- 1 - Stratigraphy and Structure of North and Middle Fork Lytle Creek Canyon and Surrounding Areas - Implications for Fault Displacement and Crosscutting Relationships By: Shaun Hunter Wilkins Department of Geological Sciences California Polytechnic University, Pomona September 23, 2004 A Senior Thesis submitted in partial fulfillment of the requirements for the Bachelor of Science Degree in Geology- 2 - Table of Contents • Introduction • Data/Observations – 10-34 o Middle Fork Lytle Creek Study Area – 11-29 § General Description § Lithology – 12-20 • Rock Sequence south of Icehouse Canyon and Middle Fork Lytle Creek Faults • Rock Sequence north of Icehouse Canyon and Middle Fork Lytle Creek Faults § Structure – 21-29 • Intrusive Relations • Metamorphic Foliations on south side of Icehouse Canyon and Middle Fork Lytle Creek Faults • Metamorphic Foliations on north side of Icehouse Canyon and Middle Fork Lytle Creek Faults • Local Foliation in the Telegraph Peak granite along shear zones • Lineation in mylonite vs. Telegraph Peak granite • Faulting o Upper North Fork Lytle Creek Study Area – 30-34 § Lithology – 31-32 § Structure – 32-34 • Intrusive Relations • Metamorphic Foliations • Faulting • Discussions/Interpretations – 35-51 o Locations of Major Faults – 36-40 § Quaternary San Jacinto Fault Complex § Pliocene-Quaternary sinistral faults § Late Miocene San Gabriel Fault system § Paleocene Vincent Thrust o Maximum Displacement – 40-43 § San Jacinto Fault Complex § Pliocene-Quaternary sinistral faults § San Gabriel Fault system o Speculation and Interpretation of cross-cutting relations – 43-51 • Suggestions for Further Work – 51-52 • Conclusions – 52-53 • Acknowledgements – 53 • References Cited – 54-55- 3 - Figure 1. Taking a well-earned break on the north side of Lytle Creek's Middle Fork. Introduction The eastern portion of the San Gabriel Mountains has never been studied in sufficient detail. Many of the maps that currently exist were produced wholly or in part through the use of aerial photography. Difficult terrain and time restrictions have not allowed for the production of very many detailed maps within this portion of the San Gabriel Mountains. My study focuses on the Icehouse Canyon/Middle Fork Lytle Creek and the Upper North Fork Lytle Creek areas, where major left-lateral and right-lateral fault zones intersect. This report describes my efforts to delineate lithology and structure of distinctive fault blocks with the intent of ascertaining locations of major faults and the maximum amount of displacement along those faults. There have been various uncertainties attempting to reconstruct this region of the San Gabriel- 4 - Mountains and this study endeavors to resolve some of those uncertainties. I accomplished this by detailed field mapping using 1:12,000 base maps and basic mapping equipment during 2001 and 2002. The area that I mapped extended onto the Telegraph Peak, Cucamonga Peak, Mt. Baldy, and Mt. San Antonio quads. Primarily, I focused on the areas surrounding Icehouse creek, Middle Fork Lytle Creek, Upper North Fork Lytle Creek, and Telegraph Peak. This is an area that has been heavily faulted and undergone severe stresses on numerous occasions. The major fault zones that exist within my study area are the San Antonio Canyon Fault, the Icehouse Canyon Fault, the Scotland Fault, the Stoddard Canyon Fault, the Middle Fork Lytle Creek Fault, the Vincent thrust. Bounding the area to the north are the San Jacinto and Punchbowl Fault complexes. The Vincent thrust is the oldest known fault in the region as it is displaced a number of times over a wide area. The east-west trending Icehouse Canyon and North Fork Lytle Creek Faults are the next oldest as they are believed to be original traces of the San Gabriel Fault. These faults are offset by a series of northeast-southwest trending faults typified by the San Antonio Fault and the Stoddard Canyon Fault. Finally, the recent Scotland Fault, San Jacinto Fault, and Glen Helen Fault share traces with the San Andreas Fault, just to the north, and generally display the most current activity, including seismicity. Please see Figure 3 below for a map displaying the locations and abbreviations of the faults discussed in this thesis. This study area has been the subject of much debate and has been studied by numerous people over the last several decades. These include, but are not limited to: Dibblee (1971), Ehlig (1975, 1981), Morton (1981), May (1986), Bortugno and Spittler (1986), May and Walker (1989) and Jacobson (1990), Matti and Morton (1993), Morton and Matti (1993), Norum (senior thesis, 1996-97), Nourse (2002a), Nourse (unpublished mapping, 1990-2003). Their previous work has provided a foundation on which others, like me, can build and expand. One of the complications in building a detailed map of this region is the difficulty finding proper access to the area. The topography is extremely rocky and steep and there are very few roads invading large portions of the study area. Often times, a vehicle could only travel to within several miles of the area that is to be mapped and the additional distance had to be traveled over rugged mountain trails on foot. Water was sparse much of the time but heat and sun could always be found in abundance. The map area covers numerous valleys, ridges, and peaks that would all have to be accessed from different incursion points. Due to the distances required just- 5 - to reach the map areas many excursions would be combined to cover several days at a stretch to limit wasted travel time. The mapping was accomplished almost exclusively on foot using 1:12,000 1:6,000 topographic base maps, basic hand-held writing utensils and drawing instruments. The mapping portion of this project spanned the spring and summer seasons of 2001 and 2002 with occasional mapping continuing into 2004. Figure 2. Mt. San Antonio to the northwest as seen from the top of Cucamonga Peak.- 6 - Figure 3. Geologic Maps of the central and eastern San Gabriel Mountains. Reproduced from Figure 3A in Nourse (2002a). Major fault abbreviations are as follows: CF-Cucamonga fault, DF-Duarte fault, GHF-Glen Helen fault, ICF-Icehouse Canyon fault, LF-Lytle Creek Fault, MFLCF-Middle Fork Lytle Creek fault, NSGF-north branch San Gabriel fault, PMF-Pine Mountain fault,