ARC-SCALE INCREMENTAL STRAIN AND FINITE STRAIN HISTORIES RECORDED BY MAGMATIC FABRICS AND HOST ROCKS, RESPECTIVELY IN THE MESOZOIC CENTRAL SIERRA NEVADA

Chamber-wide magmatic fabrics in plutons and finite strain in host rocks are compared to evaluate the characteristics of the incremental and finite strain histories in the Mesozoic central Sierra Nevada (CSN).

Magmatic foliation that is related to regional strain (S_R) in Mesozoic plutons in the CSN features: (1) alignment of euhedral to subhedral magmatic minerals and (2) NNW-NW-strike and steep-dip. The S_R can be treated as the X-Y plane of the incremental strain cumulated through a short period in the late stage of the emplacement with its orientation approximately perpendicular to the direction of incremental shortening. Magmatic lineation plunges steeply parallel to incremental extension. The S_R in the CSN rotates counterclockwise with time in the Cretaceous. The formation of the most WNW-striked S_R is contemporary with regional dextral transpression. Such rotation is interpreted to reflect the change of direction of incremental shortening possibly caused by a transition of subduction kinematics.

We have compiled finite strain data of host rocks based on ~650 samples from the CSN. Preliminary results show that (1) Rock type influences the strain shape and intensity; (2) Strain data in an W-E corridor across the Western Metamorphic Belt through the Mesozoic arc haven’t shown a notable difference in shape on a Hsu diagram although more are oblate; (3) Strain values from across the arc suggest that <10% volume loss occurred, that average arc-scale strain is -50% shortening and ~100% vertical extension; and (4) an age-intensity correlation is weak. We haven’t observed much higher strain in Triassic rocks than Cretaceous rocks. Relatively high periods of strain intensity occur at Late Triassic, Middle-Late Jurassic and Late Cretaceous. Host rock foliation associated with these strains strikes NNW-NW and lineation plunges steeply.

The observations suggest that: (1) total strain is caused by repeated subparallel contraction events although incremental strain may differ from time to time influenced by regional kinematics; (2) Since maximum strain is independent of rock age we are exploring options that either higher strain zones have been removed or episodes of strain are thermally regionally controlled, and (3) the measured crustal thickening requires a deep crustal root to form during the Mesozoic.