1. New views on the structure and rheology of the lithosphere
by JAMES JACKSON, DAN McKENZIE, KEITH PRIESTLEY, and BRIAN EMMERSON
Journal of the Geological Society
Volume 165(2):
February 1, 2008
© 2008 The Geological Society of London

2.  Histograms of earthquake centroid depths constrained by body-wave seismogram modelling for events larger than mb c. 5.3.
 Histograms of earthquake centroid depths constrained by body-wave seismogram modelling for events larger than mbc This figure updates figure 1 of Maggi et al. (2000a), with additional data from our own work, and that of Emmerson et al. (2006) for the Mongolia–Baikal region. The India histogram distinguishes earthquakes south of the Ganges basin (white) from those within and north of the Ganges basin, but within the Indian shield (in red). In the Tibet histogram the white bars are earthquakes on the frontal Himalayan thrust system. Green bars show Moho depths, which are typically 40–50 km in Iran, 25–35 km in the Aegean and deeper than 60 km in Tibet; in all three regions earthquakes are confined to the upper crust. In parts of Africa, India, the Tien Shan and Mongolia–Baikal earthquakes occur throughout the crust, but not substantially in the mantle (the deepest earthquakes are also where the Moho is deepest). A few earthquakes at depths of 70–90 km beneath southern and NW Tibet (Fig. 8) are omitted here, and are considered separately.
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London

3.  (a) Curves showing misfit of observed to calculated gravity anomalies across the Himalayan foreland basin as a function of Te. The curve labelled McKenzie is from Jackson (2002) and used free-air gravity; the curve from Burov & Watts (2006) used Bouguer gravity.
 (a) Curves showing misfit of observed to calculated gravity anomalies across the Himalayan foreland basin as a function of Te. The curve labelled McKenzie is from Jackson (2002) and used free-air gravity; the curve from Burov & Watts (2006) used Bouguer gravity. The vertical axis has no units and has been scaled to show the Te positions of the minima in each curve. (b) The reason for the disagreement in (a) is because the position where the flexed Indian plate is broken was specified by Burov & Watts (2006), but not by McKenzie & Fairhead (1997), Maggi et al. (2000a) or Jackson (2002). The break is defined as the location on the profile where the bending moment is zero. The plot shows the effect of fixing the location of the break for the north Indian average profiles. The actual values for the minima are Te=78km when the break is at +100 km, and Te = 39km when it is at −100 km. When the position of the break is not fixed, the best-fitting value of Te is 50 km, with the break at −50 km. (The origin is the location of the gravity minimum on the stacked profiles.) Clearly, the trade-offs between Te, break-point location and bending moment produce a broad minimum whose true upper bound is not well resolved.
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London

4.  Temperature profiles through the lithosphere based on pressure–temperature estimates from mantle nodules at Jericho (NW Canada) and Udachnaya (Siberia), calculated using the method described by McKenzie et al.
 Temperature profiles through the lithosphere based on pressure–temperature estimates from mantle nodules at Jericho (NW Canada) and Udachnaya (Siberia), calculated using the method described by McKenzie et al. (2005). Pressures and temperatures estimated from nodules were calculated from the expressions of $Finnerty & Boyd (1987). The lithosphere consists of the crust (pink), a rigid mechanical boundary layer (dark green) and a thermal boundary layer (light green). Also shown are lines indicating the onset of melting (solidus) for peridotite and the position of the graphite–diamond transformation. The estimated temperatures at the Moho are c. 500 °C at Jericho and c. 630 °C at Udachnaya.
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London

5.  Isotherms and depths of intraplate earthquakes in a cooling oceanic plate, adapted from McKenzie et al.
 Isotherms and depths of intraplate earthquakes in a cooling oceanic plate, adapted from McKenzie et al. (2005). Black contours show isotherms every 100 °C calculated using temperature-dependent thermal parameters and a plate thickness of 106 km; the 600 °C and 1000 °C contours are emphasized by thicker lines. The grey shaded area is the region of the 600–750 °C temperature range calculated in the older model of Parsons & Sclater (1977), which uses constant thermal parameters and a plate thickness of 125 km. In the new model of McKenzie et al. (2005), the limiting temperature for earthquakes is about 600 °C. In the older model, it is about 750 °C. The sole exception is an earthquake in 1964 on the outer rise of the Chile trench in lithosphere about 45 Ma old, reported by Chinn & Isacks (1983), whose depth we are unable to verify.
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London

6.  (a) Shear-wave velocity structure beneath the Pacific Ocean, obtained from surface-wave tomography by Priestley & McKenzie (2006), averaged as a function of age.
 (a) Shear-wave velocity structure beneath the Pacific Ocean, obtained from surface-wave tomography by Priestley & McKenzie (2006), averaged as a function of age. Noteworthy features are the cooling of the lithosphere away from the ridge, and the almost constant velocity at its base, which corresponds to a temperature variation of less than c. 20 °C. (b) The three black lines show Vs at 50 km depth beneath a cooling oceanic plate as a function of temperature, with 1σ upper and lower error bounds, obtained from (a) using the thermal model of McKenzie et al. (2005). The dashed line is the fit of the parameterized expression derived by Priestley & McKenzie (2006) to those data at 50 km depth. The red lines show the observed Vs (continuous line) and parameterized fit (dashed line) at 75 km. The rapid change in Vs above 1200 °C should be noted.
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London

7.  Lithosphere thickness beneath North America, calculated by fitting geotherms to temperature estimates obtained from shear-wave velocities, by Priestley & McKenzie (2006).
 Lithosphere thickness beneath North America, calculated by fitting geotherms to temperature estimates obtained from shear-wave velocities, by Priestley & McKenzie (2006). The yellow line shows the approximate edge of the North American craton, determined from surface geological observations. Purple dots are the locations of diamond-bearing kimberlites, and yellow–green dots are kimberlites with no diamonds. The adjacent numbers are lithosphere thicknesses in kilometres estimated from mantle nodule geochemistry using the method of McKenzie et al. (2005).
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London

8.  Changes to mantle composition and physical properties as a result of melting, from Priestley & McKenzie (2006).
 Changes to mantle composition and physical properties as a result of melting, from Priestley & McKenzie (2006). The dramatic change in density is caused by the depletion in garnet (see mineral mode fraction) and loss of iron (see magnesium number, mg#). The density variation shown here is the same as that produced by a temperature rise of about 500 °C. By contrast, the corresponding change in Vs is equivalent to a temperature change of only 96 °C, when the temperature is below 1100 °C.
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London

9. Fault-plane solutions of earthquakes larger than mb c. 5
 Fault-plane solutions of earthquakes larger than mb c. 5.5 beneath Tibet.
 Fault-plane solutions of earthquakes larger than mbc. 5.5 beneath Tibet. The red focal spheres are for earthquakes with centroid depths in the range 70–90 km, discussed in the text and by Priestley et al. (2008). The grey focal spheres are all for earthquakes with centroids shallower than 15 km.
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London

10.  Schematic cross-section through the Himalaya to illustrate the relations between structure and earthquake distribution.
 Schematic cross-section through the Himalaya to illustrate the relations between structure and earthquake distribution. The actual data have been discussed in detail by Priestley et al. (2008). The extra vertical exaggeration above sea level should be noted. Black dots are earthquakes within the underthrusting Indian shield, and occur down to the Moho. White dots are shallow normal faulting events, mostly in Tibet but also at the top of the flexing Indian shield beneath the Ganges foreland basin. Blue dots are shallow-dipping thrusts at c. 15 km depth on the main underthrusting interface(s) beneath the Himalaya. The box outlines the area where Schulte-Pelkum et al. (2005) and Monsalve et al. (2006) located microearthquakes in the uppermost mantle as well as in the lower crust. The rigid, strong, granulitic lower crust of India is underthrust beneath the Himalaya, and to an unknown distance further north, indicated by question marks. This Indian lower crust helps support the elevation of Tibet, because the only way Tibet can collapse is by flow of the warm, weak lower Tibetan crust over this rigid base, which is a slow, dissipative processes (Copley & McKenzie 2007). MCT, Main Central Thrust; MBT, Main Boundary Thrust. The precise configurations of geological boundaries indicated by question marks beneath the Indus–Tsangpo suture zone (ISZ) are unknown.
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London

11.  SW–NE tomographic profile showing Sv deviations, obtained from surface waves, relative to a regional reference model (adapted from Priestley et al. 2006, 2008).
 SW–NE tomographic profile showing Sv deviations, obtained from surface waves, relative to a regional reference model (adapted from Priestley et al. 2006, 2008). The positions of the Main Central Thrust (MCT), Bangong suture (BS) and Kunlun fault (KF) are also shown. Mantle velocity deviations are shown only below 100 km, because of the extreme thickness of the Tibetan crust. The almost continuous high-velocity lid at depths of 150–250 km is clear, although it is more pronounced in the south. The low velocity of the uppermost mantle shallower than 150 km in northern Tibet is also clear in this image.
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London

12.  (a) Evolution of temperature within the lithosphere after sudden thickening by shortening.
 (a) Evolution of temperature within the lithosphere after sudden thickening by shortening. Before shortening by a factor of three at time t = 0, the thicknesses of the lithosphere and crust were 80 km and 30 km. The dashed line is the location of the Moho after thickening. Adapted from McKenzie & Priestley (2007). It should be noted how the temperatures both above and below the Moho increase substantially after a few tens of million years. (b) Thick crust above thick lithosphere can be generated by uniform thickening, as long as the mantle lithosphere is already buoyant through earlier depletion during melting, as it then does not delaminate when thickened. The thickened crust above it becomes internally heated and dehydrates at depth to form granulite. The hydrated upper crust can then be removed by erosion to leave a craton. Variations on this theme, with different initial structures and thickening modes, have been discussed by McKenzie & Priestley (2007).
JAMES JACKSON et al. Journal of the Geological Society 2008;165:
© 2008 The Geological Society of London