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Basin and Range extension, which began in the Tertiary and continues today, is well documented in Mexico north of the Trans-Mexican volcanic belt. In contrast, evidence for Basin and Range extension in southern Mexico is limited. Tertiary structures of western and central Mexico north of the Trans-Mexican volcanic belt are predominantly north-northwest-trending basins, which formed during regional west-southwest-east-northeast extension in the Miocene. The extensional province is separated into two belts by the Sierra Madre Occidental plateau (Figure 1): (1) a narrow western belt, which formed in response to motion of the Jalisco block and rifting in the Gulf of California and (2) an eastern belt, which is a continuation of the Basin and Range province of the southwestern United States. Extension began approximately 30 Ma. South of the Trans-Mexican volcanic belt, evidence for Basin and Range extension is largely confined to the Oaxaca basin, a north-northwest-trending Tertiary graben in south-central Mexico (Figure 1). Using spectral stratigraphic methods in combination with field mapping, we discovered another north-northwest-trending Tertiary basin, the Arcelia graben, approximately 200 km west of the Oaxaca basin and 50 km south of the Trans-Mexican volcanic belt. Arcelia graben subsidence began in the early Tertiary and mostly ended prior to accumulation of upper Oligocene volcanic rocks, indicating Basin and Range extension in this area was limited to a < 32 m.y. interval in the Tertiary. Extension near Arcelia is among the oldest Basin and Range style deformation documented in Mexico. Most subsidence in the Arcelia graben preceded onset of Oaxaca basin subsidence by at least 8 m.y. This suggests the intriguing possibility of eastward migration of Basin and Range extension in southern Mexico during the middle Tertiary.

The base of the uppermost Cretaceous and Tertiary section throughout southern Mexico is defined by a regional unconformity, which records cessation of marine sedimentation and onset of continental deposition and volcanism. The end of marine conditions coincides temporally with a west-southwest-east-northeast-directed compressional event, which is kinematically equivalent to the Laramide orogeny of the Mexican Sierra Madre Oriental and the North American Cordillera. Volcanism commenced shortly after Laramide folding and thrusting and continued throughout the Tertiary in southern Mexico and is ongoing in the Trans-Mexican volcanic belt.

Our study area is 50 kms south of the Trans Mexican volcanic belt in northern Guerrero State between the towns of Ciudad Altamirano and Nuevo Copaltepec (Figure 2). Volcanic, volcaniclastic, and continental sedimentary rocks of the Tertiary Balsas Group, the Oligocene Tilzapotla Rhyolite, and unnamed Miocene units crop out in the area. The Balsas includes three mappable sequences separated by two unconformities: the basal Galeana Member and two younger, unnamed units. Galeana beds are tightly folded about north-northwest-trending axes, whereas the younger Balsas rocks are only gently warped. The upper Oligocene Tilzapotla Rhyolite overlies Balsas strata unconformably and begins a sequence of upper Oligocene and Miocene volcanic rock. Cretaceous shallow marine strata of the Morelos, Mal Paso, and Mexcala Formations, which lie below the regional unconformity at the base of the Tertiary section, are exposed north and west of Ciudad Altamirano. Older metamorphic rocks equivalent to the Taxco Schist and the Roca Verde Taxco Viejo Formations crop out north and east of Nuevo Copaltepec.

The age of the Galeana Member is constrained by its unconformable overlap of the Upper Cretaceous Mal Paso Formation along the western edge of the study area. Furthermore, the style and kinematics of the tight east-northeast-verging folds in the Galeana Member are similar to Late Cretaceous and early Tertiary structures mapped in Cretaceous marine strata to the west and east. Deposition of the siliciclastic strata, therefore, either preceded or coincided with regional west-southwest-east-northeast shortening during the latest Cretaceous and earliest Tertiary. In contrast, the middle Balsas rocks above the Galeana Member are only gently warped, compatible with accumulation after cessation of Laramide-age folding. The ages of the middle and younger Balsas sequences, therefore, are younger than earliest Tertiary, but older than Oligocene, as required by their position below the Tilzapotla Rhyolite.

Dates from the continental beds of the Balsas Group are sparse. The Balsas, however, is assumed to be Upper Eocene to Oligocene throughout southern Mexico. The regional change from shallow marine to continental sedimentation is consistent with a Tertiary age for Balsas strata. The Oligocene date comes from fossils within conglomeratic beds ~ 50 kms northeast of Nuevo Copaltepec. Radiometric ages for volcanic rocks of the Balsas Group include 42 ± 1 Ma for basalt from an unknown locality and 36 ± 2 Ma to 18 ± 1 Ma for volcanic rocks ~ 60 kms north of Ciudad Altamirano. Unfortunately, the stratigraphic affinity of these strata is unknown.

To map Upper Cretaceous and Tertiary rocks, we used spectral stratigraphic techniques to interpret Landsat TM data covering the area. The image used in this study was a color composite which portrays TM bands 7 (2.08µm-2.35µm), 3 (0.63µm-0.69µm), and 4 (0.76µm-0.90µm) as blue, green, and red, respectively (Figure 3). In this image, red beds are yellow and vegetation is red. The image scale of 1:50,000 was chosen to conform to the scale of topographic maps issued by the Instituto Nacional de Estadistica Geografia y Informatica of Mexico. The four topographic maps which cover the area are the Ciudad Altamirano, Arcelia, Palmar Chico, and Amatepec quadrangles.

Rocks in the area were divided into seven units based on spectral and morphological criteria (Figure 4). Layers within units were identified by image color. The minimum stratigraphic thickness, that could be resolved on the 1:50,000 image, was approximately 10 m in areas of shallowly dipping beds on shallow slopes that yield wide outcrop widths Three-point solutions were determined with the aid of the topographic maps in order to derive strikes and dips. Because the choice of points is critical and dips frequently are overestimated from those measured in the field, the minimum dip configuration was used.

Stratigraphic columns (Figure 5) and cross sections (Figure 6) were constructed from the geologic interpretation of the image to allow correlation, to estimate unit thicknesses, and to constrain structural styles. Depths to unit contacts in the subsurface were based on maximum unit thicknesses and the identification of marker horizons at the surface. Constant unit thickness was assumed in the creation of cross-sections, the estimation of displacements across faults, and the continuation of folds in the subsurface. This assumption may not be valid in areas dominated by volcanic and volcaniclastic rocks, which typically exhibit rapid facies and thickness variations along strike. The assumption, however, allowed construction of cross-sections which were geologically reasonable.

Spectral stratigraphic units were initially classified purely by color, morphology, and texture. No age or lithologic information was included. Because direct determination of lithology is not possible from TM and topographic data alone, lithologic inferences about the units were based initially on correlation with previously interpreted spectral stratigraphic units to the south in the Mesa Los Caballos region where units were field-checked. After delineation of the spectral stratigraphic units, interpretation of the geologic map, and construction of cross sections, relative age assignments were made. From these, lithologic units were matched to spectral stratigraphic units (Figure 7). The image interpretation was checked in the field.

A north-northwest-trending fault-bound basin, here named the Arcelia graben, controls the distribution of Tertiary rocks between Ciudad Altamirano and Nuevo Copaltepec (Figure 4). The width of the graben is ~ 20 km and its length is > 45 km. The western boundary of the graben is a single, east-dipping fault across which basal beds of the Galeana Member in the footwall are juxtaposed against strata of the upper Balsas Group in the hanging wall. Throw is estimated at > 2.0 km. The eastern graben boundary is a complex zone of steep, west-dipping faults with a collective vertical displacement of ~ 5.0 km based on exposure of Mesozoic metamorphic rocks in the footwall of the easternmost fault of the zone and lower Tertiary strata of the upper Balsas in the hanging wall of the westernmost fault (Figure 6).

The stratigraphic section in the graben is continuous from the middle Balsas Group through the Tilzapotla Rhyolite and younger Miocene volcanics (Figure 8). West of the graben, however, strata of the upper Balsas are above beds of the Galeana Member and are the youngest rocks exposed. Approximately 2.0 km of the lower Tertiary and 4.0 km of the upper Tertiary section, therefore, are absent on the western flank of the Arcelia graben. This requires either (1) accumulation of middle Balsas rocks exclusively in the graben or (2) selective erosion of middle Balsas rocks from topographically elevated regions along the western edge of the graben. We favor the first alternative because the base of the upper sequence of the Balsas is the same elevation across the western-bounding fault of the graben. If middle Balsas rocks existed west of the graben, they must have been deposited and eroded between deformation of Galeana beds and accumulation of upper Balsas strata. A simpler explanation is graben subsidence concurrent with volcanism within the basin, as represented by the middle Balsas sequence, during post-Laramide early Tertiary extension. The end of graben subsidence in the west is constrained by overlap of the western-bounding fault of the graben by upper Balsas rocks. In addition, an intrusion, correlated with Miocene rocks to the east, pierces the western graben-bounding fault at its southern extreme.

In contrast, the eastern-bounding faults of the graben cut rocks as young as Miocene. Deformation style, therefore, changed from graben subsidence during the Oligocene to half-graben formation during the Miocene. The gentle dips of Miocene strata in the hanging walls of the eastern graben-bounding faults are consistent with only a small amount of normal displacement during the late Tertiary.

Assuming that normal fault strike is perpendicular to the minimum compressive stress, s3, the north-northwest strikes of the graben-bounding faults are compatible with basin formation during east-northeast-west-southwest-oriented extension. Additional evidence for near east-west extension comes from the occurrence of numerous north-striking dikes ~ 25 kms southeast of Ciudad Altamirano. The maximum age of the dikes is constrained as Oligocene by the upper Balsas rocks they intrude. The minimum age, however, is not known. The dikes are cut by a west-northwest-striking vertical fault, which may be related to late Tertiary and Quaternary strike-slip faulting in southern Mexico and the Trans-Mexican volcanic belt to the north. Minor left-lateral faulting of late Eocene to Miocene age has been documented in southern Guerrero and southern Oaxaca. The dikes attest to continued near east-west extension after normal displacement along the western graben-bounding fault had stopped during the early Tertiary. Extension was superseded by strike-slip tectonics along east-west striking faults by the late Tertiary.

The east-northeast-west-southwest orientation of extension, formation of north-northwest-trending grabens, and coeval volcanism are characteristic of the Basin and Range province of northern Mexico. The similar geologic history of the Arcelia graben and environs, therefore, argues for continuation of Basin and Range style deformation south of the Trans Mexican volcanic belt. Additional evidence for Basin and Range style extension in southern Mexico comes from central Oaxaca, where north-northwest-striking grabens exist in Miocene rocks. Unlike the Arcelia graben, however, where subsidence was largely complete near the beginning of the Oligocene, subsidence in the Oaxaca Basin occurred ca. 19 to 12 Ma during the Miocene.

The questions we address are:
Is extension south of the Trans Mexican Volcanic Belt related to Basin and Range style extension to the north?
Does extension south of the Trans Mexican Volcanic Belt young toward the east?
What is the role of the Chortis block in the evolution of southern Mexico?

Funding provided by NASA.

Spectral stratigraphy of central Mexico: Neogene tectonics south of the Mexican Volcanic Belt

Basin and Range extension, which began in the Tertiary and continues today, is well documented in Mexico north of the Trans-Mexican volcanic belt. In contrast, evidence for Basin and Range extension in southern Mexico is limited. Tertiary structures of western and central Mexico north of the Trans-Mexican volcanic belt are predominantly north-northwest-trending basins, which formed during regional west-southwest-east-northeast extension in the Miocene. The extensional province is separated into two belts by the Sierra Madre Occidental plateau (Figure 1): (1) a narrow western belt, which formed in response to motion of the Jalisco block and rifting in the Gulf of California and (2) an eastern belt, which is a continuation of the Basin and Range province of the southwestern United States. Extension began approximately 30 Ma. South of the Trans-Mexican volcanic belt, evidence for Basin and Range extension is largely confined to the Oaxaca basin, a north-northwest-trending Tertiary graben in south-central Mexico (Figure 1). Using spectral stratigraphic methods in combination with field mapping, we discovered another north-northwest-trending Tertiary basin, the Arcelia graben, approximately 200 km west of the Oaxaca basin and 50 km south of the Trans-Mexican volcanic belt. Arcelia graben subsidence began in the early Tertiary and mostly ended prior to accumulation of upper Oligocene volcanic rocks, indicating Basin and Range extension in this area was limited to a < 32 m.y. interval in the Tertiary. Extension near Arcelia is among the oldest Basin and Range style deformation documented in Mexico. Most subsidence in the Arcelia graben preceded onset of Oaxaca basin subsidence by at least 8 m.y. This suggests the intriguing possibility of eastward migration of Basin and Range extension in southern Mexico during the middle Tertiary. The base of the uppermost Cretaceous and Tertiary section throughout southern Mexico is defined by a regional unconformity, which records cessation of marine sedimentation and onset of continental deposition and volcanism. The end of marine conditions coincides temporally with a west-southwest-east-northeast-directed compressional event, which is kinematically equivalent to the Laramide orogeny of the Mexican Sierra Madre Oriental and the North American Cordillera. Volcanism commenced shortly after Laramide folding and thrusting and continued throughout the Tertiary in southern Mexico and is ongoing in the Trans-Mexican volcanic belt. Our study area is 50 kms south of the Trans Mexican volcanic belt in northern Guerrero State between the towns of Ciudad Altamirano and Nuevo Copaltepec (Figure 2). Volcanic, volcaniclastic, and continental sedimentary rocks of the Tertiary Balsas Group, the Oligocene Tilzapotla Rhyolite, and unnamed Miocene units crop out in the area. The Balsas includes three mappable sequences separated by two unconformities: the basal Galeana Member and two younger, unnamed units. Galeana beds are tightly folded about north-northwest-trending axes, whereas the younger Balsas rocks are only gently warped. The upper Oligocene Tilzapotla Rhyolite overlies Balsas strata unconformably and begins a sequence of upper Oligocene and Miocene volcanic rock. Cretaceous shallow marine strata of the Morelos, Mal Paso, and Mexcala Formations, which lie below the regional unconformity at the base of the Tertiary section, are exposed north and west of Ciudad Altamirano. Older metamorphic rocks equivalent to the Taxco Schist and the Roca Verde Taxco Viejo Formations crop out north and east of Nuevo Copaltepec. The age of the Galeana Member is constrained by its unconformable overlap of the Upper Cretaceous Mal Paso Formation along the western edge of the study area. Furthermore, the style and kinematics of the tight east-northeast-verging folds in the Galeana Member are similar to Late Cretaceous and early Tertiary structures mapped in Cretaceous marine strata to the west and east. Deposition of the siliciclastic strata, therefore, either preceded or coincided with regional west-southwest-east-northeast shortening during the latest Cretaceous and earliest Tertiary. In contrast, the middle Balsas rocks above the Galeana Member are only gently warped, compatible with accumulation after cessation of Laramide-age folding. The ages of the middle and younger Balsas sequences, therefore, are younger than earliest Tertiary, but older than Oligocene, as required by their position below the Tilzapotla Rhyolite. Dates from the continental beds of the Balsas Group are sparse. The Balsas, however, is assumed to be Upper Eocene to Oligocene throughout southern Mexico. The regional change from shallow marine to continental sedimentation is consistent with a Tertiary age for Balsas strata. The Oligocene date comes from fossils within conglomeratic beds ~ 50 kms northeast of Nuevo Copaltepec. Radiometric ages for volcanic rocks of the Balsas Group include 42 ± 1 Ma for basalt from an unknown locality and 36 ± 2 Ma to 18 ± 1 Ma for volcanic rocks ~ 60 kms north of Ciudad Altamirano. Unfortunately, the stratigraphic affinity of these strata is unknown. To map Upper Cretaceous and Tertiary rocks, we used spectral stratigraphic techniques to interpret Landsat TM data covering the area. The image used in this study was a color composite which portrays TM bands 7 (2.08µm-2.35µm), 3 (0.63µm-0.69µm), and 4 (0.76µm-0.90µm) as blue, green, and red, respectively (Figure 3). In this image, red beds are yellow and vegetation is red. The image scale of 1:50,000 was chosen to conform to the scale of topographic maps issued by the Instituto Nacional de Estadistica Geografia y Informatica of Mexico. The four topographic maps which cover the area are the Ciudad Altamirano, Arcelia, Palmar Chico, and Amatepec quadrangles. Rocks in the area were divided into seven units based on spectral and morphological criteria (Figure 4). Layers within units were identified by image color. The minimum stratigraphic thickness, that could be resolved on the 1:50,000 image, was approximately 10 m in areas of shallowly dipping beds on shallow slopes that yield wide outcrop widths Three-point solutions were determined with the aid of the topographic maps in order to derive strikes and dips. Because the choice of points is critical and dips frequently are overestimated from those measured in the field, the minimum dip configuration was used. Stratigraphic columns (Figure 5) and cross sections (Figure 6) were constructed from the geologic interpretation of the image to allow correlation, to estimate unit thicknesses, and to constrain structural styles. Depths to unit contacts in the subsurface were based on maximum unit thicknesses and the identification of marker horizons at the surface. Constant unit thickness was assumed in the creation of cross-sections, the estimation of displacements across faults, and the continuation of folds in the subsurface. This assumption may not be valid in areas dominated by volcanic and volcaniclastic rocks, which typically exhibit rapid facies and thickness variations along strike. The assumption, however, allowed construction of cross-sections which were geologically reasonable. Spectral stratigraphic units were initially classified purely by color, morphology, and texture. No age or lithologic information was included. Because direct determination of lithology is not possible from TM and topographic data alone, lithologic inferences about the units were based initially on correlation with previously interpreted spectral stratigraphic units to the south in the Mesa Los Caballos region where units were field-checked. After delineation of the spectral stratigraphic units, interpretation of the geologic map, and construction of cross sections, relative age assignments were made. From these, lithologic units were matched to spectral stratigraphic units (Figure 7). The image interpretation was checked in the field. A north-northwest-trending fault-bound basin, here named the Arcelia graben, controls the distribution of Tertiary rocks between Ciudad Altamirano and Nuevo Copaltepec (Figure 4). The width of the graben is ~ 20 km and its length is > 45 km. The western boundary of the graben is a single, east-dipping fault across which basal beds of the Galeana Member in the footwall are juxtaposed against strata of the upper Balsas Group in the hanging wall. Throw is estimated at > 2.0 km. The eastern graben boundary is a complex zone of steep, west-dipping faults with a collective vertical displacement of ~ 5.0 km based on exposure of Mesozoic metamorphic rocks in the footwall of the easternmost fault of the zone and lower Tertiary strata of the upper Balsas in the hanging wall of the westernmost fault (Figure 6). The stratigraphic section in the graben is continuous from the middle Balsas Group through the Tilzapotla Rhyolite and younger Miocene volcanics (Figure 8). West of the graben, however, strata of the upper Balsas are above beds of the Galeana Member and are the youngest rocks exposed. Approximately 2.0 km of the lower Tertiary and 4.0 km of the upper Tertiary section, therefore, are absent on the western flank of the Arcelia graben. This requires either (1) accumulation of middle Balsas rocks exclusively in the graben or (2) selective erosion of middle Balsas rocks from topographically elevated regions along the western edge of the graben. We favor the first alternative because the base of the upper sequence of the Balsas is the same elevation across the western-bounding fault of the graben. If middle Balsas rocks existed west of the graben, they must have been deposited and eroded between deformation of Galeana beds and accumulation of upper Balsas strata. A simpler explanation is graben subsidence concurrent with volcanism within the basin, as represented by the middle Balsas sequence, during post-Laramide early Tertiary extension. The end of graben subsidence in the west is constrained by overlap of the western-bounding fault of the graben by upper Balsas rocks. In addition, an intrusion, correlated with Miocene rocks to the east, pierces the western graben-bounding fault at its southern extreme. In contrast, the eastern-bounding faults of the graben cut rocks as young as Miocene. Deformation style, therefore, changed from graben subsidence during the Oligocene to half-graben formation during the Miocene. The gentle dips of Miocene strata in the hanging walls of the eastern graben-bounding faults are consistent with only a small amount of normal displacement during the late Tertiary. Assuming that normal fault strike is perpendicular to the minimum compressive stress, s3, the north-northwest strikes of the graben-bounding faults are compatible with basin formation during east-northeast-west-southwest-oriented extension. Additional evidence for near east-west extension comes from the occurrence of numerous north-striking dikes ~ 25 kms southeast of Ciudad Altamirano. The maximum age of the dikes is constrained as Oligocene by the upper Balsas rocks they intrude. The minimum age, however, is not known. The dikes are cut by a west-northwest-striking vertical fault, which may be related to late Tertiary and Quaternary strike-slip faulting in southern Mexico and the Trans-Mexican volcanic belt to the north. Minor left-lateral faulting of late Eocene to Miocene age has been documented in southern Guerrero and southern Oaxaca. The dikes attest to continued near east-west extension after normal displacement along the western graben-bounding fault had stopped during the early Tertiary. Extension was superseded by strike-slip tectonics along east-west striking faults by the late Tertiary. The east-northeast-west-southwest orientation of extension, formation of north-northwest-trending grabens, and coeval volcanism are characteristic of the Basin and Range province of northern Mexico. The similar geologic history of the Arcelia graben and environs, therefore, argues for continuation of Basin and Range style deformation south of the Trans Mexican volcanic belt. Additional evidence for Basin and Range style extension in southern Mexico comes from central Oaxaca, where north-northwest-striking grabens exist in Miocene rocks. Unlike the Arcelia graben, however, where subsidence was largely complete near the beginning of the Oligocene, subsidence in the Oaxaca Basin occurred ca. 19 to 12 Ma during the Miocene. The questions we address are: Is extension south of the Trans Mexican Volcanic Belt related to Basin and Range style extension to the north? Does extension south of the Trans Mexican Volcanic Belt young toward the east? What is the role of the Chortis block in the evolution of southern Mexico? Funding provided by NASA.