Lithosphere
Last modified December, 2013; irina@ign.ku.dk
Crustal structure and Moho depth
Lithosphere
THE CONTINENTAL LITHOSPHERE
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The Lithosphere book by Artemieva. Cambridge
The lithosphere:
approach
.

Cambridge University Press,
Monograph,
794 pp., ISBN 9780521843966.

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My studies of the crustal structure focus on:
  • regional crustal models,
  • mechanisms that control the depth to Moho,
  • and mechanisms of crustal underplating.
1. Crustal underplating
2. Mechanisms that control the depth to Moho
(with a case study of crust-mantle interaction in Europe)
Underplating was originally proposed as the process of magma ponding at the base of the crust and was inferred from
petrologic considerations. This process not only may add high density material to the deep crust, but also may contribute low
density material to the upper parts of the crust by magma fractionation during cooling and solidification in the lower crust. its
characteristic low average density, also during the early evolution of the Earth.

Despite the assumed importance of underplating processes and associated fractionation, the available geophysical images of
underplated material remain relatively sparse and confined to specific tectonic environments. Direct ponding of magma at the
Moho is only observed in very few locations, probably because magma usually interacts with the surrounding crustal rocks which
leads to smearing of geophysical signals from the underplated material. In terms of processes, there is no direct discriminator
between the traditional concept of underplated material and lower crustal magmatic intrusions in the form of batholiths and
sill-like features, and in the current review we consider both these phenomena as underplating.

In this broad sense, underplating is observed in a variety of tectonic settings, including island arcs,wide extensional continental
areas, rift zones, continental margins and palaeo-suture zones in Precambrian crust. We review the structural styles of magma
underplating as observed by seismic imaging and discuss these first order observations in relation to the Moho.
Thybo H. and Artemieva I.M., 2013.
Moho and magmatic underplating in continental lithosphere.
Tectonophysics, 609, 605-619
Artemieva I.M. and Meissner R., 2012.
Crustal thickness controlled by plate tectonics: a review of crust-mantle
interaction processes illustrated by European examples.
Tectonophysics, v. 530-531, 18–49
Click on figures to enlarge images
Moho and crustal underplating: Kurils and Izu-Bonin arcs
Moho and crustal underplating
Moho and crustal underplating: cratons
Moho and crustal underplating: Baikal and Kenya rifts
Location map for discussed seismic profiles
Examples of underplating (dark blue) in different tectonic
settings. See paper for references.

Top: within the cratons (Wyoming, Baltic Shield, and
Volga-Uralia)
Left: in rift zones (Baikal, and 2 examples from Kenya)
Bottom: across the Kuril arc and along the Izu-Bonin arc.
The continental crust on Earth cannot be extracted directly from the mantle, and the primary crust extracted directly from an early magma ocean is not preserved on Earth.
We review geophysical and geochemical aspects of global crust-mantle material exchange processes and examine the processes which, on one side, form and
transform the continental crust and, on the other side, chemically modify the mantle residue from which the continental crust has been extracted. Major mechanisms that
provide crust-mantle material exchange are oceanic and continental subduction, lithosphere delamination, and mafic magmatism. While both subduction and
delamination recycle crustal material into the mantle, mafic magmatism transports mantle material upward and participates in growth of new oceanic and continental   
We discuss the role of basalt/gabbro-eclogite phase transition in crustal evolution and the links between lithosphere recycling, mafic magmatism, and crustal
underplating. We advocate that plate tectonics processes, together with basalt/gabbro-eclogite transition, limit crustal thickness worldwide by providing effective
mechanisms of crustal (lithosphere) recycling.
 internal structure of Earth, Moon, Mars, Venus, Mercury
Sketch of the internal structure of the
terrestrial bodies and the relative volume of
the crust, mantle and core as percent of the
total volume of the bodies.
Plate tectonics processes may have
a strong control on the crustal
thickness.
crust-mantle interaction processes
Sketch of major processes controlling crust-mantle material
exchange. Vertical and horizontal dimensions are not to scale
formation of continental and oceanic crust
Gabbro/basalt - eclogite phase transitions in the crustal rocks
The processes of crust-mantle interaction have created very dissimilar crustal styles in Europe, as seen by its seismic structure, crustal
thickness, and average seismic velocities in the basement. Our special focus is on processes responsible for the formation of the thin crust of
central and western Europe, which was largely formed during the Variscan (430-280 Ma) orogeny but has the present structure of an “extended”
crust, similar to that of the Basin and Range province in western USA. Major geophysical characteristics of the Variscan lithosphere are
discussed within the frame of possible sequences of crust-mantle material exchange mechanisms during and after main orogenic events in the
European Variscides.
Sketch illustrating geochemical relations between mantle, oceanic crust and
continental crust. Melting of the depleted convecting upper mantle generates
mid-ocean ridge basalts and produces oceanic crust. A significant amount of
the oceanic crust together with the associated residual depleted mantle is
recycled back in subduction zones refertilizing the mantle and producing
island-arc magmatism which plays an important role in formation of the
continental crust. The enriched upper mantle is the source of ocean-island
basalts. Large-scale mantle upwellings (plumes) as well as small-scale
convective instabilities (not shown) transport mantle material into the
continental lithosphere and lead to crustal growth, particularly notable in LIPs.
Vertical and horizontal dimensions are not to scale.

Gabbro/basalt – eclogite phase transitions in the crustal rocks.
Rainbow shading – eclogite stability field, colors refer to lithospheric
temperatures (purple for cold, red for hot). Pressure-depth conversion is made
assuming crustal density of 2.90 g/cm3.
(a) Bold black lines - phase diagram (after Spear, 1993). Shaded area and gray
boxes - extrapolated stability fields of eclogite, garnet granulite, and pyroxene
granulite-gabbro based on experimental data for the quartz tholeiite composition
(Ringwood and Green, 1966). Thin dashed lines – typical continental reference
mW/m2.
(b) Depth to gabbro/basalt – eclogite phase transition (thick gray line) in different
continental settings plotted versus continental reference geotherms labeled in
heat flow values (after Artemieva, 2011). Tectonic provinces are marked on the top
in accordance with typical heat flow values. Gabbro/basalt – eclogite phase
transition limits crustal thickness to 40-45 km in cold stable platforms and to ~30
km in Phanerozoic basins.

Two cross-sections through the European crust constrained by allavailable
seismic data averaged within 600 km-wide corridors along the profiles.
Upper plots (a, c) show the subdivision of the lithosphere into compositional
layers as based on P-wave seismic velocities (Mengel et
al., 1991; Wedepohl, 1995): granites and gneisses (upper crust) Vp<6.4-6.5
km/s; felsic granulites (middle crust) Vp~6.4-6.8 km/s; mafic granulites (lower
crust) Vp~6.8-7.2 km/s; pyroxenites and eclogite (lowermost crust) 7.2-7.6
km/s; spinel lherzolites and harzburgites (lithospheric mantle) Vp>7.8 km/s. For
data sources see Pavlenkova (1996), Artemieva et al. (2006), Ziegler and
Desez (2006), Artemieva (2007), Kelly et al. (2007), Artemieva and Thybo (2011).
Lower plots (b, d) show variations in mean P wave velocity in the basement
of the European crust (i.e. the crust without the sediments) based on seismic
data. Dashed lines refer to in situ conditions (as sampled by seismic methods)
and reflect variations in both crustal composition and average crustal
temperatures. Solid lines - Vp variations corrected for lateral temperature
variations in the crust (based on Artemieva, 2003; 2006), which reflect
variations in the average crustal composition and anisotropy (in case it is
present). Zero corresponds to average in situ Vp=6.6 km/s in a region with a
platform geotherm (surface heat flow ~55 mW/m2). TESZ= Trans-European
Suture Zone; DDR= Dnieper-Donets paleorift; NGB= North German basin.
(e) P-wave seismic velocity structure of the European Variscides and
Caledonides (North German Basin) along the profile DEKORP/ BASIN9601.
Seismic velocities are derived from wide-angle seismic data and shown in
relation to the line drawing of the seismic reflection data (based on Bayer et al.,
1999).

(a) Seismic lamellae in the lower crust in various tectonic provinces where
normal incidence and wide-angle observations are available (based
oncompilation of Meissner et al., 2006).

Four boxes refer to different tectonic settings:
  • "Variscides" include both the Variscan European crust and Cenozoic
    crust of the Basin and Range province (USA); both tectonic provinces
    have undergone significant lithosphere extension;
  • "Pt cratons" include seismic data from the Paleo-Mesoproterozoic
    structures of the Canadian and Baltic Shields;
  • "Central Tibet" (only some selected measurements) and
  • "Alps" are based on seismic data for two Cenozoic collisional
    structures.

(b) Typical temperatures in the lithosphere of different continental tectonic
structures (based on Artemieva and Mooney, 2001).
Colors match the
corresponding structures in plot (a).
 Cold lithospheric temperatures in the
Tibet and the Alps are associated with subducting lithospheric slabs.
Gray shading approximately marks the depths where seismic reflectivity is
observed. As the plot illustrates, seismic reflectivity is commonly restricted to a
depth with temperatures between 300 and 500 oC
Irina M. Artemieva
Geology Section, IGN
University of Copenhagen
Øster Voldgade 10
Copenhagen DK-1350
Denmark
Email:
irina@ign.ku.dk
www.ign.ku.dk