Geologic Process Identification through Landform Analysis

Ian Thomsen

Topographic Position Index

From Jenness 2006

Inherently Scalable Adjusts to capture features of different scales

• Quickly

• Easily

• Repeatable

Jenness, 2006

• Comparing elevation of each cell to the mean of all cells in an annulus of radius 100 cells (3000 meters) around it.

• Only interested in large, regional-scale features

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Slope Tool

TPI < -6 = Canyon bottom

-4 < TPI < 3 , Slope < 12° = Gentle Slope

-4 < TPI < 3 ; Slope > 12° = Steep Slope

3 < TPI < 6 = Steep Slope

6 < TPI = Ridge top

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Focal Statistics Operations

Areas of statistical operation can be customized to the scale of interest

For example: • 4500m square window

discards signal from smaller features

• 19500m window only captures largest features

Identify all points which are classed as Steep Slope or Ridge Top at 4500x4500m scale but only class as Shallow Slope at 19500x19500m scale • Filters out all features too large to be fault generated • Removes smaller topographic relief

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Pamir

Results

• Method successfully marks landforms which match characteristics of fault scarp structures

• Successfully removes other topography from consideration

But…

• Issues of scale

Results

• Defines SOME topographic relief as fault related, but not all

• MORE problems of scale

Results Obvious problems:

• Where are the faults?! They SHOULD be around here!

Two possibilities:

• There is no fault here

Or

• The scale of features of interest is lost in more recent features

Problems identifying fault features with TPI

Extremely scale dependent

Scale must be uniform

Difficult to detect topography of interest within other geologic/erosional features

Implications of work

Check the accuracy of map data

Identify sites where fault-generated features are more and less likely to be found

Can be applied to any geological features whose scale can be isolated from “background” topography

Future work

Develop statistical methods to quickly combine

TPI

Lithologic changes

Seismic activity

Test method accuracy in the field

Refine algorithm

Apply TPI analysis to other geologic mapping projects