Title: Geologic evolution of the Martian dichotomy in the Ismenius area of Mars and implications for plains magnetization

Author(s): Smrekar SE, McGill GE, Raymond CA, Dimitriou AM

Source: JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 109 (E11): Art. No. E11002 NOV 13 2004

Document Type: Article

Language: English

Abstract: [1] The origin of the Martian dichotomy, which divides highlands from lowlands, is unknown. We examine a section of the dichotomy ( 50 - 90E) defined by steep scarps and normal faults. Stratigraphy and age relationships preclude the formation of the 2.5 km high boundary via erosion. The abrupt disappearance of topographic knobs similar to 300 - 500 km to the northeast is interpreted as a buried fault. Alignment of the buried fault with grabens, stratigraphy, and age determinations using crater counts indicate that the lowland bench is down faulted highlands crust. The estimated local strain (3.5%) and fault pattern are broadly consistent with gravitational relaxation of a plateau boundary. Magnetic and gravity anomalies occur on either side of the buried fault. Admittance analysis indicates isostatic compensation. Although nonunique, a model with a 10 km thick intracrustal block under the lowland bench, a 20 km thick block under the plains, and an excess density of 200 kg/m(3) provides a good fit to the isostatic anomaly. A good fit to a profile of the magnetic field perpendicular to the dichotomy is produced using uniformly polarized intracrustal blocks 10 - 20 km thick, an intensity of 6 Am/m, a field inclination of - 30degrees, and gaps aligned with the isostatic anomalies. One interpretation is that high-density intrusions demagnetized the crust after dynamo cessation and that low-lying magnetized areas could be down faulted highlands crust. Another model ( inclination of 30degrees) has magnetized crust beneath the isostatic anomalies, separated by gaps. The gaps could result from hydrothermal alteration of the crust along fault zones.

Author Keywords: Mars; dichotomy; crustal magnetism; gravity; isostasy; faulting; gravitational relaxation; extension

KeyWords Plus: CRUSTAL DICHOTOMY; HEMISPHERIC DICHOTOMY; INTERNAL STRUCTURE; MANTLE CONVECTION; PHASE-TRANSITIONS; CRATERED TERRAIN; PLATE-TECTONICS; GIANT IMPACT; FIELD; TOPOGRAPHY

Addresses: Smrekar SE (reprint author), CALTECH, Jet Prop Lab, Mail Stop 183-501,4800 Oak Grove Dr, Pasadena, CA 91109 USA
CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA
Univ Massachusetts, Dept Geosci, Amherst, MA 01003 USA
SLR Alaska, Anchorage, AK 99503 USA

E-mail Addresses: ssmrekar@jpl.nasa.gov

Publisher: AMER GEOPHYSICAL UNION, 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA

IDS Number: 872ES