Craton, distribution

Figure 7.2. A) Contrasting scales of continental margin and intracratonic margin associated carbonate platforms. B) The distribution of epicontinental (long lines) and cratonic-based (short lines) platform margins during different times in the Phanerozoic. (Based on James and Mountjoy, 1983 After Moore, 1989.)...

Transport and distribution of sediments derived from Andean erosion across the cratonic landscape of Brazil have caused development of an upstream to downstream trend in geomorphic and hydrologic character of the channel-floodplain system (Mertes et al. 1996). In upstream reaches, scroll bar topography on the floodplain tends to... 

Figure 37 Frequency distribution plots for Os, Nd, and Sr isotope compositions of cratonic and noncratonic peridotite xenoliths. Upper right plots give the range for ocean island basalts (OIB) and arrows show the direction of isotopic evolution for melt depletion and enrichment events. Data compiled from sources cited in Menzies (1990b), Pearson (1999a,b), and those given in Figure 21 (after Pearson and Nowell, 2002).

Cratonic mantle peridotites. Over 230 whole-rock cratonic xenoliths have now been analyzed for Re-Os isotope compositions. Given that many peridotite xenoliths have experienced relatively recent rhenium introduction, it is generally best to use rhenium-depletion model ages (Trd) that do not rely on the measured rhenium content of the rock for model age calculation. For cratonic peridotite xenoliths, the frequency distribution of rhenium-depletion ages shows a wide range, with a pronounced mode at 2.5-2.75 Gyr and some samples that have ages of >3.5 Gyr... 

Figure 44 Frequency distribution plots of Re-Os isotope Trd rnodel ages (calculated assuming Re/Os = 0) for cratonic and off-craton peridotite xenoliths and massifs. The light-shaded field in the off-craton plot shows the range for kimberlite-derived peridotites from Namibia and East Griqualand. Data sources as in Figure 21 plus Pearson (unpublished).

Saltzer R. L., Chatterjee N., and Grove T. L. (2001) The spatial distribution of garnets and pyroxenes in mantle peridotites pressure-temperature history of peridotites from the Kaapvaal craton. J. Petrol. 42, 2215-2229. 

Another curious feature of A-types is their uneven distribution throughout the geological record. Rocks of this nature are encountered in Archean cratons (Section 3.11.3.2.1), though they are far subordinate to the TTG and other granitic types. Similarly, A-type magmas are volumetri-cally minor in Phanerozoic orogenic belts they comprise only 0.6% of the vast granitic batholiths of the LFB (Chappell et al., 1991), discussed in Section 3.11.4.4. 

Figure 9 Depositional model for the Kuruman-Griquatown transition zone in a plan view, illustrating lithofacies distribution during drowning of the Kaapvaal craton (source Beukes and Klein, 1990).

Peridotites from the Newlands kimberlite, in the southern part of the Kaapvaal Craton, have a similar, but less tightly clustered mean Tru age of 2.6 0.4Ga, median 2.6 Ga (Table 2) compared with the Northern Lesotho peridotites. Those from the Kimberley area, in the SW of the craton, show a tightly clustered distribution of Tro and Tma ages with a mean Trd age of 2.6 0.2 Ga, median 2.7 Ga, and a mean Tma age of 2.9 0.1 Ga (Table 2). Peridotites forming lithosphere in the north of the craton (Letlhakane, Bots-... 

We design our studies of plume-hthosphere interactions to (1) predict the distribution in space and time of hot, buoyant plume material beneath cratons of various shapes (2) determine the physical conditions favourable for the lateral distribution of plume material beneath cratonic keels, which may give rise to small-volume melts (e.g. kimberlites) (3) evaluate the longevity of cratonic keels beneath large and small cratons (4) predict the behaviour of viscous plume material at the edges of the plume in terms of thickness and temperature. As we show below, a significant thickness of plume material flows beneath a cratonic keel only where the plume rises beneath part of the craton, providing a viable mechanism for the emplacement of kimberlites. 

We assume that a single plume impinged on the lithosphere beneath central Ethiopia, and that the African plate has moved slowly NNE over the stationary plume since 40 Ma (George et al. 1998) (Fig. 4). Below we discuss the general results, rather than comparison with observations from East Africa, which have been summarized by Ebinger Sleep (1998). Specifically, we use these models to evaluate the effects of plume placement and cratonic size on the distribution of plume material susceptible to melting. 

Our models include the movement of the lithosphere over a stationary hotspot, thereby allowing us to evaluate the effects of plume proximity on the distribution of plume material susceptible to melting. We have considered models with 150 km (Ebinger Sleep 1998) and 220 km thick cratonic keels, and varied the loca-... 

Numerical and analytical models of the vertical and lateral flow of hot, buoyant plume material beneath variable thickness continental lithosphere show that steep gradients at the lithosphere-asthenosphere boundary (LAB) strongly influence the distribution of plume material, and, consequently, the location and volume of melt. Cratonic keels deflect the plume material, with only minor thinning and heating of the mantle lithosphere beneath the cratons. 

Fig. 1. Relative probability histograms of Slave craton detrital zircons (continuous curve with black infill below based on data from Sircombe et al. 2001), Ar/ Ar ages of impact spherules in lunar soil samples (dash-dot curve after Culler et al. 2000), and Ar/ Ar ages of impact glasses in lunar meteorites (dashed curve after Cohen et al. 2000). Time interval spans from 4500 Ma, the approximate age of formation of the Moon, to 2500 Ma, the defined Archaean-Proterozoic boundary. Vertical scales of the three curves are independent. Shaded age bars with roman numerals represent main events in basement of the Slave craton that were initially defined on the basis of individual rock age and their inheritance (see Bleeker Davis 1999). The detrital zircon data represent c. 300 zircon grains from five widely distributed samples of a c. 2800 Ma quartzite unit overlying the Mesoarchaean to Hadean-age basement complex of the Slave craton. These data represent a least-biased record of pre-2.8 Ga components of the Slave craton. The broad complementarity in the datasets should be noted. With the first major peak in Slave crustal ages (event V 3100-3200 Ma) immediately following the last major peak in the lunar spherule data. Both lunar soil and meteorite data sets support a lunar cataclysm or late heavy bombardment that appears to have erased or swamped out the pre-4.0Ga lunar record.

Many Archaean cratons show marked lateral heterogeneity. This heterogeneity can be expressed by many different attributes ages and distribution of greenstone belts, plutonic and (or) metamorphic age domains, contrasting struc-... 

Sagoerson, E. P. Turner, L. M. 1976. A review of the distribution of metamorphism in the ancient Rhodesian Craton. Precambrian Research, 3, 1-53. 

The Permian Phosphoria Formation in the northwestern Interior United States contains two phosphatic and organic-ncarbon-rich shale members, the Meade Peak Phosphatic Shale Member and the Retort Phosphatic Shale Member. Ihese rocks were formed at the periphery of a foreland basin between the Paleozoic continental margin and the North American cratonic shelf. The concentration, distribution, and coincidence of phosphorite, organic carbon, and many trace elements within these shale members probably were coincident with areas of optimum trophism and biologic productivity related to areas of upwelling. In the Phosphoria sea upwelling is indicated to have occurred by sapropel that was deposited adjacent to shoals near the east flank of the depositional basin. 

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