Top-basement detachment faults ( TBDF) represent exhumed segments
of downward-concave detachments accommodating tens of kilometers
of offset. TBDFs are key structures of magma-poor rifted margins,
but are difficult to identify in seismic profiles, because they coincide
with top-basement reflections. We have constructed realistic seismic
models of an Alpine example, which were used to simulate full marine
seismic surveys and to identify the seismic fingerprint of TBDFs
in CMP-stacked and time-migrated sections. Minor extensional allochthons,
which locally cap TBDFs, give rise to pronounced diffractions, which
are in stark contrast to continuous reflections from uncapped segments
of TBDFs. In time-migrated synthetic sections we find a continuous
and constant amplitude reflection alternating with a laterally discontinuous
reflection of variable amplitude. These diagnostics were also identified
in the Lusigal 12 seismic profile imaging the Hobby High detachment,
a TBDF in the Iberia Abyssal Plain. Using these diagnostics, small
extensional allochthons and high-angle faults cutting the Hobby High
detachment are observed in Lusigal 12 and the entire sedimentary
cover can be shown to be postrift.