PALEOMAGNETISM OF THE SASSAMANSVILLE DIABASE, NEWARK BASIN, SOUTHEASTERN PENNSYLVANIA - SUPPORT FOR MIDDLE JURASSIC HIGH-LATITUDE PALEOPOLES FOR NORTH-AMERICA

A paleomagnetic study of the Sassamansville diabase, which intrudes th e Passaic Formation red beds of the Newark Basin in southeastern Penns ylvania, was conducted to constrain the age of the diabase's magnetiza tion. Nine sites were collected from the diabase around the Sassamansv ille 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 isolate d a stable, internally consistent characteristic remanence at eight si tes. Using the bedding tilt of the nearest baked or unbaked Passaic Fo rmation sedimentary rocks for a fold test, the site means showed best clustering at 65% unfolding. This magnetization is a statistically sig nificant (at 93% confidence) synfolding magnetization. The diabase's m agnetization fails a baked contact test conducted at one of the sites, indicating that the magnetization is secondary and not a primary ther mal remanent magnetization (TRM). Isothermal remanent magnetization (I RM) and partial anhysteretic remanent magnetization (pARM) acquisition experiments and af demagnetization results indicate that the magnetiz ation 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-southwe st horizontally lineated fabric carried by low-coercivity magnetite gr ains that do not carry the remanence. The lineated fabric may have bee n caused by flow of the diabase from the northeast to the southwest du ring emplacement. When the rock magnetic results are considered in lig ht of previous petrographic observations (Davidson and Wyllie, 1968) o f these rocks, the rock magnetics suggest that the magnetization is ca rried by secondary magnetite, which has grown as rims on primary magne tite. The secondary magnetite may have formed during a hydrothermal ev ent proposed by Sutter (1988) based on geochronologic evidence and dat ed at 175 Ma. If the diabase magnetization is considered to be coeval to a widespread remagnetization (B component) of the Newark Basin sedi mentary rocks (Witte and Kent, 1991), the diabase magnetization indica tes 15-degrees of counterclockwise rotation for the Sassamansville syn cline. This is consistent with either left-lateral motion along the bo rder fault or along the intrabasinal Chalfont Fault. The presence of c ounterclockwise block rotations along the border fault could weaken pr eviously reported evidence that the Newark Basin (B component) remagne tization occurred after the Newark Basin strata had acquired most of t heir northwesterly tilt (Witte and Kent, 1991). Correction for the til t would move the Newark (B component) remagnetization paleopole to low er paleolatitudes but necessitate significant post-175 Ma tectonic act ivity in the Newark Basin. This would contradict the long sedimentary record of rifting and border fault activity for the preceding 50 my. A partial tilt correction may be the best geologic and paleomagnetic in terpretation because it would minimize the amount of post-175 Ma tecto nic activity required while bringing the Newark (B component) remagnet ization paleopole into paleolatitudinal agreement with a paleopole der ived from the rotation of European, South American, and African data i nto North American coordinates (Van der Voo, 1992).