Lower crust

Blundy JD, Brooker RA (2003) Trace element partitioning during melting and ciystalhzation of mafic rocks in the lower crust. Contrib Mineral Petrol (submitted). 

The so-called late heavy bombardment, discussed more fully in Chapter 14, was a period of intense impacts by large planetesimals, concentrated in the Nectarian and Early hnbrian periods. The Procellarum and Imbrium impacts exposed subsurface rocks in the PKT. The SPA terrane formed during this time and represents not only the largest impact basin on the Moon, but also the biggest basin in the solar system. It exposes lower crust and perhaps mantle materials. 

Mohorovicic Discontinuity The boundary between the lower crust and the upper mantle. 

A central tenet to the step-wise model is that the continental lithosphere deforms as a series of small plates, and that plate boundary stresses are sufficient to reactivate old suture boundaries as sites of crustal thickening and mantle subduction far from the plate boundary. The continental crust thickens homogeneously above internally undeformed mantle lithosphere that is partially subducted at these boundaries. Localized strain at sites of strike-slip and mantle subduction also requires a mechanically competent lower crust and mantle lithosphere such that stresses are efficiently transmitted from the mantle lithosphere through the crust. 

Beaumont C, Jamieson RA, Nguyen MH, Lee B (2001) Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation. Nature 414(6865) 738-742 Block L, Royden LH (1990) Core complex geometries and regional scale flow in the lower crust. Tectonics 9(4) 557-567... 

Bird P (1991) Lateral extrusion of lower crust from under high topography, in the isostatic limit. J Geophys Res 96(B6) 10,275-10,286... 

Kruse S, McNutt M, Phipps-Morgan J, Royden L, Wernicke B (1991) Lithospheric extension near Lake Mead, Nevada - A model for ductile flow in the lower crust. J Geophys Res 96(B8) 4435-4456 Kutzbach JE, Guetter PJ, Ruddiman WF, Prell WL (1989) Sensitivity of climate to late Cenozoic uplift in Southern Asia and the American West Numerical experiments. J Geophys Res 94 18,393-18,407 Kutzbach JE, Prell WL, Ruddiman WF (1993) Sensitivity of Eurasian climate to surface uplift of the Tibetan Plateau. J Geol 101 177-190... 

We applied this technique to make an initial estimate of the timing and extent of Cenozoic uplift of the Colorado Plateau (Sahagian et al. 2002a). Because the technique measures paleoatmospheric pressure, it is not subject to uncertainties stemming from the use of proxies that depend on environmental factors other than elevation alone. Vesicular lavas preserve a record of paleopressure at the time and place of lava emplacement because the difference in internal pressure in bubbles at the base and top of a lava flow depends on atmospheric pressure and lava flow thickness. The modal size of the vesicle (bubble) population is larger at the top than at the bottom. This leads directly to paleoatmospheric pressure and thus elevation because the thickness of the flow can easily be measured in the field, and the vesicle sizes can be measured in the lab. All proxies have their limitations and hence are not applicable in all places and at all times. Vesicular basalts are no exception. For a lava flow to record atmospheric pressure, the flow thickness must remain constant between the time the upper and lower crusts (10-20 cm) form, and the time of complete solidification of the flow. 

Natural Gas Release from Lower Crust and Mantle Domains... 

Of particular note is that methane, and natural gas, if from great depths, should contain He from radioactive decay of the very dense uranium and thorium in the lower crust and mantle domains. This has been documented in most of the above instances. Natural gas reservoirs producing gas containing over 5%, sometimes as much as 10%, helium are long known, and they provide this purified gas for many industrial uses and lighter-than-air transportation craft. 

This means that the lead paradox is alive and well, and the search for the unradiogenic, hidden reservoir continues. The lower continental crust remains (in the author s opinion) a viable candidate, even though crustal xenolith data appear to be, on the whole, not sufficiently unradiogenic (see review of these data by Murphy et al. (2003)). It is not clear how representative the xenoliths are, particularly of the least radiogenic, Precambrian lower crust. Another hypothetical candidate is a garnetite reservoir proposed by Murphy et al. (2003). 

Berckhemer H. (1969) Direct evidence for the composition of the lower crust and the Moho. Tectonophysics 8, 97—105. 

Constantin M., Hekinian R., Ackermand D., and Staffers P. (1995) Mafic and ultramafic intrusions into upper mantle peridotites from fast spreading centers of the Easter microsplate (South East Pacific). In Mantle and Lower Crust Exposed in Oceanic Ridges and in Ophiolites (eds. R. L. M. Vissers and A. Nicolas). Kluwer Academic, Dordrecht, Boston, London, pp. 71-120. 

Seyler M., Paquette J. L., Ceuleneer G., Kienast J. R., and Loubet M. (1998) Magmatic underplating, metamorphic evolution and ductile shearing in a Mesozoic lower crust-upper mantle unit (Tinaquillo, Venezuela) of the Caribbean Belt. J. Geol. 106, 35-58. 

Vissers R. L. M. and Nicolas A. (ed.) (1995) Mantle and Lower Crust Exposed in Oceanic Ridges and in Ophiolites. Kluwer Academic Press, Dordrecht, Boston, London. 

Rudnick R. L. and Fountain D. M. (1995) Nature and composition of the continental crust a lower crust perspective. Rev. Geophys. 33, 267-309. 

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