Piercing Points Research Articles

Restoration of Laramide right-lateral strike slip in northern New Mexico by using Proterozoic piercing points: Tectonic implications from the Proterozoic to the Cenozoic

Early Proterozoic rocks in northern New Mexico can be used to document 100-170 km of right-lateral slip along a network of north-striking Laramide (Late Cretaceous-early Tertiary) faults that formed the precursor of the Rio Grande rift. Piercing lines are defined by the intersection of a subborizontal 4 kbar isobaric surface with steeply dipping stratigraphic markers and regional structures. Restoration of slip provides new insight into the nature of Proterozoic orogenic and crustal province boundaries. The Yavapai crustal province (1.75-1.72 Ga juvenile volcanogenic rocks) is imbricated with overlying Hondo Group sedimentary cover in an originally east-trending ductile thrust belt that formed near the southern margin of the Yavapai province. A series of offset magnetic anomalies (Jemez lineament) is interpreted to mark the thrust-buried boundary between older (Yavapai province) and younger (Mazatzal province) crust. Our microplate tectonic model suggests that Laramide strike-slip movement (New Mexico) was kinematically linked with coeval extensional collapse (Arizona) and foreland shortening (Wyoming) during decoupling and northward movement of the rigid Colorado Plateau relative to the undeformed midcontinent. Crustal-scale strike-slip faults in New Mexico were reactivated during later Tertiary extension and continue to localize high heat flow in the Rio Grande rift and northward into Colorado.

Distinctive Triassic megaporphyritic monzogranite: Evidence for only 160 km offset along the San Andreas Fault, southern California

Distinctive megaporphyritic bodies of monzogranite to quartz monzonite that occur in the Mill Creek region of the San Bernardino Mountains and across the San Andreas fault on Liebre Mountain share identical modal and chemical compositions, intrusive ages, and petrogenesis and similar thermal histories. Both bodies are strontium‐rich and contain large potassium feldspar phenocrysts and hornblende. U‐Pb determinations on zircon from both bodies indicate Triassic intrusive ages (215 Ma) and derivation, in part, from homogeneous Precambrian continental crust. U‐Pb analyses on apatite and sphene and K‐Ar analyses on hornblende and biotite show that the bodies suffered a Late Cretaceous thermal event (70–75 Ma). The strong similarities between the two bodies suggest that they constitute segments of a formerly continuous pluton that has been offset about 160 km by movement on the San Andreas fault, about 80 km less than the generally accepted distance. Plutons having monzonitic compositions, reassembled with the megaporphyritic bodies are used as a piercing point, form a relatively coherent province within the varied suite of Mesozoic batholithic and prebatholithic rocks in southern California.

Rhyolite Clast Populations and Tectonics in the California Continental Borderland

ABSTRACT Exotic, purple-red rhyolitic clasts are abundant constituents in upper Paleocene and Eocene conglomerates in the California Continental Borderland. Four families of rhyolitic rocks have been recognized. Owl Creek clasts have low percentages of phenocrysts that never include quartz; they are primarily found in northern localities such as the Santa Ana Mountains. Poway rhyolites are packed with phenocrysts and invariably include quartz; they dominate conglomerates in the San Diego and northern Channel Islands areas. Las Palmas clasts have a distinctive micropore-microgranular groundmass; they abound in Baja California strata. Black Rhyodacites are much darker and more brittle and were derived from a closer source terrane of bedrock and Upper Cretaceous conglomerates; they are most abund nt in the earlier deposited conglomerates of the San Diego and northern Channel Islands areas. The striking similarities between San Diego and northern Channel Islands conglomerates demonstrate that they were once part of an integrated, east-west-oriented depositional system that acts as a piercing point to document major post-Eocene strike slip in the offshore California Continental Borderland. The sudden appearance of exotic rhyolitic clasts in mixed suites in upper Paleocene conglomerates, and the subsequent increases in their abundance and decreases in their variety testify to the growth of fewer, longer, and less overlapping rivers through the Eocene. The requisite integration of fluvial drainages followed the eastward-moving front of the Laramide Orogeny, caused by a shallowing ang e of Farallon Plate subduction.

Piercing points on a special arc

We describe a cellular wild arc A A in S 3 {S^3} that is the closure of the union of a null sequence of Alford arcs and show that if S S is an arbitrary 2 2 -sphere containing A A and tame modulo A A , then each point of A A is a piercing point of S S .

Piercing points of crumpled cubes

McMillan [20] proved that a free 2-sphere in S3 can be pierced by a tame arc at each of its points. Since each complementary domain of a free 2-sphere is an open 3-cell [22], it seems natural to attempt to prove some theorem analogous to McMillan's with this weaker hypothesis. We show that a crumpled cube C in S3 has at most one nonpiercing point if Int C is an open 3-cell. A crumpled cube C is the union of a 2-sphere and one of its complementary domains in S3, and a point p of Bd C is a piercing point of C if there is a homeomorphism h of C into S3 such that h(Bd C) can be pierced by a tame arc at h(p). It follows that a 2-sphere S in S3 has at most two points where S cannot be pierced by a tame arc if each component of S3 - S is an open 3-cell (Corollary 3). McMillan [21] obtained the same result independently using an entirely different approach. Our proof follows from Lemmas 1-3 and the main result of [8]. Other results follow from the methods used in the proofs of Lemmas 1-3. For example, if a Cantor set W lies in a 2-sphere S in S3 such that each component of S3 - S is an open 3-cell, then W is tame (Theorem 3). Thus each Cantor set on a free 2-sphere is tame. We also show that a continuum F is cellular if F lies in the boundary S of a cellular 3-cell and F does not separate S (Theorem 4).