Michelangelo Martini

Sandstone Provenance of the Arperos Basin (Sierra de Guanajuato, Central Mexico): Late Jurassic-Early Cretaceous Back-Arc Spreading as the Foundation of the Guerrero Terrane

Three paleogeographic scenarios have been proposed for the Mesozoic volcano-sedimentary successions that compose the Guerrero terrane, western Mexico. In the type 1 scenario, the Guerrero terrane is an exotic Pacific arc accreted to nuclear Mexico by the consumption of a pre-Cretaceous oceanic basin, named Arperos Basin. The type 2 scenario considers the Guerrero terrane as a fringing multiarc system, accreted by the closure of pre-Cretaceous oceanic basin substrates at multiple subduction zones with varying polarities. In the type 3 scenario, the Guerrero terrane is interpreted as a North American west-facing para-autochthonous arc, which was drifted in the paleo-Pacific domain by the opening of the Cretaceous back-arc oceanic Arperos Basin. To test these reconstructions, we present here a combined study that includes geologic mapping, stratigraphy, U-Pb geochronology, and sandstone provenance data from the Arperos Basin in the Sierra de Guanajuato, central Mexico. Our data document that the Arperos Basin developed in a back-arc setting and evolved from continental to open oceanic conditions from the Late Jurassic to the Early Cretaceous. Sandstone provenance analysis shows an asymmetric distribution of the infill sources for the Arperos Basin: continent-recycled sedimentary rocks were deposited along its northeastern side, whereas magmatic arc-recycled clastic rocks developed at its southwestern side. Such asymmetric distribution closely fits with sedimentological models proposed for present-day continent-influenced back-arc basins. On the basis of this evidence, we favor a type 3 scenario for the Guerrero terrane, which is then considered to represent a detached slice of the Mexican leading edge that drifted in the paleo-Pacific domain during back-arc extension and subsequently accreted back to the Mexican craton.

Kinematics of the Guerrero terrane accretion in the Sierra de Guanajuato, central Mexico: new insights for the structural evolution of arc–continent collisional zones

The Guerrero terrane comprises Middle Jurassic–Early Cretaceous arc successions that were accreted to the North American craton in the late Early Cretaceous, producing closure of the Arperos oceanic basin and the formation of an approximately 100 km-wide fold–thrust belt. Such a suture is key to investigating the structural evolution related to Guerrero terrane accretion and, in general, to arc–continent collisional zones. The Sierra de Guanajuato is an exposure of the Guerrero terrane suture belt and consists of a complex tectonic pile that formed through at least three major shortening phases: D1SG, D2SG, and D3SG (SG, Sierra de Guanajuato). During the D1SG and D2SG phases, the Upper Jurassic–Lower Cretaceous successions of the Arperos Basin piled up, forming a doubly vergent imbricate fan of thrust sheets that accommodated substantial NE–SW shortening. Mylonite microtextures, as well as syntectonic minerals, indicate that the D1SG and D2SG deformation events took place under low greenschist-facies metamorphic conditions. We relate these deformation phases to the progressive NE migration of the Guerrero terrane, which triggered the collapse and closure of the Arperos Basin. During D3SG, the El Paxtle arc assemblage of the Guerrero terrane was tectonically emplaced onto the previously deformed successions of the Arperos Basin. However, D3SG structures indicate that during this deformational stage, the main shortening direction was oriented NW–SE and that contraction was accommodated mostly by SE-vergent ductile thrusts formed under low greenschist-facies metamorphic conditions. We suggest that the top-to-the-SE emplacement of the El Paxtle assemblage may be a result of the tectonic escape of the arc produced by the continuous NE impingement of the Guerrero terrane during its collisional addition to the Mexican mainland.

Late Cretaceous‐Oligocene magmatic record in southern Mexico: The case for a temporal slab window along the evolving Caribbean‐North America‐Farallon triple boundary

Continental magmatism in southern Mexico is expected to record the eastward displacement of the Farallon-North America-Caribbean triple junction. However, a trench-transversal belt of magmatism in the central western Guerrero State does not fit into a regular pattern of arc migration and reorientation following the formation of the WSW trending Acapulco trench in the Cenozoic. We revised the magmatic pattern of southern Mexico using an updated database and new laser ablation inductively coupled plasma–mass spectrometry sensitive high-resolution ion microprobe, Ar-Ar ages, and geochemical and geologic data for the coastal part of the anomalous Guerrero belt. Our data reveal a persistent magmatic activity between ~75 and 35 Ma, with a changing character at the Paleocene-Eocene boundary (circa 56 Ma). Late Cretaceous-Paleocene granitoids have an adakitic signature imprinted by stable garnet in the source and show no plagioclase fractionation, indicative of wet and oxidized magmas. Eocene rocks consist of an almost bimodal suite of plutonic bodies covered by a succession of mafic lavas. Granitic plutons show plagioclase fractionation and flat middle rare earth element-heavy rare earth element; gabbros have a tholeiitic character, indicative of dryer and more reduced magmas. They appear later and were emplaced at shallow depth in an extensional sedimentary basin. We interpret the magmatic record of the Guerrero belt as the response to two concurrent processes: (1) a temporal window in the Farallon slab induced by the concurrent subduction along the two noncollinear trench segments of southern Mexico (WNW trending) and of the Chortis block (NNW trending) and (2) a scissor-like transtensional rifting associated to counterclockwise rotation and eastward motion of the Chortis block.

Correlating the Arperos Basin from Guanajuato, central Mexico, to Santo Tomás, southern Mexico: Implications for the paleogeography and origin of the Guerrero terrane

The Guerrero terrane has been interpreted either as a Mesozoic Pacific multi-arc system accreted to North America, or as a detached slice of the North American continental margin, which was rifted during backarc spreading and subsequently accreted back to the continental mainland. In order to test these two scenarios, we present here a petrologic study of metasandstones from the Santo Tomas area, southern Mexico. Our data document that the Guerrero terrane suture belt contains the remnants of the Tithonian–Cenomanian Arperos Basin. This basin displays a marked provenance asymmetry. Its eastern margin is composed of metasedimentary rocks derived from sources in the North American continental mainland, whereas its western margin consists of a metasedimentary succession derived from volcanic sources of the Guerrero terrane. Sedimentation in the Arperos Basin was coeval with the emplacement of Tithonian–Barremian felsic dikes and lava flows with volcanogenic massive sulfide deposits and Aptian–Cenomanian intraplate-like and mid-ocean ridge basalts. This suggests that the Arperos Basin evolved progressively from continentally to oceanic floored during the Early Cretaceous and that a mature oceanic crust was generated only ca. 15 Ma before the accretion of the Guerrero terrane, which took place in the late Cenomanian. On the basis of this evidence, we favor a North American origin for the Guerrero terrane, which is then considered to represent a west-facing North American arc that was rifted from the continental mainland during backarc spreading and subsequently accreted back to nuclear Mexico.

From back-arc rifting to arc accretion: the Late Jurassic–Early Cretaceous evolution of the Guerrero terrane recorded by a major provenance change in sandstones from the Sierra de los Cuarzos area, central Mexico

The Guerrero terrane is composed of Middle Jurassic–Lower Cretaceous arc assemblages that were rifted from the North American continental mainland during Late Jurassic–Early Cretaceous back-arc spreading within the Arperos Basin, and subsequently accreted back to the continental margin in the late Aptian. The Sierra de los Cuarzos area is located just 50 km east of the Guerrero terrane suture belt and, therefore, its stratigraphic record should be highly sensitive to first-order tectonic changes. Two Upper Jurassic–Lower Cretaceous clastic units were recognized in the Sierra de los Cuarzos area. The Sierra de los Cuarzos Formation is the lowermost exposed stratigraphic unit. Petrographic data and U-Pb zircon ages suggest that the Sierra de los Cuarzos Formation was derived from quartz-rich sedimentary and igneous sources within the North American continental mainland. The Sierra de los Cuarzos Formation is overlain by the Pelones Formation, which is composed of volcanoclastic sandstones derived from a mix of sources that include the mafic arc assemblages of the Guerrero terrane and quartz-rich sedimentary and volcanic rocks exposed in the continental mainland. The provenance change documented in the Sierra de los Cuarzos area suggests that the Pelones Formation was deposited when the Arperos Basin was closed and the Guerrero terrane was colliding with the North American continental mainland. Based on these data, we interpret the Pelones Formation as the syn-tectonic stratigraphic record associated with the accretion of the Guerrero terrane.

Style and chronology of the Late Cretaceous shortening in the Zihuatanejo area (southwestern Mexico): Implications for the timing of the Mexican Laramide deformation

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 Pacifi c coast to the Mexican mainland. However, such migration has been inferred based on paleon tologic and isotopic data from the central and eastern parts of the fold-and-thrust belt, without considering the western part of the orogen. The identifi cation 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 fi rst 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 infi ll 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 fi rst 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 presentday Pacifi c coast to the continental mainland, probably as the result of the increasing subduction rate or collision of a Caribbean terrane along the Mexican Pacifi c trench.

The missing half of the subduction factory: shipboard results from the Izu rear arc, IODP Expedition 350

ABSTRACT IODP Expedition 350 was the first to be drilled in the rear part of the Izu-Bonin, although several sites had been drilled in the arc axis to fore-arc region; the scientific objective was to understand the evolution of the Izu rear arc, by drilling a deep-water volcaniclastic section with a long temporal record (Site U1437). The Izu rear arc is dominated by a series of basaltic to dacitic seamount chains up to ~100-km long roughly perpendicular to the arc front. Dredge samples from these are geochemically distinct from arc front rocks, and drilling was undertaken to understand this arc asymmetry. Site U1437 lies in an ~20-km-wide basin between two rear arc seamount chains, ~90-km west of the arc front, and was drilled to 1804 m below the sea floor (mbsf) with excellent recovery. We expected to drill a volcaniclastic apron, but the section is much more mud-rich than expected (~60%), and the remaining fraction of the section is much finer-grained than predicted from its position within the Izu arc, composed half of ashes/tuffs, and half of lapilli tuffs of fine grain size (clasts <3 cm). Volcanic blocks (>6.4 cm) are only sparsely scattered through the lowermost 25% of the section, and only one igneous unit was encountered, a rhyolite peperite intrusion at ~1390 mbsf. The lowest biostratigaphic datum is at 867 mbsf (~6.5 Ma), the lowest palaeomagnetic datum is at ~1300 mbsf (~9 Ma), and the rhyolite peperite at ~1390 mbsf has yielded a U–Pb zircon concordia intercept age of (13.6 + 1.6/−1.7) Ma. Both arc front and rear arc sources contributed to the fine-grained (distal) tephras of the upper 1320 m, but the coarse-grained (proximal) volcaniclastics in the lowest 25% of the section are geochemically similar to the arc front, suggesting arc asymmetry is not recorded in rocks older than ~13 Ma.

Evidence for two Cretaceous superposed orogenic belts in central Mexico based on paleontologic and K-Ar geochronologic data from the Sierra de los Cuarzos

The continental interior of Mexico is characterized by a Late Cretaceous–Eocene fold-thrust belt named the Mexican Fold-Thrust Belt, which shows characteristics of an eastward-tapering orogenic wedge. The juxtaposition of the Guerrero terrane to the west of the Mexican Fold-Thrust Belt has motivated many to propose the accretion of this terrane as the cause for the regional shortening in the Mexican continental interior. The Sierra de los Cuarzos is located in the westernmost Mexican Fold-Thrust Belt, directly to the east of the boundary with the Guerrero terrane. Based on its position, the Sierra de los Cuarzos is key in reconstructing the possible propagation of shortening deformation from the Guerrero terrane suture belt into the Mexican continental interior. Our new paleontologic and K-Ar geochronologic data from the Sierra de los Cuarzos show that shortening in the westernmost Mexican Fold-Thrust Belt started ca. 83 Ma, which is ∼30 m.y. later than the late Aptian age of the Guerrero terrane suture boundary. Therefore, the integration of our new paleontologic and geochronologic determinations with previous data suggests that the Guerrero terrane was already accreted and amalgamated with the Mexican continental interior at the time of the formation of the Mexican Fold-Thrust Belt. In light of this, the shortening structures in the western Mexican Fold-Thrust Belt do not show any in-sequence relationship with the adjacent Guerrero terrane suture belt. Therefore, we favor the idea that the Guerrero terrane suture boundary and the Mexican Fold-Thrust Belt are two distinct orogens that mark two distinct stages of tectonic evolution of the North American Pacific margin.

Provenance analysis of Jurassic sandstones from the Otlaltepec Basin, southern Mexico: Implications for the reconstruction of Pangea breakup

The structural evolution that accompanied the breakup of Pangea during Jurassic time has been constrained in Mexico only at the regional scale on the basis of global plate tectonics and geometric considerations. According to available regional-scale reconstructions, the Jurassic tectonic evolution of Mexico was characterized by: (1) anticlockwise rotation of the Yucatan block along NNW-trending dextral faults and (2) sinistral block motions along W- to WNW-trending faults, which are geometrically needed to restore southern and central Mexico to the northwest of its present position during early Mesozoic time and avoid the overlap between North and South America in the reconstruction of Pangea. Reports of W- to WNW-trending sinistral faults that were active in Mexico during Jurassic time are presently few, and the existence, extension, and age of some of these structures have been questioned by many authors. In this work, we present the provenance analysis from a Jurassic clastic succession deposited within the Otlaltepec Basin in southern Mexico. Whole-rock sandstone petrography integrated with chemical analysis of detrital-garnet and U-Pb detrital-zircon geochronology documents that the analyzed stratigraphic record was deposited during rapid exhumation of the Totoltepec pluton along the Matanza fault, which is a W-trending sinistral normal fault that extends along the southern boundary of the Otlaltepec Basin. U-Pb zircon ages and biostratigraphic data bracket the age of the Matanza fault between 163.5 ± 1 and 167.5 ± 4 Ma. This indicates that the Matanza fault was involved in the crustal attenuation that accompanied the breakup of Pangea and that sinistral motion of continental blocks along W-trending structures was taking place in southern Mexico as predicted by global plate tectonic reconstructions.