A paleomagnetic study of the Sassamansville diabase, which intrudes the Passaic Formation red beds of the Newark Basin in southeastern Pennsylvania, was conducted to constrain the age of the diabase's magnetization. Nine sites were collected from the diabase around the Sassamansville syncline. A tenth site was collected at an associated dike 12 km to the north. Eight to ten individually oriented cores were drilled at each site. Alternating field (af) and thermal demagnetization isolated a stable, internally consistent characteristic remanence at eight sites. Using the bedding tilt of the nearest baked or unbaked Passaic Formation sedimentary rocks for a fold test, the site means showed best clustering at 65% unfolding. This magnetization is a statistically significant (at 93% confidence) synfolding magnetization. The diabase's magnetization fails a baked contact test conducted at one of the sites, indicating that the magnetization is secondary and not a primary thermal remanent magnetization (TRM). Isothermal remanent magnetization (IRM) and partial anhysteretic remanent magnetization (pARM) acquisition experiments and of demagnetization results indicate that the magnetization is carried by high-coercivity magnetite. Anisotropy of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) results indicate that this magnetite has a different fabric from the northeast-southwest horizontally lineated fabric carried by low-coercivity magnetite grains that do not carry the remanence. The lineated fabric may have been caused by flow of the diabase from the north-east to the southwest during emplacement. When the rock magnetic results are considered in light of previous petrographic observations (Davidson and Wyllie, 1968) of these rocks, the rock magnetics suggest that the magnetization is carried by secondary magnetite, which has grown as rims on primary magnetite. The secondary magnetite may have formed during a hydrothermal event proposed by Sutter (1988) based on geochronologic evidence and dated at 175 Ma. If the diabase magnetization is considered to be coeval to a widespread remagnetization (B component) of the Newark Basin sedimentary rocks (Witte and Kent, 1991), the diabase magnetization indicates 15° of counter-clockwise rotation for the Sassamansville syncline. This is consistent with either left-lateral motion along the border fault or along the intrabasinal Chalfont Fault. The presence of counterclockwise block rotations along the border fault could weaken previously reported evidence that the Newark Basin (B component) remagnetization occurred after the Newark Basin strata had acquired most of their north-westerly tilt (Witte and Kent, 1991). Correc-tion for the tilt would move the Newark (B component) remagnetization paleopole to lower paleolatitudes but necessitate significant post-175 Matectonic activity in the Newark Basin. This would contradict the long sedimentary record of rifting and border fault activity for the preceding 50 m.y. A partial tilt correction may be the best geologic and paleomagnetic interpretation because it would minimize the amount of post-175 Ma tectonic activity required while bringing the Newark (B component) remagnetization paleopole into paleolatitudinal agreement with a paleopole derived from the rotation of European, South American, and African data into North American coordinates (Van der Voo, 1992).