Lateral Extension in Response to Late Stage Transpression, An Example from the Zhdanov Field, Caspian Sea - An Alternative to Flower Structures.

Abstract

The Zhdanov Field is located on the eastern margin of the Caspian Sea and is geologically a part of the Apsheron-PreBalkan Fault Zone which crosses the Caspian Sea from Azerbaijan to Turkmenistan. Structurally, the field is part of an east-west orientated plunging anticline (hereafter referred to as the Cheleken Nose) that plunges westwards from the Cheleken Dome for over 25 km into the Caspian Sea. The structure of the Zhdanov Field formed in response to Pleistocene to Recent (post Apsheron) transpressive reactivation of a pre-existing fault at the Miocene and deeper levels (hereafter referred to as the Zhdanov Fault). Transpressive reactivation of the deep-seated fault created a southward verging reverse offset at the Miocene level with up to 600m of vertical displacement, with an unknown amount of lateral or strike slip offset. Data from the orientations of associated fault structures and borehole breakout data indicate that the stress field associated with the reactivation was a north-south compression with the maximum horizontal stress orientated 007 degrees N which is consistent with regional Caspian Sea tectonics. The overlying Pliocene to Recent, Red Series stratigraphy accommodated the reactivation and vertical displacement on the Zhdanov Fault on a series of extensional faults which strike NNE-SSW, orthogonal to the strike of the underlying fault. Because the extensional faults are dominantly post sedimentary, the faults have a planar geometry which form a set of domino style faults that show rigid rotation of the Red Series stratigraphy between faults. The extension direction of the planar faults is sub-parallel to the major east-west fold axis and orthogonal to the southward maximum dip direction of the fold structure which is to the south. The extension direction is therefore orthogonal to the extension direction that would be expected to occur due to gravity driven extension. Instead, the extensional faults represent lateral extension, or expulsion along the strike of the fold that was driven by the ongoing north-south compression which effectively prevented gravity driven extensional movement to the south. The extensional fault set all downthrow to the west aligning with the down plunge direction of the Cheleken Nose. The extensional fault set is likely to have been formed as a response to vertical offset in a structurally confined compressional / transpressional setting. The orientation and movement direction of the resulting extensional fault set is in direct contrast to that which would be expected in either gravity driven extension, or a positive Flower Structure that is commonly associated with transpressive strike slip faults. Lateral expulsion on extensional faults associated with transpression is valid from a kinematic and stress orientation stand point and is therefore a valid alternative to Flower Structures. At the Zhdanov Field, the series of tilted fault blocks created by the lateral extension may lead to new exploration opportunities with potential fault traps throughout the Red Series stratigraphy.