270 



MT. McGEE 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



MT. EMERSON 



ZONE OF GRANODIORITE 



NORTH PALISADE 



DEVIL'S CRAGS 



^•r. ■■ ' -..: 



5 BASIC COMPLEX 



TABLE MTN 



LOOKOUT 



"1 — ^T~— — ■ 



Gr 5 Gr 3 Di Gr 



COYOTE RIDGE 





Gr 



ROUND MTN. 



6r 



. ' ' T.i.'l ■■!■■■•' ■ jl - 



Gr 



5 3 Di 



Gr Gr 



SCALE IN MILES 



5 6r 5 



Fig. 17.10. Structure sections across southern Sierra Nevada Mountains. Upper section is north 

 of Connecting Link; second section is south of it. Lower two sections are across the northern part 

 of Coyote Salient, s, septum; Gr, granitic rocks; Di, Diorite and gabbro. After Mayo, 1941. 



basic intrusions now appear widely distributed as dark zones, strips, and 

 masses of various shapes and simulate the remnants of the metamorphic 

 rocks. According to Mayo, the bulk of the Sierra Nevada core ranges in 

 composition from granodiorite to granite, with quartz monzonite predomi- 

 nating. All members of the intrusive sequence are penetrated by dikes of 

 aplite and pegmatite. Some basic dikes were late comers also. 



The groups of intrusions are separated at many places by long, narrow 

 strips and by local broad areas of metamorphic rocks. The metamorphic 

 rocks are divisible into two groups: an older series of metasediments of 

 probable Paleozoic age, and a series of metavolcanics, part of which 

 Knopf has assigned to the Triassic. 



The metamorphic rocks are remnants of septa (Fig. 17.10) that divided 

 the intrusions to unknown depths. During the earliest recorded deforma- 



tion, the original bedding and other layered structures were thrown into 

 a series of closely appressed, nearly vertical-sided, isoclinal folds. Cleavage 

 developed approximately parallel to the axial planes of the folds, and 

 was followed by many small shears and a few upthrusts. Linear structures 

 that vary greatly in pitch were formed in the planes of cleavage, bedding, 

 shears, and upthrusts. These metamorphic rock structures are separated 

 from the intrusions by contacts that are usually very steep and sharp. 

 Gradational contacts are suggested in a few places. 



Within the granitic rocks, a parallel arrangement of inclusions, min- 

 erals, and schlieren reveals layered and linear traces of flow that are as- 

 signed to the plastic stage of intrusion. These structures of the plastic 

 stage, by grading into fractures, locally record the stage of transition. 

 The stage of transition was followed by the solid stage, when adjustments 

 resulted in fracturing. 



In the Huntington Lake area of the western slope of the central Sierra 

 Nevada, Hamilton (1956a) has concluded that the crystalline rocks there 

 consist of ten separate, sharply bounded, plutons which range in size 

 from one square mile, approximately, to several hundred square miles. 

 Only small parts of this area consist of metamorphic rocks. See Fig. 17.11. 



The granite rocks range from alaskite to quartz diorite, but it is impor- 

 tant to note that a rock type does not constitute a separate intrusion, but 

 rather, each intrusion may be made up of two or more rock types. Two of 

 the plutons range from quartz diorite through granodiorite to quartz 

 monzonite. In another, the content of ferromagnesian minerals varies from 

 2 to 19 percent. The abundance of ferromagnesian minerals and of the 

 dark inclusions are closely parallel. The inclusions are xenolithic, and 

 some and possibly most of the mafic minerals are products of assimilation 

 of metamorphic rocks. Most of the granite rocks are believed to have 

 formed from the upward intrusion of mobile materials. 



The western group, consisting of the Tamarack Creek, Huntington 

 Lake, Sheepthief Creek, and Kaiser Peak plutons, is considered the older, 

 and eastern group, consisting of the Mt. Givens, Red Lake, Rodeo 

 Meadow, Dinkey Lake, Coyote Creek, and Helms Creek plutons, the 

 younger. 



The relative aerial abundances of the rock types are as follows: 



