INTRODUCTION

Arabia Terra is a large region of cratered terrane extending from about 20° W. longitude eastward across the prime meridian to about 300° W. longitude for an average east-west width of about 5,000 km. The northern boundary ranges from 40° N. to 45° N.; the southern boundary is a poorly defined zone at about 0° N. Thus, the north-south width is about 2,500 km. Except for the westernmost part, Arabia Terra has an albedo higher than surrounding terranes. The four quadrangles mapped (30332, 35332, 40332, 45332) provide a north-south strip from highland terrane in the south to lowland terrane in the north.

The northern portion of Arabia Terra is the type region for both fretted terrane and fretted valleys (Sharp, 1973; Sharp and Malin, 1975) and, along with the immediately adjacent northern plains, is also the site of some of the best examples of putative flow deposits present as aprons around isolated knobs and mesas or as deposits on the floors of fretted valleys and on the lowland surface (Squyres, 1978, 1979). Mass wasting, eolian erosion or deposition, glacial scouring, fluvial or shoreline erosion, deposition from an ocean, hydrovolcanism, plateau volcanism, and faulting all have been proposed to account for the topography and crater characteristics in northern Arabia Terra (Sharp, 1973; Lucchitta, 1978; Greeley and Guest, 1987; Maxwell and McGill, 1987; Parker and others, 1989; Dimitriou, 1990; Moore, 1990; Grant and Schultz, 1990, 1993; Craddock and Maxwell, 1993; Edgett and Parker, 1997; Carruthers and McGill, 1998; Tanaka, 2000; McGill, 2000). Although underlain by what appears to be typical highland terrane, Arabia Terra is anomalously low, with elevations generally below the planetary reference (Smith and others, 1999).

Probably the most important question concerning the global-scale tectonic history of Mars is the origin of the crustal dichotomy. The northern lowland is not only several kilometers lower than the southern highland, it also is surfaced by materials that are significantly younger than surface materials in the southern highland (Scott and Tanaka, 1986; Greeley and Guest, 1987; Tanaka and Scott, 1987). The young surface materials in the lowland rest unconformably on basement material having an age comparable to the exposed ancient highland terrane to the south (Scott, 1978; Maxwell and McGill, 1987; McGill, 1989; McGill and Dimitriou, 1990; Schultz and Frey, 1990). The age of the dichotomy continues to be controversial, as is the mechanism for its formation, as reviewed by McGill and Squyres (1991), although gravity and topography data from Mars Global Surveyor appear to favor early formation due to internal processes (Zuber and others, 2000). Because complex depositional and erosional events affected the boundary since its formation, the cause and history of these events must be unraveled before we can directly attack the fundamental question of the reason for the dichotomy.