[BR-Crater] Black Rock Range fault

[Scot Wilcoxon]

Brad Douglas wrote:
> On Sat, 2007-03-10 at 12:24 -0600, Scot Wilcoxon wrote:
>   
>> In the Slashdot discussion, someone pointed out the Black Rock Range was 
>> a linear feature rather than the central uplift which happens in some 
>> craters.  I responded that there has been a lot of erosion, and the 
>> desert floor is not the crater floor.  The crater floor may have been 
>> hundreds of feet higher (I think one jointed column bluff is 200 feet?).
>>     
>
> I consider the central uplift area to be the lower peninsula part of the
> Black Rock Range that includes the smaller playas.  Satellite imagery
> has demonstrated that the composition of that area is different from
> anything else in the region.
>
> Most other features, I generally consider unrelated unless it lies at
> the boundaries.
>
>   
I agree the composition is more important than the location.  There's 
been a lot of time for things to happen, and more of the local geologic 
record needs to be untangled.
>> So one issue is whether the top of the Black Rock Range is no higher 
>> than the crater floor.  If so, then the Range could have formed by 
>> erosion of its surroundings.  And crater floor formations might be on top.
>>     
>
> I don't understand the point that's trying to be made here.  What do
> they consider the crater floor and top of BR Range?  I apologize in
> advance for my sarcasm: All sensors I know of (including own eyes) show
> a marked elevation change from the valley floor to surrounding areas
> (often known as mountains; so called because they show dramatic
> elevation increases).
>
>   
That was my speculation.  If the columnar jointed bluffs are part of the 
crater floor, that is a hint at the location of the impact-caused 
formations.  The simplest situation would be if there were a large 
continuous layer of "crater floor" under other deposits, with much of 
the original top surface.  Finding continuations of that layer around 
the region would be useful, and if the top of the Black Rock Range 
happened to line up with that layer (and have rock from that layer) then 
that would be interesting.  However, as the Black Rock Range is much 
higher then than the columnar bluff,  any crater floor samples which 
avoided erosion have probably been moved up.

And as the columnar jointed bluff candidates all are protruding above 
the surrounding floor, there is no "crater floor" layer which is 
connected to them.  Maybe there is some within the surrounding hills, 
but we haven't found someone who looked.
>> Another possibility is that the Black Rock Range is due to an uplift 
>> which is separate from the force which created the crater.  Such as 
>> tectonic compression of the area causing a weakness along the Range to 
>> fold upward.
>>
>> "Isostatic rebound, active faulting, and potential geomorphic effects in 
>> the Lake Lahontan basin, Nevada and California", Figure 3, page 1742
>> http://neotectonics.seismo.unr.edu/CNS_pdfs/Adams99isostatic.pdf
>>     
>
> I theorize that the current ellipse shape is due to the rifting that
> occurred from 25MYa to 15MYa.  There was more "stretching" on the north
> end than the south end of the ranges.  There is also a real lack of
> reliable data and conflicting theories to what really happened during
> this period.
>
>   
>> There is a fault line along the Black Rock Range.
>>     
>
> I don't believe in curved faults. IMO, those are inferred faults.  The
> literature doesn't say one way or the other.  If the faults are curved,
> then there is a major blockage in the way, and the fault is really a
> series of connected lineaments.
>
> Also, I'm not interested in anything that more recent than 25MYa, which
> most of the faults are.  IIRC, almost all faults listed are Quaternary,
> having no bearing on impact potential.
>
>   
Table 1 in the Adams paper mentions some Holocene motion in the BR 
fault, with an estimated average recent vertical offset of 1-2 m.   I 
think the fault line might be based on limited study (although I haven't 
looked at the mine reports yet).  The Adams paper mentions:
"Dodge, R. L., 1982, Seismic and geomorphic history of the
    Black Rock fault zone, northwest Nevada [Ph.D. disserta-
    tion]: Golden, Colorado School of Mines, 213 p."
Searching for that paper I find in "U.S. DEPARTMENT OF THE INTERIOR U.S. 
GEOLOGICAL SURVEY, Documentation of changes in fault parameters for the 
2002 National Seismic Hazard Maps---Conterminous United States except 
California" mention of:
"BLACK ROCK FAULT ZONE, #1485, 0.1 mm/yr (1996), 0.19 mm/yr (2002).
The assigned slip rate is based on 4 m of offset of a tephra bed thought 
to be 18-25 ka
(Dodge, 1982). "
"Dodge, R.L., 1982, Seismic and geomorphic history of the Black Rock 
fault zone,
northwest Nevada: Golden, Colorado School of Mines T-2593, unpublished 
Ph. D.
dissertation, 271 p., scale 1:24,000. "

Do you suppose the Black Rock Fault was discovered by Dodge and is the 
only description of it?  It would be nice if there have been more 
studies of it, but at least such a thesis should have a good amount of 
work done on it.  It is a Ph.D. thesis, so should be loaded with 
information.

>> Another thing to investigate is whether there has been enough 
>> compression to change a circular crater to the current elliptical shape.
>>     
>
> Yes, that is an avenue to pursuit.  So far, I haven't seen convincing
> evidence of that, but we're also looking at 30MYa+ of erosion.
>
>   
Apparently there has been extension, not compression, with the Black 
Rock fault being a normal fault.  So Black Rock Range might be only 
upward motion or the wedge of Range might have stretched out the north 
end of the area.  However, cutting out a wedge only makes it less 
circular...but the crust is not a rigid pie crust so someone will have 
to figure out what moved where.
> An interesting fact that Ian hasn't put on the page yet, is the
> unusually high density of the crust immediately below Black Rock.  The
> data was taken along the northern boundary of the impact zone (IIRC),
> but I expect it to extrapolate well to the south and north.  This data
> was a real surprise to me.
>
> The impact *has* to be more than 25MYa or it will be nearly impossible
> to prove with all the geologic activity in recent periods.
>   
There are some studies mentioned in the above Adams paper about the Lake 
Lahontan Basin:
   "The Lahontan basin is also characterized by
relatively thin crust (Thompson et al., 1989) and
high heat flow (Morgan and Gosnold, 1989).
These geophysical estimates confirm that the
basin is undergoing active lithospheric extension
(Stewart, 1978; Thompson et al., 1989). Wallace
(1984a) referred to most of the Lahontan basin as
the "Black Rock--Carson Sink zone of extension"
and inferred that the area has undergone greater
and possibly more rapid extension than sur-
rounding areas in late Tertiary time. "

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.thunder.net/pipermail/br-crater/attachments/20070316/42e8090e/attachment.html