The definition of Terra Cimmeria used by the 2019 JGR paper is a triangular region of the cratered southern highlands bounded on the west by Hesperia Planum (~120 E longitude) and on the west by the Ma'adim Vallis channel system (~175 E). The northern edge is along the dichotomy boundary (running more or less along the equator), and its southern edge is ~40 S (the southern limit of mapping) in the west and ~28 S in the east. The Terra Cimmeria Region has 34 mapped bedrock plains. Of particular interest are a cluster of seven bedrock plains (CI20, 21, 22, 23, 24, 26, 27) in central Terra Cimmeria which were interpreted as young volcanic plains. These plains contain unusual silica-rich surfaces, raised flow-like surfaces. One bedrock plain (CI22) also contains vent-like features and a likely diatreme-maar.
The definition of Terra Cimmeria used here is much smaller than the the USGS definition. It corresponds roughly to the northwestern Terra Cimmeria. Southeastern Terra Cimmeria is defined as the Eridania Paleolake region. My rationale for splitting these two regions is that they have very different geomorphologic characteristics. The landscape of northwestern Terra Cimmeria is relatively uniformly high-standing, and is heavily cratered. In contrast, southeastern Terra Cimmeria has broad domes and basins, many of which were flooded by an inland sea approximately the size of the Gulf of Mexico (e.g. Irwin et al 2002). These factors suggested that the two regions experienced somewhat different geological histories.
One recent study has suggested that Terra Cimmeria is a block of crust that has existed since the formation of the planet and has gone relatively little modification ever since (Bouley et al. 2020).
Eastern Terra Cimmeria is notable for the presence of the large magnetic lineaments in Mars' magnetic field (Connerney et al. 1999). The source of the magnetic lineation is unknown, but hypotheses include a fossilized record of incipient plate tectonics (Connerney et al. 1999), magnetized dike swarms (Nimmo, 2000), a fragment of crust that escaped demagnetization by impacts (Kletetschka et al. 2010), and formation of iron-oxide minerals (particularly magnetite and hematite) by groundwater alteration of the Martian crust (Hood et al. 2007).