Lunar Floor-fractured craters : models of sill intrusion and prediction of associated gravity anomalies

1. Lunar Floor-fractured craters: models of sill intrusion and prediction of associated gravity anomalies Lauren Jozwiak James Head Department of Geological Sciences, Brown University Providence, RI, USA Third Moscow Solar System Symposium Moscow, Russia October 8th, 2012

2. Floor- Fractured craters (FFCs) • Anomalously shallow craters with fractured floors [Schultz, 1976]. • Other Characteristics • Floor Moats • Ridges • Patches of Mare Material • Dark Halos • Characteristics Define the 8 Morphologic Classes Crater Gassendi, LROC-WAC

3. Characteristics of FFCs Floor-Fractured Crater, Gassendi Copernican-Aged Crater, Tycho Crater Gaudibert, Class 4b “V” Moat with Inner Ridge Crater Vitello, Class 2 Concentric Central Uplift Crater Humboldt, Class 1 Mare Patches

4. Distribution of Lunar FFCS N= 170 Jozwiak and Head [2012] JGR-Planets (accepted)

5. Distribution Trends • FFCs identified using LOLA and LROC-WAC data and imagery • Observed Relationships between FFC class and Areal Distribution • Craters closer to basin edges and interiors have flatter floors, more uplift, and more pronounced fractures • Craters farther into the highlands have convex up floor profiles, and less overall uplift • Could be due to 1) Thermal Effects close to Impact Basins 2) Intrusion Effects close to Maria and sources of magma

6. Proposed Formation Mechanisms • Two proposed formation mechanisms: • Viscous Relaxation • Magmatic Intrusion and Sill Formation • Morphologic investigations using LOLA data support a formation via magmatic intrusion and sill formation • Significant change in floor depth • Unaltered Rim Crest Height • Lack of crater symmetry • Moat features • Location far from basin edges • Significant population of craters with D< 30 km

7. Testing the Mechanics of Magmatic Intrusion • Dike propagates from the mantle, driven by a certain pressure • Dike stalls at a density barrier caused by the brecciated lens beneath the crater • Dike propagates laterally forming a sill beneath the brecciated lens • a. Sill inflates, forming a laccolith and bowing the overlying crater floor b. If the yield stress is exceeded at the edges of the laccolith, faulting occurs and uplifts the crater floor

8. Dike Propagation- Step 1 Dike propagates from mantle driven by a driving pressure equal to the total magma pressure minus the lithostatic pressure

9. Breccia Lens Boundary – Step 2 ρ m = 3300 kg/m3 ρb = 2750 kg/m3 ρc = 2900 kg/m3 [Huang and Wieczorek, 2011]

10. Breccia Lens as a Barrier • Magma Propagation is governed by • Driving Pressure to reach surface: • 27 MPa through lunar crust material • 32 MPa through breccia lens material • Driving Pressure to Continue Propagation at Breccia Lens material • 21 MPa to continue into lunar crustal material • 29 MPa to continue propagation into breccia material • Dike requires an addition 8 MPa of driving pressure to continue propagation through a breccia lens, greater than the pressure required to reach the surface • Crater Floor Breccia Lens is a viable means of halting vertical propagation

11. Lateral Propagation • Unable to continue vertical propagation, pressure builds below the dike tip • The pressure then exceeds lithostatic pressure , and the dike begins to fracture laterally Rubin and Pollard [1987]

12. Sill Formation – Step 3 Sill propagates until it reaches the edge of the breccia lens Beyond the breccia lens, overburden pressure increases, pinching off further dike propagation

13. Sill Inflation (Laccolith formation) – Step 4a Magma continues to fill sill Crystallization of magma at periphery, causes concentration of magma in the center of the intrusion Extreme flexure in the overlying crater floor, and a convex up floor profile

14. Sill Inflation and faulting-step 4b Rapid Filling of Sill distributes magma throughout the entire sill volume Increased edge stresses overcome breccia yield stress, and faulting occurs, uplifting the crater floor Therefore: Both Piston-like uplift and Flexural Doming are consequences of sill intrusion

15. Morphologic evidence of uplift styles Faulting and Piston Uplift Flexure and Flexural Doming

16. Using Gravity to Determine Formation mechanism • Gravity data can help distinguish between the two formation mechanisms • If formed by magmatic intrusion, the large amount of magma in the sill will contribute a positive gravity anomaly • Thus a crater with a subsurface magma-filled sill would have a more positive Bouguer anomaly than a crater without such an intrusion • Use LOLA data to model the intrusion dimensions • Uplifted central region of the crater yields the diameter • Difference in crater depth from the crater depth predicted by Pike [1981] yields the intrusion thickness • Expected signal of 10s to several 10s of mGals

17. Observed Gravity anomaly • Crater Humboldt • Class 1 FFC • D=207 km • Displays positive Bouguer gravity anomaly beneath the central region of the crater floor Data courtesy of GRAIL team

18. Complications and future analysis • The GRAIL mission is providing a powerful data set that will allow us to further investigate the gravity anomalies associated with FFCs • Accurate analysis will require an understanding of complex crater gravity signatures, against which we can compare FFC gravity signatures • Complications arise with FFCs located close to basin edges • Basin Mascons will dominate the more subtle FFC signal • We will also investigate further the interplay between the gravity contribution of the magmatic intrusion, and how that may be balanced by the gravity contribution of the less dense breccia lens