LATE EARLY Research Articles

Mesozoic metamorphic and middle to late Tertiary magmatic events on Magdalena and Santa Margarita Islands, Baja California Sur, Mexico: Implications for the tectonic evolution of the Baja California continental borderland

An 40 Ar/ 39 Ar study of metamorphic and volcanic rocks from Magdalena and Santa Margarita Islands, Baja California Sur, Mexico, provides evidence for at least four events that can be related to the tectonic evolution of this portion of the southern Baja California continental borderland prior to, and during, the transition from a convergent to a transform plate boundary system. (1) Metamorphism of amphibolites and quartz-mica schist from upper plate units occurred in Late Jurassic and Early Cretaceous time (∼158–138 Ma). Analogous metamorphic rocks from upper plate rocks in the Cedros–Vizcaino–San Benito terranes of west-central Baja California have not yet been identified. However, the Santa Margarita amphibolites are similar in age to amphibolite blocks from melange of the Puerto Nuevo terrane. (2) Metabasite and metapelitic float blocks associated with serpentinized ultramafic rocks were metamorphosed and subsequently cooled in Early Cretaceous time (129–115 Ma). Their corresponding 40 Ar/ 39 Ar age spectra do not show pronounced age gradients, and they are dissimilar in age, as compared to Late Jurassic (160–170 Ma) amphibolite and late Early Cretaceous (100–115 Ma) blueschist blocks from melange in the Cedros–San Benito terranes. (3) A 30.7 Ma lamprophyre dike that crosscuts normal faults within ophiolitic rocks indicates that synsubduction extension of the upper plate on Santa Margarita Island occurred prior to mid-Oligocene time. (4) Latest Miocene–early Pliocene age (5–6 Ma) adakites were erupted into serpentinite-matrix melange on Santa Margarita Island and indicate derivation by partial melting of a young subducted slab prior to collision of ancestral East Pacific Rise ridge segments. The discovery of adakites in the continental borderland of Baja California Sur provides additional constraints for the Neogene thermal evolution of this convergent margin.

Spacing between simian virus 40 early transcriptional control sequences is important for regulation of early RNA synthesis and gene expression.

We have analyzed the effect of insertion mutants between the simian virus 40 (SV40) 21-base pair (bp) repeats and the early-early (EE) TATA sequence. Insertion of 4, 42, or 90 bp of DNA at the SV40 NcoI site (map position 37) has been analyzed for its effect on expression of the SV40 early gene and positioning of the RNA 5' ends. Insertion of 4 bp reduced SV40 early promoter-dependent chloramphenicol acetyltransferase (CAT) expression by six- to eightfold. Increasing the size of the insertion to 42 or 90 bp resulted in a further drop in early gene expression to basal levels. At the RNA level, the 4-bp insertion reduced EE RNA synthesis approximately 10-fold. No concomitant increase in late-early (LE) RNA synthesis was observed. Insertion of 42 or 90 bp of DNA resulted in a decrease of EE RNA synthesis and a stimulation of LE RNA synthesis. Deletion of the SV40 72-bp repeats from the insertion mutants demonstrated that some, but not all, of the LE RNA depends upon the presence of these sequences. These studies suggest that the ability of RNA polymerase II to utilize the EE (TATA-directed) transcriptional control sequence requires an interaction with the upstream 21-bp repeats or the 72-bp repeats or both. That LE RNA levels in pJI1-in42 CAT and pJI1-in90 CAT were equivalent to the level of EE RNA in pJI1-CAT, yet the level of CAT gene expression was decreased greater than 10-fold, suggests that LE mRNA is under translational control and probably prefers a 5' initiation codon proximal to that of the CAT gene.

Transcription from the SV40 early-early and late-early overlapping promoters in the absence of DNA replication.

Transcription for a hybrid SV40 promoter-beta globin coding sequence recombinant initiates from both early-early (EE) and late-early (LE) SV40 start sites (EES and LES) in the absence of DNA replication. The 72-bp repeat is essential to potentiate the elements of the two overlapping EE and LE promoters (EEP and LEP). Two current models, which can account for the EE to LE shift in RNA chain initiation during the SV40 replication cycle, are that LE transcription is linked to replication and occurs on newly replicated DNA molecules or that there are two promoter elements, a stronger EEP and a weaker LEP, T antigen repressing the EEP late in infection. Our results support the second model. A 5'-TATTTAT-3' to 5'-TATCGAT-3' mutation in the putative SV40 TATA box decreases transcription from EES, increases transcription from LES, and inhibits DNA replication. Therefore, this element acts as a classical TATA box for transcription, and yet is also important for DNA replication.

Stratigraphy of upper Paleozoic volcanic rocks and regional unconformities in part of the northern Sierra terrane, California

Upper Paleozoic volcanic rocks in the southern part of the Taylorsville strike belt are unconformably truncated at the North Fork of the American River by a lens of Upper Triassic conodont-bearing limestone and Lower and Middle Jurassic graywacke-slate flysch deposits of the Sailor Canyon Formation. The volcanic sequence consists of submarine felsic pyroclastic debris flows and tuffaceous slate of the Late Devonian Sierra Buttes Formation, andesitic turbidites of the Taylor Formation, a lower tuff-siltstone member and an upper chert member of the Peale Formation, and fine-grained tuff of the Reeve Formation. Newly discovered radiolarians date the chert member of the Peale as latest Late Mississippian and(or) Early Pennsylvanian. Shelly fossil fragments found 50 m above the chert member date the lower part of the Reeve in this area as late Early Permian. These rocks formed in the distal part of the submarine volcanic apron, presumably in an island-arc setting, and were deposited unconformably on quartz-rich clastic rocks and tectonically interlayered chert of the pre–Late Devonian Shoo Fly Complex. The Peale marks a period of quiescence in the late Paleozoic volcanism that culminated in deep-water chert deposition during latest Mississippian to the Middle Pennsylvanian. The Peale-Reeve contact is a regional hiatus, probably nondepositional in the south and becoming erosional to the north, that spans most or all of the Late Pennsylvanian and Early Permian. Post-late Early Permian to pre-Late Triassic deformation produced northeast-trending folds and a southeast-trending fault in the late Paleozoic volcanic rocks that are truncated nearly orthogonally by the Late Triassic unconformity and northwest-trending folds in the Jurassic Sailor Canyon Formation.