Arc plutonism in a transtensional regime : the late Palaeozoic Totoltepec pluton, Acatlán Complex, southern Mexico

The ENE-trending, ca. 306-287 Ma, Totoltepec pluton is part of a Carboniferous-Permian continental magmatic arc on the western Pangaean margin. The 15 km x 5 km pluton is bounded by two N-S Permian dextral faults, an E-W thrust to the south, and an E-W normal fault to the north. Thermobarometric data indicate that the main, ca. 289-287 Ma, part of the pluton was emplaced at ≤20 km depth and ≥700°C and was exhumed to 11 km and 400°C in 4 ± 2 million years. We have documented the following intrusive sequence: (1) the 306 Ma northern marginal mafic phase; (2) the 287 Ma main trondhjemitic phase; and (3) ca. 289-283 Ma sub-vertical dikes that vary from (a) N39E, undeformed with crystal growth perpendicular to the margins, through (b) ca. N50-73E, foliated and folded with sinistral shear indicators, to (c) N73-140E and boudinaged. The obliquity of the boundary between the folded and stretched dikes relative to the N-S dextral faults suggests sequential emplacement in a transtensional regime (with 20% E-W extension), followed by different degrees of clockwise rotation passing through a shortening field accompanied by sinistral shear into an extensional field. The ca. 289-287 Ma intrusion also contains a steep ENE-striking foliation and hornblende lineations varying from sub-horizontal to steeply plunging, probably the result of emplacement in a triclinic strain regime. We infer that magmatism ceased when some of the dextral motion was transferred from the western to the eastern bounding fault, causing thrusting to take place along the southern boundary of the pluton. This mechanism is also invoked for the rapid uplift and exhumation of the pluton between ca. 287 Ma and 283 Ma. The distinctive characteristics of the Totoltepec pluton should prove useful in identifying similar tectonic settings within continental arcs.24 page(s