5 Km/Pixel Geologic Maps: Notes and References

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Scale: 1 pixel = 5 km. 5-degree grid. Maps centered on the equator are Mercator projection with true scale along the equator. Maps centered at 30 and 60 degrees latitude are Lambert Conformal Conic projection. Standard parallels for maps centered 30 degrees north or south are at 20 and 40 degrees. Standard parallels for maps centered 45 degrees north or south are at 35 and 55 degrees. Standard parallels for maps centered 60 degrees north or south are at 50 and 70 degrees.

Meridians at high latitudes converge so sharply that maps centered along 60 degrees would have absurdly large overlap if they were 30 degrees apart, or large overlap at the top and large gaps at the bottom if 60 degrees apart. So high latitudes are covered by two sets of maps, one at 60 degrees latitude and longitudes 0, 60, 120 and 180, and another at 45 degrees latitude centered at longitudes 30, 90 and 150.

Notes

Ocean floor ages in the open oceans are mostly interpreted from magnetic anomaly data and validated by sea floor sampling and drilling. Ages in smaller closed basins like the Gulf of Mexico, Caribbean and Mediterranean appear to have been inferred from plate-tectonic reconstructions.

The Black Sea and southernmost Caspian Sea are widely believed to be floored by trapped oceanic crust of unknown age, probably early Mesozoic or late Paleozoic. On these maps they have been assigned the color for 230-240 my.

After numerous experiments, it seemed that the most effective way to show the relationship between bathymetry and sea floor geology was to draw labeled age contours on a bathymetric map. Drawing bathymetric contours on a colored age map of the sea floor resulted in unintelligible visual clutter.

Units are colored in the USGS style with yellows for Cenozoic rocks, green for Mesozoic, blue and purple for Paleozoic and reds, pinks, browns and grays for Precambrian. My personal preference would be to use red and orange for igneous rocks but there are just not enough shades available. So sedimentary, volcanic, intrusive and metamorphic rocks all use broadly the same hue grading from light to dark.

Since oceanic and continental crust are two separate systems, colors for oceanic crust reflect age and are not connected in any way with the same colors for continental rocks.

References

Ocean floor age data are from Mueller, R.D., M. Sdrolias, C. Gaina, and W.R. Roest 2008. Age, spreading rates and spreading symmetry of the world's ocean crust, Geochem. Geophys. Geosyst., 9, Q04006, doi:10.1029/2007GC001743. (http://www.ngdc.noaa.gov/mgg/ocean_age/data/2008/grids/age/) Data were contoured at 10 Ma intervals using Global Mapper 14.0. Some artifacts created by the original data gridding process were removed manually.

Land geologic data were derived from Chorlton, L. B. (Compiler), 2007: Generalized Geology of the World: Bedrock Domains and Major Faults in GIS Format; Geological Survey of Canada, Open File 5529, (© Her Majesty the Queen in Right of Canada 2007). Cenozoic units in Eurasia are not subdivided and were classified by reference to the USGS Energy Resources Program, World Geologic Maps. A few selected features (like the Sierra Nevada Fault) were manually added.

Coastlines and major lakes were simplified from the CIA World Data Bank II. Rivers and international boundaries are also from the CIA World Data Bank II, supplemented by recent boundary changes.

Topography and bathymetry were based on the ETOPO1 data set: Amante, C. and B. W. Eakins, ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis. NOAA Technical Memorandum NESDIS NGDC-24, 19 pp, March 2009. (http://www.ngdc.noaa.gov/mgg/global/global.html), displayed using Global Mapper 14.0.

Fracture zone and magnetic anomaly data are based on The Global Seafloor Fabric and Magnetic Lineation Data Base Project, http://www.soest.hawaii.edu/PT/GSFML/; Published Magnetic Picks for Tectonic Reconstruction, http://www.soest.hawaii.edu/PT/GSFML/ML/index.htm and Matthews, K. J., R. D. Mueller, P. Wessel, and J. M. Whittaker (2011), The tectonic fabric of the ocean basins, J. Geophys. Res., 116(B12109); http://www.soest.hawaii.edu/PT/GSFML/SF/index.htm.

Volcano locations are from the Smithsonian Global Volcanism Program spreadsheet. Volcanoes active since 2000 BCE are solid red, Holocene volcanoes without recent eruptions are black with red borders. Seamounts are from the SOEST Global Seamount Database (P. Wessel, ftp://ftp.soest.hawaii.edu/pwessel/Global_Seamounts_JGR2001_revised.txt).

Gravity and depth to Moho maps are from the International Gravimetric Bureau WGM 2012 model (http://bgi.omp.obs-mip.fr/data-products/Grids-and-models/wgm2012; Balmino, G., Vales, N., Bonvalot, S. and Briais, A., 2011. Spherical harmonic modeling to ultra-high degree of Bouguer and isostatic anomalies. Journal of Geodesy. DOI 10.1007/s00190-011-0533-4).

Gravity measurements must be corrected for the earth's overall gravitational field, including its equatorial bulge, and centripetal acceleration due to the earth's rotation. Any values remaining after those corrections are termed anomalies. Not that there's anything wrong with anomalies - they're what we want because they reveal clues about the structure of the earth. The traditional units of gravitational acceleration are milligals, .001 cm/sec-squared or about one-millionth of the average gravitational acceleration. (A gal, named for Galileo, is one cm/sec-squared. Gals and milligals are not standard SI units. Tough.)

The IGB gravity measurements are based on a 2-minute grid. Unfortunately, the spherical harmonic method used for creating the maps created a pebbly texture of artifact noise over large regions of Asia. To eliminate the artifacts, the grid was smoothed to 0.1 degrees (6 minutes) for the Bouguer maps. This correction was not applied to the free-air and isostatic maps, and the texture is particularly evident on some areas of the isostatic maps like the Tibetan Plateau and central China.

The depth to Moho map is based on a one-degree grid and therefore contains many abrupt steps, especially in the Sahara.


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Created 11 April 2014, Last Update 09 January 2015
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