2. The Basin-Range
Faulting Process
Mick
and Bob then discussed the Basin-Range faulting process of
15 MYa, the process which produced the major landscapes
we see in our area today.
[In the
following discussion, you may want to refer to an authoritative
geological time scale; to see one adapted from the work of the
University of California Berkeley Museum of Paleontology (cited
in Credits
& Bibliography)
click here.]
Mick illustrated
how the contrast of Basin-and-Range developed through a process
of extension of the continental
crust, causing faults which made major blocks of formations
drop thousands of feet ("grabens"), while other blocks
remained higher or even rose ("horsts"). The model he
shows below illustrates this, the one at left showing
the land when compressed, the one at right when the land
is pulled apart, breaking along various fault lines, some parts
dropping and others rising.
Bob applied
this to the map (below right) of the lower part of the
Basin-Range Province, his hands stretching apart to reflect an
East-West direction of spreading for the entire West and Southwest
east of the Sierra Nevada. He observed that this stretching, block-faulting
process is still occurring in the further northern part of the
Basin-Range Province, in British Columbia. In our area, it began
at 15 MYa, stretches into northern Mexico, but has now
almost ceased. (Click on the right-hand
image for an enlargement.)
This stretching and
fault-block breakage produced the large array of roughly north-south
running mountain ranges, separated by parallel valleys that you
can see depicted on the map at right above.
Viewed from
more or less southeast-to-northwest, one can see that the continental
crust of the Basin-Range Province has about half the thickness
of the continental crust elsewhere in North America: (Click
on the image to enlarge it):
This diagram
projects a roughly southeast-to-northwest cross-section of the
continental crust in our area, running from the Pinacate Mountains
in Mexico to the Whetstone Mountains (both are at far left,
under Bob's right hand), across the Santa Catalinas, Sierra Ancha,
and the Mogollon Rim to the further uplands of the Colorado Plateau
at right. The double arrow shows a "100% extension"
(with its associated faulting process) that took place over a
period of 2-3 MY, accommodated by numerous faults, one earthquake
at a time, which occurred in an east-west axis as well as the
one shown southeast-northwest. The entire area remains more seismically
active than in the areas of thicker continental crust, because
of the hotter, shallower mantle lying beneath this part of the
region.
Part of
this heating (and the "taffy-pull spreading" associated
with it) may have been due to subduction of the East Pacific
Rise -- the oceanic spreading center to the west of North
America in Cretaceous times -- it being pulled under (pulled beneath)
the western edge of North America beginning about 30 Ma. In the
map image below, see the purple spreading center running
from the bottom upward, leading into the Gulf of California (Click
on the image to enlarge it):
Dickinson and others
have modeled this crustal thinning as due to the changing angle
of subduction of the Pacific Plate under the North American Plate.
At any rate, the thinning and stretching of the crust, which first
caused the volcanism of the Mid-Tertiary
Orogeny, later
produced the extensional faulting that created the Basin-Range
structures.
Basin-Range faulting
in Hot Springs Canyon is nicely shown at the Yellow Cliffs,
where our outcropping of Galiuro Volcanics now sits side by side
with the younger San Manuel Formation. The San Manuel formerly
was deposited above this part of the Galiuro Volcanics, but dropped
down with the fault somewhat more than 10 MYa:
Above,
on the left, gray San Manuel Formation; on the right,
Yellow Cliffs Galiuro Volcanics. Both formations, originally
deposited in more horizontal layerings, were sharply tilted by
Basin-Range faulting, which began about 15 MYa. The
Teran Wash Fault, part of the more widespread faulting process
of those times, runs as Mick Meader indicates toward the left-upper-middle
of the picture, to the right of the whitish pyramid which stands
in the far distance (below the line of Soza Mesa; we're looking
more or less northwest in this image.) The whitish pyramid is
part of San Manuel Formation which has fallen away from the main
SM Formation to the left; the purplish hill to its right is Galiuro
Volcanics. (This photo was taken on the earlier Geology Walk in
November 2000.)
Below,
a close-up view of a piece of sharply-tilted San Manuel Formation
along the Teran Wash Fault pointed to by Mick Meader in the photo
above. This big chunk appears to have fallen down from the main
body of San Manuel Formation in a distinctive alignment of its
own.
Below, a better
view of the Yellow Cliffs Galiuro Volcanics themselves from a
ways further downstream. We see here several layers, the lower
yellowish, higher pink, higher still dark purple. In this photo,
the sharp, fault-block tilt of the Formation, dipping downward
away from the viewer, is more clearly evident: (Click
on the image to enlarge it)
Below, a portion
of the geological map of our area (compiled by Goodlin 1985 and
Mark 1985 -- see Credits
& Bibliography),
showing how the Teran Wash Fault runs through the Hot Springs
Canyon from NNW to SE. (Click on the
image to enlarge it):
In the image above,
the yellow "Qal" means Quaternary Alluvium -- the contemporary
floodplain; the beige "Tq" means "Tertiary Quiburis
Formation"; the darker brown "Tsm" means Tertiary
San Manuel Formation; and the pink formation is Galiuro Volcanics.
The dark line running diagonally from upper-left-center, then
reappears across the canyon to lower right marks where the Fault
can be observed on the ground, the dotted line where it is presumed
to run, but is buried under the more recent formations.
An aeial view of
the same formations is useful here, with the red lines roughly
marking the position of the Fault::
Mick Meader
has provided us with the following model of this fault: (Click
on the image to enlarge it):
Taking a more macroscopic
perspective, note the tilt of the Galiuro Mountains as a whole
when viewed from the West. This aerial photograph, below,
shows the Fore-range in the foreground, while the higher, Back-range
appears in a more blue, aerial cast, with its down-and-away tilt
quite pronounced (Click on the image
to see this detail more clearly):
The whole extent
of the Galiuros Mountains forms a Fault-block Range of this kind.
See This
link for a panoramic
view of the Galiuros from South to north, as seen from the top
of the plutonic granite core-complex of Mount Graham further east..
For a summary placing
the Basin-Range process in its wider historical context, see Meader:
Geological History of our Area.
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