Amabel Ortega-Rivera

Forearc-basin sedimentary response to rapid Late Cretaceous batholith emplacement in the Peninsular Ranges of southern and Baja California

The eastern Peninsular Ranges batholith is dominated by voluminous La Posta–type tonalite-granodiorite intrusions that compose half of the magmatic arc at present erosion level. Zircon U-Pb and hornblende 40 Ar/ 39 Ar results from these intrusions indicate that they were emplaced in a remarkably narrow interval (99–92 Ma) that closely followed cessation of west-directed compression of the arc system. Emplacement of the La Posta suite coincided with a major pulse of coarse-grained sediment into the adjacent forearc basin in early Cenomanian to middle Turonian time. Paleontologic control, and plutonic age and detrital zircon U-Pb data demonstrate the virtual absence of a time lag between magma emplacement and sedimentary response. The tight linkage between magmatism, arc exhumation, and sediment delivery to the forearc indicates that development of major erosional topography in the arc was driven by thermal and mechanical effects associated with large-volume batholith emplacement.

Geochronological constraints on the tectonic history of the Peninsular Ranges Batholith of Alta and Baja California: Tectonic implications for western México

A compilation of existing age data and new age determinations (U/Pb zircon, 4 0 Ar/ 3 9 Ar step-heating, K/Ar, Rb/Sr, and apatite fission-track dates) for the Peninsular Ranges batholith of Alta and Baja California provides geochronological constraints on the tectonic history of the Peninsular Ranges batholith and tectonic implications for western Mexico. The plutons of the Peninsular Ranges batholith ofAlta and Baja California between the 28°N and 34°N parallels were emplaced from west to east between ca. 140 to 80 Ma (U/Pb zircon dates) and display 4 0 Ar/ 3 9 Ar hornblende and biotite plateau dates that range from 118 to 83 Ma and 116 to 80 Ma, respectively, and biotite K/Ar dates as young as 65 Ma. Rapid cooling is indicated by the small differences in U/Pb zircon and cogenetic 4 0 Ar/ 3 9 Ar dates. Mineral pairs having mainly concordant dates yield 4 0 Ar/ 3 9 Ar cooling ages that also systematically decrease from southwest to northeast. Also, monotonic eastward younging of ages across various plutons was found as was previously reported for the Sierra San Pedro Martir pluton. This systematic regional and local eastward younging may be explained by the systematic tilting of pluton-sized crustal blocks containing individual plutons. The northeastward decrease in ages across the Peninsular Ranges batholith and across some of its plutons is therefore attributed to regional eastward migration of granitic intrusion foci combined with rapid differential exhumation histories and superimposed east-side-up tilting of crustal-sized blocks containing some of the plutons and beautifully exemplifies the short time lapse between batholith intrusion and unroofing. Additionally, apatite fission-track dates from samples collected across the Peninsular Ranges, from Ensenada to San Felipe, Baja California, establish that there is also an eastward younging of ages from 104 Ma to 51 Ma from west to east and also across plutons. The fission-track dates indicate that from west to east, present exposures of much of the western Peninsular Ranges batholith have been within «3 km of the Earths surface (assuming a normal geothermal gradient) since Early Cretaceous time, and that by early Tertiary time, much of the eastern Peninsular Ranges had cooled to temperatures of <110 °C, indicating rapid exhumation histories and superimposed east-side-up tilting shown by younger ages at higher elevations within plutons. The batholith was sufficiently uplifted during the Late Cretaceous such that 10-15 km of material was eroded off the eastern part of the batholith by early Paleocene time. Furthermore, a regional compilation of the available age data for western Mexico provides geochronological constraints for the testing of controversial paleomagnetic and geological models for the tectonic evolution of the Mesozoic Peninsular Ranges of Alta and Baja California. The controversy involves different perceptions of the geologic development of western Mexico and southernmost California as derived from paleomagnetic studies and regional geology. Whereas the geological data and plate tectonic reconstructions seem to indicate a northward motion between 300 and 500 km with respect to the rest of North America since the Cretaceous, paleomagnetic data suggest much greater movement involving at least 2500 km of northward translation for the Baja peninsula during the same period.

Mid-Jurassic Tectonothermal Event Superposed on a Paleozoic Geological Record in the Acatlán Complex of Southern Mexico: Hotspot Activity During the Breakup of Pangea

U-Pb isotopic analyses of zircon from the lowest structural units of the Acatlan Complex of southern Mexico indicate that Paleozoic tectonothermal events are overprinted by mid-Jurassic (175±3 to 171±1 Ma), low pressure migmatization (5–6 kb), polyphase deformation, and intrusion of felsic and mafic magmas. Ensuing rapid cooling recorded by 40Ar/39Ar muscovite, biotite and K-feldspar ages is estimated to have taken place at 21±3°C/my at exhumation rates of 0.6 mm/yr. Such rapid exhumation requires a combination of erosion and tectonic unroofing that is recorded by top-to-the-west kinematic data. Synchronous tectonic unroofing is also recorded 100 km to the east in the adjacent Oaxaca terrane, where top-to-the-north, extensional shear zones occur in Paleozoic strata. This pattern of extension suggests tectonic unroofing in response to domal uplift (radius >100 km) like that associated with core complexes, slab windows, and hotspots. Most tectonic analyses for the Jurassic place the Acatlan Complex in the forearc region of an arc in Colombia lying 600–800 km inboard of the subduction zone, presumably in response to flat-slab subduction. Modern analogues suggest that flat-slab subduction reflects subduction of young buoyant oceanic lithosphere adjacent to either a mid-oceanic ridge, or a plume. Since core complexes are typical of arc-backarc regions, and slab windows generally produce metamorphic belts, the forearc setting and associated domal uplift suggest a plume to be the most likely cause of this Jurassic tectonothermal pulse in southern Mexico. This plume activity is synchronous with the opening of the Gulf of Mexico during the breakup of Pangea, to which it may have contributed.

Geochronology and Geochemistry of the ~917 Ma, Calc-alkaline Etla Granitoid Pluton (Oaxaca, Southern Mexico): Evidence of Post-Grenvillian Subduction along the Northern Margin of Amazonia

The post-tectonic Etla pluton intrudes the ~1 Ga granulitic Oaxacan Complex that cooled through 450°C by ~945 Ma. The Etla pluton consists of massive, coarse, porphyritic granodiorite-monzogranite (plagioclase, K-feldspar, quartz, biotite ± hornblende) with fine-grained felsic rocks along the margin. Geochemistry indicates that it is a peraluminous, I-type, medium-K, calc-alkaline, volcanic-arc granite-trondjemite with relatively low contents of high-field-strength elements and flat REE patterns. U-Pb zircon isotopic analyses fall on a chord with intercepts at 180 ± 50 Ma and 920 ± 25 Ma: the latter is similar to the 207Pb/206Pb age of 917 ± 6 Ma of the least discordant (1%) analysis and is inferred to date the time of intrusion. This pluton is synchronous with similar igneous activity in Avalonia (eastern Appalachians) and in Tocantins Province of central Brazil, which may form parts of a peri-Amazonian magmatic arc. 40Ar/39Ar laser step-heating analyses of biotite and K-feldspar yielded plateau ages of 207 ±5 Ma and 221 ± 3 Ma, respectively, that may be related to Phanerozoic reheating.

Geology and geochronology of Paleozoic rocks in western Acatlán Complex, southern Mexico: Evidence for contiguity across an extruded high-pressure belt and constraints on Paleozoic reconstructions

The Acatlan Complex straddles a high-pressure belt previously interpreted as either: (1) a suture zone within the Iapetus or the Rheic oceans, which would have a contrasting geological record across the suture; or (2) a tectonic slice extruded into the upper plate, which would imply contiguity across the complex. Distinguishing between these hypotheses is critical to paleogeographic reconstructions. Examination of the western Acatlan Complex reveals the following: (1) deposition of clastic rocks between 654 and 464 Ma; (2) intrusion of bimodal Ordovician bodies at ca. 464 Ma; (3) high-grade deformation with cooling through 400 °C by ~360–335 Ma; (4) deposition of clastic rocks and pillow lavas after ~350–400 Ma; (5) deformation accompanied by greenschist facies metamorphism at ca. 335 Ma; (6) deposition of clastic and bimodal volcanic rocks at ca. 327 Ma; (7) ~320–270 Ma subgreenschist deformation; (8) deposition of the Middle-Upper Permian sedimentary rocks; and (9) intrusion of a 61 ± 1 Ma diorite followed by early Cenozoic (Laramide) ENE folding and faulting. Zircon ages (~350–400, 570–505, 827–890 Ma, 0.9–1.3 Ga) suggest both local and Amazonian sources with deposition above a local Mesoproterozoic (Oaxacan) basement on the southern margin of the Rheic Ocean. This geological record is very similar to that of the eastern Acatlan Complex, which supports the extrusion hypothesis, a model that may be applicable to other orogens.

Geochronology and Geochemistry of the Francisco Gneiss: Triassic Continental Rift Tholeiites on the Mexican Margin of Pangea Metamorphosed and Exhumed in a Tertiary Core Complex

Migmatized amphibolite-facies gneisses and amphibolites of the Francisco Gneiss exposed in the northern part of the Guerrero composite arc terrane have been interpreted as either Precambrian basement, a Triassic metamorphic complex, or a distinct terrane. Field observations suggest exposure in a metamorphic core complex of a protolith composed of interleaved bimodal igneous and sedimentary rocks. Geochemical data indicate that the amphibolites are within-plate, continental tholeiites. Recalculated TDM ages for the rhyolites are consistent with partial melting of the Grenvillian basement of North America projected beneath the area. U-Pb isotopic analyses of zircon from two felsic rocks yielded concordant ages between 216 and 197 Ma due to a combination of inheritance and Pb-loss: the best estimate of protolith age is ∼206 Ma—i.e., Norian, Late Triassic. Concordant U-Pb titanite ages range from 112 to 98 Ma, whereas nearly concordant U-Pb xenotime ranges from 91 to 51 Ma. These are inferred to result from partial-complete resetting during the high-grade metamorphic event. 40Ar/39 Ar analyses from the gneisses yielded plateau ages of 16.5 ± 1 Ma (muscovite) and 13 ± 1 Ma (biotite), which date cooling through ∼370°C and ∼300°C, respectively (early Middle Miocene). Biotite from the granitic sheet yielded a plateau age of 13 ± 2 Ma. These data are interpreted in terms of Miocene exhumation in a core complex of high-grade metamorphic rocks developed either over a slab window or as a result of tectonic burial during the Laramide orogeny. The extrusion of Upper Triassic, continental rift tholeiites is consistent with emplacement in a back-arc environment.

Deformational History of the Granjeno Schist, Ciudad Victoria, Mexico: Constraints on the Closure of the Rheic Ocean?

Exposed in the core of a NNW-trending frontal anticline of the Laramide fold-thrust belt of northeastern Mexico, the Granjeno Schist comprises a polydeformed assemblage of Paleozoic metasedimentary and metavolcaniclastic rocks and serpentinized mafic-ultramafic units. The earliest deformation (D1) predates emplacement of a leucogranite at 351 ± 54 Ma and may record obduction of this oceanic unit. Subsequent deformations (D2a-D2c) record tectonic juxtapositioning of the Granjeno Schist against the ∼1 Ga Novillo Gneiss by NNW-directed dextral shear under conditions of decreasing temperature. Cooling ages of 313 ± 13 Ma and 300 ± 4 Ma are considered to date the onset of dextral motion, which continued into the Permian. These events are linked to the Late Paleozoic closing of the Rheic Ocean.

Late Cretaceous subduction of the continental basement of the Maya block (Rabinal Granite, central Guatemala): Tectonic implications for the geodynamic evolution of Central America

The Rabinal Granite is a peraluminous S-type composite pluton formed upon partial melting of a metasedimentary source region that fringes the southernmost North America plate in central Guatemala. It is therefore considered, together with the intruded metasedimentary sequences, to be part of the continental basement of the Maya block. This leucocratic K-feldspar–plagioclase–quartz–muscovite ± biotite granite shows increasing deformation along its southern margin, where it is cut across by the dextral, Late Cretaceous, top-to-the-NE Baja Verapaz shear zone. Although it has been recently dated at 562–453 Ma (isotope dilution–thermal ionization mass spectrometry), the new data presented here, including laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb and 40 Ar- 39 Ar geochronology and electron-probe mineral chemistry, allow us to more precisely establish the timing of intrusion and metamorphic overprinting of the Rabinal Granite. The zircons dated by LA-ICP-MS indicate a crystallization age of 471 +3 / −5 Ma (Early Ordovician), as well as abundant inherited cores with Pan-African and Mesoproterozoic dates. Laser total fusion Ar-Ar analyses of magmatic low-silica muscovite (Si = 6.2–6.4 atoms per formula unit) indicate cooling following magmatic crystallization during the mid-late Paleozoic and variable extents of resetting of Ordovician micas during Cretaceous metamorphism and deformation. The pressure-temperature ( P - T ) conditions of the inferred Ordovician metamorphism that produced partial melting of the metasedimentary source of the Rabinal Granite and the ascent and crystallization of the granitic melt are uncertain, but a clockwise P - T -time path with maximum P and T of P and T of ∼8.5 kbar and ∼300 °C, respectively. This second event, dated at 70.1 ± 0.6 Ma by means of laser total fusion 40 Ar- 39 Ar analyses on high-Si muscovite grains, is interpreted to be the result of subduction and accretion of the basement of the Maya block during the latest Cretaceous, likely in a transpressional tectonic regime related to the lateral collision of the Maya block with the Pacific (Farallon)–derived Caribbean arc. This finding represents the first direct evidence for latest Cretaceous subduction of the metamorphic Paleozoic basement of the Maya block, north of the Baja Verapaz shear zone.

A Carboniferous high-pressure klippe in the western Acatlán Complex of southern México: implications for the tectonothermal development and palaeogeography of Pangea

High-pressure (HP) rocks are critical for palinspastic restorations because they mark inferred subducted/extruded oceanic crust; knowledge of their geometric, geodynamic, and age relationships provide essential constraints on palaeogeographic reconstructions. The westernmost HP belt (Ixcamilpa) in the Acatlán Complex of southern Mexico has been inferred to be a mid-Late Ordovician backarc basin on the southern Iapetan margin that was subducted beneath eastern Laurentia and extruded up the subduction zone during the Early Silurian. Re-examination of Ixcamilpa HP rocks has revealed that they comprise lower Palaeozoic rift-passive margin protoliths and occur in a W-vergent klippe (not a suture) formed during polyphase deformation. Peak metamorphic mineral assemblages of blueschist-amphibole eclogite facies underwent retrogression through epidote amphibolite to greenschist facies. 40Ar/39Ar dating of various rocks yielded plateau ages of 344–339 Ma for calcic amphibole, 318 ± 4 Ma for glaucophane, and 329–325 ± 2 Ma for muscovite (excess argon), which clearly indicate a Carboniferous tectonothermal event. We interpret the 20 million years range in amphibole ages as reflecting progressive unroofing. The terminal stage of progressive thrusting placed the HP rocks above the middle Mississippian Zumpango Unit, during which a single penetrative sub-greenschist fabric was produced. Subsequent Permian or Laramide deformation refolded all the rocks about NE-trending upright folds. We postulate that the root zone of the HP nappe lies to the east in the median HP belt, which has a structure consistent with an extrusion zone. Inasmuch as similar units of the Acatlán Complex bound this HP root zone on either side, it is inferred to have been extruded into the upper plate above the subduction zone, and thus is not an oceanic suture. Our new data provide constraints for a Carboniferous palaeogeographic reconstruction, whereby subduction erosion of passive margin rocks took place along the western margin of Pangea and were subsequently extruded into the upper (Acatlán) plate.

Palaeozoic structures in the Xayacatlán area, Acatlán Complex, southern Mexico: transtensional rift‐ and subduction‐related deformation along the margin of Oaxaquia

The Xayacatlán area (eastern Mixteca terrane, southern Mexico) was previously inferred to preserve the Ordovician‐Silurian thrust contact between vestiges of the Iapetus Ocean and the para‐autochthon bordering Oaxaquia. Detailed remapping indicates that the rocks occur in four vertically‐bounded, NS fault blocks. The latter record the following tectonothermal events that post‐date Iapetus and occurred along the margins of the Rheic (1) and Pacific (2 and 3) oceans: (1) dextral transtension accompanying intrusion of an NS, tholeiitic dike swarm at ∼442 Ma; (2) penetrative, greenschist‐facies deformation during the Mississippian related to extrusion of high‐pressure rocks; and (3) subgreenschist‐facies dextral transtension on NS faults during the generation of Middle Permian fabrics.