Forearc structure and morphology along the Sumatra‐Andaman subduction zone

Abstract

The Sunda subduction margin, the location of recent magnitude 8-9 megathrust earthquakes, is characterized by major changes in prism and forearc morphology and structure along its 5000 km length. On the Sumatra-Andaman section, measurements of prism width and surface slope (α) indicate along-strike segments, inclu1ding abrupt changes, of prism morphology: (1) a narrow and steep prism between Burma and the Andamans; (2) broad with an averaged gentle slope in the Andamans, Nicobars, and North Sumatra; (3) steep and narrow in Central Sumatra; and (4) wider and less steep offshore South Sumatra, decreasing in width to West Java. Prism width varies from~90 to 180 km and average surface slope from~1 to 3°with a strong inverse correlation between width and slope, also observed globally. The prism deviates from typical taper geometry in parts of the margin, notably offshore North Sumatra where it is characterized by a steep toe and broad plateau. Along-strike changes in morphology are strongly linked to input sediment thickness. Sections of the prism toe represent key global examples of unusual landward vergent thrusting. These sections correspond to a thick sediment input and to a wide prism with shallow surface slope. A low basal shear stress or backstop mechanism may drive this style of faulting. Prism morphology and structure appear to be driven predominantly by input sediment thickness linked to oceanic basement topography, with sediment properties, plate smoothness, and orthogonal subduction rate and obliquity also contributing, and no clear role of plate age or dip.

A 15 year slow‐slip event on the Sunda megathrust offshore Sumatra

“…To model deformation from spatial variations in fault coupling, we assumed a plate subduction rate of 40 mm/yr [Simons et al, 2007;McNeill and Henstock, 2014] and calculated surface displacements with the Okada model of elastic dislocations [Okada, 1985], in a back slip framework [Savage, 1983] (other fault and model parameters are described in Text S4). In this framework, fully locked regions have a coupling ratio of 1, while parts of the fault that slip freely at the full plate subduction rate (40 mm/yr) have a coupling ratio of 0.…”

Sumatra Subduction Zone

“…In other parts of the margin, such as offshore Central Sumatra , the sediment/basement interface acts as the décollement, and this interface may become important as a detachment surface at greater depth within the subduction zone, including offshore North Sumatra. In addition, the internal and basal properties of the north Sumatran forearc are thought to drive the unusually wide prism plateau and the landward vergence of outer prism thrust faults (e.g., McNeill and Henstock, 2014;Fisher et al, 2007;Gaedicke et al, 2006). Postcruise experimental analyses and numerical models will be used to extrapolate the input section properties-constrained by core samples and logging data-to the higher stresses and temperatures that IODP Proceedings 5 V o l u m e 3 6 2 these materials would experience during burial and subduction through time.…”

Deformasi Kompleks Di Pulau Simeulue, Sumatra: Interaksi Antara Struktur Dan Diapirisme

“…Ujung dari prisma akresi yang dekat dengan palung menunjukkan sesar anjak yang berarah ke palung dan ke darat (McNeill & Henstock, 2013). Sementara itu di bagian baratlaut Pulau Simeulue, perkembangan prisma akresi menunjukkan sesar anjak yang mengarah ke palung dimana bagian yang naik adalah blok yang berada di timurlaut (Berglar et al, 2010).…”

“…Asumsi ini didasarkan kepada topografi Pulau Simeulue sebagai salah satu pulau yang terangkat ke permukaan dan berdasarkan kepada model ilustrasi fitur cekungan mukabusur (McNeill & Henstock, 2013). Pada daerah lepas pantai Simeulue puncak busur luar ini adalah tinggian Simeulue (Simeulue plateau), Singh et al, 2008).…”

“…Pada peta geologi regional juga terpetakan sesar-sesar berarah baratlaut-tenggara dan baratdayatimurlaut (Gambar 2) dimana pada peta geologi regional diinterpretasikan adanya sebuah sesar mendatar dekstral yang juga memiliki komponen naik, dimana bagian yang naik adalah blok utara, sesar ini dinamakan sesar Pagaja (Endharto dan Sukido, 1994). Pada kompleks prisma akresi, sesar yang dominan berkembang adalah sesarsesar anjak yang berarah palung dengan dip berarah cekungan (McNeill & Henstock, 2013). Pada penampang seismik di sekitar lepas pantai Simeulue tergambarkan bahwa kompleks prisma akresi Simeulue dominan berkembang sesar anjak berarah palung (Berglar et al, 2010;Singh et al, 2008;Shulgin et al, 2013) .…”