Wulf A. Gose

Paleoproterozoic intraplate magmatism and basin development on the Kaapvaal Craton: Age, paleomagnetism and geochemistry of ~1.93 to ~1.87 Ga post-Waterberg dolerites

We report U–Pb baddeleyite crystallization ages of ~1927 and ~1879 to ~1872 Ma for dolerite sills intruding the Waterberg Group in Botswana and South Africa. These data increase the known extent of ~1.9 Ga intraplate magmatism in southern Africa and place tighter age constraints on the Waterberg Group than previously available. In South Africa, ~1.88 Ga dolerite intrudes upper Waterberg strata, constraining most, if not all, of the succession to have accumulated between ~2.06 Ga (age of the underlying Bushveld Complex) and ~1.88 Ga. This is consistent with derivation of much of the group from uplifted sources in reactivated segments of the Limpopo Belt. The dolerites are typical continental tholeiites, but their trace-element contents discriminate them from dolerite sills of the 1.1 Ga Umkondo Igneous Province, which occur in the same region. Paleomagnetic samples from dolerite intrusions in the Waterberg Group in South Africa (including one sill with a U–Pb baddeleyite age of ~1872 Ma), and from dolerite sills and basalt flows in the Soutpansberg Group to the east-northeast, yield antipodal directions with a site mean pole at 15.6°north, 17.1°east, A95 = 8.9°. These data are interpreted to indicate that the ~1879 to ~1872 Ma dolerites were intruded into the Waterberg Group during voluminous magmatism associated with development of the Soutpansberg rift basin. Older, ~1927 Ma dolerite in Botswana is similar in age and geochemistry to basalts in the craton-margin Olifantshoek Supergroup, suggesting that the mafic magmatism in those two regions is genetically related.

The Patagonian Orocline: New paleomagnetic data from the Andean magmatic arc in Tierra del Fuego, Chile

The Hardy Formation is a 1300-m-thick succession of Upper Jurassic-Lower Cretaceous volcaniclastic sedimentary rocks interbedded with lava flows on Hoste Island at the southernmost tip of South America (55.5°S, 291.8°E). The strata are gently folded and metamorphosed to the prehnite-pumpellyite grade. A well-defined characteristic direction of magnetization, carried by magnetite, was readily identified in 95 samples from seven sites. At a given site, the directions group slightly better without structural correction. However, the means of the seven sites cluster better without tilt correction at the 99% significance level, implying that the magnetization postdates the folding event. It is most likely that the magnetization was acquired during the mid- to Late Cretaceous Andean orogeny that involved the folding and emplacement of the Patagonian Batholith. The fact that all samples are normally magnetized supports this age assignment. The pole position of 42.9°N, 156.6°E, α95=3.3° implies that the sampling area has rotated counterclockwise relative to cratonic South America by 90.1±11.9° with no significant flattening of inclination (F=1.9 ± 3.7°). Geologic considerations indicate that the rotation involved the entire Andean magmatic arc in Tierra Del Fuego. The results support interpretation of the Hardy Formation as part of the Andean magmatic arc deposited on the Pacific side of the Late Jurassic-Early Cretaceous Rocas Verdes marginal basin. Oroclinal bending of the arc in southernmost South America accompanied inversion of the marginal basin and the development of a Late Cretaceous-Cenozoic left-lateral transform system (South America-Antarctica) that later developed into the North Scotia Ridge.

Paleomagnetic data from the Redding section of the eastern Klamath belt, northern California

A paleomagnetic analysis of units of Devonian to Triassic age within the Redding section of the eastern Klamath Mountains indicates clockwise rotation of 83°–116° with respect to stable North America. In the kinematic model presented here, shear zones within the Franciscan Complex, with separation antithetic to the San Andreas, are used to effect this rotation.

Paleomagnetic and geochronological evidence for large-scale post–1.88 Ga displacement between the Zimbabwe and Kaapvaal cratons along the Limpopo belt

Proterozoic reconstructions of the Kaapvaal and Zimbabwe cratons have been limited by the scarcity of precisely dated paleomagnetic poles for the Zimbabwe craton. We present new U-Pb baddeleyite and apatite dates from two diabase sheets that have previously yielded paleomagnetic data from the Mashonaland igneous province in the Zimbabwe craton. Discordant baddeleyite analyses yield upper intercept dates of 1871.9 ± 2.2 and 1882.7 +1.6/–1.5 Ma. Apatite data from the same samples give less precise but statistically indistinguishable dates, providing direct constraints on the post-magmatic thermal history of the diabases. The new U-Pb dates and other recently published baddeleyite dates from the Mashonaland province are coeval with mafic magmatism in the adjacent Kaapvaal craton (1879–1872 Ma), but paleomagnetic poles from the two intraplate suites differ by 39°, suggesting that the two cratons underwent substantial relative motion after ca. 1.88 Ga. Paleomagnetic reconstructions are consistent with >2000 km of lateral displacement being accommodated in the Limpopo orogenic belt that separates the two cratons.

Heinrich H1 and 8200 yr B.P. climate events recorded in Hall's Cave, Texas

Measurements of magnetic susceptibility (MS) have been performed on a continuous set of samples from a well-dated ∼3 m sequence of sediments collected from Halls Cave, Edwards Plateau, central Texas. Three major climatic events are represented by distinctive MS increases in the sequence: (1) the Heinrich event H1 (from 17,500 to 17,000 yr B.P.); (2) the end of full glacial conditions on the Edwards Plateau at 14,200 yr B.P.; and (3) the 8200 yr B.P. climatic event (from 8250 to 8050 yr B.P.). In addition, a minor event at 4400 yr B.P. is also well defined here and in Canada, indicating that it is a broadly regional event. These data indicate that all four events represent a shift toward milder climatic conditions accompanied by increased rainfall at this location. The general climatic trends interpreted from the MS data are consistent with independent vertebrate assemblages, floral indicators, and isotopic data recovered from the cave.

Paleomagnetic results from Mesozoic strata in the Mérida Andes, Venezuela

We report paleomagnetic results from Jurassic through Cretaceous sedimentary rocks (nine sites) in the Merida Andes, Venezuela. Detailed thermal and alternating field demagnetization yielded well-grouped directions of both polarities. The mean of the normal directions is statistically identical to the mean of the reversed directions and the site means clusters better after structural correction, both at the 95% significance level, indicating that the primary magnetization has been isolated. The mean pole position lies at 86.5°N, 151.1°E, A95 = 5.6°. Combining this pole position with 10 other poles from the southern part of cratonic South America establishes a mean pole for the Late Jurassic through Cretaceous at 87.6°N, 211.0°E, A95 = 3.3°. These results imply, within the error limit of the paleomagnetic data, that South America is indeed one rigid plate and that the motion of this plate was restricted to east-west translation since the opening of the South Atlantic Ocean.

Caribbean Tectonics from a Paleomagnetic Perspective

The geologic definition of Central America encompasses the area from the Isthmus of Tehuantepec in southern Mexico to the Atrato lowlands in western Colombia (Fig. 1) and was arrived at “more on a geographic basis than on geologic grounds” (Dengo and Boh-nenberger, 1969). The fundamentally different nature of northern and southern Central America has long been recognized (e.g., Vaughan, 1918; Schuchert, 1935; Sapper, 1937). In northern or nuclear Central America, metamorphic and plutonic rocks of Paleozoic and possibly late Precambrian age are overlain by sedimentary strata as old as late Paleozoic (e.g., Weyl, 1980). By contrast, the oldest rocks in southern Central America are of upper Jurassic age (Nicoya Complex; Schmidt-Effing, 1979) and the overlying sedimentary rocks are upper Cretaceous and younger. The boundary between the two areas lies somewhere in Nicaragua. The extensive Tertiary volcanic cover prevents a specific delineation based on surface geology.

Evidence for episodic basin dewatering in salt-dome cap rocks

A detailed record of episodic basin-dewatering events is preserved in the anhydrite cap rocks of two Gulf Coast salt domes. Metalrich brines were intermittently expelled from geopressured zones deep in the stratigraphic section and were channeled upward along escape structures bounding the salt diapirs. Overhanging anhydrite cap rock helped to focus some escaping fluid into the zone of dissolution between the top of salt and overlying residual anhydrite cap rock. Iron, lead, and zinc sulfide solubilities were exceeded in this zone, possibly in response to dissolution and reduction of cap-rock sulfates. Because the metalliferous brines entered the dissolution zone intermittently, they were recorded as relatively thin horizontal bands of sulfide sandwiched between thicker accumulations of anhydrite. Continued dissolution of salt and underplating of residual anhydrite caused the sulfide bands to be displaced upward relative to the base of the cap, leading to an inverted stratigraphic record of basin-dewatering events. Paleomagnetic data from the Winnfield salt dome suggest that sulfide-producing basin-dewatering events and anhydrite cap-rock accumulation occurred between 157 and 145 Ma.

Clockwise block rotations in the Perija Mountains, Venezuela

Paleomagnetic analyses of Mesozoic sedimentary rocks (8 sites, 115 samples) from the Perija Mountains, Venezuela, yield well-grouped directions of both polarities. Fold tests imply that the magnetization of the Jurassic La Quinta Formation is pre-mid-Cretaceous in age and that the Lower Cretaceous Cogollo Group acquired its magnetization prior to its mid-Tertiary deformation. These data as well as the results from the Cretaceous Apon and La Luna Formations yield consistent northeast or southwest declinations and inclinations corresponding to the present latitude of the sampling area. The clockwise rotations indicated by these data are interpreted to be the result of rotation of fault-bounded blocks in a left-lateral strike-slip zone. Faults such as the Cogollo, La Ge, and Totumo faults which intersect the Perija-Tigre fault at low angles, had initially a north-northwest strike. The Late Oligocene northwest-southeast compression which initiated the uplift of the Perija Mountains rotated these faults to their present north-northeast orientation imparting the observed left-lateral slip. Our paleomagnetic data measure the corresponding rotation of the fault-bounded blocks. Model calculations suggest that a displacement on these faults of less than ten kilometers would be sufficient to cause the observed approximately 45[degrees] clockwise rotations.

Preliminary Paleomagnetic Investigations of Winnfield Salt Dome Cap Rock, Louisiana: ABSTRACT

Quarrying operations at Winnfield salt dome in the North Louisiana basin provide access to calcite, gypsum, and anhydrite cap-rock zones. Sulfide laminae in the anhydrite zone are comprised dominantly of pyrhotite with lesser amounts of sphalerite, galena, pyrite, and marcasite. Sulfides cement euhedral anhydrite grains and represent the products of the episodic introduction of metalliferous waters along the salt-anhydrite interface during halite dissolution and residual anhydrite accumulation. Thus, sulfide laminae provide a chronological record of anhydrite cap-rock accretion. Two hundred oriented samples were collected in stratigraphic sequence, covering much of the exposed anhydrite section. Alternating field demagnetization readily revealed the magnetic polarity of most samples. Using only reversely magnetized samples with a well-defined stable magnetization (N = 50, ^agr95 = 6.2°) yields a pole position at 71.4°N, 125.7°E, which implies that the sampled cap-rock sequence formed in the Late Jurassic. This age is consistent with geologic evidence indicating that cap-rock formation began in the late Jurassic and was most intense during the Early Cretaceous. A densely sampled 45-ft stratigraphic interval contains a sequence of normal and reverse polarity zones. Assuming a constant formation rate, these zones can be compared with the M-anomaly sequence. A growth rate of about 67 ft/m.y. (20 m/m.y.) is indicated. This value is about 30 times less than estimates of salt dome growth rates. Because a 10 to 50-fold decrease in volume is associated with halite dissolution and anhydrite accretion, the paleomagnetically determined value for cap-rock formation rate is reasonable. This investigation is the first such study undertaken. The results are encouraging and offer a unique approach for investigating the timing of various geologic processes related to salt dome formation. End_of_Article - Last_Page 1422------------