Transcript Powerpoint

UNDERSTANDING
STRAIN ACCOMODATION
IN SPACE AND TIME
(AND WITH DEPTH)
ACROSS THE NORTHERN
BASIN AND RANGE
Questions and Problems generated
by an evolving database
Over the years we have worked on establishing the
timing, style and amount of extension across parts of
the northern Basin and Range:
·Geologic mapping and cross-section construction
·Cross-section restoration
·Studies of faults and fault zones
·Stratigraphy and geochronology of Tertiary sedimentary and
volcanic rocks
·Apatite Fission Track thermochronology studies
·U-Th/He dating of apatite
Fission-Track Thermochronology of Apatite
Trevor Dumitru, Danny Stockli, Ben Surpless, Joe Colgan
• Based on the spontaneous fission
of 238U
• Resulting damage to crystal is
visible as a “track” after chemical
etching.
·Number of tracks is proportional
to the cooling age (~65-120ºC), and
the U content of the grain.
•Track lengths are proportional to
the cooling rate of the grain.
Fission Track and He
Thermochronology
Transects
Rocks residing at depth
beneath the apatite and UTh/He PR zones have zero
ages.
When exhumed by
normal fault slip, rocks
cool and apatite begins
accumulating tracks .
Cooling age is a proxy
for age of fault motion.
FT and (U-Th)/He data from Wassuk Range plotted against pre-extensional Miocene paleodepth.
There are clear inflections in both data sets at ~15 Ma, marking onset of rapid cooling and
exhumation. Ages are invariant over a wide range of subsequent depths, indicating very rapid
cooling rates. Both data sets provide some evidence for a younger, post-Miocene cooling event
perhaps associated with younger, steep, range-bounding faults (Surpless, 1999).
In general, younger Basin
and Range faulting ripped through
the central part of the province
between 14-19 Ma (orange area).
Extensional faulting continued to
broaden the province after this
time, cutting into the Colorado
Plateau on the east and the Sierra
Nevada to the west. The borders
of the province are the most active
sites of faulting today. (Note:
there IS older faulting in orange
zone)
In detail, the amount of
extension by faulting is highly
heterogeneous, fitful, happens
quickly in one place and stops,
moves around in space and time,
can be the result of multiple
discreet events and has complex
relations to regional and local
magmatic activity.
Despite this complexity, the history and magnitude
of strain by normal faulting (and its relation to
magmatism) is a crucial data set for
1. the interpretation of deeper crustal structure
2. understanding of the ultimate driving forces and mechanisms
of extension across the province
3. understanding the distribution/accomodation of strain across
the province today (there’s a lot of preconditioning going
on).
……and we still have a lot of work to do!!
Some of the implications of supracrustal
faulting/extension history for crustal-scale processes
1. There are clear mass flux/balance problems when %
extension is compared to crustal thickness. How much is
related to flow, how much is addition by magmatism?
2. What do the complicated space-time patterns of faulting
mean for the evolution of the province? What controls the
changing locus of strain? Diffuse versus focused? Preconditioning due to multiple extension/magmatic events?
3. Contemporary strain measurements. What sets of faults
are actually accomodating this strain? Is there strain
partioning?
4. Fault slip rates-why would geologic/geochronologic
determinations of averaged fault slip rates be so fast?
(Lahren and Schweikert, 1995; from Surpless, 1999).
Apatite FT and (U-Th)/He data from the Carson Range. Heat flow here is either
too low and/or fault slip has not been sufficient to exhume rocks from beneath the
Tertiary partial annealing zone. Extrapolation of the integrated data set suggests
that faulting along the Carson Range front is younger than 10 Ma and probably
began about 4 ±3 m.y. ago (Stockli, 1999).
These sequential diagrams illustrate the nature of the transition zone between the Sierra
Nevada and the Basin and Range at the scale of the crust.
0%
>150% Extension
Sources of data: Klemperer et al. (1986), McCarthy and Thompson
(1988), Vetter et al. (1983), Hill et al. (1991), NCEDC,CNSS,
Blackwell et al. (1991), Blakely (1995), Jarchow and Thompson(93)
Surpless et al. TECTONICS 2002
In the Basin and Range, multiple sets of normal faults have operated over time to thin the upper,
brittle crust. The youngest normal faults cut older, rotated normal faults (red) and interact with
a modern ductile-brittle transition zone that lies between 6 and 10 km depths (Surpless, 1999).
Morton and Black (1975) suggested that normal faulting (Afar depression of
Ethiopia) should cause increasing stratal tilts and structural complexity towards the
rift axis. The similarities of this “cartoon” to the supracrustal geology of the Sierra
Nevada-Basin and Range transition zone is astonishing! But we need much more
crust in the Basin and Range! Magmatism! Another big difference is that extension
is migrating outwards as the province enlarges, rather than being focused in its
center part.
Profett (1977)
Most extension happened very quickly,
syn 15 Ma Lincoln Flat Andesite, magmatism is important!
Summary of geologic history of the Yerington District, Singatse Range. a) Middle Miocene at ~15
Ma, Lincoln Flat andesite erupted. b) Middle Miocene at ~13.8-extension underway, c) and d) major
east-west extension, e)most extension has taken place by 13-12.6 Ma and between 10-7 Ma, basalt
flows deposited, e) present. From Proffett (1977) and Dilles and Gans (1995). Note similarities to
cross-sections of Wassuk Range.
16.5 Ma (mid-Miocene)
·Columbia River - Oregon
Plateau Basalts
·Initiation of the
Yellowstone “hotspot” along
the eastern Snake River
Plain
Colgan (2004)
Columbia
Plateau
Another example
of a mass balance
problem
16.5 Ma (mid-Miocene)
·Columbia River - Oregon
Plateau Basalts
Columbia
Plateau
·Initiation of the
Yellowstone “hotspot” along
the eastern Snake River
Plain
?
Colorado River
Extensional
Corridor
Colgan (2004)
Big pulse
of extension
15-200%
Superimposed
on older %
Stockli (1999) Main faulting events based on apatite FT ages
16.5 Ma (mid-Miocene)
·Columbia River - Oregon
Plateau Basalts
Columbia
Plateau
·Initiation of the
Yellowstone “hotspot” along
the eastern Snake River
Plain
Colgan (2004)
< 10 Ma
Only 15%
Pine Forest
Santa Rosa
Colgan (2004)
Catchings (1992)
little extended
50-100% extended
Catchings ‘92
15%
50-100%
September Seismic Experiment
To Understand Extension at the Scale of
the Crust in the Basin and Range
Our challenge is to fully integrate:
-Geologic History
-Magmatism and Petrology
….with geophysics
(Oldow, 2003):
Regional Global Positioning
System velocities in fixed North
American frame; 95%
uncertainty is shown by ellipses
(see text for sources). Dashed
lines mark velocity boundaries.
How has
deformation
played out through
geologic time and
how does it
compare to today?
Geologic map of the central Wassuk Range by Ben Surpless (1999). At this scale, note pink and
red-orange granitic basement, Mz metaseds (green), Miocene volcanic (grey) and sedimentary (lt.
orange) rocks above a tilted unconformity. Gently-dipping tilted normal faults are cut by higher
angle normal faults, including the youngest range-bounding fault on the east side of the map and
section. Original pdf version of map can be obtained from Ben Surpless. Zoom for fuzzy detail.
First 3 steps of cross-section restoration remove motion on younger,
higher-angle faults. Note 15-13.8 Ma Lincoln Flat andesite (grey)
and overlying orange-yellow sediments. Black bar charts distance
between two points before and after restoration and is used to
calculate amount of extension.
200% E-W extension
150% 15-14 Ma
Restorations of fault motion at ~14 Ma (top) and ~15 Ma (bottom), and rotation of
Tertiary unconformity to horizontal. Total extension greater than 150%.
The region from the Sierra Nevada to Walker Lake consists of a series of fault bound
mountain ranges with east-dipping faults on their eastern flanks. The Wassuk and
Carson Range faults are the most seismically active.
Wassuk Range fault plane measurements. There is inherent waviness
or curvature in the attitude of normal faults. The important relation is
That the pole to the plane defined by poles to faults should be parallel
to the slip direction along the fault.
And it is…
The region from the Sierra Nevada to Walker Lake consists of a series of fault bound
mountain ranges with east-dipping faults on their eastern flanks. The Wassuk and
Carson Range faults are the most seismically active. I don’t think any of these mapped
faults has accomodated much strike-slip motion. So where is the strike-slip?
Tectonics and
Topography today:
Why is central
Nevada elevated?
There must be a younger
Basin-Range wide extensional
Faulting event.
But when did it begin and
why did it happen? Is it
Province-wide?
U-Th/He dating of apatite is
particularly well-suited to
answering these questions.
Miller et al., 1999
Total fault slip/ time interval of slip indicate slip rates of 2-4mm/year..
Yellowstone “hotspot” as a
consequence of regional
tectonics?
CRB
17 Ma - eruption of CRB
and related rocks coincides
with rapid extension in BR
to the south.
Did rapid extension in
central Nevada prevent
basaltic magma from
reaching the surface?
Or was it already “burned out”
Further extension, thinning,
and lower crustal flow in the
BR, but not to the north
…But we still have to explain the CRBs…
?
?
extension
?
Today the Basin and Range is deforming on its edges
From Bennett et al., 2003
Modeled Apatite ages in a Tilted Normal Fault Block
• 40º Rotation from 20 to 15 Ma (horiz. velocity = 3mm/y)
• Initial Geothermal Gradient = 30ºC/km
• Block Dimensions = 10 x 15 km
Thermal modeling by Greg Houseman, Phil Gans
And Gordon Lister (unpublished)