It appears the nature of the magnetic anomalies on Mars is due more to due tectonic events after the magnetic dynamo ceased.
The southern highlands of Mars display zones of intense crustal magnetization. The magnetic anomalies are weak or absent in the vicinity of large impact basins, the northern plains, and in volcanic regions, indicating that magnetization in these areas have been erased by thermal events. The presence of magnetic anomalies on Mars suggests that the planet maintained an intrinsic magnetic field early on in its history. The anomalies are linear in shape and of alternating polarity, which some authors have interpreted as a sequence of reversals and a process akin to seafloor spreading. The stripes are ten times wider than those found on Earth, indicating faster spreading or slower reversal rates. Although no spreading center has been identified, a map of the magnetic anomalies on Mars reveals that the lineations are concentric to the south pole.
Alternate theories suggest
that these lineations could have been formed at a convergent plate margin through collision and accretion of terranes. A similar process produced banded magnetic anomalies, similar in geometry and even in size to those in Earth’s North American Cordillera. Because only sparse and generally weak anomalies have been detected in the martian northern lowlands, which could constitute an analog to the terrestrial oceanic crust, it is possible that the magnetic field stopped its activity while crustal recycling was still active in Mars.
A third reference tends to agree with the first reference here and states the largest anomalies
detected with the narrow band of latitudes sampled thus far (near 30°N) do not appear to be randomly distributed in longitude. These appear to be concentrated in the 120° longitude sector between 270° and 30°W longitude, roughly coincident with the most heavily cratered and oldest terrain at this latitude. This is contrary to some earlier proposals (27) that assumed that the largest magnetic anomalies would be found in association with relatively young volcanically emplaced material in the Tharsis region (90° to 150°W longitude).
A fourth reference also supports this view
The absence of magnetism indicates that the underlying crust was either formed and/or modified (igneous and/or metamorphic) after the magnetic dynamo had ceased. These events may represent remelting and/or reheating of large portions of the crust by rock-forming processes or by impact-related demagnetization or physical removal of magnetized crustal material. The magnetic anomaly distribution outlines two different age epochs of Mars crust. The oldest crust (> 3 billion years) is associated with the significant magnetic anomalies (greater than 15-20 nT at 400 km altitude) and the younger modified crust with magnetic signatures less than 15 nT to lower than the instrument detection threshold (± 4nT).