GEOLOGICAL RECONNAISSANCE OF THE STIRLING RANGES. 



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Feet 

 3000 



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lOOO 



5po 





Stirl 



mq Range 







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"vl 2-. 



Peneplain be-fore uplift 









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Fig. 4. Fig. 5. 



Figures 4 and 5 indicate, diagrainmatically, the tectonic history 

 of the Stirling Range. Before peneplanation the sediments had 

 dropped into a "senkungsfeld." This was bounded by reversed 

 faults, since overthrusting was contemporaneous with quartz injec- 

 tion, and therefore, probably, Pre-Cambrian. 



During the uplift of the peneplain, and its conversion into a 

 plateau, the formations on both sides of the fault moved upwards. 

 The sedimentary series, however, underwent greater uplift; so that 

 the granite areas are relatively depressed. In this way the faults, 

 originally reversed become normal after rejuvenation. 



Compression and overthrusting are characteristic of late Pre- 

 Cambrian time, while tension and normal faulting were associated 

 with Cainozoic epeirogenic movements. 



Porongrups is also opposed to such an explanation, and the 

 peculiarities of river development indicate that something 

 more than passive resistance to erosion must be postulated. 

 It seems probable, then, that the fault planes which origin- 

 ally let down the substance area at some very distant 

 geological epoch, and so preserved the weak sedimentary 

 masses in the trough, became planes of weakness again in 

 more recent times, and, when the plateau area of Western 

 Australia commenced that long series of upward move- 

 ments which culminated in the development of the Darling 

 Plateau, the Stirling Ranges and the Porongrups were 

 thrust upwards faster than the rest of the area, and, what 

 was formerly a senkungsfeld has now become a horst. 



