The tectonic interpretation of basement structures in seismic reflection
profiles from ocean-continent transitions (OCT) of magma-poor rifted
margins is notoriously difficult due to the scarcity of borehole
information. Low-angle intra-basement reflections are frequently
interpreted as detachment faults, and in certain locations the drilled
top of the basement is interpreted as exhumed detachment fault. The
seismic expression of such detachment faults is, however, poorly
understood. We address this problem by comparing synthetic seismic
data from the Tasna OCT, an exposed remnant of a Tethyan margin,
with seismic reflection data from Hobby High, a drilled basement
high within the west Iberian margin. Both sites are widely considered
as being representative of OCT zones. Their geological similarity
and the complementary nature of the data enable us to perform a detailed
investigation of the seismic structure and response of these OCT
zones. This provides insights into the seismic imaging of OCT zones
in general and the tectonic evolution of the associated detachment
systems in particular. On the basis of the Tasna OCT models and their
seismic responses we have identified some potential characteristics
of intra- and top-basement detachments: (i) variable amplitudes and
numerous diffractions from the top of exhumed subcontinental mantle,
(ii) a continuous and strong reflection imaging the top of exhumed
lower crustal rocks, and (iii) a weak and discontinuous reflection
of inverse polarity representing a shallow intra-basement crust-mantle
detachment. Similar features are consistently observed at geologically
equivalent positions in the seismic data from Hobby High and may
thus serve as guidelines for interpretation of seismic data from
un-drilled OCT zones.