T he Tectonically A ctive M argin o f th e W estern U ．S *A . parallel to a series o f transform faults in the fl oor of the by John C. C row ell G ulf betw een active sea-floor spreading centers. These continental transform s include the Elsinore, San Jacinto and T he m ountainous Pacific m argin of the U .S.A . has been the N ew port- Inglew ood faults of southern California, w hich are site of repeated interactions betw een the N orth A m erican subparallel to the San A ndreas fault itself and part of its lithosphere and plates that have com e in against the transform system . The northern end of the Baja Peninsula, continent from Pacific O cean regions to the w est. The therefore, extends well into southern C alifornia w here it is record of these com plex interactions is now overprinted by sliced by these faults betw een w hich the blocks are tilted, the presently active divergent plate m otions at the head of w arped, and folded . For exam ple, betw een the Elsinore the G ulf of C alifornia, along the San A ndreas transform fault and the Pacific O cean, the block has been tilted so belt, and in the C ascadian convergent zone. A ctive tectonic that its w estern m argin is depressed. A s a consequence, a realm s to the east include the extensional Basin and R ange prism of M esozoic and T ertiary strata is preserved along the p r o v in c e . coast, w hereas their equivalents to the east have been eroded aw ay so that only basem ent rocks are exposed at the s u r fa c e . In tr o d u c t io n The Pacific m argin of the U.S.A. today lies along the join T he G ulf o f C a lifo rn ia na rro w s w ith in th e U .S .A . to fo rm betw een the N orth A m erican lithospheric plate and the the Salton trough, a com plex graben now occupied by the Pacific and Juan de Fuca plates (Fig. I）． T he diverse S alton S ea w ith a su rfa c e ele va tion a t a bo u t 7 0 in b elo w sea topography and geology along a broad belt w ell over a level. The rugged and steep borders of the trough are h un d red k ilo m etres in la nd fro m th e c oa st is the result o f th e defined by faults w ith large com ponents o f dip slip. T he tectonic interaction betw een these tw o huge plates, sum m its of bordering ranges lie close to the m argins, overprinted upon the products of previous plate interactions reaching heights on the w est of 3293 m and on the east of (see H am ilton, this issue). In fact, along this continental 1163 m . In latest M iocene tim e before the Salton trough m argin, three m ajor styles of plate junction now occur: opened, the crust w as stretched and the surface sagged so convergent, divergent, and transform . that a m arine sea invaded the region. In the Early Pliocene, the Salton trough proper opened, accom panied by regional O n the southeast, plate divergence is responsible for the arching, w hich is interpreted as the result of therm al G ulf of C alifornia, w hich extends into the U .S.A . as the expansion of the crust and upper m antle above the northern Salton Trough. A s the G ulf w idens obliquely, associated c o n t in u a t io n o f t h e E a s t P a c ific R ise . w ith active spreading in its sea floor, the Peninsula of Baja C alifornia m oves relatively northw estw ard. F rom near the R ocks on either side of the Salton trough consist larg ely of b orde r w ith M ex ico on no rth w estw a rd , the Sa n A nd reas ancient granites, gneisses, and schists, now deeply eroded transform boundary takes over, a long w hich lateral and w ell exposed in the desert environm ent. Their structure displacem ents predom inate associated w ith crustal largely reflects geological events of the M esozoic and early splintering and rising and falling of crustal blocks. The San Tertiary, and only sparse shreds of rocks, now m etam or- A ndreas transform belt, w hich includes m ost of w estern phosed, form ed during M esozoic and early Tertiary displace- C alifornia, extends from the Salton Sea region to tha t of the m ents (see H am ilton, this issue). O n the eastern side of the M endocino triple junction (Fig. 1). troug h, m id-Tertiary crustal extension is especially docu- Plate convergence prevails farther north along coastal m ented by low -angle detachm ent faults betw een basem ent C alifornia, O regon, and W ashington. A long this border the and overlying blocks of T ertiary volcanic rocks and non- South G orda and Juan de Fuca pla tes on the w est dip to m arine strata. Late C enzoic faults and folds, w hich form ed depth beneath the C oast R anges, and belts of m ountains and during the opening of the G ulf of California and the Salton valleys and the row of volcanoes surm ounting the C ascade Trough, overprint these M iocene and older structures. R ange are the result. W ithin the Salton trough itself, Pleistocene and Recent The active coastal m argin m erges eastw ard w ith several strata consist largely of alluvial and playa deposits and are other tectonic realm s. T hese include the still-deform ing folded and faulted, attesting to ongoing deform a tion. C olum bia Plateau, consisting m ainly of thick flow s of Sedim ent carried into the grow ing trough from the ancestral M iocene basalt, and the Basin and Range province that C olorado R ivers (Fig. 1,) form s the bulk of the fill, w hich reaches all the w ay into northern M exico and that is beneath the Salton Sea is over 6 km thick, as show n by deep characterized by extensional tectonics. Previous plate seism ic profiling. A ctive spreading centers are inferred to tectonics regim es have left their im print upon the geology lie at depth, because volcanoes have erupted here during the of the Pacific m argin and adjacent regions, how ever, and last few hundred years. G ravity studies indicate dense their effects shine through the recent tectonic overprintings rocks, probably upper m antle m aterial, at relatively high (see H am ilton, this issue). c ru s ta l le v e ls . D ivergence in the G ulf o f C alifornia: the Salton Trough The region is now hot, w ith therm al gradients so high that at places there are geotherm al pow er plants. Pleistocene B eginning about 5 M a ago, the Last Pacific R ise beca m e so sedim ents are undergoing m eta m orphism , and greenschist positioned benea th w estern M exico that the peninsula of facies m inerals, including sulfides, are recovered from Baja C alifornia, consisting of thick continental rocks, w as deeper w ells. G eodetic m easurem ents and first-m otions on broken from the North A m erican lithospheric plate. This frequently recurring earthquakes also docum ent the oblique block (Fig. 1) began to m ove obliquely northw estward w idening of the trough. Several recent earthquakes (1940, 2 7 8 E PISO D ES, Vol. 10, N o. 4, D ecem ber 1987 12 0 , C A N A D A 吮，～ 半一 ．一UNI花。妥矛履犷 一 一厂oults , a ctiv e and late Cenozoic, including S'N AS H I N G T 0 IN 飞tr a n s fo r m fa u lts ‘，兰. T. RANIER. 439 M Spreading center w ith J U A N mt 针 bordering transform faults D E F U C A fa u lts I0 胖 O 洛 大、一芦了 ︸A c tiv e s u b d u c tio n z o n e o 士 d 厂 z o n e PLA0%TE ﹃In a c tiv e o r fo s s il s u b d u c fi o n z o n e E。。N ＼ V o lc a n o M E N D O C IN O S O U T H 诊片 G O R D A B A S I IN P L A T E 40' TR A NSFOR11 A N D 厂牙R A N G E 1｛／。R 0 V I N C 〔 PIONZEZE}R- }rpNSFORM 认 4 0 o 闰 0 K ilom e tre S 40 0 依 今 、 护 ： ！ 吞 、 飞 、 M ile s 2 0 0 抓 。 毛 O 卫 ， \1 、 气飞协 W H IT N E Y 火 C 0 L 0 尺 A D 0 a 4 4 1 8 m P L A T E A U 、 ／ ！ 夕 夕 、0 ·、 尹 之 矛 &I " O 扮 了 1 M UR R A Y TR A N S F O R M 堆厂M%O JA V E 闷 了 、共 ＼ ｜ 户 劝 理鑫之 t 人 丫 13 0 .