Anomalous stress field and active breaching at relay ramps: a field example from Gediz Graben, SW Turkey

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Relay ramps form at different scales in extensional terrains to accommodate the differential stretching among the overlapping bounding-normal fault segments. The major boundary structure of the Gediz Graben (SW Turkey) is segmented, with formation, evolution and active breaching of relay ramps. The Akçapınar relay ramp is an example of these processes that forms within a 2 km wide overlap zone between ∼ E–W-trending segments on the southern margin of the Gediz Graben. Change of structural style at the ramp area is clearly evident from variations in the orientation of fractures and faults of the relay ramp. As a site of local stress field anomaly, ∼ N–S-trending structures in the Akçapınar relay ramp are conformable with the extension that is parallel to the bounding faults of the relay ramp. These structures are also superimposed onto the ∼ E–W-trending regional structural style by an obvious cross-cutting relationship, suggesting both spatial and temporal variation of the state of stress at the ramp area during the relay ramp formation. The regional stress field and the resulting regional structural style characterize the earlier stages of ramp evolution, during which segment interaction is limited. As segment interaction and relay ramp formation advances towards the breaching phase, an anomalous local stress field arises and the relay ramp area experiences extension parallel to the bounding normal faults, that is, transverse to the regional direction of extension. This creates a new structural style at the ramp area.