R. David Dallmeyer

Mechanisms of continental growth in extensional arcs: An example from the Andean plate-boundary zone

During the Late Triassic, Jurassic, and Early Cretaceous, an extensional magmatic arc was formed in the Andean margin of northern Chile. Plutons emplaced at ramps within a hinterland-propagating extensional duplex were fed by dikes that transferred magma through the lower crust from a reservoir in the mantle Phases of volcanism separated phases of plutonism, and our model demonstrates that plutonism was favored only when upper-plate extensional fault systems were active. When they were not, dikes cut across inactive faults, and magma rose directly to the surface during volcanic phases of arc growth.

Late Cretaceous exhumation of the metamorphic Gleinalm dome, Eastern Alps: kinematics, cooling history and sedimentary response in a sinistral wrench corridor

Abstract The metamorphic Gleinalm dome, Eastern Alps, was uplifted and exhumed within a releasing structure in a sinistral wrench corridor during the Late Cretaceous. The dome is confined by a system of ductile shear zones including low-angle normal faults and steep sinistral tear faults which define a large releasing structure with the metamorphic dome in its center. The fabrics developed within all ductile shear zones record processes which were operating during decreasing temperatures from initial epidote-amphibolite/upper greenschist facies conditions (with crystal plastic fabrics in quartz) to temperatures below ca. 300°C (with predominantly cataclastic fabrics). A cooling path based on 40Ar/39Ar amphibole (95.4 ± 1.2 Ma) and muscovite ages (87.6 ± 0.6; 84.3 ± 0.7 Ma) together with sphene, zircon and apatite fission track data indicate cooling through ca. 500°C at ca. 94 Ma to below ca. 250-200°C at 65 Ma. Subsidence of the adjacent Late Cretaceous Kainach Gosau basin occurred synchronously with cooling and uplift of the Gleinalm dome. Internal depositional patterns record rapid subsidence at the time of cooling with internal synsedimentary block rotation above an intra-crustal ductile normal fault. The sinistral wrench corridor of the Eastern Alps developed by sinistral displacement of the Austroalpine units against a relatively stable Europe during the Late Cretaceous.

Age of amphibolitic metamorphism in the ophiolitic unit of the Morais allochthon (Portugal): implications for early Hercynian orogenesis in the Iberian Massif

Structurally lower portions of the allochthonous Morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault. The lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro. Most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions (c. 500°C and 5 kbar). Amphiboles formed during this metamorphism are typically magnesio-hornblende. An amphibole concentrate from the lower structural unit records a 39Ar/40Ar plateau age of 397.3 ± 4 Ma. An amphibole concentrate from the upper structural unit displays an internally discordant 39Ar/40Ar age spectrum corresponding to a total gas age of 431.1 ± 27.3 Ma. Gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent K/Ca ratios and yield well-defined 36Ar/40Ar v. 39Ar/40Ar isotope correlations which result in similar ages of 384.2 ± 5.2 and 392.4 ± 7.3 Ma. These are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains. The associated tectonothermal event represented a first stage of Hercynian orogenesis in the Iberian massif. It was probably related to oceanic closure and associated obduction of granulitic/eclogitic massifs which at present structurally overlie the ophiolitic unit.

Character of the Alleghanian orogeny in the southern Appalachians: Part III. Regional tectonic relations

Geological and geochronological studies in the eastern Piedmont of the southern Appalachians have documented a polyphase late Paleozoic deformational chronology (D 2 –D 4 ) that is contemporaneous with the Alleghanian orogeny recorded in the western Appalachian foreland. The scale and vergence of D 3 anticlinoria in the eastern Piedmont are similar to those of anticlinoria in the western Appalachians which are related to ramping of underlying thrust surfaces. We suggest that the Appalachian decollement extends southeastward beneath the entire Piedmont Province and that all of the Piedmont rocks were displaced at least 175 km northwestward relative to North America during the Alleghanian orogeny. Late Paleozoic deformational effects in the eastern Piedmont thus are considered cogenetic with Alleghanian foreland deformation. We interpret the Alleghanian orogeny to be the result of oblique, dextral convergence and collision of Gondwana and Laurentia.

Mesozoic Magmatic and Tectonic Events within the Andean Plate Boundary Zone, 26°-27°30'S, North Chile: Constraints from

In North Chile, Jurassic-Cretaceous arc magmatism is represented by several narrow belts of plutons and two separate volcanic sequences. New

Mineral Ages

^{40}Ar/^{39}Ar

Contrasting Thermal Evolution within the Ross Orogen, Antarctica: Evidence from Mineral

mineral ages confirm five distinct episodes of magma emplacement as plutonic complexes (c. 202-188 Ma, c. 160-153 Ma, c. 142-138 Ma, c. 130-127 Ma, and c. 106-103 Ma). Conjugate dike sets were emplaced immediately before periods of plutonism, and each distinct episode of magmatism prior to c. 127 Ma was located successively eastward, inboard from the subduction boundary, features interpreted to result from regional extension within the magmatic arc. Steeply dipping, ductile mylonitic shear zones were developed in wall-rocks along western contacts of Late Jurassic and Early Cretaceous plutons. Steeply-plunging mineral stretching lineations and a variety of kinematic indicators record an east-side-down extension. Along eastern faults of the Atacama Fault System steeply dipping, ductile mylonitic shear zones display subhorizontal mineral stretching lineations, and a variety of kinematic indicators record sinistral strike-slip displacement. Hornblende separated from mylonitic am-phibolite yields