Abstract The day starts with a short trip to the Calcite mine area (Stop S2), a few miles west of Salome at the foot of the Granite Wash Mountains (figs. 13) Figure 15 shows the localized geology around the mine. A general roadlog follows (partly taken from Spencer and others (1990):

The Caribbean Plate was created as the North and South American Plates began to separate about 140 m.y. ago, allowing the Phoenix/Farallon spreading ridge to extend eastward. The Caribbean Plate was separated from the Phoenix/Farallon spreading ridge, about 110-100 m.y. ago, by a subduction zone near present-day Central America, connected by transform faults to the older Greater Antilles subduction zone. Geologic data from the margins of the Caribbean Plate indicate six important discontinuities in the history of the plate. Near the beginning of the Albian (110 m.y. B.P.), northeast-dipping subduction in the Greater Antilles zone may have been blocked by underthrusting of part of the Chortis Block of Central America, causing subduction to flip to southwest-dipping, perhaps followed by the beginning of subduction beneath Central America. A Santonian (85 m.y. B.P.) discontinuity may be the result of thickened oceanic crust formed at the Galapagos hot spot reaching the Central American subduction zone and blocking or modifying subduction. At the beginning of the Tertiary (66 m.y. B.P.), the Caribbean Plate changed its relative motion from northeastward to eastward and began to underthrust northern South America. In the Late Oligocene (27 m.y. B.P.), the Farallon/Phoenix Plate separated into the Nazca and Cocos Plates. The present-day emergence (5-0 m.y. B.P.) has not yet been correlated with plate motion changes. Major changes in plate arrangement and motion are thus reflected in the Caribbean Plate by major geologic discontinuities such as unconformities. Geologic structures along the plate margins are the resultants of the direction of relative plate motion, and of the type of lithosphere. Oblique plate motion produced horizontal slip accompanied by slow subduction without volcanism, by uplift and erosion, or by a combination of processes. Oceanic lithosphere forms relatively simple plate boundaries, as in the Greater Antilles, but continental lithosphere forms complex border zones composed of old structural blocks moving along ancient zones of weakness, as in northern South America.

Abstract The Scotia–Antarctic plate boundary extends along the southern branch of the Scotia Arc, between triple junctions with the former Phoenix plate to the west (57°W) and with the Sandwich plate to the east (30°W). The main mechanism responsible for the present arc configuration is the development of the Scotia and Sandwich plates from 30–35 Ma, related to breakup of the continental connection between South America and the Antarctic Peninsula. The Scotia–Antarctic plate boundary is a very complex tectonic zone, because both oceanic and continental elements are involved. Present-day sinistral transcurrent motion probably began 8 Ma ago. The main active structures that we observed in the area include releasing and restraining bends, with related deep extensional and compressional basins, and probable pull-apart basins. The western sector of the plate boundary crosses fragmented continental crust: the Western South Scotia Ridge, with widespread development of pull-apart basins and releasing bends deeper than 5000 m, filled by asymmetrical sedimentary wedges. The northern border of the South Orkney microcontinent, in the central sector, has oceanic and continental crust in contact along a large thrust zone. Finally, the eastern sector of the South Scotia Ridge is located within Discovery Bank, a piece of continental crust from a former arc. On its southern border, strike-slip and normal faults produce a 5500-m-deep trough that may be interpreted as a pull-apart basin. In the eastern and western South Scotia Ridge, despite extreme continental-crustal thinning, the basins show no development of oceanic crust. This geometry is conditioned by the distinctive rheological behaviour of the crust involved, with the bulk concentration of deformation within the rheologically weaker continental blocks.

The upper Mesozoic to lower Cenozoic sequence in the region of James Ross Island is the only exposed marine succession of that age in Antarctica. The sequence makes up part of the fill of the James Ross Basin and includes: (1) an upper Jurassic mudstone-tuff sequence, the Nordenskjöld Formation; (2) a Lower to Upper Cretaceous conglomerate-sandstone-mudstone-tuff assemblage, the Gustav Group and equivalents; (3) an Upper Cretaceous to Paleocene poorly consolidated sand, silt, mud and tuff sequence, the Marambio Group and equivalents; and (4) an Eocene sequence of weakly consolidated, nonvolcanic fine sands and silts—the La Meseta Formation. Sedimentary facies include proximal submarine fans, shelf settings, and deltaic environments. Sea-floor anomaly data from the Pacific Ocean suggest that development of Upper Mesozoic to Cenozoic fore-arc, magmatic arc, and back-arc terrains of the Peninsula resulted from the subduction of the Phoenix Plate until the early Tertiary, and, after reorganization of spreading centers in Late Cretaceous time, subduction of the Aluk Plate. Strata in the James Ross Island region constitute the sedimentary and volcanic fill of an ensialic back-arc basin developed on the Weddell Sea flank of the Antarctic Peninsula. Broad correlations can be made between the strata and evolution of the James Ross Basin, the tectonic and magmatic history of the peninsula, and plate subduction.