DETAILED FRACTURE HISTORY OF THE BRIGHT PLAINS REGION, EUROPA: IMPLICATIONSFROM NONSYNCHRONOUS ROTATION MODELS. S. A. Kattenhorn, Department of Geological Sciences,University of Idaho, Moscow ID 83844-3022 ([email protected]).Introduction: The complex fracture history of theBright Plains region of Europa's trailing hemisphere(~15N/273W) has been unraveled using detailed frac-ture maps created from Galileo SSI images obtainedduring the E6 orbit. Features identified include doubleridges, complex ridges, smooth bands, and surfacefractures, each of which formed in multiple fracturingepisodes. The fracture evolutionary sequence reveals aconsistent clockwise rotation of fracture orientationsthrough time and hence an ongoing clockwise rotationof the principal stresses in the stress fields responsiblefor each fracturing episode. This behavior is consistentwith nonsynchronous rotation (NSR) models for thenorthern hemisphere of Europa. The amount of rota-tion of the stress field implies at least 720º (and per-haps >900º) of NSR of Europa's decoupled outer crustwith respect to the moon's interior.Fracture Mapping of the Bright Plains: Detailedfracture mapping was carried out using images of theBright Plains, in the Conamara Chaos region of Eu-ropa's trailing hemisphere. The mapped region is im-mediately north of the intersection point of AgaveLinea and Asterius Linea and covers an area approxi-mately 60 km long and 33 km wide. High resolutionGalileo SSI images (down to 20m/pixel) enabled finedetails of fracture lineament relationships to be unrav-eled. Most lineaments are either "mode I" fractures(tensile fractures and intrusive dikes) or normal faults.Such structures developed perpendicular to regionalmaximum principal tensile stresses where the drivingstress was of sufficient magnitude to break Europa'sice crust. The fracture lineaments thus provide directevidence of the history of principal stress orientationsand driving stress magnitudes in the Bright Plains.Geologic Features: A number of lineament typeswere differentiated [e.g., 1], with several different agedfracture episodes for each class of feature. Each frac-ture episode is shown in the geologic map in Figure 1.The fracture types are described below.Ridged Plains. These are the oldest portions of thesurface, comprised of a dense network of ridges uponwhich younger fracture lineaments are superimposed.Smooth Bands (SB). These are generally linearbands (2-13 km wide) of smooth, featureless materialinternally dissected by normal faults. Smooth bandsprobably represent crustal spreading where icy mate-rial was intruded from below to form new crust. Fiveseparate episodes of smooth band development can beidentified, typically with an east-west orientation.Double Ridges (DR). This is the most commonclass of fracture lineament in the Bright Plains. Doubleridges are essentially ice dikes that constructed flank-ing ridges to either side of the central fracture due toextrusion of icy material. At least five separate epi-sodes of double ridge development can be identified.Complex Ridges (CR). The two episodes of com-plex ridge development represent repeated intrusion ofadjacent parallel dikes/double ridges within linearbands having similar dimensions to smooth bands [cf.,2, 3].Medial-trough Ridges (MTR). These features maybe similar to double ridges but have wider flankingridges of featureless material to either side of a centralfracture, thus resembling smooth bands [cf., 1, 2].Proto-ridges (PR). Transitional features betweensurface fractures and double ridges. They generallyhave poorly developed flanking ridges and representthe youngest intrusive features in the Bright Plainsarea.Surface fractures. These are the youngest linea-ments at the surface and do not extend through the en-tire crust. Individual fractures are <12 km in length. Atleast five separate fracture set orientations can be dis-cerned, although relative ages are difficult to gauge.Faults. Both strike-slip and normal faults arecommon in the Bright Plains. Strike-slip faulting oc-curs along pre-existing lineaments such as doubleridges. Ancient transform faults define sudden termi-nations of complex ridges. Normal faults are generallyrestricted to smooth bands, and are sometimes reacti-vated with a strike-slip component of motion.Fracture Sequence: Cross-cutting relationshipswere used to identify a fracturing sequence (Figure 2).The history of fracturing is generally unambiguous,with uncertainties where similar aged features do notcross-cut directly. Smooth bands, double ridges andcomplex ridges formed throughout the fracture se-quence in a range of orientations. Successivelyyounger fractures are consistently oriented in a moreclockwise sense compared to older fractures. This pat-tern of fracture development provides strong supportfor NSR models in order to explain the repeated epi-sodes of fracturing of the crust in the Bright Plains.Nonsynchronous Rotation Models: Tensilestresses can be induced by diurnal tidal flexing of Eu-ropa's crust in response to the variable gravitationalpull of Jupiter during Europa's elliptical orbit [4, 5].However, the range of fracture orientations in theLunar and Planetary Science XXXII (2001)1238.pdfFRACTURE HISTORY OF THE BRIGHT PLAINS: S. A. KattenhornBright Plains is inconsistent with the stress field thatcurrently exists at this longitudinal position (red box inFigure 2). A plausible explanation for these observa-tions is rotation of a decoupled crust with respect toEuropa's interior in response to the spin rate beingslightly faster than synchronous [4]. The resultant NSRstress field is superimposed on the diurnal stress field(Figure 2). NSR causes the global stress pattern to mi-grate relatively westwards across the Europan surfacethrough time, resulting in a progressive rotation offracture lineaments during the fracture sequence(clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere). Successivelyolder fracture sets can thus be placed into a time se-quence of older stress states that existed when theBright Plains region was positioned at successivelymore westerly longitudes. This technique is demon-strated in Figure 2.