Thoughts on the CoED

Following Jean's inspiring opening gambit, I have put down some thoughts in an attempt to congeal my interests in the potential CoED relating to questions of global mechanics and the intraplate tectonic record. They are intended simply to give some advance warning of where I am coming from when we meet together.

The romance of the Indo-Australian Plate : a tragedy in 3 parts

Geosat gravity and topography

An attempt at a scientific justification for what follows can be found at this link.

The union of the Indian and Australian plates was a marriage of convenience, born of India's northward flight. Without the union the Himalayan collision would have been stillborn. From the Australian perspective the allure of India was obvious. Prior to the collision, it was stagnating; bound by nascent ridges and unwilling to sever its ties with Antarctica.

Distance was always going to test a long-term allegiance. Local interests, the necessity of strategic alliances and inevitable dalliances, provided more than enough temptation. In the Australian realm, flirtations with the north (New Guinea, Timor) and to the east (New Zealand) increased the stress levels. The Pacific was inevitably going to demand Australia's long-term attention focussed to the north and east as much as to the west. Tensions were apparent by about 20 Ma, and by 8 Ma divorce proceedings were filed as the Indian Ocean began to fail. By 5 Ma, Australian discontent was manifest. Settlement appears imminent, although it appears little love will be lost in the wash up over the next few million years. The final separation will be will marked by the catastrophic collapse of Tibet with unimaginable consequences for global climate. In the scheme of plate tectonics the union will have proved tragically brief but, nevertheless, incandescent.

Despite the inappropriate analogies, I am convinced that we can learn much about "global mechanics" from the plate scale record of the Indo-Australian flirtation. My primary interest is in unraveling the way this record is manifested inside the various parts of the plate, particularly in continental Australia.

Intraplate stress

after Coblentz et al 1998

My interest in this story started with our attempts to understand the controls on the Australian in situ stress field (Coblentz et al, 1995, 1998). The Indo-Australian plate shows a surprising degree of intraplate seismicity, reflecting unusually high levels of intraplate stress. The stress field is dominantly reverse to strike slip, and shows a regionally organized pattern suggesting a profound role played by collisions in balancing the torque's that drive the plate northwards. The important implication is that as the collisions have evolved so has the intraplate stress field. The test of the notion is in therefore in the neotectonic record.

Neotectonics and seismicity in Australia

Milendella Fault, eastern Mount Lofty Ranges, in digital elevation model (left) and outcrop (right), the red rock in the footwall is fanglomerate outwash containing the Brunhes-Matuyama palaeomagnetic reversal at ~780,000 BP, The white in the footwall is the Miocene(>15 Ma) Mannum Limestone, the blue grey in the hangingwall is Cambrian metamorphic of the Kanmantoo group.

Australia has an astonishingly rich neotectonic record. For example, the current slip rates of the faults bounding the Mount Lofty Ranges and northern Flinders Ranges are in the range 20-100 m/myr. The neotectonic record is consistent with the in-situ stress field and, at least for south eastern Australia, show a surprising correlation with the seismic record (Sandiford, 2002). The modern neotectonic regime can be confidently traced back to about 5-6 Ma, to the time when the southern Alps were initiated due to changing Australia, Pacific plate motion. The implication is that the intraplate stress field and neotectonic record is sensitive to distant plate boundary interactions. Nevertheless, our knowledge of the Australian neotectonic record is very poor. For example, there is some emerging evidence in the flexural response around the Flinders Ranges, for a dramatic increase in stress levels in the last 250 kA

A separate issue relates to the spatial controls on the distribution of intraplate deformation in Australia. The neotectonic record shows that regions such as the Flinders Ranges and the southern uplands of Victoria are locus of neotectonic activity, and have probably been so since the early Pliocene. In the case of the Flinders Ranges, the active deformation correlates with a region of anomalous heat flow (Neumann et al, 2001), suggesting substantial regional variations in lithospheric strength are governed to first order by the distribution of heat sources in the crust. Importantly, the ongoing mild tectonic activity will lead to a significant redistribution of the heat sources, and consequently to changes in the the long-term thermal regime of the lithosphere. I see a profound record in continental interiors which links intraplate tectonic activity with the geochemical organization of the crust as reflected in the heat producing elements (Sandiford & Mclaren, in press). The Phanerozoic intraplate orogens in Central Australia are the archetypes of this type of "tectonic feedback" (Sandiford et al, 2001). Interestingly, the rates of deformation associated with the Alice Springs Orogen (Haines et al, 2001) may well have been similar to the current rates of deformation in the Flinders Ranges-a notion that has profound implications for the mechanics of these remarkable orogens.

The lumpy mantle

Etopo digital elevation image of the Indo-Australian plate warped onto the Geoid

The Australian continent dances not only to the tune of its neighbouring plates, but also to the mantle beneath. Australia is currently traversing an extraordinarily lumpy mantle as reflected in the increase in geoid height of about 70 m across the continent from south to north. The imprint of this lumpy mantle is reflected (I believe) in the starkly contrasting Neogene stratigraphic records of the northern and southern margins consistent with a dynamic (north-down) tilting of the continent of about 70 m over the last 5 Ma. Apart from this tilting, we have little knowledge of how this lumpy mantle imprints on the tectonic response of the continent through the Neogene.

Prenuptial Australia and the modern African continent

To understand the pre-Eocene Australian plate, when it was largely surrounded by active ridge systems, we could do little better than study the factors governing the dynamics of contemporary Africa. The modern ridge-bound plates (Africa and Antarctica) are slow moving because of the low net driving torque's (Coblentz et al, 1995). Plate-scale potential energy distributions in such ridge-bound plates naturally engender extensional stress regimes (Coblentz & Sandiford, 1994; Sandiford & Coblentz, 1994), but the record of plate interactions and episodicity is evident in the thermochronological record of the continental interior - as I trust Rod Brown will elucidate.

In other words, we need to contemplate the record and dynamics of the very different styles of plates on the modern Earth if we are to achieve anything of a global mechanistic view.

 

Some of my related contributions on these ideas

Sandiford, M., McLaren, S., Thermo-mechanical controls on heat production distributions and the long-term evolution of the continents, "Evolution and differentiation of the continental crust" (eds, Brown, M. & Rushmer, T.). html, pdf, figures

Sandiford, M., Neotectonics of southeastern Australia: linking the Quaternary faulting record with seismicity and in situ stress, submitted toG.S.Australia and G.S.America, Joint Special Publication, Evolution and dynamics of the Australian Plate , August, 2001. pdf, html, figures

Haines, P, Hand, M., Sandiford, M., 2001, Palaeozoic syn-orogenic sedimentation in central and northern Australia: a review of distribution and timing with implications for the evolution of intracontinental orogens, Australian Journal of Earth Sciences, 48,

Sandiford, M., Hand, M.,McLaren, S., 2001, Tectonic feedback, intraplate orogeny and the geochemical structure of the crust: a central Australian perspective, In "Continental Reactivation and Reworking", (eds, Miller, J., Holdsworth, R., Buick, I., Hand, M.), Geological Society Special Publication No. 184, 195-218 html, pdf, figures

Neumann, N, Sandiford, M., Foden, J., 2000, Regional geochemistry and continental heat flow: Implications for the origin of the South Australian heat flow anomaly. Earth and Planetary Science Letters. 183, 107-120. pdf, figures

Coblentz, D., Zhou, S., Hillis, R., Richardson, R., Sandiford, M., 1988, Topography, plate-boundary forces and the Indo-Australian intraplate stress field, Journal of Geophysical Research, 103, 919-931, figures

Coblentz, D., Sandiford, M., Richardson, R, Zhou, S., Hillis, R., 1995, The origins of the Australian stress field, Earth and Planetary Science Letters, 133, 299-309.

Sandiford, M., Coblentz, D., Richardson, R.M., Focusing ridge-torques during continental collision in the Indo-Australian plate, Geology, 23, 653-656.

Coblentz, D., Sandiford, M., 1994, Tectonic stress in the African plate: Constraints on the ambient stress state, Geology, 22, 831-834.

Coblentz, D., Richardson, R.M., Sandiford, M., 1994, On the gravitational potential of the Earth's lithosphere, Tectonics, 13, 929-945.

Sandiford, M., Coblentz, D., 1994, Plate-scale potential energy distributions and the fragmentation of ageing plates, Earth and Planetary Science Letters, 126, 143-159.