INENCO   05446
INSTITUTO DE INVESTIGACIONES EN ENERGIA NO CONVENCIONAL
Unidad Ejecutora - UE
artículos
Título:
Yardangs in terrestrial ignimbrites:Synergistic remote and field observations on Earth with applications to Mars.
Autor/es:
DE SILVA S; BAILEYJ.E; MANDT K.E; VIRAMONTE J.G.
Revista:
PLANETARY AND SPACE SCIENCE
Editorial:
PERGAMON-ELSEVIER SCIENCE LTD
Referencias:
Año: 2009 vol. 58 p. 459 - 471
ISSN:
0032-0633
Resumen:
The conditionsofformationandtheformofyardangsinignimbritesintheCentralAndesofChile, Bolivia, andArgentinamaybethemostconvincingterrestrialanalogtotheprocessesandlithologythat produce theextensiveyardangsoftheMedusaeFossaeFormation(MFF)ofMars.Throughremoteand field studyofyardangmorphologiesintheCentralAndeswehighlighttherolethatvariablematerial properties ofthehostlithologyplaysintheirfinalform.Here,ignimbritestypicallyshowtwomain facies:aninduratedandjointedfacies,andaweaklytopoorlyindurated,ash-andpumice-richfacies. Both faciesareverticallyarrangedinlarge(eruptedvolume 4100’sofkm3) ignimbritesresultingina resistant cappinglayer,whilesmaller(10’sofkm3) ignimbritesaremadepredominantlyoftheweakly induratedfacies.Thetwofacieshavequitedifferentmechanicalproperties;theinduratedfacies behavesasstrongrock,failsbyblockcollapseandsupportssteep/verticalcliffs,whilethenon-indurated facies ismoreeasilyerodedandformsgentleslopesandmanifestsasmoresubduederosionalforms.In response toaeolianaction,thepresenceofanupperinduratedfaciesresultsinlarge,elongate,high aspect ratio(1:20–1:40)megayardangsthatformtall(100m),thinridgeswithsteeptoverticalwalls. Thesearebuiltonabroadapronoftheweaklyinduratedfacieswithabundantfallenblocksfromthe upper induratedfacies.Theseterrestrialmegayardangsappeartobeanalogoustomegayardangswith associatedblockfieldsseenonMars.Smaller-volume,weaklyinduratedignimbritesaresculptedinto smaller,stubbierformswithaspectratiosof1:5–1:10andheightsrarelyexceeding10m.Excavationof a windwardbasalmoatsuggestsanerosionalprogressionlikethatseeninincipientyardangsonMars. Excavationratesof0.007–0.003cm/yeararecalculatedfortheweaklyinduratedignimbrites.Whilea persistentstrongunidirectionalwindisthedominantparametercontrollingyardangformationand orientation,aroleforflowseparationandvorticityisalsosuggestedbyourobservationsatbothyardang types. Whiletheinduratedfaciesiscommonlypervasivelyjointed,jointingisofsecondaryimportance in controllingyardangorientation.Serratedmargins,acommonfeatureonMars,resultfromoblique intersections ofjointingwithyardangflanksorscarpsofignimbrite.Theprocessesofyardangformation we describefromignimbritesfromtheCentralAndesarenotnecessarilyspecifictoignimbrites,butdo connote thatdegreeanddistributionofindurationisamajorcontrolinyardangformationandthishas implicationsforthelithologyoftheMFFonMars. 3) ignimbritesresultingina resistant cappinglayer,whilesmaller(10’sofkm3) ignimbritesaremadepredominantlyoftheweakly induratedfacies.Thetwofacieshavequitedifferentmechanicalproperties;theinduratedfacies behavesasstrongrock,failsbyblockcollapseandsupportssteep/verticalcliffs,whilethenon-indurated facies ismoreeasilyerodedandformsgentleslopesandmanifestsasmoresubduederosionalforms.In response toaeolianaction,thepresenceofanupperinduratedfaciesresultsinlarge,elongate,high aspect ratio(1:20–1:40)megayardangsthatformtall(100m),thinridgeswithsteeptoverticalwalls. Thesearebuiltonabroadapronoftheweaklyinduratedfacieswithabundantfallenblocksfromthe upper induratedfacies.Theseterrestrialmegayardangsappeartobeanalogoustomegayardangswith associatedblockfieldsseenonMars.Smaller-volume,weaklyinduratedignimbritesaresculptedinto smaller,stubbierformswithaspectratiosof1:5–1:10andheightsrarelyexceeding10m.Excavationof a windwardbasalmoatsuggestsanerosionalprogressionlikethatseeninincipientyardangsonMars. Excavationratesof0.007–0.003cm/yeararecalculatedfortheweaklyinduratedignimbrites.Whilea persistentstrongunidirectionalwindisthedominantparametercontrollingyardangformationand orientation,aroleforflowseparationandvorticityisalsosuggestedbyourobservationsatbothyardang types. Whiletheinduratedfaciesiscommonlypervasivelyjointed,jointingisofsecondaryimportance in controllingyardangorientation.Serratedmargins,acommonfeatureonMars,resultfromoblique intersections ofjointingwithyardangflanksorscarpsofignimbrite.Theprocessesofyardangformation we describefromignimbritesfromtheCentralAndesarenotnecessarilyspecifictoignimbrites,butdo connote thatdegreeanddistributionofindurationisamajorcontrolinyardangformationandthishas implicationsforthelithologyoftheMFFonMars.