The early Mesozoic Talbotton diabase dike set in west-central Georgia consists of four en-echelon, multiply intruded segments with an overall N17°W strike. Compared to other known dikes in Georgia and Alabama, dikes in the Talbotton set are unusually wide, and have pronounced magnetic anomalies due to their relatively high Fe-Ti oxide content. Both chemical and modal compositions of the Talbotton rocks are remarkably homogeneous along strike as well as in cross section. Total variation for a given element or mineral commonly does not exceed analytical error. No compositional differences were observed within the dikes for large-ion-lithophile (LIL) elements across three pre-Mesozoic lithotectonic belts. The Talbotton dikes appear to be similar to dikes of the north-south swarm in the Carolinas and Virginia. Dikes of the Talbotton set and the north-south swarm are comparable with respect to their high-Fe quartz-tholeiite (HFQ) compositions, orientations, and individual dike widths and lengths. The Talbotton magma, however, does not fall upon the fractionation trend of this swarm and cannot be associated by mineral control as evidenced by observed phenocryst assemblages. Major- and trace-element relationships between diabases in the Talbotton dikes and other diabases in Georgia and Alabama cannot be reasonably explained by crystal fractionation or partial melting. A distinct Eu anomaly in the Talbotton dikes, however, indicates fractionation of plagioclase. Mafic index values suggest that ferromagnesian minerals, most probably clinopyroxene, were also part of the fractionation assemblage. The Talbotton dikes were apparently derived from a parental magma not represented at the current level of erosion in the Georgia-Alabama Piedmont.

ABSTRACT Georgia, the westernmost part of the southern Caucasus, is located at the junction of the European and Asiatic branches of the Alpine-Himalayan orogenic belt and represents a unique area where the Tethys Ocean was completely closed only in the late Cenozoic as a result of prolonged convergence between the Eurasian and Africa-Arabian plates. During the Neoproterozoic–early Cenozoic, the territory of Georgia and the adjacent area of the Caucasus were parts of the Tethys Ocean and its northern margin. The Tethys was not a single continuous oceanic plate, but rather developed in branches separating continental terranes of different sizes, which rifted and drifted away from the Gondwana margin and eventually collided with Laurasia. Prior to the final collision in the late Cenozoic, the region hosted systems of islandarc, intra-arc, and back-arc basins located between the East European continent and the oceanic basins of the Tethys. Integrative geological and paleogeographical studies show a collage of several tectonic units (terranes) in Georgia and adjoining areas that have distinctive geological histories with marine Tethyan, Eurasian, or Gondwanan affinities. These include the Scythian platform, the Great Caucasus, the Transcaucasus-Pontides, and the Lesser Caucasus–Alborz–West Iran regions. Their position between the Africa-Arabian and Eurasiatic continents provides a reason for grouping them into the Northern Tethyan (Eurasian) and Southern Tethyan (Gondwanan) domains. The Scythian platform, Great Caucasus, and Transcaucasus-Pontian belts are of North Tethyan origin, while Anatolia, Taurus, Iran, and the southern Lesser Caucasus belong to the South Tethys. The Arabia-Nubian Shield, at the end of the Proterozoic, experienced basement consolidation related to the final stages of the Pan-African cycle of tectogenesis. In contrast to the southern Lesser Caucasus (Daralagöz), the Transcaucasus did not undergo this process because it broke away from the Arabia-Nubia Shield and, during Cambrian–Devonian times, drifted deep into the Prototethys toward the northern (Baltica) continent. During the early–middle Paleozoic, in the wake of northward-migrating Gondwanan fragments, the Paleotethyan basin formed, and, in the Ordovician, along its border with the Transcaucasus, subduction of oceanic crust occurred, accompanied by suprasubduction volcanic eruptions. Northward migration of the Transcaucasus throughout the Paleozoic caused narrowing of the Prototethys and its transformation into an oceanic back-arc (Dizi) basin. Fragments of Paleotethyan crust are found along the southern border of the Transcaucasus, within accretionary complexes of the Lesser Caucasus ophiolite suture, and in the Pontides. Relicts of Paleotethyan crust crop out in Iranian Garadagh. During the late Paleozoic–early Mesozoic, the oceanic basin separating the Africa-Arabian continent from the Taurus-Anatolian-Iranian platformal domain gradually extended. During this phase, only the Central Iranian terrane separated from Gondwana, drifted northward, and collided with the Eurasian continent in the Late Triassic. The Taurus-Anatolian terranes separated from Gondwana later, in the Early–Middle Jurassic. During the Mesozoic–Cenozoic, Daralagöz represented the northwesternmost margin of the Central Iranian platform and was separated from the North Anatolian platform by an oceanic or back-arc basin (Khoy basin), which within the modern structure is represented by Mesozoic–Cenozoic ophiolites of Urumieh-Khoy and Van. The Paleozoic–Paleogene evolution of the North Tethyan domain was marked by major magmatic events corresponding to the Pacific-type and Mediterranean stages of Tethyan development. The precollisional magmatic assemblages reflect a variety of paleogeographic environments. They are indicative of a west Pacific-type oceanic setting in which a mature, Andean-type continental arc developed. There were several episodes of oceanic lithospheric obduction onto the continental terranes of the region; these included (1) the middle–late Paleozoic, during which basite-ultrabasite complexes were thrust over the island-arc system of the Transcaucasus and the Main Range zone of the Great Caucasus; (2) pre–Late Triassic obduction in the Lesser Caucasus; and (3) pre–Late Jurassic obduction during which ultrabasic rocks were thrust over the continental block of the Artvin-Bolnisi zone. The metabasites apparently represent Paleotethyan fragments. Final closing of the oceanic and back-arc basins, head-on continent-continent collision, topographic inversion, and formation of the present-day structures in Georgia and adjacent countries took place in the late Cenozoic.

Abstract Oligo-Miocene outcrops along the southern margin of the western Greater Caucasus preserve a record of sediments shed from the range into the northern and central parts of the Eastern Black Sea. Sandstones in the Russian western Caucasus are significantly more quartz-rich than those located farther SE in western Georgia. The latter contain appreciably more mudstone and volcanic rock fragments. Oligo-Miocene turbidite systems derived from the Russian western Caucasus in the Tuapse Trough and central Eastern Black Sea may therefore form better-quality reservoirs at shallow to moderate depths than sediments derived from west Georgian volcaniclastic sources in the easternmost part of the basin. Palynomorph analysis indicates sediment derivation predominantly from Jurassic and Cretaceous strata in the Russian western Caucasus and from Eocene strata, and an increasing proportion of Cretaceous strata upsection, in western Georgia. An Eocene volcaniclastic source is proposed for the increased rock fragment component in west Georgian sandstones. Eocene volcaniclastic rocks are no longer exposed in the Greater Caucasus, but similar rocks form the inverted fill of the Adjara–Trialet Basin farther south in the Lesser Caucasus. The former presence of a northern strand of this basin in the west Georgian Caucasus is supported by earlier thermochronological work. Supplementary material: A sample data table, petrographic data table, petrographic key, QFL sandstone compositional plot and palynomorph reworking Stratabugs™ charts are available at www.geolsoc.org.uk/SUP18662 .

Geologic mapping of the Kigluaik gneiss dome on Seward Peninsula, Alaska, was conducted at 1:24,000 scale and compiled with thesis maps and published maps as a colored map covering eight 15″ quadrangles at 1:63,360 scale. The Kigluaik dome consists of an exceptionally well-exposed 15-km-thick structural section of metasedimentary rocks and orthogneisses. A 91 ± 1 Ma upper-amphibolite-to-granulite facies metamorphic event that was syntectonic with gneiss dome fabrics overprints a pre–120 Ma blueschist-to-greenschist facies event. Peak conditions associated with younger metamorphism increase from greenschist facies in the structurally higher rocks of the Nome Group, through biotite, staurolite, sillimanite, and second sillimanite isograds to granulite facies in the rocks of the Kigluaik Group. The high-grade overprint is coeval with mantle-derived magmatism at 90 ± 1 Ma. Mafic magmatism provided heat for metamorphism and induced partial melting, also enabling flow of gneisses. Deformation thinned the crust as the gneiss dome was exhumed, collapsing isograds and juxtaposing rocks that equilibrated at dramatically different structural levels. Lineations in the gneiss dome record a rotation of stretching directions with structural depth. The continuous transition of lineation azimuth and the lack of overprinting relationships indicate orthogonal stretching directions at different structural levels in the gneiss dome during its formation. Based on regional geologic relations, it is likely that the Bering Strait gneiss domes developed within a continental arc that was undergoing extension, probably as the result of rollback of the north-dipping subducting slab along the Pacific margin during Late Cretaceous time. Additional mapping contributions were made by: Phillip B. Gans, Department of Geological Sciences, University of California at Santa Barbara; Andrew T. Calvert, U.S. Geological Survey, Menlo Park, California; Timothy A. Little, School of Earth Sciences, Victoria University of Wellington, New Zealand; Kimberly A. Hannula, Department of Geology, Fort Lewis College, Durango, Colorado; Jeffrey Lee, Department of Geological Sciences, Central Washington University, Ellensburg, Washington; Charles M. Rubin, Department of Geological Sciences, Central Washington University, Ellensburg, Washington; Jaime Toro, Department of Geology and Geography, West Virginia University, Morgantown, West Virginia; and James E. Wright, Department of Geology, University of Georgia, Athens, Georgia.

ABSTRACT Existing interpretations of the major rock assemblages of the Florida basement (Cambrian Osceola Complex, Ordovician-Silurian Suwannee Basin sedimentary rocks, and Mesozoic Southwest Florida Volcanic Field) and basement structural features (Jay Fault-Bahamas Fracture Zone extensions and South Georgia Rift) indicate a history of initial evolution at a Gondwana continental margin, late Paleozoic coupling with North America, and eventual appendage as a peninsula following a fortuitous geometry of Triassic rifting. Reconciliation of this scenario, however, with the demonstrated spatial distribution of basement rock units and proposed concepts of motion on the Jay Fault is elusive without invoking unlikely initial configurations or tectonic behavior. Apprehensions concerning the shape of the extension into Florida of the South Georgia Rift, the presence of apparent elements of the Osceola Complex in the panhandle and southeastern Georgia, and lack of deformation within the Suwannee Basin sedimentary units can be tempered with a reconstruction of basement response to the closure of North America and Gondwana. A proposed sequence of events is contingent on deformational collision of Gondwana at and north of the North America Alabama Promontory followed by clockwise rotation and northwestward motion of Gondwana. En echelon right-lateral faults, including a precursory segment of the Jay Fault, progressively developed from east to west, facilitating closure at the Ouachita Orogen and displacing elements of a once-continuous continental margin to accommodate pivoting of Gondwana about the promontory. Thus, adjacent portions of the central Florida Osceola Complex to the northeast and southwest were fixed closer to the suture and bracketed the Suwannee Basin sequence. The subsequent development of Triassic basins in the Florida basement was constrained by the presence of offset basement blocks. Divergent motion of South America was accommodated by reactivated, but left-lateral and normal, motion along some of the initial Paleozoic faults. What previously had been described as the Jay Fault was actually a series of subparallel features, linked by embryonic spreading centers and perhaps converging near the present-day Osceola Complex. Accordingly, the Florida basement is a reflection of tectonic events attributable to a disruptive Paleozoic suturing process as well as Triassic rifting.