Radiolarian rich sediments of the décollement zone: Trying to understand why major earthquakes and tsunamis occur

Abstract

It was previously hypothesised that thick sediments would limit earthquake and tsunami size in the Sumatra seismogenic zone. However, expectations were defied when in 2004 a magnitude 9.2 shallow slip earthquake produced a tsunami that killed more than 250,000 people. Analysis of the fault-forming sediments pouring into a subduction zone at the North Sumatran subduction zone was the subject of the International Ocean Discovery Program (IODP), Expedition 362 drilling project. In an attempt to test what drives shallow seismogenesis and to elucidate the origin of the HANP reflector, a freshening anomaly linked to amorphous silica dehydration was identified. Thus, shallow slip offshore North Sumatra occurred on a clay-rich fault that was strengthened by silica diagenesis. Contrary with prevailing models for subduction seismogenesis calling for fluid production during subduction, complete dehydration of biosilicates is expected before plate subduction. This increases fault strength and appears to be driven by rapid burial-induced temperatures from thick Nicobar Fan sediments.

The majority of the amorphous silica comes from tiny, siliceous-skeletons of marine organisms called radiolarians. Radiolarians can occur in such abundances that they make up the majority of the sediment fraction. In the Sumataran décollement zone, radiolarians are diagenetically recrystallized (transformed from clear to opaque), where stratigraphic repletion of Miocene through late Eocene radiolarians can be observed. Furthermore, in this zone, the dominance of orosperids (“fat rad zone”) and large nassellarians like Cyclampterium and Calocycletta are recognised to aid fluid transportation.

This work is of great importance because the model derived from our analysis of the Sumatran core samples may be applicable to other thickly sedimented subduction zones with limited earthquake records (such as Makran, Cascadia, southern Lesser Antilles, Eastern Aleutians). strong similarity to North Sumatra in terms of state of diagenesis and potential for megathrust earthquake rupture and coseismic slip is the Lesser Antilles in the northern Barbados Ridge. Here, the thickness and thermal state of sediments affected by input of submarine fan at the southern that shows signs of this major diagenetic phenomenon with a coeval “fat rad zone”.

Biography

Sarah Kachovich has just completed her PhD at UQ in “Capturing initial skeletal growth in Palaeozoic radiolarians”. Three-dimensional scans of microscopic-fossils from Newfoundland were analysed to understand the internal morphology of complex radiolarians in hope to improve the biostratigraphical potential of Palaeozoic forms. This work has then been further applied to geological problems of the eastern Australian accretionary complexes.

Sarah’s current research focuses on her IODP work where she is linking radiolarian biostratigraphy to tectonic problems, such as large earthquakes, the Himalayan collision and the onset of the Indian monsoon.