It is polished on one side and has been left rough on the opposite side. It has a deep red, coloration and many people consider the Indonesian plume agates to be some of the finest in the world. Below northern Sumatra, we find that the slab is folded at depth, exhibiting geometry similar to that of the volcanic arc and the trench at the surface. It was collected from Karang Jaya in Northern Sumatra. New data provided by the 20042005 SumatraAndaman great earthquake sequences allow us to image with improved detail the Pwave velocity structure beneath Sumatra and adjacent regions. Off the Sunda Strait, northward bending branches of the MFZ are connected with the right‐lateral Sumatra fault zone (SFZ) along the volcanic arc segment on Sumatra. This is a gorgeous, 7' wide, polished section of red, Indonesian plume agate/jasper. The arc‐parallel transpressional Mentawai strike‐slip fault zone (MFZ) was correlated from the Sumatra forearc basin to the northwest Java forearc basin. The rotation created transtensional pull‐apart basins along the western Sunda Strait (Semangka Graben) as opposed to transpression and inversion on the eastern Sunda Strait, within the new detected Krakatau Basin. The missing outer arc high of the southern Sunda Strait is explained by a combination of Neogene transtension due to a clockwise rotation of Sumatra with respect to Java and by arc‐parallel strike‐slip movements. Today, wedge I forms the outer arc high and the backstop for the younger outer wedge II. The inner wedge I is composed of tectonic flakes stretching from southeast Sumatra across the Sunda Strait to northwest Java, implying a similar plate tectonic regime in these areas at the time of flake development during upper Oligocene. Multichannel reflection seismic profiles along the active Sunda Arc, where the Indo‐Australian plate subducts under the overriding Eurasian margin revealed two accretionary wedges: The inner wedge I is of assumed Paleogene age, and the outer wedge II is of Neogene to Recent age. Tectonic features of the southern Sumatra‐western Java forearc of Indonesia Tectonic features of the southern Sumatra‐western Java forearc of Indonesia
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