Geology of the Thaumasia region, Mars: plateau development, valley origins, and magmatic evolution by James M. Dohm, Kenneth L. Tanaka

Geologic features of the Thaumasia region HIGHLANDS AND INNER PLATEAU n Scattered massifs and volcanoes n Mountain belts n Numerous large impact craters n Successions of lava-flow fields n Systems of rifts, narrow grabens, wrinkle ridges, and folds and thrusts EROSION OF THE OUTER MARGIN n Extensive valley networks n Canyons n Troughs n Tilted, eroded rock layers forming cuestas

Geologic Setting

Stratigraphy

Stratigraphy n Relative ages are based on a time-stratigraphic scale from Tanka (1986) which reflects material ages and stratigraphic relations n This indicates 5 main stages of geologic activity (oldest to youngest) –Stage 1: Noachian –Stage 2: Late Noachian to Early Hesperian –Stage 3: Early Hesperian –Stage 4: Late Hesperian to Early Amazonian –Stage 5: Amazonian

Stage 1: Noachian n The more rugged units formed during this stage n Complex modification due to cratering, tectonic deformation, erosional processes, and volcanic and sedimentary burial has degraded or destroyed older craters (making it difficult to determine onset)

Stage 2: Late Noachian to Early Hesperian n n Some materials have been heavily modified by folding or faulting and/or valley development during this stage n n Rock materials from stage 1 & 2 cover more than half of the Thaumasia region

Stage 3: Early Hesperian n n Extensive rigid plains volcanism in Sinai Planum and Corprates East (CE) n n as well as local valley formation in southern Thaumasia Plateau

Stage 3 & 4: Hesperian to Early Amazonian n n Extensive lava sheets were erupted from crest and flanks of Syria Planum and Tharsis Montes (in NW) n n 40% rock from these stages

Stage 5: Amazonian n n relatively small outcrops of volcanic materials and colluvial and eolian deposits n n = 3% of outcrop from region

Tectonic History n n The analysis of Thaumasia’s faults and grabens, rift systems and wrinkle ridges indicate tectonic activity began and reached it’s peak during Noachian (stage 1) and declined during the Hesperian and Amazonian periods (stages 2-5) n n Additionally, intense extensional and contractional deformation appears to have largely resulted from: Tharsis- and Syria-centered volcanotectonic activity (1-5) uplift and associated rifting of that plateau, including local centers of volcanotectonic activity (mainly 1 and 2) formation and tectonic relaxation of the Argyre impact basin (1 and 2) 4. Other. Tectonic deformation of region, such as compression due to rapid planetary cooling

Volcanic History n n The Thaumasia region preserves some of the best examples of early highland volcanoes and intermediate-age plateau lavas on Mars n n Many of the prominent, isolated mountains that occur along or near faults and commonly exhibit summit depression and highly dissected flanks have been interpreted as ancient volcanoes n n complex geomorphologies may indicate interbedded lava and pyroclastic deposits

Volcanic History n n Mapped 14 volcanoes - using stratigraphic relations as well as morphology n n Formed throughout the Noachian and into Early Hesperian   STAGE volcanoes   STAGE volcanoes, and widespread volcanism continued to resurface local areas throughout the map area   STAGE 3 & 4 - Sheet lavas erupted from the crest and flanks of of Syria Planum and Arsia Mons (outside the study) and cover several provinces   STAGE 5 - Continued Tharsis volcanic activity resulted in lava flows through the Daedalia Planum province

Erosional History n n they mapped and dated > 4000 branching and single valleys n n branching or networked valleys strongly indicate water- or ice-based erosion n n single, nonbranching valleys may also form by eolian, tectonic, and/or volcanic processes n n Valleys tend to concentrate in areas of volcano flanks and near rift systems of the central part of the Thaumasia Highlands province and southern part of the Coprates province   STAGE 1 - valleys are obscure and discontinuous, probably due to later resurfacing   STAGE 1 & 2 - overall valley formation was most extensive   STAGE 3 & 4 - valleys tend to be more pristine and mostly occur in the map region   STAGE 5 - no stage 5 valleys occur on this map

Additional Discussion n Thaumasia Plateau Development n Valley Origins n Intrusions beneath the Thaumasia plateau n Syria Planum volcanotectonic development n Thaumasia and Tharsis evolution

Further Studies n The Thaumasia region is a major, complex volcanotectonic province of Tharsis for which comprehensive reconstruction of geologic history has been lacking n This is an interpretive geologic history of the Thaumasia region based on geologic mapping and topical analysis n This detailed geologic study provides a basis to address tectonic, magmatic, and resurfacing issues in greater depth