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Title: Late Miocene to Quaternary extension at the northern boundary of the Jalisco block, western Mexico: The Tepic-Zacoalco rift revised
Author: Ferrari, L.
Rosas-Elguera, J.
Issue Date: 2000
Abstract: In the last decade several tectonic models have considered the Jalisco block (JB) as an incipient microplate that is rifting away from mainland Mexico since Pliocene time due to an eastward "jump" of the East Pacific Rise. These models predict normal and right-lateral faulting along the northern boundary of the JB, called the Tepic-Zacoalco rift (TZR). However, the Plio-Quaternary kinematics of the Jalisco block has remained unclear due to the scarcity of structural data along its boundaries. We present a new picture of the structure, the kinematics and time of deformation along the TZR obtained by geological and structural mapping integrated with subsurface stratigraphic data provided by deep geothermal drilling. What has previously been defined as the TZR is actually a combination of different fault systems developed during Late Miocene (12-9 Ma), Early Pliocene (5.5-3.5 Ma) and, to a lesser extent, in Late Pliocene to Quaternary times. These structures can be grouped in three branches: 1) a northwestern branch, named the Pochotitán fault system, consisting of listric faults belonging to the Gulf Extensional Province; 2) a central branch made of en echelon grabens which reactivated the boundary between the JB and the Sierra Madre Occidental; 3) a southern branch constituted by detachment faults located inside the Jalisco block. The Pochotitán fault system is composed of north-northwest-trending, high angle normal faults which tilt up to 35° towards east-northeast blocks of the Sierra Madre Occidental succession. These faults accommodate at least 2,000 m of vertical displacement related to 12-9 Ma "Protogulf" extension. The central branch consists of two composite grabens developed along an older transcurrent deformation zone. The western one, the Compostela-Ceboruco graben, is a complex asymmetrical depression developed during Late Miocene and Pliocene time with vertical displacement exceeding 2,000 m. Toward the east is the Plan de Barrancas-Santa Rosa graben, a west-southwest-trending and 30-km-wide depression, bounded to the north by the Santa Rosa-Cinco Minas fault and to the south by the Plan de Barrancas fault and its buried southeastern prolongation detected by geophysical studies under the Tequila volcano and the southwestern part of La Primavera caldera. The graben displays a total vertical displacement of ∼550 m mainly achieved during early Pliocene time. The southern branch is formed by the Amatlán de Cañas half-graben and the Ameca-San Marcos detachment fault. They are south- to southwest-dipping listric normal fault systems with a minimum of 1,400 m of vertical displacement largely produced during the Pliocene. Only the San Marcos faults show clear geologic evidence of Quaternary tectonic activity. The great majority of the 295 measured mesofaults of Late Miocene to Quaternary age have pitches higher than 45° and inclinations ranging between 45° and 75°, typical of normal faults. The paleo-stress field has been computed by fault-slip data inversion and cinder cone alignment at 40 locations and the computed stress tensors are always extensional (vertical maximum principal stress). The average direction of extension (sHmin) is 72° for the Late Miocene extension in the Gulf area, whereas for Pliocene and Quaternary time, it ranges from 35° to 2°. Displacement of dated geologic units constrains an average minimum deformation rate for each fault system which decreases from 0.75 mm/yr for the Late Miocene to 0.1 mm/yr for the Quaternary. These results confirm the absence of strike-slip deformation along the TZR in Plio-Quaternary times and indicate that the JB is not actively separating from the Mexican mainland. In our view, the TZR represents an intraplate deformation zone which reactivated the tectonic boundary between the Sierra Madre Occidental and the JB. These deformations are more likely related to plate boundary forces rather than to an eastward relocation of the East Pacific Rise under continental Mexico. The small divergent motion between the Rivera and Cocos plate and the steep subduction of the Rivera plate can account for the deformation observed at the boundaries of the Jalisco block.
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