The Palaeozoic motion of the future Arctic continents is presented in the animation found in the accompanying CD-ROM. The animation shows snapshots of the motion of the tectonic blocks from 550 to 250 Ma in 3 million year steps. The locations of the blocks are controlled mainly by palaeomagnetic pole values for the blocks tied to known geological events, particularly the three main Arctic orogenies: the Scandian Caledonian which began in the Silurian, the Ellesmerian in the Late Devonian and the Uralian that began in the Late Pennsylvanian. Perhaps the most significant observation to come out of the animation is that the future Arctic continents were never very far from one another during the Palaeozoic. The maximum distance from Baltica to Laurentia may have reached 6000 km during the Middle Cambrian but the Arctic continents all surrounded the same eastern Iapetus Ocean and, by Silurian, they were quite close. Reliance on the palaeomagnetic data causes extremely rapid motion of Gondwana during the Silurian. Consequently the X-path for that period is used. The palaeomagnetic poles for 422 and 406 Ma have been eliminated so that Gondwana motion is within the bounds of present day plate motion.

Extension in Death Valley is usually interpreted as a combination of low-angle Basin and Range style extension and strike slip associated with the developing Pacific-North America plate boundary in western North America, with these two tectonic regimes operating synchronously in Death Valley. Examination of structural, stratigraphic, and timing relationships in the region suggests that this interpretation needs revision. Evolution of Death Valley is best described as a two-stage process. In the first stage, lasting from ca. 18 to 5 Ma, low-angle Basin and Range extension transported allochthons consisting of Late Proterozoic through Early Paleozoic miogeoclinal section along detachment surfaces that, as extension continued, were exhumed from mid-lower crustal levels to the surface. Near the end of this extensional phase and lasting until ca. 3 Ma, deposition of a thick sequence of volcanics, clastics, and some lacustrine carbonates signaled a period of relative tectonic quiescence, with sediments in some areas covering the exhumed detachment surfaces. At ca. 3 Ma, initiation of the East California Shear Zone started development of the present-day topographic depression of Death Valley, formed as a pull-apart basin associated with this strike slip. Faulting broke the older, inactive, Basin and Range detachment surfaces, with high-angle transtensional faulting along the Black Mountains front. The Black Mountains were elevated as a result of footwall uplift, with the well-known turtleback structures being megamullions along these bounding faults. These megamullions are similar to those seen at oceanic spreading centers. The Panamint Range has previously been interpreted as an extensional allochthon, with the entire range transported from on top of or east of the Black Mountains. A new interpretation presented here is that the range is a large core complex similar to the core complex at Tucki Mountain, at the northern end of the range. The Basin and Range extensional detachment tracks over the top of the range, with extensional allochthons perched on the eastern flanks of the range. This modified model for evolution of Death Valley suggests a strong link between timing and style of deformation in the basin with the developing Pacific-North America plate boundary, particularly eastward propagation of this boundary.

No direct information about the age and composition of rift-related igneous activity associated with the Late Jurassic opening of the Gulf of Mexico exists because the igneous rocks are deeply buried beneath sediments. Three salt diapirs from southern Louisiana exhume samples of alkalic igneous rocks; these salt domes rise from the base of the sedimentary pile and overlie an isolated magnetic high, which may mark the position of an ancient volcano. Three samples from two domes were studied; they are altered but preserve relict igneous minerals including strongly zoned clinopyroxene (diopside to Ti-augite) and Cr-rich spinel rimmed with titanite. 40Ar/39Ar ages of 158.6 �± 0.2 Ma and 160.1 �± 0.7 Ma for Ti-rich biotite and kaersutite from two different salt domes are interpreted to represent the time the igneous rock solidified. Trace element compositions are strongly enriched in incompatible trace elements, indicating that the igneous rocks are low-degree melts of metasomatized upper mantle. Isotopic compositions of Nd and Hf indicate derivation from depleted mantle. This information supports the idea that crust beneath southern Louisiana formed as a magma-starved rifted margin on the northern flank of the Gulf of Mexico ca. 160 Ma. These results also confirm that some magnetic highs mark accumulations of mafic igneous rocks buried beneath thick sediments around the Gulf of Mexico margins.

Several years ago, samples of mafic igneous rock were found incorporated in salt diapirs within Avery Island and Weeks Island mines, Louisiana. At the time, several alternative hypotheses were proposed for the source of this material. More recently, an additional, slightly larger sample was recovered from the Avery Island mine which has been re-examined, together with the original samples. The rock types have been identified as high-Mg ultrabasic lavas that plot as �continental-rift basic rocks� with volcanic textures. Xenolithic quartz grains within the igneous rocks suggest contamination from metamorphic continental crust. A newly obtained 40Ar/39Ar date of 160.1 ± 0.7 Ma from separated amphiboles indicates an Oxfordian (early Late Jurassic) age. This age helps eliminate several of the early hypotheses and enables a more confident interpretation for the origin of the igneous rocks. Weeks Island and Avery Island salt domes are part of a linear trend of shallow domes, known as the Five Islands, striking northwest-southeast. This particular lineament is, in turn, a component of a sub-regional system of northeast-southwest and northwest-southeast features that may be a surface expression of basement faults and fractures dating from crustal extension (transitional crust) during the early opening of the Gulf of Mexico Basin. It is now possible to propose that igneous activity accompanied the opening and that magmatic rocks were picked up by the salt as it started to form diapirs. The Five Island trend is believed to consist of diapirs whose initial movement was triggered by salt thickness irregularities along a basement fault, and the fault may also have provided a pathway for the picritic magma through the stretched contental crust. It is noted that the end of salt deposition was essentially coeval with the start of formation of oceanic crust in the central Gulf of Mexico. The new radiometric date fits closely with this event.