Structural development of the Jurassic Magnetic Quiet Zone off Morocco and identification of Middle Jurassic magnetic lineations

Abstract

Geophysical investigations carried out during two Meteor cruises have revealed weak linear magnetic anomalies in those parts of the Jurassic Magnetic Quiet Zone (JMQZ) off Morocco that are not affected by Cenozoic igneous activity. The linear magnetic anomalies are not correlated with variations in relief or structure of oceanic crust. Using the reversal sequence M25‐M41, the anomalies of the JMQZ can be modeled with a half spreading rate of 2.2 cm/yr. Extrapolation of the weak lineations back to the slope anomaly S1 results in a breakup age of 170 Ma. According to this interpretation the crust of the JMQZ off Morocco has formed during the time 170–155 Ma (Bajocian to Oxfordian). However, much lower spreading rates (around 1 cm/yr) cannot be excluded. S1 coincides with a poorly developed “seaward dipping reflector sequence” (SDRS), which is most likely its source. The SDRS corroborates the earlier claim that S1 marks the ocean‐continent transition. Seaward of S1 is a 70‐km‐wide strip of horizontal reflectors that become landward dipping to the west in the uppermost part of the basement; it parallels S1 over a distance of 200 km and may indicate excessive magma supply within the first 2 m.y. of seafloor spreading. Similar landward dipping reflectors are observed also in the conjugate Sohm Abyssal Plain off Nova Scotia. The average magnetization of the landward dipping reflectors is much lower than that of the SDRS. A lower crustal body with a seismic velocity of ∼7.3 km/s, which formed during the Neogene beneath the eastern part of the Essaouira Rise, has only a weak magnetization (<0.5 A/m).