New insights about formation mechanisms of fracture zones from detailed geologicaland structural map of Aphrodite Terra

(15S-20S / 110E-124E)David Tovar, Vicki L. Hansen & John B. Swenson

Department of Earth and Environmental Sciences

VEXAG Meeting #14Washington D.C., NASA HQNov 28th – Dec 1st , 2016

8 / 18Tovar, et. Al. 2016

Problem

Rift zones have been suggested as one of the major geological settings on Venus; however they look completely different if we compare them to rift zones on Earth.

Topographic signatures and geological settings observed on rift zones on Earth are hard to reconcile to those on Venus. Therefore an alternative mechanism of formation associated to the observed lineaments must be explored.

Tovar, et. Al. 2016 2 / 18

The map area has excellent coverage by MagellanSAR data (right-look, left-look and stereo)

Resolution: 100m/pixel

Fracture Zones – No coronae beltFracture Zones –Coronae beltStudy area

Cartoon (not to scale)

5000 km

200 km

3 / 18

• Detailed structural analysis

1. Understand the architectural evolution throughtime and space. Geologic Mapping.

2. Construct models for fractures formation

3. Fracture Zones vs Rift Zones

4. Gain insight into possible mechanisms of heattransfer on Venus

Tovar, et. Al. 2016

4 / 18

Right-look + left-look (C1, C2 & C3)

100 km 100 km

Tovar, et. Al. 2016

6 / 18

• Structural and geological mappingmethodology

1. Paired lineaments – single structure1. 1. Troughs – topographic contrast

2. Pit chains – clear sequence of pits

3. Undefined lineaments – under resolution limit

4. Architectural domains – lineaments density

Tovar, et. Al. 2016

5/ 18

Length: ~ 500

Width: ~ 1400 km

Total area: 700,000 km2

4 detailed areas

Tovar, et. Al. 2016

10 / 18Tovar, et. Al. 2016

D

B

C

A

CA

11 / 18Tovar, et. Al. 2016

D

B

C

A

B

10 / 18Tovar, et. Al. 2016

D

B

C

A

C

10 / 18Tovar, et. Al. 2016

D

B

C

A

50 kmD

9 / 18Tovar, et. Al. 2016

8 / 18Tovar, et. Al. 2016

Observations

15 / 18

Thought experiment

If extension is the main process that controls fractures formation on Venus, how much extension can we accommodate across each detailed area?

13 / 18Tovar, et. Al. 2016

Extension = (transect length – Li) / Li

Initial length (Li) = transect length – (# of lineaments * opening)

12 / 18Tovar, et. Al. 2016

Cushing, et.al. 2015

17 / 18

Cushing, et.al. 2015

Formation of mechanismsthat could produce AtypicalPit Craters (APC’s)

A and B: collapse above adike-induced void. Materialmay plug laterally extendingpassages.

C and D: collapse above anactive magma dike. Materialis transported away from thecollapse zone

Conclusions• Despite the resolution limitation to identify some secondary structures

(lineaments), it is possible to address the geomorphic of major fractures, allowing us to interpret them.

• Density of lineaments, as well as spacing between them, are crucial parameters at the time of further interpretations of fracture zones.

• Based on the observations made on the study area, even small amounts of extension are hard to accommodate along several hundreds of kilometers; therefore we construct an alternative model (stoping) where extension is not required, providing not just a reasonable explanation for the formation of lineaments, but also being consistent with the idea of a planet that lacks plate tectonics.

• Changes of trend, cross-cutting relationships, and buried lineaments are strong indicators of events that are not contemporaneous.

18 / 18Tovar, et. Al. 2016