking subsidiary faults that lie along

the axis of the Wagner and Consag basins (Figs. 1 and 2A). Farther south, in

the Upper Delfín and Lower Delfín basins, the shallow sediments are cut by

dense fault arrays that crown and splay away from the northern strands of

the Canal de Ballenas fault (Persaud et al., 2003; Aragón-Arreola, 2006).

The data clearly image the acoustic basement in the northeastern

Gulf (Fig. 1). Here the basement is depicted as a sharp reﬂ ector that out-

lines the rhombochasmic Adair-Tepoca and Upper Tiburón basins. The

Adair-Tepoca basin has a stratal thickness of as much as ~3.8 s (~4.8 km)

and is bounded by the Amado fault and several minor faults to the east,

a structural high to the west (see below), and terminates northward into

the Adair graben (Figs. 1, 2A, and 2B). This basin contains two seismic

sequences; the lower is formed by growth strata and aggradation patterns

that accrete eastward against the basin-bounding faults, suggesting the

syntectonic evolution of faulting and sedimentation (B in Figs. 2A, 2B).

The upper sequence is formed by nearly parallel strata that onlap to the

east and drape the basement relief (C in Figs. 2A, 2B).

The Upper Tiburón basin has a stratal thickness of as much as ~4.8 s

(~6.0 km) and is bounded by the De Mar fault to the east and the Tiburón fault

to the west (Figs. 1 and 2D). This basin contains three seismic sequences; the

lower is formed by a hammock reﬂ ector pattern cut by the basin-bounding

faults (A in Fig. 2D). The middle sequence is built by growth strata that

accrete on the basin-bounding faults, revealing the syntectonic sedimenta-

tion (B in Fig. 2D). The upper sequence is formed by nearly parallel strata

that onlap and drape the shallow (<1.5 s) and faulted basement offshore

Sonora; toward the top this sequence includes local unconformities, but they

do not separate changes in the stratal pattern (C in Figs. 2A, 2B).

The lateral continuity of seismic reﬂ ectors across the northeastern

Gulf reveals the correlation of sequences B and C of the Adair-Tepoca

and Upper Tiburón basins. The sequence C forms condensed sections

that drape the Amado, De Mar, and Tiburón faults and ﬁ lls the basement

relief offshore Sonora. The data also show that reﬂ ectors of sequence C

are locally truncated at the sea bottom, but do not record active faulting

(Fig. 2C), indicating that this region is now structurally inactive.

The basement in the central-west margin of the northern Gulf is

poorly deﬁ ned, but suggests a broad rhombochasmic depression limited

by the Cerro Prieto and Canal de Ballenas faults. This depression contains

the Wagner, Consag, and Upper and Lower Delfín basins (Fig. 1) that

form a set of shallow sags oriented N-NE (Persaud et al., 2003). Here the

stratal thickness is poorly estimated, but exceeds 4.0 s (~4–5? km; Figs.

2A, 2C). The reﬂ ectors can be traced across contiguous sags and display

multiple truncations, suggesting that the active basins tend to coalesce.

Our data also show growth strata within the Wagner and Consag basins

that form aggradation patterns against the Wagner fault (Fig. 2A), indicat-

ing syntectonic sedimentation.

The boundary between the active and inactive basins consists of a

broad anticline. This ﬂ at-topped, ~20-km-wide, doubly hinged structure

trends NNW-SSE and extends ~120 km from the Tiburón to the Cerro

Prieto fault (Figs. 1, 2B, and 2C); the data suggest that this anticline is

cored by a basement high. This anticline is built by condensed sequences

cut by numerous small-throw faults. A conspicuous feature is that the B-C

sequence boundary in the inactive basins can be traced across the entire

anticline and deepens to the west into the active basins (Figs. 2A, 2C).

These seismic relationships indicate that once subsidence and sedimenta-

tion shifted from the eastern to the west basins.

DATABASE

Our database contains ~3600 km of multichannel seismic reﬂ ec-

tion data (Fig. 1B) surveyed by PEMEX during 1978 and 1979 (Aragón-

Arreola , 2006). These 48-fold data were acquired with a 48-channel, 2400-m-

long streamer, a 21.98 L (1341 cu. in.) air gun array, and a shot interval of

25 m. Recording time was 6.1 s, with a sampling rate of 2 ms. Our proc-

essing included ﬁ ltering, semblance velocity analysis, spherical spreading

and normal move-out corrections, predictive deconvolution, stacking, and

post-stack time migration. Depths are given in seconds of two-way travel-

time; some absolute depths were estimated using the calculated interval

velocities, such that they should be taken as rough estimates due to the lack

of well constraints. Our geologic map (Fig. 1C) also presents the geologic

information shown by Persaud et al. (2003) for the active basins.

RESULTS: STRUCTURE OF THE NORTHERN

GULF OF CALIFORNIA

The main structural fabric of the northern Gulf consists of NW-SE–

striking faults of moderate dip (Fig. 1) with >1 s of throw (>1 km). The

larger faults offshore Sonora include the west-dipping Amado fault and

the conjugate De Mar and Tiburón faults (Fig. 2 1 ; see GSA Data Reposi-

tory 2 ); these faults do not cut the uppermost strata, and are now inactive.

The major faults display an en echelon array with intervening basins that

mimics the active structural fabric of the Gulf (Fenby and Gastil, 1991;

Lonsdale, 1989), which suggests that they had accommodated signiﬁ cant

dextral shear. Our data show that the active Cerro Prieto fault extends into

the northern Gulf; however, its southernmost segment is inactive and draped

by undeformed strata (Figs. 2A, 2B). The data also reveal that numerous

minor faults develop in the shallow section of the major structures.

Several NW-striking secondary faults were identiﬁ ed near the coast of

Sonora. These structures display <1 s of throw and deﬁ ne horsts and grabens

of moderate relief that are draped by sediments. One of these faults paired

with the Amado fault, and bounds the ~10-km-wide Adair graben, which

extends >50 km (Figs. 1, 2A, and 2B). In the north-central Gulf, the Wagner

fault zone branches out from the Cerro Prieto fault and dips to the west at

a moderate angle that tends to shallow with depth, suggesting listric fault

geometry. The strata overlying the Wagner fault deﬁ ne fault-propagation

DISCUSSION

We interpret that the lower sequence in the Upper Tiburón basin (A in

Fig. 2D [see footnote 1]) was deposited in a sag-shaped depression with

no obvious association to any major fault. In contrast, sequence B in the

Adair-Tepoca and Upper Tiburón basins comprises growth strata associated

with the Amado, De Mar, and Tiburón faults, which are parallel to the main

1 Figure 2 is provided on a separate insert.

2 GSA Data Repository item 2007130, Figures DR1–DR4, seismic pro-

ﬁ les prepared for large-format printout, is available online at www.geosociety.

org/pubs/ft2007.htm, or on request from editing@geosociety.org or Documents

Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

572

GEOLOGY, June 2007

structural fabric of the Gulf of California rift (Fig. 1). The syntectonic evolu-

tion of sequence B reveals that the major faults accrued >1 km of throw, and

possibly large strike-slip motion, based on the en echelon pattern (Fig. 1).

The recognition of linked seismic sequences in the Adair-Tepoca and Upper

Tiburón basins associated with growth faults suggests that sequence B

records the onset of focused extension in the northern Gulf, while sequence

A represents pre-rift deposition probably linked to the proto-Gulf Stage

(Stock and Hodges, 1989; Henry and Aranda-Gomez, 2000).

Our results indicate that sequence C records the abandonment of

the eastern basins. This sequence drapes and is posttectonic to the major

faults; in addition, the unconformities and stratal truncation within this

sequence (Figs. 2B, 2D) suggest erosional stages probably tied to the cease

of tectonic activity. Moreover, the correlation of the B-C sequence bound-

ary from the Adair-Tepoca and Upper Tiburón basins into the Wagner,

Consag, and Upper Delfín basins (Figs. 2A, 2B, 2C) supports that strain

and associated subsidence waned in the eastern basins as they relocated

to the west. This sequence boundary was traced down to ~3.0 s into the

Wagner and Consag basins (Figs. 2A, 2B); thus, most of the stratigraphic

section in the active basins postdates the shift of deformation.

Here we infer prerift and synrift deposition, and the migration of strain

and subsidence, but we lack time constraints for these events. The recon-

struction of conjugate margins in the Upper Delfín basin segment suggests

that rifting began ca. 6–7 Ma (Oskin et al., 2001; Oskin and Stock, 2003).

However, microfossil data from an ~4800 m well drilled by PEMEX in the

Upper Tiburón basin (W in Fig. 1) show that the upper ~3500 m postdate the

late-middle Miocene; the lower section that mostly correlates with sequence

A lacks biostratigraphic control due to poor faunal preservation (Helenes

et al., 2005). Thus, the prerift sequence A predates or is at least late-middle

Miocene age, which suggests that it was deposited during the proto-Gulf

stage (Stock and Hodges, 1989; Henry and Aranda-Gomez, 2000). The

upper 800 m in well W that correlate with sequence C were deposited in

Pliocene–Pleistocene time (Helenes et al., 2005), suggesting that migration

of strain occurred in that time. Thus, our interpretation agrees with recon-

structions in which, ca. 3.3–2.0 Ma, the Upper Tiburón basin and Tiburón

fault became inactive as the Lower Delfín basin and Canal de Ballenas fault

initiated (Nagy and Stock, 2000; Stock, 2000).

The shift of subsidence in narrow basins has been explained by the

lateral contrast of heat ﬂ ow caused by attenuation of continental crust dur-

ing rifting (Sandiford et al., 2003). In addition, large sediment piles likely

play a roll in the lithospheric weakening, because sediments thermally

insulate the lower crust (Lavier and Steckler, 1997). This insulation favors

the lateral heat transfer that may trigger the migration of deformation in

the vicinity of rift basins (Sandiford et al., 2003). The northern Gulf meets

both conditions: it contains kilometric sedimentary columns and its west-

ern shoulder records volcanic activity since the early Miocene (Sawlan,

1991; Martín-Barajas, 2000). Moreover, the onshore volcanism and the

numerous volcanic knolls in the active basins (Fig. 1) also suggest a sus-

tained heat source near Baja California.

The Wagner, Consag, and Upper and Lower Delfín basins are in

a large stepover of the Canal de Ballenas and Cerro Prieto faults; here,

subsidence is diffusely controlled by shallow fault arrays that branch out

from the major faults (Lonsdale, 1989; Nagy and Stock, 2000; Stock,

2000; Persaud et al., 2003). However, our data reveal that the likely listric

Wagner fault roots the shallow deformation and yields the formation of

thick growth strata; thus, this fault is likely the main control of subsidence

in the Wagner and Consag basins (Figs. 1 and 2A). We argue that vertical

propagation of the Wagner fault results in the distributed faulting that con-

trols the modern depocenter of these basins. Furthermore, subsidence of

the Upper Delfín and Lower Delfín basins is related to horsetail structures

splaying away the Canal de Ballenas fault (Fig. 1; Persaud et al., 2003).

We speculate that these shallow faults may also be linked to the propaga-

tion of deep structures, similar to the Wagner fault.

The pattern of inactive basins is a conspicuous feature along the east-

ern Gulf of California rift (Fig. 3). North of the Adair-Tepoca and Upper

Tiburón basin system, the Altar and East Mesa basins contain deltaic suc-

cessions that onlap late Miocene marine deposits; these basins are above

the base level and are now inactive (Pacheco-Romero et al., 2006). In the

central Gulf, the Yaqui basin is an inactive half-graben abandoned during

late Pliocene time (Aragón-Arreola et al., 2005). Farther south, the eastern

margin contains the inactive San Blas, Tamayo, Nayarit, and Tres Marías

troughs (Brown et al., 2006; Sutherland et al., 2006). We propose that the

inactive basins along the eastern Gulf of California constitute an aban-

doned rift margin that is forming an incipient drift margin.

CONCLUSIONS

Our results indicate that focused extension in the northern Gulf of

California occurred through a process of localization and relocalization

of strain that resulted in two diachronous basin systems controlled by

large dextral-oblique faults. The inactive margin includes the Adair-

Tepoca and Upper Tiburón basins, controlled by the Amado, de Mar, and

Tiburón faults. During middle to late Pliocene time, the strain localized

to the west, giving rise to the Wagner, Consag, and Upper and Lower

Delfín basins, which are controlled by the Wagner fault and the fault

arrays that branch away the Cerro Prieto and Canal de Ballenas faults.

The pattern of inactive fault-basin systems in the east is present along

the entire length of the Gulf and forms an abandoned rift margin, while

active rifting is located in the western Gulf.

Figure 3. Eastern Gulf of California contains abandoned rift basins,

while active rifting occurs in the western Gulf (Lonsdale, 1989;

Fenby and Gastil, 1991; Persaud et al., 2003; Aragón-Arreola et al.,

2005; this study). Eastern Gulf constitutes abandoned rift margin

(see inset). Abbreviations as in Figure 1.

GEOLOGY, June 2007

573

ACKNOWLEDGMENTS

This research was funded by PEMEX and CONACyT. We thank PEMEX and

Ing. A. Oviedo for permission to publish these results. J. Mojarro provided techni-

cal assistance. Discussions with J. Stock and J. Contreras, and reviews by G. Axen,

P. Umhoefer, and two anonymous reviewers greatly improved this manuscript.

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Manuscript accepted 25 January 2007

Printed in USA

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GEOLOGY, June 2007

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