Since identification of the late Cenozoic Strimon Valley detachment in northern Greece there has been controversy about whether detachment faults exist in SW Bulgaria. The Paleogene Mesta half-graben lies east of the Strimon Valley detachment and contains evidence for an early Cenozoic detachment system in SW Bulgaria. The Ribnovo low-angle normal fault dips 8-18° W at the south end of the Mesta graben. Northward the fault zone dips more steeply west (up to 70°) where it cuts younger Paleogene strata. The geometry of the fault zone coupled with the fact that the half-graben strata dip progressively more steeply toward the base of the section (up to 70°) indicates that the fault zone is a synsedimentary listric normal fault. The detachment fault is unconformably overlain by Middle Miocene sediments, thus the time of faulting and rotation of hanging wall rocks began by at least Priabonian, and ceased by Middle Miocene time. Syn-detachment volcanic and shallow intrusive rocks are present in the southern Mesta graben and present day exposure is an oblique section through the tilted volcanic and subvolcanic complex. Steep foliations in hanging wall basement metamorphic rocks of the Rila Mountains indicate the range has been rotated east relative to gently dipping foliations in footwall metamorphic rocks in the Rhodope Mountains. Other east-dipping Paleogene half-grabens lie to the W and NW of the Mesta graben and may form part of a extensional region underlain by one or more west-dipping low-angle detachment faults. The Paleogene half-grabens and their associated faults trend oblique to a contemporaneous magmatic arc indicating regional Paleogene extension was oblique, to the arc. The Mesta detachment is partly responsible for unroofing of the W Rhodope Mountains and marks the beginning of west-dipping detachment faulting that progressed westward into the younger Strimon Valley detachment fault.