Abstract

Terrane accretion, variations of the convergence rate, and interaction with the Caribbean plate have been proposed as the causes of the Mexican Laramide orogeny. However, the mechanism that triggered this regional deformation event remains unknown. Based on available data, some authors supported the notion that the Laramide shortening migrated from the present-day Pacific coast to the Mexican mainland. However, such migration has been inferred based on paleontologic and isotopic data from the central and eastern parts of the fold-and-thrust belt, without considering the western part of the orogen. The identification of a chronologic pattern of the deformation is crucial to understand the cause of the Laramide orogeny, because it is a direct consequence of the tectonic process that triggered this regional shortening.

Here we present the first structural study of the Zihuatanejo area, which is located in southwestern Mexico, within the interior of the Guerrero composite terrane. Our data document that this region underwent progressive shortening during the Late Cretaceous, which resulted in regional uplift and unconformable deposition of continental red beds over a Lower Cretaceous marine arc succession. We interpret the continental rocks as the infill of a piggyback basin related to the early evolution of the Laramide orogeny. According to this scenario, the ∼94 Ma age obtained for the oldest continental strata constrains the beginning of the Laramide shortening in the Zihuatanejo area, which was thus the first area of the Laramide belt to be deformed. Considering that during terrane accretion the deformation propagates from the suture zone to the continent and terrane interiors, the timing of the Laramide deformation in southern Mexico cannot be explained as a direct consequence of the accretion of the Guerrero terrane. In fact, we document that the Laramide shortening started at the Cenomanian–Turonian boundary within the interior of the terrane, and migrated progressively eastward involving the suture belt and the continental mainland.

Based on the chronologic pattern documented for the Late Cretaceous–Paleogene shortening in southern Mexico, the Laramide deformation front can be envisioned as a tectonic wave that propagates from the present-day Pacific coast to the continental mainland, probably as the result of the increasing subduction rate or collision of a Caribbean terrane along the Mexican Pacific trench.

INTRODUCTION

Based on the style and age of deformation, the Late Cretaceous–Early Paleogene shortening structures in southern Mexico have been assimilated to the Laramide orogeny of the North American Cordillera (Campa et al., 1976; Dickinson et al., 1988; Salinas-Prieto et al., 2000; Nieto-Samaniego et al., 2006). Currently, terrane accretion, variations of the convergence rate, and interaction with the Caribbean plate have been proposed as the causes for the Mexican Laramide orogeny (Mendoza and Suástegui, 2000; Salinas-Prieto et al., 2000; Keppie, 2004; Cerca et al., 2007; Solari et al., 2007). However, the driving force for this regional shortening event has never been discussed in detail and remains a matter of debate. Only in the past decade the timing, geometry, and kinematics of the structures related to the Laramide orogeny began to be established in southern Mexico (Cabral-Cano et al., 2000a, 2000b; Elías-Herrera et al., 2000; Salinas-Prieto et al., 2000; Nieto-Samaniego et al., 2006; Cerca et al., 2007; Solari et al., 2007; Martini et al., 2009). Some workers support the notion that the Laramide shortening migrated from west to east during the Late Cretaceous–Early Paleogene (De Cserna et al., 1980; Nieto-Samaniego et al., 2006; Martini et al., 2009). However, such migration has been constrained by paleontologic and isotopic data from the eastern and central fold-and-thrust belt, but no structural study has been presented for the westernmost part. The timing of the Late Cretaceous shortening is crucial to test the terrane accretion model as a possible cause for the Laramide deformation. In fact, during the initial stage of terrane accretion the compressive stresses are confined along the collisional boundary, resulting in the formation of a strain-localized suture. Subsequently, the deformation eventually propagates away from the suture, producing a wide fold-and-thrust belt that involves the continental mainland and the interior of the terrane. Following such a scenario, rocks of the suture belt should record the oldest deformation, whereas the continental mainland and the terrane interior would be shortened in a subsequent stage, as the result of the progressive impingement of the terrane along the continental margin.

In this paper we present new structural and U-Pb geochronologic data from the Zihuatanejo area, which is located in southwestern Mexico, within the interior of a large Mesozoic terrane accreted to the North American Pacific margin. Our data improve the definition of the Laramide chronologic pattern in southern Mexico, and permit us to test the terrane accretion hypothesis.

OVERVIEW OF THE LARAMIDE DEFORMATION IN SOUTHWESTERN MEXICO

The Laramide orogeny is represented in southern Mexico by a NNW-SSE to NE-SW fold-and-thrust belt, which extends for ∼700 km from the present-day Pacific coast to the Veracruz basin (e.g., Nieto-Samaniego et al., 2006) (Figs. 1A and 1B). This paper will focus on the western part of the Laramide belt, where the timing and style of deformation have never been studied in detail. In the following sections we summarize the available data constraining the age of the Laramide shortening, as well as the mechanism proposed for this orogeny.

The Age of the Laramide Deformation

In southwestern Mexico, the timing of the Laramide shortening is best constrained by the age of the infills of the foreland basins, which were later deformed and incorporated into the Laramide wedge. According to paleontologic data from the foreland successions, the main shortening took place during the Early Maastrichtian–Paleocene between Temalac and Atenango del Río, in the Coniacian–Campanian in the Axaxacoalco and Teloloapan areas, and during the Turonian in the Arcelia region (Figs. 1B and 1C) (Guerrero-Suástegui et al., 1991; Hernández-Romano et al., 1997; Lang and Frerichs, 1998; Mendoza and Suástegui, 2000; Perrilliat et al., 2000).

In the Huetamo region (Fig. 1B), rocks deposited in a foreland basin are apparently lacking (Centeno-García et al., 2008; Martini et al., 2009). However, two major Laramidic folding pulses have been recognized in this area (Centeno-García et al., 2008; Martini et al., 2009). The first one caused regional uplift, which produced the end of calcareous sedimentation and the unconformable deposition of continental red beds over the marine strata. Scarce paleontologic and isotopic data from the marine and continental successions constrain this event between the late Cenomanian and Santonian (Buitrón-Sánchez and Pantoja-Alor, 1998; Mariscal-Ramos et al., 2005). Although the age of this shortening phase is poorly constrained, available data do not preclude an eastward migration of the Laramide contractional front. During the Paleocene, a second shortening event gently folded the continental strata and amplified the preexisting folds within the marine succession (Martini et al., 2009).

The kinematics and age of the shortening structures along the coastal region, between Zihuatanejo and Manzanillo (Fig. 1A), have not been studied in detail. However, the age of the deformation in this area is critical to define the migration pattern of the Laramide front and, consequently, to test the terrane accretion hypothesis.

The Cause for the Laramide Orogeny in Southern Mexico

The mechanism that produced the Laramide orogeny in southern Mexico is poorly understood. Several authors considered this regional deformation event to be a result of the accretion of a large Mesozoic terrane, the Guerrero composite terrane (Fig. 1), to the Mexican mainland (Salinas-Prieto et al., 2000; Mendoza and Suástegui, 2000; Keppie, 2004). According to these authors, the suture between the Guerrero terrane and nuclear Mexico is represented by the Teloloapan thrust system, along which the Guerrero terrane was overthrusted onto the Mesozoic cover of the Mexican craton (Fig. 1).

An alternative scenario suggests that, by analogy with the USA and Canadian Cordillera, the Laramide shortening in southern Mexico could be the result of a period of flat-slab subduction, related to an increase of the convergence rate along the Pacific trench (Solari et al., 2007).

Finally, an intriguing scenario has been proposed by Cerca et al. (2007). These authors consider that the Laramide shortening may be the result of the entering of the Caribbean plate between North and South America, producing the collision of an arc, an oceanic plateau, or an aseismic ridge with the Mexican Pacific margin.

STRATIGRAPHY OF THE ZIHUATANEJO AREA

A detailed stratigraphic description of the Zihuatanejo area has been recently presented in Martini et al. (2010). Here we present an upgrade of such stratigraphic framework, based on new field observations. The areal distribution and geometric relation between the main lithologic units are shown in the geologic maps of Figure 2. A synthesis of the stratigraphy is presented in Figure 3.

The oldest rocks exposed in the Zihuatanejo area are pre-Cretaceous, polydeformed, quartz-rich metasandstone, phyllite, and schists of the Las Ollas Complex (Centeno-García et al., 2008; Martini et al., 2009). They show a block in matrix texture, and contain decameter- to kilometer-scale slices of banded metagabbro, serpentinite, metabasalt, dunite, plagiogranite, and quartzite (Talavera-Mendoza, 2000; Martini et al., 2010). These rocks are unconformably overlain by a late Lower Cretaceous marine arc succession (Posquelite and Playa Hermosa formations), capped by Albian limestone with rudists (Ixtapa Formation) (Vidal-Serratos, 1986; Martini et al., 2010). The Lower Cretaceous rocks are moderately folded, and are in turn unconformably overlain by an Upper Cretaceous gently folded subaerial to shallow-marine arc succession, made up of widespread red beds (La Unión Formation) and scarce marine volcaniclastic sandstone (Zihuatanejo Formation), interbedded with andesite lava flows and rhyolitic tuff (Talavera-Mendoza et al., 2007; Martini et al., 2010). The Upper Cretaceous sandstone and conglomerate are made up of fragments of volcanic rocks and minor rock fragments from the Lower Cretaceous marine units. Detrital zircons from volcaniclastic sandstone of the Zihuatanejo Formation yielded a main U-Pb age peak at 85 Ma (Talavera-Mendoza et al., 2007).

We introduce here a new stratigraphic unit named Playa Larga Formation, which crops out between the homonymous beach, at the outer eastern side of the Zihuatanejo bay, and the village of El Coacoyul (Fig. 2A). This formation is made up of ∼250 m of horizontally bedded polymictic conglomerate, which unconformably overlies the Upper Cretaceous folded arc succession. The conglomerate is matrix supported, very poorly sorted, and is composed of subangular to angular boulders and cobbles of granite, granodiorite, diorite, and scarce porphyritic andesite, in a sandy matrix composed essentially of quartz and feldspar. The textural immaturity of the conglomerate indicates scarce transport, and suggests proximal supplying sources.

The Zihuatanejo volcano-sedimentary succession is cut and unconformably overlain by 48–39 Ma batholiths and undeformed volcanic rocks (El Camalote Formation), respectively (Valencia et al., 2009; Martini et al., 2010).

U-PB GEOCHRONOLOGY

Detrital zircons separated from two clastic rock samples were dated by the U-Pb method, in order to improve the chronostratigraphic framework of the Zihuatanejo area and, consequently, obtain major control on the timing of deformation. Mineral separation was carried out using the standard methodology (crushing, sieving, density and magnetic separations, and handpicking) at the mineral separation facility of Centro de Geociencias, Universidad Nacional Autónoma de México (UNAM). To assist interpretation, zircons were observed and imaged under cathodoluminescence, using an ELM3R luminoscope connected to a digital camera. Individual zircon ages were obtained by laser ablation–inductively coupled plasma mass spectrometry (LA–ICPMS) at the Laboratorio de Estudios Isotópicos (LEI), Centro de Geociencias, UNAM. Ablation of zircons was performed with a Resolution M-50/Lambda Physik LPX220 excimer laser, operating at a wavelength of 193 nm, and coupled to a Thermo X Series II quadrupole ICPMS. Details of the analytical methodology can be found in Solari et al. (2010) and http://www.geociencias.unam.mx/∼solari/index_files/LEI/LA-ICPMS.html. Tera and Wasserburg's (1972) concordia plots as well as error calculation were obtained using Isoplot v. 3.06 (Ludwig, 2004). Analyses that yielded >15% or <–5% discordant results are considered meaningless and thus discarded. Sample locations and details of the analytical data are shown in Figure 2 and the Supplemental Table1, respectively.

Sample UN1 is a lithic arkose collected near the base of the La Unión Formation, at the Barranca de San Diego ranch (Fig. 2B). This sample yielded abundant colorless, yellowish, and pinkish zircons that vary from 65 to 180 μm in size; 32% of them are euhedral, well to moderately elongated, suggesting low rates of transport and a local provenance. The rest of the grains are elongated to stubby, well rounded to subrounded, indicating extensive transport. Cathodoluminescence images show the predominance of oscillatory zoning, in some cases developed around xenocrystic cores. Th/U ratios are >0.1 for all crystals (Supplemental Table [see footnote 1]), indicating a magmatic origin (Rubatto, 2002). We performed 80-point ablation analysis, 73 points of which produced acceptable ages. Most of the euhedral zircons yielded concordant to slightly discordant ages between 88 and 185 Ma, with main peaks at 94, 123, 160, and 185 Ma (Fig. 4). More rounded grains yielded concordant ages that span from 237 to 1387 Ma, and define peaks at 247, 282 Ma, and minor peaks between the Silurian and the Late Mesoproterozoic (Fig. 4). Considering that the deposition of the continental succession was coeval with arc volcanism in the Zihuatanejo area (Martini et al., 2010) and that zircon morphology of the youngest population indicates a local provenance, we take the 94 Ma age as the best approximation for the beginning of the continental sedimentation in the study area.

Sample PL is a pebble conglomerate from the Playa Larga Formation (Fig. 2A). Zircons separated from this sample are yellowish to colorless and vary in size from 150 to 320 μm. The great majority of them are euhedral and prismatic, suggesting low rates of transport. Zircons are highly elongated to stubby, with a width/length ratio that spans from 1:5 to 1:1.5. Cathodoluminescence images show a continuous oscillatory zoning for all crystals. Th/U ratios are >0.09 (Supplemental Table [see footnote 1]), indicating a magmatic origin (Rubatto, 2002). We performed 89-point analysis, 72 points of which produced concordant to slightly discordant ages from 62 to 1948 Ma, with a single major peak at 67 Ma (Fig. 4). Jurassic to Proterozoic ages represent 26% of the determinations, and produce subordinate peaks (Fig. 4). Based on these data, the deposition of the Playa Larga Formation is constrained between 67 Ma, age of the youngest zircons population, and the 42 Ma U-Pb age reported for a granitic intrusive cutting the conglomerate (Valencia et al., 2009).

SHORTENING DEFORMATION IN THE ZIHUATANEJO AREA

The volcano-sedimentary succession of the Zihuatanejo area records multiple phases of shortening during the Mesozoic. In this section we present a detailed geometrical description and a statistical analysis of the main shortening events. Major structures are depicted in the geologic maps of Figure 2. Structural sections and stereographic projections are shown in Figures 5, 6, and 7 to illustrate the geometry of the deformation.

D1: Top to SW Shearing

The D1 deformation phase is represented by folds and thrusts that are developed exclusively in the pre-Cretaceous rocks of the Las Ollas Complex. F1 folds are cylindrical, and vary in size from the decameter to millimeter scale. They are recumbent isoclinal to tight folds (Figs. 5A and 5B), and show a constant vergence toward the SW. F1 folds vary from the type 2-1C of Ramsay (1967) (Fig. 5C), suggesting that they formed, at least in part, by a simple shear mechanism. Fold axes are horizontal to subhorizontal and trend N132° on average. Poles to bedding define single main clusters that plot close to the center of the nets (Fig. 5B), reflecting the parallelism of the flanks and the horizontal attitude of the axes. F1 folds are accompanied by an S1 axial plane foliation, which developed parallel to the bedding surface (S0) (Figs. 5B and 5D). S1 is spaced disjunctive, rough to anastomosing (Twiss and Moores, 1992) in metasandstone and schist, and is defined by the preferred orientation of neoblasts of actinolite, epidote, titanite, scarce hornblende, and quartz ribbons. Quartz shows undulating extinction, subgrain domains, and grain boundary migration, suggesting crystal-plastic deformation and recrystallization. Phyllites are characterized by a continuous fine foliation (Twiss and Moores, 1992) (Fig. 5D), defined by the preferred orientation of white mica, scarce quartz ribbons, and oxides.

T1 thrusts developed subparallel to the S1 foliation, with a N142° average direction. They dip both toward NE and SW, as the result of the following phase of shortening that folded previous T1 structures. L1 mineral lineations on T1 surfaces show a dominantly NE-SW direction, indicating that measured F1 axes are subperpendicular to the XZ plane of the finite strain ellipse (Fig. 5B). The vergence of the T1 faults is somewhat ambiguous at observed outcrops, because the deformation is disharmonic along the overriding surfaces, and kinematic indicators are refolded during the following D2shortening event. On the other hand, the vergence of F1 folds suggests a top to SW tectonic transport during the D1 deformation event.

D2: NE-SW Shortening

The most remarkable features of the study area are NW-SE–trending folds that involve both the Cretaceous volcano-sedimentary succession and the Las Ollas Complex. F2 folds are cylindrical and mostly symmetrical upright (Figs. 6, 7A, and 7B). In the Las Ollas Complex and the Lower Cretaceous marine succession, they vary in size from the millimeter to the kilometer scale, and show interlimb angles between 60° and 115°. In contrast, in the Upper Cretaceous rocks they developed from the decameter to the kilometer scale, with interlimb angles varying from 120° to 140° (Fig. 6). Sandstone and metasandstone competent layers develop 1B to 1C fold type of Ramsay (1967), whereas 1C to 2 types are characteristic of incompetent lutite and phyllite layers (Fig. 7C). F2 axial planes are vertical to moderately inclined, with a N141° main direction. Scarce NW-SE moderately recumbent folds are also observed, showing a constant vergence toward the NE. F2 axes are horizontal to gently SE dipping, and show an average direction of N138° (Figs. 6 and 7B). In the surroundings of La Unión and Zihuatanejo, F2 axes display a sigmoidal geometry in plan view (Fig. 2), which results from the local clockwise rotation of the D2 structures during a subsequent post-D2 right-lateral deformation. The latter is not the subject of this work and will be treated in a separate paper. Poles to bedding of kilometric F2 folds at Feliciano and La Salitrera areas (Figs. 2 and 6) cluster symmetrically in the NE and SW quadrants, reflecting the cylindrical nature of these structures and the horizontal attitude of the fold axes. Stereographic projections of these major structures are similar, suggesting that F2 folds formed during a homogeneous NE-SW regional shortening. Scarce mechanic striae on the fold limbs surfaces indicate that F2 structures formed, at least in part, by flexural slip.

F2 folds are accompanied by an S2 axial plane foliation. In the Cretaceous volcanic and sedimentary rocks, S2 is penetrative at the meter to decimeter scale, and is expressed as a subvertical spaced cleavage, slightly convergent toward the intradox of the folds. In contrast, in the metamorphic rocks of the Las Ollas Complex, S2 is as a subvertical zonal to discrete crenulation cleavage (Twiss and Moores, 1992), penetrative from the centimeter to submillimeter scale (Figs. 7D and 7E). The concentration of oxide and other insoluble minerals along the S2 surfaces indicates that this foliation formed mostly by a pressure-solution mechanism. Scarce neoblasts of white mica grew along the S2 foliation in metasandstone layers. The superposition of F2 on F1 folds developed type 3 interference structures (Ramsay, 1967) in the rocks of the Las Ollas Complex (Fig. 8).

DISCUSSION

The integration of our new U-Pb ages and structural analysis with previous data provides a more coherent interpretation of the deformation history and tectonics of southwestern Mexico.

The Shortening Deformation in the Zihuatanejo Area

Previous studies briefly described the Mesozoic rocks of the Zihuatanejo region as essentially undeformed, showing only a constant westward dip (Vidal-Serratos, 1986; Mendoza and Suástegui, 2000). However, we document at least two major shortening events in this area.

The D1 phase produced isoclinal to tight recumbent folds and thrusts in the metamorphic rocks of the Las Ollas Complex. The Albian fossil fauna from the Ixtapa Formation predates the age of this deformation event. Comparable structures have been reported within the Triassic rocks of the Arteaga Complex (Centeno-García et al., 2003) (Fig. 1B), which is interpreted to be the lateral equivalent of the Las Ollas Complex (Martini et al., 2010). The main deformation in the Arteaga area took place between the Late Triassic and the Middle Jurassic (Centeno-García et al., 2008), which permits a correlation with the D1 phase recognized in the study area. The mechanism that triggered the D1 deformation event is unknown. However, the block-in-matrix texture of the Las Ollas and Arteaga complexes, and the coexistence of oceanic and continental blocks suggest that D1 folds and thrusts may be related to the progressive accretion at the front of an accretionary prism, which is also consistent with the style documented for these structures.

The D2 shortening phase produced NE-SW upright folds that involved both the Las Ollas Complex and the Cretaceous volcano-sedimentary succession. The age of such a deformation event is constrained between 94 and 48 Ma, which are the ages of the youngest dated rocks affected by the D2 shortening (sample UN1, this work) and the oldest U-Pb age for a batholith cutting the F2 folds, respectively (Martini et al., 2010). Based on the age range and the style of deformation, we consider the D2 deformation phase as part of the Mexican Laramide orogeny.

It is worth noting that a major angular unconformity has been documented between the Lower Cretaceous marine rocks and the overlying Upper Cretaceous continental to shallow marine successions (Martini et al., 2010). Such unconformity, together with the abrupt change from marine to mostly continental sedimentation, documents a regional uplift in the Zihuatanejo area. Effects of the progressive uplift are recorded by the clastic rocks of the La Union, Zihuatanejo, and Playa Larga formations. In fact, sandstone and conglomerate of the La Union and Zihuatanejo formations are dominantly composed of fragments of volcanic rocks and other less abundant lithologies proceeding from the Lower Cretaceous marine succession. Detrital zircons from two sandstones of these formations show Th/U ratios typical of magmatic origin, and main peaks at 94 Ma (sample UN1, this work) and 85 Ma (Talavera-Mendoza et al., 2007). In addition, the Playa Larga conglomerate mainly consists of granite to diorite cobbles and boulders, and contains detrital zircons with ages mostly comprised between 95 and 62 Ma (sample PL, this work). These data indicate that the clastic rocks that unconformably overlie the Lower Cretaceous marine succession were derived mostly from the erosion of the Late Cretaceous Zihuatanejo arc. The progressive uplift initially produced the erosion of the Late Cretaceous volcanic edifices that supplied detritus to the La Union and Zihuatanejo formations. Subsequently the exhumation and erosion of the arc roots resulted in the deposition of the Playa Larga Formation. Based on these considerations, the beginning of the uplift can be constrained at ∼94 Ma, which is the age that best approximates the beginning of the continental sedimentation in the study area.

The uplift documented in the Zihuatanejo region fits temporally with the age range estimated for the D2 deformation event. In this sense, the F2 folds of the study area can be interpreted in terms of a progressive NE-SW shortening and thickening of the crust. In this scenario, the shortening of the Lower Cretaceous marine succession started at ∼94 Ma, producing regional uplift in the Zihuatanejo area and the beginning of the continental sedimentation. Subsequently, the progressive shortening produced the flattening and amplification of the earlier F2 folds and the gentle folding of the Upper Cretaceous red beds. This interpretation explains the increase in the F2 interlimb angle from the Lower to the Upper Cretaceous successions and justifies the occurrence of a major unconformity separating rocks with the same style of deformation.

The Laramide Continental Sedimentation in the Zihuatanejo Area

Our data document that the Laramide deformation in the Zihuatanejo area started at ∼94 Ma, producing progressive shortening and crustal thickening that triggered widespread continental sedimentation. A comparable tectonic history has been documented in the Huetamo region, ∼70 km northeast of Zihuatanejo (Martini et al., 2009) (Fig. 1B). Considering that the westernmost Laramide regional thrust system documented in southern Mexico is located in the Arcelia area (Figs. 1B and 1C), we suggest that the Zihuatanejo and Huetamo Lower Cretaceous successions were part of a single tectonic block that overthrusted the marine rocks of the Teloloapan area at the Arcelia thrust system (Fig. 9). This is supported by the lack of foreland-related successions from Zihuatanejo to Arcelia. In fact, the oldest Laramidic foreland successions recognized in southern Mexico are Turonian turbidites of the Teloloapan region (Mendoza and Suástegui, 2000), exposed immediately to the east of the Arcelia thrust system. In this frame, the continental red beds of the Zihuatanejo and Huetamo regions were deposited on the back of the Turonian Laramidic front, contemporaneously with the sedimentation in the foreland basin (Fig. 9). Based on these considerations, we conclude that the Upper Cretaceous continental rocks of the Zihuatenjo-Huetamo area were likely deposited in a piggyback basin that formed during the early Laramidic shortening.

Timing of the Laramide Shortening in Southwestern Mexico

Our data from the Zihuatanejo area support the eastward migration of the Laramide contractile front from the western part of the Guerrero terrane to the Mexican mainland. In fact, the main shortening started at ∼94 Ma between Zihuatanejo and Huetamo, and migrated progressively eastward in the Teloloapan and Axaxacoalco areas during the Coniacian–Campanian, and between Atenango del Río and Temalac during the early Maastrichtian and Paleocene. Such migration is a direct consequence of the tectonic process that triggered the Laramide shortening across the Mexican Cordillera, and must be taken into account to understand the driving force that produced the Late Cretaceous–Paleogene fold-and-thrust belt. One of the causes proposed for the Laramide shortening in Mexico is the accretion of the Guerrero terrane. Terrane accretion has been recognized as a main tectonic process producing regional shortening across the North American Cordillera (Coney et al., 1980; Hildebrand, 2009). During the initial stage of terrane accretion, tectonic compressive stresses are generally confined along the collisional boundary, resulting in the formation of a strain-localized suture. Depending on the thickness and strength of the terrane and continental margin, the deformation eventually propagates away from the suture and may result in the formation of a wide fold-and-thrust belt, which involves the continental mainland and the interior of the terrane. In the case of southern Mexico, the collisional suture zone has been inferred to correspond to the Teloloapan thrust system (Campa and Coney, 1983). The main shortening at both sides of this major fault started in the Coniacian. According to previous considerations, the progressive impingement of the Guerrero terrane should have resulted in the migration of the deformation toward the east, producing the shortening of continental mainland, and toward the west, triggering deformation within the interior of the terrane. On the other hand, our data document that the Laramide shortening began in the Zihuatanejo and Huetamo areas during the Cenomanian–Turonian boundary, and become progressively younger to the east. In this sense, the timing of the Laramide deformation in southwestern Mexico cannot be explained as a direct consequence of the accretion of the Guerrero terrane.

Based on the chronologic pattern documented for the Late Cretaceous–Paleogene shortening in southern Mexico, the Laramide deformation front resembles a tectonic wave that propagates from the present-day Pacific coast toward the interior of to the Mexican mainland. This suggests that the Guerrero terrane was already accreted to the Mexican mainland by the Late Cretaceous, and that increasing subduction rates or collision of a Caribbean terrane along the Pacific continental margin of Mexico remain available hypotheses to explain the Laramide shortening in southern Mexico.

CONCLUSION

Stratigraphic and structural data, combined with U-Pb geochronology, document a major Laramidic shortening in the Zihuatanejo area. This deformation phase produced progressive folding and regional uplift, which resulted in the finalization of the Early Cretaceous marine sedimentation, and the unconformable deposition of a 2000-m-thick continental succession during the Late Cretaceous. We interpret the continental rocks as the infill of a piggyback basin related to the early evolution of the Laramide orogeny. According to this scenario, the oldest continental strata constrain the beginning of the Laramide shortening in the Zihuatanejo area at ∼94 Ma, supporting the eastward migration of the orogenic deformation from the western part of the Guerrero terrane to the Mexican mainland. Considering that during terrane accretion the deformation propagates from the suture zone to the continent and terrane interiors, the timing of the Laramide deformation in southern Mexico cannot be explained as a direct consequence of the accretion of the Guerrero terrane. In fact, we document that the Laramide shortening started at the Cenomanian–Turonian boundary within the interior of the terrane, and migrated progressively eastward involving the suture belt and the continental mainland. Based on the chronologic pattern documented for the Late Cretaceous–Paleogene shortening in southern Mexico, the Laramide deformation front resembles a tectonic wave that propagates from the present-day Pacific coast to the continental interior, probably as the result of the increase in the subduction rates or the collision of a Caribbean terrane along the Mexican Pacific trench.

The research was funded by Consejo Nacional de Ciencias y Tecnología (CONACyT) grant SEP 2003-C02-42642 to Luca Ferrari. Juan Tomás Vasquez prepared thin sections. We thank Luigi Solari for assistance with LA-ICP-MS at Laboratorio de Estudios Isotópicos (LEI). The helpful revisions of Gabriel Chávez-Cabello and an anonymous reviewer greatly improved this manuscript.

1Supplemental Table. Excel file. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00743.S1 or the full-text article on www.gsapubs.org to view the supplemental table.