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334 W. Manspeizer and Others A LATE TRIASSIC CONTINENTAL BASINS MOROCCAN MESETA B EARLY JURASSIC PARTIAL MELT ASTHENOSPHERE HIGH ALTITUDE LAKES BASIN REWORKING C MIDDLE JURASSIC EROSIONAL SURFACE POST RIFT UNCONFORMITY EVAPORITE PLATFORM MANTLE U~WELLING THERMAL UPLIFT BAJOCIAN TRANSGRESSION SPREADING CENTER COOL UPPER PLATE SUBSIDENCE Figure 12. Crustal evolution model for the Atlantic margins based on low-angle detachment faulting and the formation of lower and upper plate margins (concept and caption modified from Klitgord and others, 1988). The line of section is taken along line A-B, Figure 3. 1, Late Triassic detachment faulting with uplift and arching of the lower American plate, as the load of the upper plate is tectonically removed and displaced laterally, thereby wedging the Moroccan plate upward so that it becomes a broad erosional surface. Late Triassic marine seas transgress the toe of the wedge, depositing evaporites and carbonates on the upper plate. 2, Early Jurassic uplift and partial melting. As tectonic thinning of the upper plate migrates eastward, the locus of partial melting and thermal uplift migrates to the proto-Atlantic axial basins, which are uplifted and eroded during the formation of the post-rift unconformity. As the cooler Moroccan plate subsides, it becoming a broad evaporite platform. 3, Bajocian cooling, subsidence, and seafloor spreading. From Manspeizer, 1988. diabase sheets and basalt flows within the basin produce short- wavelength anomalies similar to magnetic crystalline country rocks. Metamorphism can alter the magnetic character of Triassic sedimentary rocks; rocks such as siltstone and shale, for example, are relatively nonmagnetic away from diabase bodies but strongly magnetized near the diabase where they have been metamor- phosed to hornfels. Part of the hornfels zone can be more magnetic than the diabase (Sumner, 1977) because hematite in "red beds" is converted to magnetite in the contact aureole. Magnetic signatures of diabase dikes are highly variable due to magnetic susceptibility, width, depth extent, orientation, de- velopment of magnetic aureole, and occurrence of multiple dikes. The diabase dikes that cut and intrude both the crystalline basement rocks and the basins result in striking linear magnetic
contours in red are over Jurassic dikes. anomalies that clearly outline the locations of some of the dikes (Fig. 13). Typical examples of magnetic lows over exposed early Meso- zoic basins are the Danville basin in Virginia (Fig. 13) and parts of the Newark and Gettysburg basins in Pennsylvania and New Jersey. The Dunbarton basin in South Carolina-Georgia is an example of a magnetic low over a basin buried beneath Coastal Plain sediments (Marine and Siple, 1974). Ground magnetic surveys across early Mesozoic basins are rare. However, many detailed profiles across Mesozoic diabase dikes and sheets provide resolution not attainable by aeromag- netic surveys (see Daniels, 1980, for examples). GRAVITY The range of measured densities of sedimentary rocks in early Mesozoic basins is generally lower than that of most enclos- ing Precambrian crystalline or lower Paleozoic sedimentary and crystalline rocks. Thus, moderate negative gravity anomalies would be expected where the preserved strata in the basins are thick; however, the geometry of the basins is commonly not Post-Paleozoic activity I 0 20 Km readily apparent from Bouguer anomaly maps because of the dominance of large-amplitude regional gravitational fields. After regional gradients are removed, the margins of the basins are usually discriminated, especially where there are major faults and thick sedimentary sections. Interpretation of basin geometry using gravity maps is further complicated by the presence in some areas of large volumes of intrusive diabase sheets, dikes, and basalt flows; however, the large density contrast between diabase (density Â 3.0 gm/cm3) and early Mesozoic sedimentary rocks (density Â 2.5 to 2.7 gm/cm3) facilitates meaningful analysis of diabase sheets (Daniels, 1980). Bouguer gravity fields over parts of the Culpeper and Newark-Gettysburg basins are shown in Figures 14 and 15, re- spectively (Wise and Johnson, 1980). Trends in Bouguer gravity anomalies are not deflected at the eastern boundary of the Cul- peper basin because the foliated basement rocks trend nearly parallel to the basin along much of the eastern margin. Also, the density contrast between the early Mesozoic sedimentary rocks and the adjacent phyllites is small, the preserved Triassic section at the eastern basin margin is thin, and the Bouguer gravity is dominated by a regional field, the source of which is deeper in the
Page 1 and 2: The Geology of North America Vol. F
Page 3 and 4: Post-Paleozoic activity SIBERIAN Fi
Page 5 and 6: DETRITAL BASINS :'->: SALT BASINS 0
Page 7 and 8: similar orientations and/or lie alo
Page 9 and 10: Post-Paleozoic activity NEWARK BASI
Page 11 and 12: Strata within these Mesozoic basins
Page 13 and 14: Post-Paleozoic activity USGS LINES
Page 15: sic marine tongue was speculated to
Page 19 and 20: crust. A pronounced north-northeast
Page 21 and 22: for relative contributions from iso
Page 23 and 24: Post-Paleozoic activity CULPEPER LI
Page 25 and 26: the basins and also corresponding w
Page 27 and 28: tons (1963), Thayer and others (197
Page 29 and 30: ank 0 6 I I Post-Paleozoic activity
Page 31 and 32: the north side of the basin. Accord
Page 33 and 34: - 46' - 42O Post-Paleozoic activity
Page 35 and 36: activity was initiated. The initial
Page 37 and 38: percent pyroxene (chiefly augite),
Page 39 and 40: have formed under higher conditions
Page 41 and 42: lines an elongated north-south patt
Page 43 and 44: Several studies of the intrusions w
Page 45 and 46: 0 100 200 300 Miles ^ Figure 27. Ma
Page 47 and 48: tomac Formation. They also report t
Page 49 and 50: REFERENCES CITED ' Post-Paleozoic a
Page 51 and 52: Galton, P. M., 1976, Prosauropod di
Page 53 and 54: - , 1988, Triassic-Jurassic rifting
Page 55 and 56: Bulletin, v. 95, p. 1176-1 187. Res

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