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Annual Review of Earth and Planetary Sciences

Volume 43, 2015

Continental Lower Crust

Abstract

The composition of much of Earth's lower continental crust is enigmatic. Wavespeeds require that 10–20% of the lower third is mafic, but the available heat-flow and wavespeed constraints can be satisfied if lower continental crust elsewhere contains anywhere from 49 to 62 wt% SiO. Thus, contrary to common belief, the lower crust in many regions could be relatively felsic, with SiO contents similar to andesites and dacites. Most lower crust is less dense than the underlying mantle, but mafic lowermost crust could be unstable and likely delaminates beneath rifts and arcs. During sediment subduction, subduction erosion, arc subduction, and continent subduction, mafic rocks become eclogites and may continue to descend into the mantle, whereas more silica-rich rocks are transformed into felsic gneisses that are less dense than peridotite but more dense than continental upper crust. These more felsic rocks may rise buoyantly, undergo decompression melting and melt extraction, and be relaminated to the base of the crust. As a result of this refining and differentiation process, such relatively felsic rocks could form much of Earth's lower crust.

Keyword(s): continental crust, delamination, differentiation, heat flow, lower crust, relamination, wavespeeds

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Continental Lower Crust
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2015-05-30
2025-06-17
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Continental Lower Crust
Annual Review of Earth and Planetary Sciences 43, 167 (2015); https://doi.org/10.1146/annurev-earth-050212-124117
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Supplementary Data

  • Supplemental Table 1. Compositions of samples from Archean granulite-facies terrains. Data from Huang et al. (2013), supplemented by (Balasubrahmanyan 1978; Bingen et al 1988; Bridgwater & Collerson 1976; Condie et al 1991; Hansen et al 1987; Harley et al 1990; Howie 1954; Iyer & T.R.N. 1978; Jaeckel et al 1997; Janardhan et al 1982; Kröner et al 1999; Martin et al 1983; Nehring et al 2010; Percival 1991; Pride & Muecke 1982; Raith et al 1999; Rollinson 2012; Rosen 1989; Suzuki et al 2006).

    Download Supplemental Table 1 (XLS).

    Download References for Supplemental Tables 1–2 (PDF).

    Supplemental Table 2. Compositions of samples from post-Archean granulite-facies terrains. Data from Huang et al. (2013), supplemented by (Bernard-Griffiths et al 1996; Bhattacharya & Kar 2002; Brandt et al 2011; Conceição de Araújo Pinho et al 2011; Cox et al 1998; De et al 2000; GAF-BGR 2008a; b; c; Hacker unpublished data; Indares & Dunning 2001; Indares & Dunning 2004; Indares et al 1998; Iyer et al 1996; Janoušek et al 2004; Janoušek et al 2006; Janoušek et al 2007; Johannson & Moeller 1986; Knudsen & Andersen 1999; Knudsen et al 1997; Kotková & Harley 2010; Martignole & Martelat 2003; Osanai et al 2004; Ouzegane et al 2003; Raith et al 1997; Rivalenti et al 2008; Von Quadt 1993; Vrána 1989).

    Download Supplemental Table 2 (XLS).

    Download References for Supplemental Tables 1–2 (PDF).

    Supplemental Table 3. a) Radiogenic heat production in lithologies and b) heat production and heat flow in model crustal sections.

    Download Supplemental Table 3 (XLS).

    Supplemental Table 4. Calculated lower crust compositions that satisfy VP, VP VS and heat-flow constraints, divided into different SiO2 and VP ranges.

    Download Supplemental Table 4 (XLS).

    Download all Supplemental Figures as a single PDF or see below.

    Supplemental Figure 1. Mode of garnet calculated with Perple_X for the granulite-facies xenolith database of Huang et al. (2013) at 1 GPa and 700°C. Of the ~1000 samples modeled, only 0.3% are predicted to have no garnet; 5.7% have < 5 vol%.





    Supplemental Figure 2. Experimentally measured samples with VP/VS = 1.71–1.76 mostly have < 55 wt% SiO2. Experimentally measured samples with 55–65 wt% SiO2 mostly have VP/VS = 1.71–1.76. Our database of laboratory-measured samples includes (Almqvist et al 2013; Babuska & Pros 1984; Birch 1960; Boundy et al 1992; Brownlee et al 2011; Burke & Fountain 1990; Christensen 1989; 1996; Christensen & Okaya 2007; Christensen & Ramananantoandro 1971; Christensen & Szymanski 1988; Fliedner et al 2000; Fountain 1976; Fountain et al 1994; Fountain & Salisbury 1996; Fountain et al 1990; Gao et al 2001; Gao et al 2000; Jackson & Arculus 1984; Ji et al 2013; Kern et al 1999; Kern et al 1996; Kern et al 2002; Kern et al 1997; Kern et al 2009; Kern et al 2001; Kern & Richter 1981; Kern & Schenk 1988; Kern & Tubia 1993; Kern et al 1993; Kono et al 2009; Manghnani et al 1974; McCaffree & Christensen 1993; McDonough & Fountain 1993; Miller & Christensen 1994; Mooney & Christensen 1994; O'Reilly et al 1990; Punturo et al 2005; Reid et al 1989; Salisbury & Fountain 1994; Wang et al 2005a; b; Zhao et al 2011).





    Supplemental Figure 3. Difference in a) VP and b) VP/VS measured in the laboratory at 0.6 GPa and 25°C and that calculated for 300°C, 500°C, and 900°C at 1.0 GPa. Corrections laboratory measurements to lower crustal conditions is minor except when lower crustal temperatures reach the β quartz stability field. Red, green and blue filled symbols show calculations for laboratory samples done using reported mineral modes and compositions and the algorithm of Hacker and Abers (2004); pale green unfilled symbols show Perple_X calculations for the reported compositions of granulite-facies xenoliths and terrains.





    Supplemental Figure 4. VP and VP/VS for middle (top) and lower (bottom) continental crust beneath shields & platforms and Paleozoic–Mesozoic orogens. Data from (Assumpcao & Bamford 1978; Catchings 1992; Clowes et al 2005; Epili & R. F. Mereu 1991; Fernandez-Viejo et al 2005; Fliedner et al 2000; Grad et al 2006; Hauser et al 2008; Holbrook et al 1988; Holbrook et al 1992; Janik et al 2007; Juhlin et al 1996; Karplus et al 2011; Levander et al 2005; Luosto et al 1990; Mechie et al 2011; Mechie et al 2004; Mechie et al 2012a; Mechie et al 2012b; Moisio & Kaikkonen 2004; Musacchio et al 2004; Musacchio et al 1997; Rumpfhuber & Keller 2009; Starostenko et al 2013; Stratford & Thybo 2011; Tiira et al 2014; Walther & Fluh 1993; Wang et al 2000).





    Supplemental Figure 5. Relation between density and VP a) calculated using the algorithm of Hacker and Abers (2004) for samples in our database of laboratory-measured samples (Supplemental Figure 2), and b) calculated using Perple_X for granulite-facies xenoliths and terrains from Huang et al. (2013) plus our database. From 6.5–7.5 km/s, these fits differ by <0.07 km/s.

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