UNDERSTANDING STRAIN ACCOMODATION IN SPACE AND TIME (AND WITH DEPTH) ACROSS THE NORTHERN BASIN AND RANGE Questions and Problems generated by an evolving.

UNDERSTANDING STRAIN ACCOMODATION

IN SPACE AND TIME (AND WITH DEPTH)

ACROSS THE NORTHERNBASIN AND RANGE

Questions and Problems generatedby 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

Overall the extension is highly heterogeneous, fitful, happening quickly in one place and then stopping, and can be highly

complex in terms of its history of superimposed strain

Fission-Track Thermochronology of ApatiteTrevor Dumitru, Danny Stockli, Ben Surpless, Joe Colgan

• 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.

• Based on the spontaneous fission of 238U

When exhumed by normal fault slip, rocks cool and apatite begins accumulating tracks . Cooling age is a proxy for age of fault motion.

Rocks residing at depth beneath the apatite and U-Th/He PR zones have zero ages.

Fission Track and HeThermochronology

Transects

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!!

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? Pre-conditioning 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?

Some of the implications of supracrustal faulting/extension history for crustal-scale processes

(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 SierraNevada and the Basin and Range at the scale of the crust.

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).

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

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.

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.

Most extension happened very quickly, syn 15 Ma Lincoln Flat Andesite, magmatism is important!

Profett (1977)

·Columbia River - Oregon Plateau Basalts

·Initiation of the Yellowstone “hotspot” along the eastern Snake River Plain

16.5 Ma (mid-Miocene)

ColumbiaPlateau

Snake River PlainColgan (2004)

Another example of a mass balance

problem

·Columbia River - Oregon Plateau Basalts

·Initiation of the Yellowstone “hotspot” along the eastern Snake River Plain

16.5 Ma (mid-Miocene)

ColumbiaPlateau

Snake River Plain

Colgan (2004)

?

Colorado RiverExtensional

Corridor

Big pulseof extension

15-200%Superimposed

on older %

Stockli (1999) Main faulting events based on apatite FT ages

·Columbia River - Oregon Plateau Basalts

·Initiation of the Yellowstone “hotspot” along the eastern Snake River Plain

16.5 Ma (mid-Miocene)

ColumbiaPlateau

Snake River Plain

Colgan (2004)

< 10 MaOnly 15%

Santa RosaPine ForestColgan (2004)

Catchings (1992)

little extended

50-100% extended

Catchings ‘92

50-100%15%

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 hasdeformation

played out throughgeologic 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.

Restorations of fault motion at ~14 Ma (top) and ~15 Ma (bottom), and rotation of Tertiary unconformity to horizontal. Total extension greater than 150%.

200% E-W extension

150% 15-14 Ma

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 extensionalFaulting 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..

Did rapid extension in central Nevada prevent basaltic magma from reaching the surface?

CRB

Or was it already “burned out”

Yellowstone “hotspot” as a consequence of regional tectonics?

extension

17 Ma - eruption of CRB and related rocks coincides with rapid extension in BR to the south.

Further extension, thinning, and lower crustal flow in the BR, but not to the north

…But we still have to explain the CRBs…

? ? ?

From Bennett et al., 2003

Today the Basin and Range is deforming on its edges

• 40º Rotation from 20 to 15 Ma (horiz. velocity = 3mm/y)• Initial Geothermal Gradient = 30ºC/km• Block Dimensions = 10 x 15 km

Modeled Apatite ages in a Tilted Normal Fault Block

Thermal modeling by Greg Houseman, Phil GansAnd Gordon Lister (unpublished)