Gravity`s potential to contribute to deep crustal investigations of the northern Sierran and north Walker Lane region - some developmental ideas

G Oppliger , July 8, 2003.  4 pages, 2 figures  - a draft of my notes and ideas 

The deep crustal framework as suggested by existing gravity data

The NWL and the Eastern Sierran gravity low

 The central NWL is associated with a well defined 25 km wide, 100 km long, NW trending 12 milligal gravity gradient in the10 km upward continued isostatic residual gravity grid.  The NWL gravity gradient forms the northeast flank of a 100  km wide, 15 milligal gravity low which is bounded by the Sierra Nevada block on the southwest and continues on to the south. At a more regional perspective the NWL gravity low forms the north-northwest terminal projection of an elongate 25 milligal isostatic anomaly that parallels the eastern Sierra Nevada range front extensional fault zone and encompasses the Long Valley Caldara, California 250 km to the south-southeast.

The regional dimensions of NWL and eastern Sierran extensional fault zone suggest a possible lithospheric scale kinematic relation may exist between these features  - a relation that is also suggested by our basin scale gravity and geology studies.   

The 15 milligal NWL gravity low is consistent with a 7 km thick layer with -0.05 g/cc density anomaly below 10 km depth, but based on its 100 k width, part of the anomaly could originate from as deep as 30 km. (my rough estimates, not from modeling) This density low could reflect lower density felsic crust related to the Sierran Batholith, but this explanation is contradicted by a lack of gravity lows over adjacent areas of similar dimensions that are also underlain by the same type of batholiths (e.g. Diamond Mtn.). Alternatively, the low could be due to local changes in the crustal density profiles associated with Tertiary extension, but mechanism for this density change has not been clearly identified. Possible mechanisms include low density crustal intrusions and local crustal downward displacement by thinning of the underpinning crustal layers through lateral flow.  Crustal downward displacement by any mechanism reduces the density contrast at all depths.

Circumstantial support for a extension related cause of the eastern Sierran gravity low comes from a direct alignment the axis of this broad gravity low axis with the deepest and generally most currently active basins along the eastern Sierran block boundary.  

Questions that can be addressed by an integrated geophysics and geologic investigation are:

1) Is the location of NWL fault system influenced by changes in the type and strength of crust and lithosphere?

2) Are the NWL and/or Eastern Sierra extensional fault zone expressed to any degree in the deep crust or mantle?  

3) How do the eastern Sierran and NWL fault systems interact to allow relative NW movement of the Sierran block?

4) Can the geometry and depth of the NWL region eastern Sierran gravity low be better constrained using new seismic velocity models? 

5) Do the collective geophysical and geological observations support a particular crustal scale process as the cause of the eastern Sierran gravity low and its termination against the NWL?  

 6) What do the data suggest about the nature of isostatic support for the Northern Sierran block, i.e. is it compensated by a crustal root or changes in mantle density?

7) Are the observed characteristics of the deep crust and mantle well explained by  accepted theories for the evolution of the Sierran block and NWL?

Note Gravity and geologic data alone cannot address these question.

The role of gravity in further deep crustal investigations:

As a crustal investigative method, unconstrained gravity analysis is not diagnostic of mass depths associated with long wavelengths anomalies, especially if the isostatic compensation mode is also in question. Consequently, we propose to use 2-1/2D and 3-D gravity modeling only to evaluate and constrain the density variations in crustal velocity models across the study area. Specifically we will evaluate end-member models that test whether isostatic compensation can be achieved with laterally constant crust and mantle densities or if crustal and  mantle density variations or lithospheric thinning are needed. Using these methods Fliedner (1996) demonstrated that mantle density variations or lithospheric thinning were needed to model the gravity and seismic structure across the southern Sierra Nevada and basin and range. Our analysis will test these lithospheric density assumptions in three dimensions.

As a key application of the gravity data we propose to examine the density models for evidence of crust and mantle strike-slip displacements.  Horizontal depth slices across the velocity density models will be used to assess the lower crust and mantle for evidence of strike-slip displacements across the NWL to corroborate displacements inferred from crustal scale gravity 50 km northwest of Honey Lake and geologic mapping.    

The existing regional gravity data is estimated to be adequate to test density assignments to velocity models with 10 km horizontal greater resolution below 2 km crustal depths.

References:

Fliedner, M.M., Ruppert S., et at., 1996, Three-dimensional crustal structure of the southern Sierra Nevada from seismic fan profiles and gravity modeling: Geology; v. 24; no. 4; p. 367–370.

Chase, C. G., and Wallace, T. C., 1988, Flexural isostasy and uplift of the Sierra Nevada of California: Journal of Geophysical Research, v. 93, p. 2795–2802.

Additional presentations of the isostatic gravity field  made for this assessment.