Focused fluid flow and its relation to gas hydrate distribution on the outer accretionary wedge, Southern Hikurangi Margin, New Zealand: evidence from seismic data
Pecher, I.A.; Toulmin, S.; Henrys, S. A.; Kukowski, N.; Wood, W.T.; Gorman, A. R.; Crutchley, G.; Greinert, J.; Coffin, R. B. (2008). Focused fluid flow and its relation to gas hydrate distribution on the outer accretionary wedge, Southern Hikurangi Margin, New Zealand: evidence from seismic data. Eos, Trans. (Wash. D.C.) 89(53): OS33A-1305
In: Eos, Transactions, American Geophysical Union. American Geophysical Union: Washington. ISSN 0096-3941; e-ISSN 2324-9250
Large quantities of fluids are predicted to be expelled from subduction margins. We present seismic images from the outer accretionary wedge on the southern Hikurangi Margin offshore New Zealand. The data show high-amplitude anomalies beneath a thrust ridge, the Porangahau Ridge, that are most likely caused by free gas above the regional level of bottom simulating reflections (BSRs). While we cannot rule out buoyancy- driven gas invasion, several observations let us favor upwarping of the base of gas hydrate stability (BGHS) due to advective heat flow as cause for these anomalies. Estimates of advection rates indicate that the most significant source of fluids is likely to be compaction of subducted sediments. We roughly estimate that ~15% of fluid expulsion across the margin may take place at this ridge, demonstrating the significance of focusing of fluid expulsion in subduction margins. Furthermore, a north-to-south progression of the degree of upwarping of the BGHS and the lack of a pronounced heatflow anomaly in seafloor thermal data point towards transience of fluid expulsion. The amplitude anomalies disappear further south and develop into a BSR gap, which we interpret as evidence for gas depletion. Such several-hundred-meter wide BSR gaps occur beneath other ridges on this margin and we speculate they may mark locations of focused fluid flow. On regional scales i.e., ignoring such relatively small gaps, BSRs coincide with deeply rooted thrusting in the accreted wedge. Low coherency, decrease of amplitudes, and possible lowering of frequencies in the seismic data indicate elevated attenuation in the thrusted sections possibly caused by free gas and/or scattering associated with deformation. The source of methane for hydrate formation on this margin is thought to be mainly biogenic. We propose that the regional link between thrusting and BSR occurrence reflects the opening of fluid conduits that facilitate de-watering of the wedge. Upward migrating fluids "pick up" gas generated in the temperature window for biogenic methane generation in the upper ~2 km beneath the seafloor. This gas is available for BSR and hydrate formation at locations where fluids are being channeled to the seafloor, such as the thrust ridges.
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