Surge-tectonic evolution of southeastern Asia: a geohydrodynamics approach

Abstract

The repeated need for ad hoc modifications in plate-tectonic models to explain the evolution of southeastern Asia reveals their inability to fully explain the complex features and dynamics of this region. As one example, the hypothesis does not provide a mechanism to explain the 180° turns and twists along the strike of several foldbelts and island arcs in the region (e.g. Banda arc). Convection-cell configuration renders such 180° contortions and Rayleigh-Bénard-type convection impossible. However, during the last 10 years, new data bearing on the convection-cell problem have become available in the form of seismotomographic images of the earth's interior. These images show that (i) mantle diapirs as proposed by traditional plate-tectonic models do not exist; (ii) there is no discernible pattern of upper or lower mantle convection, and thus no longer an adequate mechanism to move plates; and (iii) the lithosphere above a depth of about 80 km is permeated by an interconnected network of low-velocity channels.

Seismic-reflection studies of the low-velocity channels discovered on the seismotomographic images reveal that these channels have walls with a 7.1–7.8 km s⁻¹ P-wave velocity. Commonly, the interiors of the channels are acoustically transparent, with much slower P-wave velocities, in places as low as 5.4 km s⁻¹. The author and co-workers have interpreted the low velocities as evidence for the presence of partial melt in the channels, and they postulated that this melt moves preferentially eastward as a result of the earth's rotation. They named these channels “surge channels” and their new hypothesis for earth dynamics “surge tectonics”.

Surge channels underlie every type of tectonic belt, which includes mid-ocean ridges, aseismic ridges, continental rifts, strike-slip fracture zones, and foldbelts. In southeastern Asia, surge channels—mainly foldbelts—lie between all platform and cratonic massifs. These massifs, platforms, and tectonics belts—the surge channels—form an anastomosing E-W pattern southern Asiatic Russia, Mongolia, western China, the Qinghai-Tibetan region, and northern India and Pakistan. Such an anastomosing pattern indicates that flow is an active process in the surge channels.

Surface studies of phenomena that might be associated with the surge channels soon revealed that all active channels are characterized by higher-than-normal heat flow (> 55 mW m⁻², thermal springs and elevated ground-water temperatures, volvanic phenomena, bands of microearthquakes, and linear belts of faults, fractures, and fissures. The latter are especially visible on satellite images. The bands of high heat flow, thermal springs, microearthquakes, and faults-fractures-fissures almost exactly coincide. The fault-fracture-fissure systems are interpreted to be streamlines caused by flow in the surge channels—a consequence of Stokes's Law (an expression of Newton's Second Law of Motion)-and show that Poiseuille flow must dominate in the channels. Hence, the mechanism producing the belts of linear faults-fractures-fissures is viscous drag, produced by fluid motions.

The eastward flow of the magma in the channels is demonstrated clearly in the tectonic patterns of southeastern Asia. In the northern part of the region studied, the E-W striking anastomosing surge channels (tectonic belts) splay northeastward into the coastal regions of Russia. In the south, they splay southward and southeastward through the Malay Peninsula and Indonesia. The open horsetail structures thus created prove that flow is W-E. The presence of the two splay directions, NE and S-SE, indicates in addition that a barrier to eastward flow must lie directly east of Asia. In this author's opinion, this barrier is the existing Benioff zone, because the same NE and S-SE splay patterns are present on each of the paleotectonic maps that have been prepared for nine time intervals from the beginning of Sinian (latest Proterozoic) time to the present.

The presence of the W-E flow patterns through 850 Ma of geological time, patterns that remain essentially unchanged, means simply that tectonic explanations of Asian geology need revision. The patterns that have been mapped indicate that W-E flow across Asia has persisted essentially unchanged for 850 Ma. Surge tectonics is the only hypothesis yet proposed that explains these patterns and their persistence.