3-D Architecture and the Upper Miocene-Pliocene Depositional Pattern of the Gulf of İzmir by Reflection Tomography

The ongoing tectonism in the Western Anatolia creates N-S extension and counter-clockwise rotational motion along the right-lateral North Anatolian fault (NAF) and left-lateral East Anatolian Fault (EAF). This continental extension creates predominantly E-W extending onshore grabens rarely NE to SW and NW to SE trending onshore/offshore grabens characterised by the intense seismic activity, high heat flow associated with volcanism, crustal thinning and geothermal systems. Our study area, the gulf of İzmir, has an “L” shape composing of an E-W oriented inner bay from İzmir to Urla and incompatibly NNW-SSE oriented outer bay between offshore Foça and Karaburun. It is located at the intersection of the E-W oriented onshore Gediz Graben and NE-SW oriented onshore Bakırçay graben. Geophysical evidence for fluid discharge and subsurface gas-associated structures such as gas chimneys, pockmarks, mud diapirs and acoustic turbidity zones have been detected in the inner and outer parts of the Gulf of İzmir by the previous studies. For this reason, the Gulf of İzmir and the adjacent onshore grabens are areas of great interest for further study of the region.

In this study, the 3-D stratigraphic architecture (up to 1.5 km) and the Upper Miocene-Pliocene depositional settings of the Gulf of İzmir reconstructed by reflection tomography for the first time. Three seismic stratigraphic units, labelled SSU1, SSU2 and SSU3 from bottom to top, were identified by their bounding unconformity surfaces (H1-H5). We have subdivided unit SSU1 into three subunits named SSU1c-SSU1a. The acoustic basement associated with SSU3 is likely tied to the Lower-Middle Miocene Yuntdağ Volcanics consisting of tuffs, sandstones, limestones and volcanics. The upper surface of SSU3 (horizon H5) is marked as a major regional unconformity representing a basin-ridge morphology. The first rocks deposited on top of acoustic basement (SSU2) correspond to the sandstones, limestones, volcanics and shales of the Bozköy Formation and the limestones of the Ularca Formation, dating from the Late Miocene to the Pliocene. The top of SSU2 (horizon H4) is interpreted as another unconformity and is correlated with the Pliocene unconformity. Above that, part of the Bayramiç Formation (SSU1c) is dated as Quaternary, consisting of conglomerates at the base overlain by sandstones and shales above. On top of the SSU1c are two further sub-units of the Bayramiç Formation separated by horizons H3 and H2. SSU1b consists of a similar sequence of conglomerates, sandstones and shales; SSU1a consists of Quaternary sandstones. Following the tomographic analysis, the isopach map of the Plio-Quaternary sediment fills was derived from the depth of interpreted horizons calculated using tomographic interval velocities. According to the isopach map of the sedimentary fills, thickness abruptly decreasing from NW to SE. The maximum thickness of total sedimentary succession is ~1400 m in the NW, whereas the thickness decreases through the west, east (up to ~450 m) and the southeastern flank of the basin, reaching ~150 m forming a ridge. A few local lateral velocity variations were identified within the Plio-Quaternary sedimentary succession associated with faults, fluid escape and shallow gas occurrences or a combination of these.