Australia: The Land Where Time Began
Large Igneous provinces - Essential Criteria – Distinguish a LIP from a non-LIP
LIPs – Essential attributes
A critical attribute is the volume of igneous rocks that are formed during a magmatic event. The emplacement of very large volumes of magna on the surface and throughout the profile of the crust, as well as presumably in the lithospheric mantle, characterise LIPs. The total volumes can be difficult to constrain, however, for the various constituents of LIPs, especially as it is only in the youngest LIPs that the volcanic components are intact, whereas intrusive components from which the volcanic portions have been removed by erosion dominate older LIPs. Some parts of the plumbing system can be revealed by regional geophysics, though the only way in which the distribution of underplated components at the base of the crust can be revealed is by seismic studies.
Igneous province classification
· Giant (LIP) >107 km3 (>10 Mkm3)
· Major (LIP) 106-107 km3 (1-10 Mkm3)
· Substantial (LIP) 105-106 km3 (0.1-1 Mkm3)
· Moderate 103-105 km3 (0.001-0.1 Mkm3)
· Small ≤ 103 km3 (≤0.001 Mkm3)
According to Ernst the extrusive and intrusive components and Underplate of a LIP are fundamentally interrelated, as exemplified by giant continental dyke swarms feeding flood basalts. Ernst says he rejects the approach of Sheth (2007), following instead Bryan & Ernst (2008) in subdividing LIPs into primary volcanic or plutonic types.
For many LIPs from the Mesozoic-Cainozoic the preserved thicknesses of extrusive rocks range from ~500 m to > 3 km (e.g. Bryan et al., 2002; Jerram & Widdowson, 2005; Bryan, 2007), and for any individual section the thickness is typically ≥ 1 km. Therefore many LIPs have eruptive and/or subvolcanic intrusive volumes that are well in excess of 1 Mkm3 (Courtillot & Renne, 2003). For LIP-related dyke swarms the average dyke thickness can be 10-30 m, with extents > 1,000 km, and display an overall radiating pattern. The thicknesses of LIP-related sills are 10s-100s of m, and associated layered intrusions have volumes of up to 40,000 km3.
In this volume Ernst follows the original definition (Coffin & Eldholm, 1994) as well as the revised definition (Bryan & Ernst, 2008) to conclude that LIPs should have a minimum extrusive/subvolcanic intrusive volume that exceeds 0.1 Mkm3. For the purpose of comparing estimates of LIP volume it is also important to note whether these estimates include extrusive and subvolcanic (upper crustal) intrusive volumes, and/or middle and lower crustal components that have been revealed by geophysical methods (see also Courtillot & Renne, 2003; Bleeker & Ernst, 2006).
According to Ernst, given the complexities involved in determining the volume of LIPs, areal extent is a simpler measure and there is probably a broad proportionality between LIP volume and the original areal extent for most events, though there appears to be a distinctly higher volume to surface area than is the case of continental LIPs. Ernst suggests a reasonable reconnaissance approach is to draw a line around all units of a LIP and calculate the areal extent that is enclosed then convert this to a volume by the use of an “assumed” thickness of 1 km. The Ontong Java Plateau, e.g., has an area of about 2 Mkm2 which Ernst says is approximately 1-third of the conterminous USA or equivalent to the area of western Europe.
The Columbia River flood-basalt province is the smallest LIP, that has been recognised, was originally estimated to cover about 0.16 Mkm2 (Coffin & Eldholm, 1994), has been subsequently revised upwards to >0.2 Mkm2 (Camp et al., 2003). For exposed plumbing systems and intrusive provinces of LIPs, studies have shown that their areal extents are also consistent with this about 0.1 Mkm2 minimum estimate (e.g. Yale & Carpenter, 1998; Marzoli et al., 1999; Ernst et al., 2005). Major regional continental dyke swarms, so-called giant dyke swarms, e.g., are >300 km long (Ernst & Buchan, 1997) with typical extents that also exceed 0.1 Mkm2. The Mackenzie Swarm in northern Canada, from 1,270 Ma, is the largest (Fahrig, 1987). Many LIPs are in practice much larger than 0.1 Mkm2. It has been revealed by a review of several LIPs from the Mesozoic-Cainozoic that they have areal extents of about 1 Mkm2 (see summary in Courtillot & Renne, 2003) and many oceanic plateaus also have areas of 1 Mkm2 (Kerr, 2005, 2014), and silicic LIPs (SLIPs) have also been shown to have dimensions much greater than 0.1 Mkm2 (Bryan et al., 2002; Bryan & Ernst, 2008). Ernst concluded that while many LIPs have areas greater than 0.1 Mkm2 it remains useful to keep the cutoff at the original value of 0.1 Mkm2 to avoid the loss of some LIPs that have been long-recognised, such as the Columbia River LIP from 17 Ma in North America.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|